Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Azimi Dijvejin, Zahra
Doctor of Philosophy - PhD
University of British Columbia
Metamaterials are artificially engineered materials with unique properties that are not found in nature. They have potential applications in various fields, and their development has opened up new avenues of research in physics, materials science, and engineering. Here, I designed and fabricated kirigami, a 2D mechanical metamaterial, and microwave sensing metamaterials for applications such as antifouling surfaces and sensors. The combination of the out-of-plane deformation of kirigami structures and the fast, on-demand detection of changes in microwave radiation properties of electrical metamaterials were utilized. This thesis presents two key contributions to the use of metamaterials. First, the development of a suspended kirigami surface, a flexible and deformable mechanical metamaterial that reduces the adhesion strength of various foulants including ice, mud, epoxy adhesive, and wax. Such surfaces can even be applied on complex curved geometries. Second, the investigation of electrical microwave sensors (electrical metamaterials) and their inclusion within coatings that exhibit low interfacial toughness with ice, for smart, hybrid de-icing applications. My work first revealed that both the ice adhesion strength and interfacial toughness of some materials are inversely dependent on temperature. Next, I utilized embedded heaters to modulate the surface temperature and tailor the interface properties of ice on the coating. Moreover, embedded microwave metamaterials were used to detect icing and de-icing accurately, which would help optimize the electrical energy expended during de-icing. Finally, my work explores the combination of electrical and mechanical metamaterials for strain or humidity sensing. Heterogeneous kirigami structures and an array of electrical metamaterials are designed and optimized for strain sensing. The amount of strain applied to the kirigami sheet can be correlated to the response of the electrical metamaterials as they rotate out of plane on the kirigami hinges. I extended this concept for humidity sensing by developing a kirigami structure fabricated from a humidity-absorbing material, nylon 6-6. In this case, the kirigami deforms out-of-plane in response to a humidity difference on the two sides of the kirigami sheet, demonstrating the potential of using metamaterials for humidity sensing applications. Overall, this thesis demonstrates the engineering potential of combined mechanical/electrical metamaterials.
Metamaterial Development for Mechanical Interfacial Tuning and Sensing Applications by Zahra Azimi Dijvejin B.A., The University of Tehran, 2015 M.S., Tarbiat Modares University, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) July 2023 © Zahra Azimi Dijvejin, 2023 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, the dissertation entitled: Metamaterial Development for Mechanical Interfacial Tuning and Sensing Applications submitted by Zahra Azimi Dijvejin in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering Examining Committee: Dr. Kevin Golovin, Faculty of Applied Sciences Supervisor Dr. Alexander Uhl, Faculty of Applied Sciences Co-supervisor Dr. Ian Foulds, Faculty of Applied Sciences Supervisory Committee Member Dr. Mattia Bacca, University of British Columbia – Vancouver Supervisory Committee Member Dr. Isaac Li, Department of Chemistry University Examiner Dr. Guojun Liu, Department of Chemistry, Queen's University External Examiner iii Abstract Metamaterials are artificially engineered materials with unique properties that are not found in nature. They have potential applications in various fields, and their development has opened up new avenues of research in physics, materials science, and engineering. Here, I designed and fabricated kirigami, a 2D mechanical metamaterial, and microwave sensing metamaterials for applications such as antifouling surfaces and sensors. The combination of the out-of-plane deformation of kirigami structures and the fast, on-demand detection of changes in microwave radiation properties of electrical metamaterials were utilized. This thesis presents two key contributions to the use of metamaterials. First, the development of a suspended kirigami surface, a flexible and deformable mechanical metamaterial that reduces the adhesion strength of various foulants including ice, mud, epoxy adhesive, and wax. Such surfaces can even be applied on complex curved geometries. Second, the investigation of electrical microwave sensors (electrical metamaterials) and their inclusion within coatings that exhibit low interfacial toughness with ice, for smart, hybrid de-icing applications. My work first revealed that both the ice adhesion strength and interfacial toughness of some materials are inversely dependent on temperature. Next, I utilized embedded heaters to modulate the surface temperature and tailor the interface properties of ice on the coating. Moreover, embedded microwave metamaterials were used to detect icing and de-icing accurately, which would help optimize the electrical energy expended during de-icing. Finally, my work explores the combination of electrical and mechanical metamaterials for strain or humidity sensing. Heterogeneous kirigami structures and an array of electrical metamaterials are designed and optimized for strain sensing. The amount of strain applied to the kirigami sheet can be correlated to the response of the electrical metamaterials as they rotate out of plane on the iv kirigami hinges. I extended this concept for humidity sensing by developing a kirigami structure fabricated from a humidity-absorbing material, nylon 6-6. In this case, the kirigami deforms out-of-plane in response to a humidity difference on the two sides of the kirigami sheet, demonstrating the potential of using metamaterials for humidity sensing applications. Overall, this thesis demonstrates the engineering potential of combined mechanical/electrical metamaterials. v Lay Summary In this work, I explore the use of mechanical and/or electrical metamaterials in innovative engineering applications. The mechanical properties of metamaterials have been utilized to design surfaces and structures that significantly reduce adhesion, for antifouling applications. The reduction in ice adhesion minimizes the maintenance required and improves the performance of surfaces in terms of the energy spent on electrical heating for de-icing. Additionally, electrical metamaterials have been harnessed to create advanced sensing systems, which are not only accurate but also non-contact and wireless. The hybrid metamaterial systems (electrical and mechanical) have shown broad implications in fields such as strain sensing, humidity control, and detection, contributing to more sustainable and efficient technologies. vi Preface This Ph.D. dissertation is originally conducted by Zahra Azimi Dijvejin under the supervision of Dr. Kevin Golovin and Dr. Alexander Uhl in the School of Engineering at The University of British Columbia and the Department of Mechanical & Industrial Engineering at the University of Toronto. Parts of this dissertation have produced a number of journal publications and conference presentations. The details of all publications extracted from this dissertation and the contributions of the author are listed as follows: Journal publications A version of Chapter 3 has been published in the following article. I was responsible for performing the experiments, data analysis and writing the article. Mr. Behrooz Khatir helped with adhesion measuring tests: • Azimi Dijvejin, Z., Khatir, B., & Golovin, K. (2022). Suspended kirigami surfaces for multifoulant adhesion reduction. ACS Applied Materials & Interfaces, 14(4), 6221-6229. A version of Chapter 4 has been published in the following article. I was responsible for performing the experiments, data analysis and writing the article. Mr. Mandeep Mandeep Chhajer Jain and Ryan Kozak helped with microwave sensing tests: • Azimi Dijvejin, Z., Jain, M. C., Kozak, R., Zarifi, M. H., & Golovin, K. (2022). Smart low interfacial toughness coatings for on-demand de-icing without melting. Nature Communications, 13(1), 5119. A version of Chapter 5 has been published in the following article. I was responsible for performing the experiments, data analysis and writing the article. Mr. Kasra Khorsand Kazemi helped with microwave sensing tests. Mr. Kamran Alasvand conducted numerical simulations for tensile tests: vii • Dijvejin, Z. A., Kazemi, K. K., Alasvand Zarasvand, K., Zarifi, M. H., & Golovin, K. (2020). Kirigami-enabled microwave resonator arrays for wireless, flexible, passive strain sensing. ACS Applied Materials & Interfaces, 12(39), 44256-44264. Conference papers and presentations I designed, performed, and analyzed all experiments and presented the following works: • Z.Azimi Dijvejin, K.Golovin. “Smart low interfacial toughness coating for large-scale de-icingwithout interface melting”, 46th annual meeting The Adhesion Society, 19-22 Feb 2023. • Z.Azimi Dijvejin, K.Golovin.” Icephobic kirigami metamaterial with the application for scalablestructures”, 46th annual meeting The Adhesion Society, 19-22 Feb 2023. • Z.Azimi Dijvejin, K.Golovin.” Implementation of mechanical metamaterials as anti-fouling surfaces”, 13th EUROPEAN Adhesion Conference. EUROADH, 11-14 Oct 2021 Additional Project Collaborations Also, I collaborated on a number of projects during my doctoral work, which resulted in the following journal publications: • Khatir, B., Azimi Dijvejin, Z., Serles, P., Filleter, T. and Golovin, K., 2023. Molecularly Capped Omniphobic Polydimethylsiloxane Brushes with Ultra‐Fast Contact Line Dynamics. Small, p.2301142. Contribution: Sample preparation. Designing, performing, and analyzing tribology’s experiments. • Lahiri, S.K., Azimi Dijvejin, Z. and Golovin, K., 2023. Polydimethylsiloxane-coated textiles with minimized microplastic pollution. Nature Sustainability, pp.1-9. viii Contribution: Sample preparation. Designing, performing, and analyzing tribology’s experiments. Writing the manuscript. • Golovin, K., Khatir, B., Recla, L., Dijvejin, Z.A. and Zhao, X., (2022). Polydimethylsiloxanebrushes and the search for extraterrestrial life. Surface Topography: Metrology and Properties, 10(2), p.024002. Contribution: Sample preparation. Designing, performing, and analyzing tribology’s experiments. Writing the manuscript. • Mohseni, M., Dijvejin, Z.A. and Golovin, K., (2021). Designing scalable elastomeric anti-foulingcoatings: Shear strain dissipation via interfacial cavitation. Journal of Colloid and Interface Science, 589, pp.556-567. Contribution: Sample preparation and adhesion testing. • Ghaffarkhah, A., Kamkar, M., Dijvejin, Z.A., Riazi, H., Ghaderi, S., Golovin, K., Soroush, M. andArjmand, M., (2022). High-resolution extrusion printing of Ti3C2-based inks for wearable humanmotion monitoring and electromagnetic interference shielding. Carbon, 191, pp.277-289. Contribution: Microscopic imaging and analyzing the data. • Ghaffarkhah, A., Kamkar, M., Riazi, H., Hosseini, E., Dijvejin, Z.A., Golovin, K., Soroush, M.and Arjmand, M., (2021). Scalable manufacturing of flexible and highly conductive Ti 3 C 2 Tx/PEDOT: PSS thin films for electromagnetic interference shielding. New Journal ofChemistry, 45(44), pp.20787-20799. Contribution: Microscopic imaging and analyzing the data. ix Table of Contents Abstract .......................................................................................................................................... iii Lay Summary ...................................................................................................................................v Preface............................................................................................................................................ vi Table of Contents ........................................................................................................................... ix List of Tables ............................................................................................................................... xiv List of Figures ................................................................................................................................xv List of Symbols .......................................................................................................................... xxix List of Abbreviations ............................................................................................................... xxxiii Acknowledgements ....................................................................................................................xxxv Dedication ............................................................................................................................... xxxvii Chapter 1: Background and thesis organization ..............................................................................1 1.1 Introduction ..................................................................................................................... 1 1.2 Motivation and objectives ............................................................................................... 2 1.3 Thesis organization and novelties ................................................................................... 3 Chapter 2: Background and literature review ..................................................................................9 2.1 Metamaterials .................................................................................................................. 9 2.1.1 Electrical Metamaterials ............................................................................................. 9 2.1.1.1 Split ring resonators .......................................................................................... 12 2.1.2 Mechanical metamaterials ........................................................................................ 14 2.1.2.1 Kirigami: History, Techniques, and Applications ............................................ 16 2.1.2.2 Kirigami structural design and mechanical properties...................................... 18 x 2.2 Principles of wettability and adhesion .......................................................................... 23 2.2.1 Wettability................................................................................................................. 23 2.2.2 Adhesion ................................................................................................................... 26 2.3 Antifouling surfaces ...................................................................................................... 29 2.3.1 Chemical functionalization ....................................................................................... 29 2.3.2 Tuning mechanical properties ................................................................................... 30 2.3.3 Textured surfaces ...................................................................................................... 32 2.3.4 Active methods ......................................................................................................... 33 2.3.5 Hybrid methods ......................................................................................................... 34 Chapter 3: Suspended kirigami surfaces for multi-foulant adhesion reduction .............................36 3.1 Introduction ................................................................................................................... 37 3.2 Experimental Section .................................................................................................... 39 3.2.1 Materials ................................................................................................................... 39 3.2.2 Methods..................................................................................................................... 40 3.2.2.1 Fabrication of kirigami sheets ........................................................................... 40 3.2.2.2 Preparing the superhydrophobic side of SKINS ............................................... 42 3.2.2.3 Preparing bonded foulants on the surface of the SKINS .................................. 43 3.2.2.4 Preparing the silicon coated CA sheet .............................................................. 44 3.2.2.5 Adhesion strength measurements for planar SKINS ........................................ 44 3.2.2.6 Adhesion strength measurements for curved SKINS ....................................... 45 3.2.2.7 Mechanical characterizations of SKINS ........................................................... 46 3.2.2.8 Monitoring the SKINS deformation using photoelasticity ............................... 46 3.2.2.9 Contact angle measurements............................................................................. 47 xi 3.3 Results and Discussion ................................................................................................. 49 3.4 Conclusions ................................................................................................................... 63 Chapter 4: Smart low interfacial toughness coatings for on-demand de-icing without melting ...65 4.1 Introduction ................................................................................................................... 66 4.2 Experimental Section .................................................................................................... 67 4.2.1 Materials ................................................................................................................... 67 4.2.2 Methods..................................................................................................................... 68 4.2.2.1 Characterization of LIT coating's mechanical properties ................................. 68 4.2.2.2 Preparing the LIT coating ................................................................................. 68 4.2.2.3 Ice adhesion measurement without heater ........................................................ 69 4.2.2.4 Sensor and heater design and fabrication.......................................................... 70 4.2.2.5 Smart, active de-icing using the LIT coating .................................................... 72 4.2.2.6 Durability and Scalability Analyses.................................................................. 73 4.3 Results and discussion .................................................................................................. 77 4.3.1 Effect of temperature on mechanical properties of coating, ice and their interface . 77 4.3.2 Optimization of the operating voltage for joule heating ........................................... 79 4.3.3 Using heaters to control LIT properties .................................................................... 83 4.3.4 Durability and scalability .......................................................................................... 88 4.3.5 Smart active de-icing using a microwave resonator sensor ...................................... 91 4.4 Conclusion .................................................................................................................... 96 Chapter 5: Kirigami enabled microwave resonator arrays for wireless, flexible, passive strain sensing............................................................................................................................................97 5.1 Introduction ................................................................................................................... 97 xii 5.2 Experimental Section .................................................................................................... 99 5.2.1 Materials and Methods .............................................................................................. 99 5.3 Simulations ................................................................................................................. 103 5.4 Results and Discussion ............................................................................................... 106 5.4.1 Derivation of heterogenous pattern stiffness .......................................................... 108 5.5 Conclusions ................................................................................................................. 117 Chapter 6: Flexible self-folding kirigami structures triggered by humidity and its application in sensing..........................................................................................................................................119 6.1 Introduction ................................................................................................................. 119 6.2 Experimental section ................................................................................................... 121 6.2.1 Material ................................................................................................................... 121 6.2.2 Methods................................................................................................................... 121 6.2.2.1 Preparing the silicone coated nylon 6-6 sheet ................................................. 121 6.2.2.2 Fabrication of kirigami sheets ......................................................................... 121 6.2.2.3 Mechanical characteristics of fabricated kirigami .......................................... 124 6.2.2.4 Thermal evolution of silicone-coated kirigami sheet ...................................... 124 6.2.2.5 Effect of humidity on kirigami deformation ................................................... 125 6.2.2.6 The microwave humidity sensor fabrication................................................... 126 6.3 Results and Discussion ............................................................................................... 127 6.3.1 The effect of kirigami unit geometry on deformation by humidity gradient .......... 127 6.3.2 The effect of geometric design of kirigami structure on mechanical properties .... 130 6.3.3 Thermal and mechanical properties of PDMS elastomer coated kirigami structure 132 xiii 6.3.4 Self-deforming kirigami structures ......................................................................... 136 6.3.4.1 Flexible thermal and humidity regulating substrate........................................ 136 6.3.4.2 The microwave humidity sensing using self-deforming kirigami .................. 140 6.4 Summary ..................................................................................................................... 141 Chapter 7: Conclusion and future work .......................................................................................143 7.1 Summary of main findings.......................................................................................... 143 7.2 Future work recommendations ................................................................................... 148 Bibliography ................................................................................................................................152 xiv List of Tables Table 1.1 The list of developed metamaterials in the chapters of this thesis. ................................. 7 Table 3.1 Geometric characteristics of kirigami patterns on various substrates. .......................... 42 Table 3.2. The advancing and receding water contact angles on the various base materials. ...... 48 Table 5.1 The geometric characteristics of the three kirigami patterns (also see Figure 5.1b). For all three patterns, Gl = 1 cm and the sheet thickness was t ≈ 127 µm. ....................................... 101 Table 6.1 The geometric characteristic of kirigami patterns used in this chapter. ..................... 123 xv List of Figures Figure 1.1 The thesis organization. ................................................................................................. 8 Figure 2.1 The different types of developed metamaterials. a) Electrical metamaterials: (I) the first metamaterial was fabricated by interleaving resonators and copper wires16; (II) shows photographs of electric metamaterials characterized in the study, including individual unit cells, different electric responses, and an expanded view of the sample19. (III) A nonlinear tunable magnetic metamaterial created by a square lattice of nonlinear SRRs14. (IV) Carbon nanotube metamaterial for optical application24. (V) Reconfigurable terahertz metamaterials for external stimuli detection25. (VI) The Z metamaterial antenna on its small circular copper insert26. b) The mechanical metamaterials: (I) The self-folding 2D metamaterial responding to external stimuli used in photonic applications27. (II) The metamaterial structure of Octet elastomer lattice with no damage when a metal weight was dropped onto it from a height28. (III) The compressed behavior of an elastic material with negative Poisson's ratio29. (IV) A rubber metamaterial with elliptic holes compressed between plates, activating static solitons and inducing symmetry breaking with two distinct buckling states, and localized periodic solitons28. .................................................... 10 Figure 2.2 All possible properties of isotropic natural materials and metamaterials in domain of the electrical permittivity (ε) and the magnetic permeability (µ). ................................................ 11 Figure 2.3 Split ring resonator sensor. a) A double ring SRR with its geometric parameters40,45; b) The change in sensing samples’ relative permittivity (ε) results in shifts of the S parameters45. . 13 Figure 2.4 The K-G map. Correlation of the bulk modulus, K, and shear modulus, G, with the Poisson’s ratio, υ, and the material’s mechanical stability. The stable region with abnormal properties represents mechanical metamaterials.46 ....................................................................... 15 xvi Figure 2.5 Kirigami applications. (a) Kirigami structure has enough flexibility to make a soft actuator crawl52 (b) Dynamic kirigami structures for integrated solar tracking53. (c) Kirigami enhanced adhesion, where the kirigami adhesive film maintains adhesion on the elbow during cyclic bending33 (d) Wireless passive strain sensing with kirigami embedded microwave resonators54. (e) Electromechanically reconfigurable optical nano-kirigami, showing top-view SEM images of 2D and deformed 3D pinwheels under an applied current55.Scale bar is 1 µm. . 17 Figure 2.6 Kirigami unit consist of geometric parameters that have effect on mechanical deformation of the structure upon a tensile load. .......................................................................... 19 Figure 2.7 The four stages of deformation of a simple kirigami sheet under uniaxial tension. a) The elastic modulus of the structure changes with increasing strain54. b) An elastomer kirigami structure with linear cuts shows three elastic deformation stages49. ............................................. 20 Figure 2.8 The Deflection of kirigami structure due to uniaxial tension. a) The tensile loading condition on a kirigami unit; b) Two possible boundary conditions of end points for the rectangular beam (kirigami unit). The top image shows Pinned-pinned condition, with zero moment at the end points, and at bottom the fixed-fixed boundary condition, with moment at the end points of MA and MB; c) A linear kirigami pattern mimics springs in series and parallel during tensile strain. A homogenous kirigami pattern with 10 lateral rows and 5 transversal columns is depicted. ...................................................................................................................... 21 Figure 2.9 (a) Liquid droplet on flat, homogeneous, rigid surface and Young's contact angle definition. γlv , γsv and γsl are the surface tension of the liquid, the solid surface energy, and the solid-liquid interfacial tension. (b) Liquid droplet on rough surface in Wenzel wetting state where liquid completely penetrates surface asperities. ................................................................. 24 Figure 2.10 Different modes of fracture at the interface of two different materials. .................... 27 xvii Figure 2.11 The possible fracture propagation results for mode II fracture. During cohesive fracture, the crack kinks into either the foulant or solid substrate, rather than propagating along the interface. .................................................................................................................................. 28 Figure 3.1 Schematic of the five individual aspects of Suspended Kirigami Inverted Nil-adhesion Surfaces, or SKINS. ...................................................................................................................... 39 Figure 3.2 Schematic of SKINS fabrication. a) The kirigami patterns with its geometric parameters are cut into thin sheets using craft cutter. b) The underside of the kirigami sheet is coated with a commercial superhydrophobic spray paint. c) The underside-coated kirigami sheet is adhered to a substrate using double-sided tape to confine only the edges of the sheet. ........... 41 Figure 3.3 The effect of modifying the underside of SKINS with a superhydrophobic (SH) coating; a) Adhesion strength of ice or wax on CA SKINS with and without SH coating on the underside. b) Ice adhesion strength on SKINS fabricated from both hydrophilic and hydrophobic polymers, with and without the SH coating. ................................................................................. 43 Figure 3.4. a) Shear adhesion strength measurement custom-built setup. b) Semi-cylindrical adhesion tests: a container holds the semi-cylindrical surface, and two gaskets prevent the water from leaking. The optimized SKINS is wrapped around the acrylic semi-cylinder, and then the container is filled with water and placed inside a freezer for 24 hours. A circular probe was used to uniformly end-load the acrylic semi-cylinder. .......................................................................... 45 Figure 3.5 Photoelasticity visualization of pressure-sensitive tape adhesion to SKINS when varying the dimensions of the foulant (Methods). ........................................................................ 47 Figure 3.6 The ice adhesion strength of CA when fully constrained, with a kirigami motif, suspended, and as SKINS. ............................................................................................................ 49 xviii Figure 3.7 The adhesion strength of carnauba wax on cellulose acetate when fully constrained, with a kirigami motif, suspended (no kirigami motif), and as SKINS. ........................................ 50 Figure 3.8 The visual observation of pressure-sensitive tape detachment from smooth, suspended CA and five different CA SKINS. From II to VI the stiffness of the sheet decreases, as does the wrinkle wavelength, as the strain at delamination increases. ....................................................... 51 Figure 3.9 a) Logarithmic relationship between kirigami sheet (film) elastic modulus and the wavelength of wrinkling for CA SKINS during the detachment of pressure-sensitive adhesive tape. b) Schematic of the wrinkling instability observed for SKINS. .......................................... 52 Figure 3.10 Kirigami geometry alters the adhesion strength of hard foulants. a) Ice adhesion strength on CA SKINS versus kirigami index, κ, at constant cut size C = 1.5 cm. The minimum ice adhesion strength was always κ ≈ 1. b) Ice adhesion strength of UHMW-PE SKINS utilizing three different cut sizes. ................................................................................................................ 53 Figure 3.11 Hard foulant adhesion strength on SKINS as a function of the kirigami index, which exhibited a minimum around κ ≈ 1: a) ice adhesion strength on UHMW-PE SKINS, and b) pressure-sensitive tape adhesion strength on CA SKINS. ............................................................ 54 Figure 3.12 Mechanical response of CA SKINS with changing kirigami pattern geometry. a) The kirigami patterns showed two distinguishable elastic moduli during the tensile test. These two linear regions indicated in-plane and out-of-plane deformation of the kirigami units upon increasing strain. b) The CA kirigami patterns with higher elastic moduli absorbed more strain energy during deformation, for both in-plane and out-of-plane deformation stages. Strain energy was calculated by numerically integrating the stress-strain data in (a) using MATLAB. ............ 54 xix Figure 3.13 The sheet thickness of SKINS was another parameter that affected the adhesion strength of hard foulants. Increasing the thickness of the optimized UHMW-PE SKINS decreased the elasticity of the kirigami, resulting in an increased ice adhesion strength. ............ 55 Figure 3.14 a) Ice adhesion strength versus wettability for both suspended smooth sheets and SKINS. b) Ice adhesion strength of six different materials, comparing the fully constrained, smooth material, the fully constrained material with the kirigami pattern, the smooth and suspended material, and SKINS. .................................................................................................. 56 Figure 3.15 a) Adhesion strength of six different hard foulants: paraffin wax, pressure-sensitive adhesive tape, carnauba wax, ice, dried mud, and epoxy. Whereas the epoxy adhesive was adhered on UHMW-PE SKINS, and all other foulants were evaluated using CA SKINS. b) Optical top-down image of the six hard foulants adhered to SKINS. The interfacial area was 1 cm2. ............................................................................................................................................... 57 Figure 3.16 The durability of optimized CA SKINS. The mechanical properties of CA SKINS before and after 30 cycles of a) ice and b) carnauba wax adhesion tests. The insets show stretched SKINS before and after the 30 adhesion cycles. The adhesion strength of c) ice and d) carnauba wax on the optimized CA SKINS over 30 cycles of adhesion. Each cycle for adhesion measurement includes at least 4 repeats. ...................................................................................... 59 Figure 3.17 Scalability of SKINS on flat and curved surfaces. a) The force required to detach increasing lengths of adhered ice from both suspended CA and CA SKINS. b) The impact of ice width and length on the observed ice adhesion strength of the optimized CA SKINS. ............... 60 Figure 3.18 Hard foulants’ adhesion strength on SKINS as a function of foulant size. The increase in foulant width resulted in a larger decrease of the measured adhesion strength. a) SKINS ice adhesion strength decreased with an increase in the length and width of the foulant on xx the optimized UHMW-PE SKINS, and b) similarly for the pressure-sensitive adhesive foulant adhered to the optimized CA SKINS. ........................................................................................... 61 Figure 3.19 a) Observed ice adhesion strength of SKINS wrapped around a semi-cylindrical acrylic substrate (Methods). b) Ice adhered to a semi-cylinder coated with suspended CA (left) and CA SKINS (right) before and after flexing, highlighting the ease at which ice is removed from SKINS. ................................................................................................................................. 62 Figure 3.20 CA SKINS protecting a 9-cm length of an overhead transmission cable, with an extremely low observed ice adhesion strength of 4.5 kPa. ........................................................... 63 Figure 4.1 Optical and topography images using a laser microscope. a UHMW-PE sheet (Root-mean-squared roughness, Sq = 1.53 µm); b Mini PCB heater covered by the UHMW-PE material (Sq = 1.51 µm); c The microwave sensor covered by the UHMW-PE coating (Sq = 1.50 µm). The scale bar for all images is 5 mm.................................................................................................... 69 Figure 4.2 Dimension and spacing of heaters and sensor. a) SRR sensor and resistive heaters modeled in HFSS. b) Optical image of fabricated SRR sensor and resistive heaters. .................. 72 Figure 4.3 The experimental setup consisted of a VNA, power supply, cold Peltier stage, temperature data logger, PCB heaters, and the split ring resonator sensor................................... 73 Figure 4.4 Optical and topography images after durability tests. a) UHMW-PE sheet before exposure (Sq = 1.53 ± 0.05 µm) and after b) Chemical contamination by acetone (Sq = 1.92 ± 0.08 µm); c) mechanical abrasion (Sq = 3.4 ± 0.3 µm); d) Cyclic icing/de-icing (Sq = 1.4 ± 0.2 µm); and e) outdoor exposure (Sq = 1.5 ± 0.3 µm). The parameter of Sq is the Root Mean Square Roughness. (Top images are optical and bottom images are topography heightmaps). All scale bars are 300 µm. ....................................................................................... 75 xxi Figure 4.5 De-icing force measurements in walk-in freezer at -20°C. a) The ice adhesion setup involving the UHMW-PE coated substrate, heaters, ice, and mechanical detachment instrumentation; b) Top view of LIT-coated substrate decorated with four heaters with dimensions of 2.54 cm ⨉ 8 cm. .................................................................................................... 76 Figure 4.6 The effect of temperature on the mechanical properties of the coating and its interface with ice. a) Storage and loss moduli of UHMW-PE between 25 °C and -60 °C. b) The toughness of the ice-UHMW-PE interface between -40 °C and -5 °C. c) The ice adhesion strength of UHMW-PE between -40 °C and -5 °C. Values for strength and toughness were taken from the linear and plateau regions of ice length versus removal force plots (Methods) as described previously83. Dashed lines are selected based on best poly-fit on average value of error bars. ... 78 Figure 4.7 Effect of temperature on the interfacial properties between ice and the UHMW-PE material. a) Schematic of ice adhesion measurement on the LIT-coated Al sheet. b-f) The shear force per width required to remove the ice at different temperatures as a function of ice length: b, -5 °C, c, -10 °C, d, -20 °C, e, -30 °C, f, -40 °C. ............................................................................ 79 Figure 4.8 Effect of melting ice at LIT-coated heater on de-bonding force. a) The force per width necessary to detach the ice from the LIT-coated heater for four different removal scenarios: (1) with the heater turned off at T = -20 °C; (2) with the heater turned off at T = -5 °C; (3) with the heater turned on such that TH = -5 °C although the remainder of the surface was at T = -20 °C; and (4) allowing the heater to melt the ice directly above it, also at T = -20 °C. b) Optical images of the ice when the heater was allowed to melt the ice directly above it. The top image shows the ice before applying the shear force. The middle image shows the shattered ice after cohesive failure. The bottom image shows the extent of the ice shattering with the 3D-printed mold removed......................................................................................................................................... 80 xxii Figure 4.9 De-icing force measurements. Schematic setup for monitoring the ice and coating temperature during de-icing. At the target temperature of the LIT coating above the heater, TH, the force probe impacts the ice in shear in order to remove it from the surface. The parameters T and TH represent the overall system temperature and the temperature at the surface of the heater/LIT coating, respectively. .................................................................................................. 81 Figure 4.10 Operational power supply voltage optimization at T = -25 °C. a Ice temperature, Tice, as a function of mini PCB heater voltage, directly above the heater and 50 mm away along the length direction. The temperature of the interface above the heater (TH) and the temperature of the ice above the LIT material and heater (Tice), monitored for various lengths of ice: a and b 150 mm, c 60 mm, and d 20 mm. The target temperature of TH = Tice = -5 °C was reached after 30 seconds at 4.4 V for all three lengths of ice. ................................................................................. 82 Figure 4.11 Using the mini PCB heater to reduce the de-icing force, Fice, by locally increasing the temperature from T to TH, for different lengths of ice. a T = -20 °C increased to TH = -5 °C. b T = -30 °C increased to TH = -10 °C. c The effective reduction in interfacial toughness by locally increasing the temperature from T to TH using the mini PCB heaters. ......................................... 84 Figure 4.12 Active LIT de-icing geometry optimization. a) Infrared images demonstrating that locally TH was achieved regardless of the ice thickness. b) The square root dependence of the de-icing force with the thickness of the ice. c Measured de-icing time for various lengths of ice at T = -30 °C. .................................................................................................................................... 85 Figure 4.13The de-icing force of the LIT material as a function of the percentage of: a) the heater length over the length of ice (LH/Lice), and b) the length of ice above the heater over the total heater length (LIC/LH). ................................................................................................................... 86 xxiii Figure 4.14 Durability and scalability of hybrid de-icing system. a) Cyclic icing/de-icing tests for ice lengths of 60 – 150 mm (L > Lc). Fice values before and after the icing/de-icing cycles are statistically equivalent (p-value: 0.22). b) The de-icing force or interfacial toughness required to remove various lengths of ice after mechanical abrasion, chemical contamination, and outdoor exposure for three weeks. ............................................................................................................. 88 Figure 4.15 De-icing force versus ice length of LIT material after durability characterization. a) After abrasion with 800 grit sandpaper for 15 minutes; b) After contamination with acetone residue; c) After exposure to the outdoors for 3 weeks in Toronto, ON, Canada. The parameters Fice and TH represent the force to dislodge ice and the temperature at the surface of the heater/LIT coating, respectively. .................................................................................................................... 89 Figure 4.16 Scaled up de-icing. a) De-icing force for multi-heater hybrid de-icing system, up to a length of 920 mm. The inset shows the accreted and de-iced surfaces. b) Movie stills depicting the interfacial crack propagation and adhesive fracture underneath ice with a length of 500 mm and a width of 2 cm. All tests in a-b were conducted with 2.54 cm wide heaters locally raising the temperature -20 °C to -5 °C. ................................................................................................... 91 Figure 4.17 Effective relative permittivity of UHMW-PE measured using a N1501A dielectric probe kit. The εr, eff' of LIT coating changes by increasing resonant frequency. b) The S21 spectrum response of the sensor with and without the LIT material covering the SRR. .............. 94 Figure 4.18 a) Simulated and measured transmission coefficient (S21) for the bare sensor and in the presence of a material under test (MUT). b) The smart hybrid LIT de-icing system’s sensor response to water, adhered ice, detached ice, and the bare sensor. ............................................... 92 Figure 4.19 a) Resonant sensor’s response Recorded resonant amplitude and b) resonant frequency vs. time depicting the water being frozen to the LIT coating covering the sensor, xxiv heating the surface locally from T to TH, detaching the ice at TH with a shear force, and then thawing the system. ....................................................................................................................... 95 Figure 5.1 Design of kirigami patterns. a) Schematic of the three kirigami patterns. b) Geometric properties of heterogenous Pattern AB with two different kirigami motifs (Pattern A and B). See Table 1 for the exact dimensions of each parameter in Pattern AB. The scale bar is 1 cm. c) Schematic of tensile testing apparatus. (d) A torn kirigami sheet (Pattern B) at deformation stage IV. The stress on some hinges went beyond the yield point at a strain of ΔL/L = 16%. The sheet was viewed through cross polarizers such that photoelasticity aided in visualization of the deformation process. The lateral distance between cuts for Pattern B was Gl = 1 cm. .............. 100 Figure 5.2 Sensor array design. a) Schematic of the simulation setup for this work. b) Optical image of the kirigami structure mounted with the split ring resonators at a strain of ΔL/L = 20%. c) Optical image of the experimental setup. ............................................................................... 102 Figure 5.3 (a) Perfect boundary condition on the antenna. (b) Vacuum box with radiation boundary condition. (c) Ring resonators with finite conductivity boundary condition. ............. 103 Figure 5.4 Finite element simulations. Comparison of the angle of 3D deformation, θ, versus uniaxial strain obtained experimentally and numerically for kirigami a) Pattern A, b) Pattern B, and c) Pattern AB. The inset shows the out-of-plane deformation of the kirigami sheet subjected to a maximum strain of ΔL/L = 20%. d) Rotation of Pattern AB kirigami hinges for ΔL/L = 3.3%, 8%, and 14%, showing excellent agreement with the numerical simulations. ........................... 105 Figure 5.5 Mechanical properties of kirigami patterns. a) The four stages of deformation of a kirigami sheet under uniaxial tension. b) Mechanical response of the three kirigami patterns subjected to uniaxial tension. ...................................................................................................... 107 xxv Figure 5.6 a) Experimental and predicted stiffnesses of three kirigami patterns. b) Kirigami hinge rotation angle as a function of strain for the three kirigami patterns studied in this work. ........ 110 Figure 5.7 Alteration in the mechanical response of kirigami Pattern AB when increasing the number of lateral columns, Nl, from 15 to 23. ............................................................................ 111 Figure 5.8 a) Mechanical response of kirigami Pattern AB subjected to uniaxial tension for 100 cycles of stretching and releasing to ΔL/L ≈ 18%. b) Mechanical hysteresis of kirigami Pattern AB during the first stretch-release cycle. c) Hysteresis in the rotation angle of the resonators at the one hundredth stretch-release cycle. d) Comparison of the rotation angle between the first and one hundredth stretch cycle of Pattern AB. ......................................................................... 112 Figure 5.9 Sensor array design. a) HFSS results for the SRR array rotated to different angles. b) The S11 response and c) phase response of the antenna for different amounts of strain in the kirigami sheet, over a frequency span of 2.75 GHz to 2.85 GHz. d) The S11 response of the antenna without the kirigami sheet. ............................................................................................ 114 Figure 5.10 Kirigami microwave sensor results. a) The S11 response and b) the phase response of the antenna for a kirigami sheet distance of 27 cm, for various percentages of strain. Inserts have the same labels of axis as main graphs. ...................................................................................... 116 Figure 5.11 a) The S11 response of the antenna at ΔL/L = 0.6% at 5 time points over 120 seconds. b) The change in resonant frequency and resonant amplitude as a function of kirigami sheet strain for an antenna distance of 21.5 cm. .................................................................................. 117 Figure 6.1 Design of kirigami patterns a) The different kirigami units in this work, with similar projected cuts (the dashed lines), Gt and Gl. From top to bottom, pattern A, B, C and D. b) The schematic of custom-built humidity chamber. c) A comparison of nylon (swells) and cellulose xxvi acetate (does not swell) kirigami sheets when exposed to a humidity change. The effect of flow on the deformation of kirigami units was negligible. The scale bars are 5 mm. ........................ 125 Figure 6.2 The experimental setup consisting of a microwave waveguide probe and Kirigami structure for studying humidity-based structural variations. ...................................................... 127 Figure 6.2 The 3D deformation of various nylon kirigami structures. a) Image of deformed kirigami patterns at ∆RH ~30%. All images are side views of the deformed kirigami sheet except for Pattern A which is a top view of the undeformed sheet. The scale bar is 1 cm. b) The measured angle of deformation of various kirigami units for differences in relative humidity from 30% to 80%. ....................................................................................................................... 128 Figure 6.3 The curling of kirigami structure with triangular extended cuts (Pattern C). The scale bar is 1 cm. .................................................................................................................................. 129 Figure 6.4 3D deformation of nylon kirigami structures with different lengths of extended cuts. a) The measured angle of deformation of various kirigami units for relative humidity changes from 30% to 80%. b) Schematic of deformed kirigami structure due to the humidity gradient on two sides of it. As the extended cut is longer the opening after deformation of the unit is larger..................................................................................................................................................... 130 Figure 6.5 The effect of the kirigami unit’s geometry on its tensile response. a) Tensile tests results for the four different nylon kirigami structures, using an elongation rate of 50 µm/s. b) Visual observation of the torn kirigami sheets after tensile testing. The scale bar is 2 cm. ....... 131 Figure 6.6 a) The effect of extended cut size on mechanical response of patterns B, E and F. b) The effect of nylon sheet thickness on the stiffness and ultimate strain of kirigami pattern B. . 132 Figure 6.7 The effect of various thickness of PDMS elastomer (50 µm to 450 µm) on the surface temperature of nylon with thickness of 50 µm, over 30 seconds of observation. a) Bare surface of xxvii the hotplate. The surface temperature of sample at b) 0 sec, and c) 30 sec of contact. The scale bar is 1 cm. .................................................................................................................................. 134 Figure 6.8 The effect of the PDMS coating on the mechanical properties of the nylon kirigami Pattern B. a) The comparison of mechanical response during tensile test of kirigami pattern B fabricated using coated and non-coated nylon sheets with thickness of 50 µm. b) The out-of-plane deformation of the coated nylon kirigami Pattern B sheets for different thicknesses of the PDMS layer. ................................................................................................................................ 135 Figure 6.9 The effect of non-conductive particles dispersed in the PDMS elastomer layer (450 µm) on the surface temperature of nylon with thickness of 50 µm after 30 sec of contact. The coating on nylon is PDMS elastomer a) without particles b) with 5wt% carbonyl iron particles and c) 5 wt% fumed silica particles. The scale bar is 1 cm. ....................................................... 137 Figure 6.10 The FTIR spectra for PDMS, PDMS elastomer layer with 5 % fumed silica and 5% carbonyl iron over the range of 650 cm-1 to 4000 cm-1............................................................... 138 Figure 6.11 a) The reflection and transmission of wavelengths from 200 nm to 1100 nm from different surfaces, including bare nylon, nylon coated with PDMS, nylon coated with PDMS/carbonyl iron, and nylon coated with PDMS/fumed silica. (a) Transmission and (b) Reflection. ................................................................................................................................... 139 Figure 6.12 The effect of humidity change and temperature on the self-folding behavior of the nylon coated kirigami. a) The kirigami units deform out-of-plane as relative humidity changes. b) The IR images show the drop in surface temperature as the kirigami units deformed. The temperature of the surface underneath was 40ºC. The scale bar is 5 mm. All images are taken in 45º from the substrate. ................................................................................................................ 140 xxviii Figure 6.14 The S11 response of the antenna. a) and b) display the change in resonant frequency and amplitude, respectively, in response to humidity changes within the closed chamber housing the kirigami unit. These changes are depicted for a microwave waveguide distance of 30 mm from the self-folding kirigami unit. ............................................................................................ 141 xxix List of Symbols Chapter 2 𝜏𝜏𝑐𝑐 Adhesion strength of foulant τfs Adhesion strength of the foulant-solid P Applied load on the kirigami unit 𝐼𝐼 Area moment of inertia of the cross-section K Bulk modulus 𝜃𝜃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶−𝐵𝐵𝐶𝐶𝐵𝐵𝐵𝐵𝐶𝐶𝐵𝐵 Cassie-Baxter contact angle τf Cohesive strength of the foulant τS Cohesive strength of the solid 𝐹𝐹𝑐𝑐 Critical force at the toughness-controlled region A Cross-sectional area of the dislodged foulant C Cut size 𝛿𝛿 Deflection of the kirigami unit along x-axis 𝐸𝐸∗ Effective elastic modulus of the substrate ε Electrical permittivity 𝑓𝑓𝐶𝐶 Fraction of material (i) BW Frequency bandwidth 𝛾𝛾𝐶𝐶𝑠𝑠 Interfacial energy between solid and foulant 𝛾𝛾𝐶𝐶𝑠𝑠 Interfacial energy between solid and liquid 𝛾𝛾𝐶𝐶𝑠𝑠 Interfacial energy between solid and vapor Γ Interfacial toughness Gl Lateral gap µ Magnetic permeability 𝑀𝑀(𝑥𝑥) Moment along the x-axis 𝑁𝑁𝑠𝑠 Number of lateral columns 𝑁𝑁𝐵𝐵 Numbers of transversal rows xxx FAdhesion Peak force applied to dislodge the solid foulant from a surface υ Poisson ratio 𝜃𝜃rec Receding contact angle r Roughness factor G Shear modulus 𝑘𝑘 Stiffness of the kirigami unit 𝛾𝛾𝑠𝑠𝑠𝑠 Surface energy of foulant 𝛾𝛾𝐶𝐶𝑠𝑠 Surface energy of solid 𝑟𝑟𝐶𝐶 Surface roughness of material (i) 𝛾𝛾𝑠𝑠𝑠𝑠 Surface tension of liquid t Thickness of sheet Gt Transversal gap θw Wenzel contact angle Wa Work of adhesion θY Young’s contact angle 𝜃𝜃𝑌𝑌,𝐶𝐶 Young’s contact angle of material (i) E Young’s modulus Chapter 3 τ Adhesion strength 𝜃𝜃𝐶𝐶𝑎𝑎𝑠𝑠 Advancing contact angle A Amplitude of wrinkles 𝛼𝛼 An empirical coefficient εd Delamination strain Es Elastic modulus of elastomer substrate Ef Elastic modulus of the film κ Kirigami index Lice Length of ice xxxi 𝜆𝜆 Wavelength of wrinkles Wice Width of ice Chapter 4 Fc Critical force at the toughness-controlled region a Cross section area of printed copper Fice De-icing force tD De-icing time σ Electrical conductivity R Electrical resistance of the heater Q Heater’s electrical energy consumption H Hight Lice Ice length L Length LH Length of heater LIC Length of ice above the heater l Length of printed copper R Radius of curvature 𝑓𝑓𝐵𝐵𝐶𝐶𝐶𝐶 Resonant frequency Sq Root-mean-squared roughness U Supply voltage T Temperature TH Temperature at the surface of the heater/LIT coating Tice Temperature inside the ice adhered to the LIT coating Hice Thickness of ice W Width xxxii Chapter 5 ΔL Change in length θcr Critical angle 𝜙𝜙𝐴𝐴 Fraction of pattern A GF Gauge factor θ Out-of-plane rotation angle D The largest dimension of the antenna 𝜆𝜆′ Wavelength of the antenna Chapter 6 𝜔𝜔 Cut angle 𝜙𝜙𝐴𝐴 Fraction of pattern A RH Relative humidity D The largest dimension of the antenna 𝜆𝜆′ Wavelength of the antenna xxxiii List of Abbreviations BW Band width CA Cellulose Acetate FES Finite Element Simulation FTIR Fourier Transform Infrared HFSS High-Frequency Structure Simulator LIT Low Interfacial toughness ON Ontario PC Polycarbonate PDMS Polydimethylsiloxane PET Polyethylene terephthalate PTFE Polytetrafluoroethylene PVC Polyvinylchloride PCB Printed Circuit Board SEM Scanning Electron Microscopy SLIPS Slippery Liquid-infused Porous Surface SRR Split Ring Resonator SMA Sub-Miniature A xxxiv SHS Superhydrophobic Surfaces SKINS Suspended Kirigami Inverted Nil-adhesion Surfaces UHMW-PE Ultra-High Molecular Weight Polyethylene UV-Vis Ultraviolet–visible VNA Vector Network Analyzer xxxv Acknowledgements I would like to begin by expressing my profound gratitude to my advisor, Dr. Kevin Golovin. Throughout my research journey, he has imparted invaluable knowledge, guidance, and mentorship. Dr. Golovin has not only equipped me with essential research skills but has also inspired me to approach challenges with determination and creativity. His unwavering support and high expectations have been instrumental in shaping the researcher I am today. As I embark on the next phase of my career, I will always cherish the lessons and confidence he has instilled in me. I would like to express my profound gratitude to my committee members, my co-supervisor Dr. Uhl, Dr. Bacca, and Dr. Foulds. Your insightful questions and suggestions have consistently opened new avenues of exploration. The meetings we had throughout my research journey were invaluable in refining the direction of my work and in overcoming obstacles along the way. Thank you all for your unwavering support and encouragement. I am also deeply grateful for the opportunity to collaborate with the OMEGA Lab, under the supervision of Dr. Zarifi. This exceptional experience has allowed me to broaden my research horizons and expand my skill set, particularly in the field of microwave sensing. I am also deeply grateful for my lab colleagues, who have played a significant role in my growth as a researcher. Sadaf Shabanian, Anupama Pillai, and all the other amazing lab members have shared memorable moments with me both inside and outside the lab. I eagerly look forward to our paths crossing again in the future. Behrooz Khatir, your meticulous work and willingness to lend a hand have been crucial to my success. Sudip Lahiri and Mohammad Soltani, your camaraderie and support have brightened the long days in the lab. Finally, I must express my heartfelt gratitude to my family, whose unwavering support and encouragement have been invaluable throughout this journey. My parents have consistently xxxvi believed in me and supported my dreams, even when they seemed far-fetched. My siblings have been a constant source of motivation, as we push each other to achieve our goals. Above all, I am immensely grateful for my loving husband, Ahmadreza, who has been my rock during this entire journey. His unwavering support, understanding, and love have been a beacon of light during the most challenging times. Ahmadreza, your presence in my life has made this accomplishment possible, and I am forever thankful for your partnership. This thesis is dedicated to all of you who have made such a profound impact on my life, both personally and professionally. xxxvii Dedication To mom and Ahmadreza 1 Chapter 1: Background and thesis organization 1.1 Introduction Conventional materials derive their electrical and mechanical properties from their intermolecular forces and the properties of atoms. Conventional mechanical and electrical materials have been used in various applications, but they have limitations. The mechanical properties of conventional materials exhibit limited stretchability, positive compressibility, positive Poisson ratio, etc1. Conventional electrical materials have a positive permittivity, permeability and refractive index that result in restrictions in the design of devices, such as sensors2–4. The ability to tune and switch these properties of materials, something very rarely offered by nature, is in high demand in different fields. In 2005, the first electrical material with negative permeability was developed by Smith, which helped overcome certain electrical limitations and led to significant advancements in the research and development of metamaterials. ‘Meta’ is a Greek word which stands for “beyond”, “above”, or “superior”. Metamaterials provide properties beyond what is conventionally possible. A metamaterial is a material consisting of periodic or non-periodic structures which derives its unique properties from both their cellular architecture and chemical composition. Metamaterials demonstrate exceptional electrical and mechanical properties and can be utilized to produce artificial structures with high tunability and switching capabilities. Metamaterials can exhibit unnatural properties owing to their structural design, such as a negative Poisson ratio (mechanical) or a negative refractive index (electrical)1,5–7. Accordingly, they have shown a growing importance in a wide range of applications. Mechanical metamaterials are lightweight and compact, making them suitable for weight-sensitive applications in industries such 2 as aerospace or automotive. The enhanced mechanical properties of metamaterials make them ideal for applications that require high structural integrity and durability. They can also exhibit properties such as high stiffness, strength, and toughness, which can be superior to conventional materials, making them suitable for use in structures or components that need to withstand extreme conditions or loads5. Similarly, electrical metamaterials offer unique electromagnetic properties not found in nature. These materials have gained significant attention due to their potential to provide customizable permittivity, permeability, and refractive index, making them useful for a wide range of electromagnetic applications, particularly in radiated-wave devices, across the microwave to optical spectrum3,8,9. These properties can be utilized in designing antennas, sensors, and communication devices with improved performance and functionality. Another advantage of all metamaterials is their tunable properties, which allow for dynamic and adaptive behavior. Metamaterials can be engineered to have properties that can be adjusted or controlled in real-time or through external stimuli, allowing for versatile applications where material properties can be modified based on changing conditions or requirements. The versatility of metamaterials makes them suitable for a wide range of engineering applications in fields such as aerospace, automotive, civil, mechanical, electrical, and materials engineering, among others. 1.2 Motivation and objectives The motivation for this thesis was to arrive at an enhanced understanding of mechanical and electrical metamaterials in tuning interfacial properties. To demonstrate the comparative advantage of metamaterials over conventional materials, two engineering applications were selected. Hard foulant adhesion was targeted as the application for mechanical metamaterials, as it remains a problem in many industries today. The current strategies for lowering hard foulant adhesion 3 involve different types of coatings that can exhibit low surface energy, low elasticity, low interfacial toughness, and more10–16. On the electrical metamaterial side, microwave sensing was the selected application. Conventional electrical materials have electromagnetic properties that have limited flexibility in the design of sensors, antennas, and other devices17–20, and therefore provide a good comparison for the relative advantage of electrical metamaterial structures. The main objective of this research was to seek new ways to develop metamaterials that can guide the process of tuning interfacial properties, as well as optimize the design of the developed metamaterials (Table 1.1). On the engineering side, my research aimed to develop effective and energy-efficient hard foulant removal surfaces by utilizing metamaterials (mechanical or electrical). It also explored the combination of electrical and mechanical metamaterials for the real-time sensing of humidity and strain. 1.3 Thesis organization and novelties In the literature review in Chapter 2, the principles and concepts related to the objectives of this thesis will be covered. The work is organized into specific objectives (Figure 1.1), and each chapter contributes to the overall goal mentioned above through the following more specific objectives: Objective 1: Create 2D mechanical metamaterials, kirigami structures, and explore their use as a low adhesion coating with various hard foulants. Understand the mechanical properties of the fabricated planar metamaterial by measuring its flexibility and observing its out-of-plane deformations during the shearing of an adhered foulant (Chapter 3). 4 Hypothesis: Optimizing the out-of-plane deformation in the mechanical metamaterial will decrease the adhesion strength of hard foulants regardless of their type or the base material used for planar kirigami structure fabrication. In the research project, the mechanical properties of kirigami as a planar metamaterial were investigated. Kirigami patterns with varying geometries were designed to clarify their effect on foulant adhesion strength, and the impact of substrate wettability on adhesion strength was studied. A shear adhesion setup was used to measure the adhesion strength of various materials, including ice, wax, mud, epoxy adhesive, and pressure-sensitive tape. The mechanical metamaterials were optimized on curved surfaces to measure the shear adhesion strength of ice, and the scalability of the technology was also studied when measuring the shear adhesion strength of ice of various dimensions. Objective 2: Develop an electrical metamaterial, a non-contact planar microwave resonator sensor, to precisely detect the presence or absence of water or ice on a surface (Chapter 4). Investigate the application of the microwave resonator for ice/de-icing detection in a hybrid de-icing system that utilizes a low interfacial toughness (LIT) coating and heating elements. The ultimate goal of this project is to reduce energy expenditure. Hypothesis: The embedded sensor underneath the low interfacial toughness (LIT) coating can successfully monitor the status of the coating wirelessly throughout the icing/de-icing process. The developed smart de-icing system will enable an energy efficient mitigation technique of undesired icing. The LIT coating was made 'smart' with an embedded microwave resonator sensor, enabling on-demand de-icing. This chapter investigates the influence of various factors on ice adhesion strength 5 and interfacial toughness with the LIT material. The elastic modulus of both materials varied at different temperatures and their effect on the coating’s interfacial properties was studied. Miniature printed circuit board resistive heaters were used to investigate the effect of thermal loading, and the metamaterial sensor's response to the presence and absence of ice was designed and studied. Finally, the scalability of the smart LIT coating using sensors and heaters was studied for large lengths of ice. Objective 3: Investigate the application of hybrid electrical and mechanical metamaterials in the sensing field. Fabricate a metamaterial-based strain sensor using kirigami pattern with controlled mechanical properties and an embedded array of resonators on it (Chapter 5). Develop a synergistic use of microwave sensors and kirigami structures for strain sensing. Design and study the metamaterials properties for fast, real-time, low-cost, and wireless detection of strain. Hypothesis: The out-of-plane deformation of a heterogeneous kirigami pattern could provide different rotation angles with strain, enabling the sensor to detect the change in the kirigami units. Chapter 5 aims to understand the mechanical and electrical properties of smart kirigami patterns with linear cuts. The mechanical properties of both homogenous and heterogeneous kirigami patterns were studied, and the rotation angle of kirigami units for these patterns was evaluated and compared. Finite element simulations were used to investigate the mechanical and electrical properties of the designed kirigami patterns, and microwave split ring resonators (SRR) for wireless detection of changes in phase and frequency were developed. Furthermore, finite element simulations were carried out to study the electrical properties of the designed SRR in various rotation angles with respect to the antenna. A designed heterogenous kirigami sheet was used as a flexible platform for strain sensing via this passive, wireless microwave approach. 6 Objective 4: Investigate self-deforming kirigami which can enable passive thermal and humidity regulation and humidity detection using microwave resonator sensing (Chapter 6). Study the effect of humidity changes on stimuli-responsive 2D mechanical metamaterials and use them to design a self-folding kirigami structure with a programmable direction of deformation that responds to humidity changes, to provide passive thermal/humidity regulation. Moreover, utilize microwave sensing resonators for humidity detection. Hypothesis: A kirigami pattern fabricated using a humidity-responsive material will alter its 3D structure, which can enable humidity control and sensing. Chapter 6 investigates the mechanical properties of self-folding kirigami patterns. Various geometric parameters such as lateral gap, ratio of actual cut size to projected cuts, and base material thickness are studied to optimize the kirigami pattern. The role of humidity changes on the out-of-plane deformation of the kirigami patterns is studied, and kirigami structures are fabricated by coating one side of the kirigami sheet with a PDMS elastomer to explore the coating's effect on metamaterial deformation. Furthermore, the effect of this PDMS elastomer's thickness on surface temperature is investigated when there is a temperature gradient between the two sides of the fabricated kirigami structure. The sensing properties of microwave resonators embedded in self-folding kirigami is also studied with this humidity gradient present. 7 Chapter Mechanical Metamaterial Electrical Metamaterial Application 3 Kirigami structures with linear cut patterns None Tuning interfacial properties with various hard foulants and reducing adhesion strength. 4 None Microwave split ring resonator (SRR) embedded underneath ice-phobic coating (LIT coating) Fabricating a smart LIT coating for tuning interfacial properties of the coating with ice while sensor detects the icing/de-icing status at the interface. 5 Homogenous and heterogeneous kirigami with linear cut patterns An array of SRRs embedded on kirigami structure Fabricating a hybrid metamaterial for wireless strain sensing. 6 Flexible self-folding kirigami structures with various kirigami unit designs Microwave SRR embedded on kirigami unit Fabricating a hybrid metamaterial for wireless humidity detection and control. Table 1.1 The list of developed metamaterials in the chapters of this thesis. 8 Figure 1.1 The thesis organization. Hybrid Metamaterial Electrical Metamaterial Mechanical Metamaterial Metamaterial Development for Mechanical Interfacial Tuning and Sensing ApplicationsChapter 1: Background and thesis organizationChapter 2: Background and literature reviewChapter 3. Suspended kirigami surfaces for multi-foulant adhesion reductionChapter 4. Smart low interfacial toughness coatings for on-demand de-icing without meltingChapter 5. Kirigami enabled microwave resonator arrays for wireless, flexible, passive strain sensing Chapter 6. Self-folding kirigami structures triggered by humidity change and its application in sensingChapter 7: Conclusion and future worksMetamaterials Chapter 2: Background and literature review 2.1 Metamaterials Metamaterials, i.e. artificially engineered materials with unique properties that are not found in nature, have revolutionized the field of materials science and engineering21–26. These materials are designed at the macro- or micro-scale, with intricate structures that can manipulate electromagnetic waves1, acoustic waves27,28, and mechanical properties in unprecedented ways. Among the various types of metamaterials (Figure 2.1), electrical metamaterials and mechanical metamaterials have garnered significant attention due to their exceptional properties and wide-ranging applications. In this section, we will explore the principles, properties, and applications of electrical metamaterials and mechanical metamaterials, highlighting their significance in modern research and technology. 2.1.1 Electrical Metamaterials Electrical metamaterials are materials that exhibit unique electromagnetic properties due to their engineered structures at the subwavelength scale (Figure 2.2)1. These materials are designed to control and manipulate the behavior of electromagnetic waves, such as light or radio waves, in ways that are not achievable with conventional materials. The interaction between the incident electromagnetic wave and the meta-atoms or meta-molecules in the metamaterial structure leads to unique electromagnetic properties, such as negative refractive index29, superlensing2, and perfect absorption30, among others. The principles of electrical metamaterials are based on the concept of electromagnetic wave manipulation through engineered structures. The design and arrangement of meta-atoms or meta-molecules (unit cells) can be tailored to achieve the desired electromagnetic response19,29. 10 Figure 2.1 The different types of developed metamaterials. a) Electrical metamaterials: (I) the first metamaterial was fabricated by interleaving resonators and copper wires21; (II) shows photographs of electric metamaterials characterized in the study, including individual unit cells, different electric responses, and an expanded view of the sample24. (III) A nonlinear tunable magnetic metamaterial created by a square lattice of nonlinear SRRs19. (IV) Carbon nanotube metamaterial for optical application29. (V) Reconfigurable terahertz metamaterials for external stimuli detection9. (VI) The Z metamaterial antenna on its small circular copper insert31. b) The mechanical metamaterials: (I) The self-folding 2D metamaterial responding to external stimuli used in photonic applications32. (II) The metamaterial structure of Octet elastomer lattice with no damage when a metal weight was dropped onto it from a height33. (III) The compressed behavior of an elastic material with negative Poisson's ratio34. (IV) A rubber metamaterial with elliptic holes compressed between plates, activating static solitons and inducing symmetry breaking with two distinct buckling states, and localized periodic solitons33. Electrical Mechanical Resonators Copper wire Unit cell I II Unit cell III IV V VI IV III II I a b 11 Superlensing, which refers to the ability of a material to achieve sub-wavelength resolution imaging2, is achieved through metamaterial structures that exhibit negative refractive index29,35. Perfect absorption metamaterials, on the other hand, exhibit near-complete absorption of incident electromagnetic waves across a wide frequency range, and have applications in solar energy harvesting36,37, and sensing38. Figure 2.2 All possible properties of isotropic natural materials and metamaterials in domain of the electrical permittivity (𝜺𝜺) and the magnetic permeability (µ). Metamaterials can be divided into two categories: non-resonant and resonant. Non-resonant metamaterials, like those made of I-shaped inclusions, have a resonant frequency that is much higher and offer broad bandwidth and low loss. However, they have a small dynamic range of Most nature materials Left-Handed materials or metamaterial 𝜺𝜺 µ air 12 material parameters and they need complex calibration and de-embedding procedures for measurements1. Resonant metamaterials, on the other hand, offer a significant advantage in their ability to achieve large dynamic-range material parameters. Resonators are resonant-based capacitive devices that are sensitive to dielectric loadings and when the size of the resonator changes slightly, the resonant frequency shifts, resulting in a significant change of permittivity and permeability3,39–41. This makes resonant metamaterials ideal for sensing applications. By detecting changes in the resonant frequency, it is possible to sense changes in the surrounding environment, such as the presence of a specific solid or liquid. This sensing capability has applications in fields such as environmental monitoring, medical diagnostics, and security screening. Resonant metamaterials also have the potential to enable new sensing modalities, such as metamaterial-enhanced spectroscopy, which can provide enhanced sensitivity and specificity in chemical sensing42. 2.1.1.1 Split ring resonators Split ring resonators (SRR), a 2D metallic structure proposed by Pendry et al., are extensively studied in the literature as an essential type of electrical metamaterial43. This structure consists of single ring or two concentric rings with splits on opposing sides, separated by a gap44 (Figure 2.3a). Magnetic resonance is induced by these splits on the rings and the gap between the inner and outer rings, as depicted Figure 2.37,44. The frequency of each ring is 𝑓𝑓𝐵𝐵𝐶𝐶𝐶𝐶 = 𝑐𝑐2∗𝑠𝑠√𝜀𝜀𝑟𝑟, where c is the speed of light in vacuum (3 × 1011 mm/s), 𝑙𝑙 is the length of ring, and 𝜀𝜀𝐵𝐵 is the relative permittivity of the microstrip line. The transmission characteristics of periodic and disordered SRRs, their effective parameters, and magnetic resonances have been studied through numerical simulations and analytical models45–47. The presence of a gap in the SRR structure significantly reduces the 13 system's resonance frequency. This enables the creation of a resonant structure with dimensions that are only a small fraction of the corresponding wavelength. SRR sensors use transmission or reflection parameters, denoted as S11 and S21, respectively. The changes in the resonant frequency, amplitude, or quality factor (Q-factor) are used as a form of sensing. The Q-factor can be calculated as fres/BW, where BW is the frequency bandwidth. A higher quality factor results in more sensitive SRR sensors. For sensing, a microwave generator excites the SRR to resonate at its frequency. The microwave transducer/detecting circuit converts the microwave signal, which corresponds to the sample's target parameter, into an electrical signal. Any change in effective dielectric parameters, either due to the introduction of a solid/liquid object in the vicinity of the SRR, the deformation of the resonator, or both, results in shifts in the S11 or S21 parameters (Figure 2.3b). Figure 2.3 Split ring resonator sensor. a) A double ring SRR with its geometric parameters44,48; b) The change in sensing samples’ relative permittivity (ε) results in shifts of the S parameters48. d w l' t' a b 14 2.1.2 Mechanical metamaterials Mechanical metamaterials have emerged as a promising class of materials with unique mechanical properties that are, similar to electrical metamaterials, derived from their complex geometric structures rather than their chemical composition5,49,50. Although they are commonly referred to as metamaterials, their properties are primarily derived from their unique structure rather than the material itself. They offer unprecedented mechanical properties such as low elastic modulus (E)51, negative Poisson ratio (𝜐𝜐 < 0)52, negative bulk modulus (K < 0), and negative/zero shear modulus (𝐺𝐺 ≤ 0)5. The Poisson ratio can be defined as the relative amount that a given material contracts transversally when stretched axially. Moreover, bulk modulus determines the change in volume when the shape remains the same, and shear modulus refers to a change in shape at constant volume. The K-G or the Milton map shown in Figure 2.4 depicts the various types of mechanical metamaterials based on these elastic constants. The value of K >> G and 𝜐𝜐 = 0.5 represent the Penta-mode and Kagome-type structures, where the metamaterial shows no/less compressibility while it shears easilly5. The lower right indicates an energetically stable region of the metamaterial map with negative compressibility, where although G >0, K ≤ 0. The other category of metamaterials is defined as structures with negative Poisson ratio, meaning G > K. Both origami and kirigami structures are in this category. One of the fundamental principles of mechanical metamaterials is the concept of structural hierarchy, which involves designing materials with multiple length scales of architectural features. These features can be organized in a hierarchical manner, with smaller features nested within larger features, to create materials with unique mechanical properties. The hierarchical structure allows for precise control of the mechanical behavior of the material, which is done by tuning the geometry, topology, and connectivity of the architectural features. 15 Figure 2.4 The K-G map. Correlation of the bulk modulus, K, and shear modulus, G, with the Poisson’s ratio, 𝛖𝛖, and the material’s mechanical stability. The stable region with abnormal properties represents mechanical metamaterials.5 Another aspect of mechanical metamaterials is the concept of architectured materials, which involves designing structures with specific architectures or patterns to achieve the desired mechanical properties. Architectured materials can be thought of as "building unit" materials, where the properties of the material are determined by the arrangement of the building blocks. These building blocks can be arranged in different ways to achieve specific mechanical behaviors, such as negative stiffness, auxetic properties, and low/negative compressibility. The design of mechanical materials involves careful consideration of the geometry, topology, and connectivity of these units, which determine the overall mechanical response of the material. Stable Unstable 𝜐𝜐 = −1 Stable Negative compressibility 𝜐𝜐 = 2 𝜐𝜐 = 0.5 𝜐𝜐 = 0 𝐺𝐺 ≫ 𝐾𝐾 𝐺𝐺 ≪ 𝐾𝐾 −4𝐺𝐺3 < 𝐾𝐾 < 0 𝑮𝑮 𝒌𝒌𝟐𝟐 16 2.1.2.1 Kirigami: History, Techniques, and Applications Among the different types of mechanical metamaterials/meta-structures, kirigami-based mechanical meta-structures have emerged as a fascinating area of research, combining the principles of kirigami, a cutting technique that involves folding and/or cutting paper, with the design of mechanical materials. Kirigami is a traditional Japanese art form that involves folding and cutting paper to create intricate designs53. The word "kirigami" comes from the Japanese words "kiri," which means "to cut," and "gami," which means "paper." Kirigami has a long history, dating back to at least the 7th century in Japan, and has been used for various artistic, decorative, and functional purposes. Over the years, kirigami has evolved into a highly versatile and creative art form, with various techniques and applications beyond paper art, including in science, engineering, and technology. Kirigami structure fabrication involves folding and/or cutting paper in specific patterns to create three-dimensional structures that can transform and deform in response to external stimuli, such as mechanical forces, heat, or light. These patterns can be carefully designed to achieve desired mechanical behaviors, such as stretchability, compressibility, or shape-shifting, making the concept of kirigami an ideal approach for creating mechanical metamaterials. One of the key techniques in kirigami is the use of cuts through a thin sheet, allowing it to fold and deform in specific ways. These cuts can be designed in different patterns, such as straight lines, curves, or complex geometries. The arrangement, length, and orientation of the cuts can be precisely controlled to achieve desired mechanical behaviors, such as controlled bending, stretching, or twisting. Folds can also be used in kirigami design, such as creases or ridges in the paper, allowing it to bend and deform in specific directions. The combination of cuts and folds in kirigami allows 17 for the creation of complex and dynamic three-dimensional structures with unique mechanical properties. Figure 2.5 Kirigami applications. (a) Kirigami structure has enough flexibility to make a soft actuator crawl54 (b) Dynamic kirigami structures for integrated solar tracking55. (c) Kirigami enhanced adhesion, where the kirigami adhesive film maintains adhesion on the elbow during cyclic bending37 (d) Wireless passive strain sensing with kirigami embedded microwave resonators56. (e) Electromechanically reconfigurable optical nano-kirigami, showing top-view SEM images of 2D and deformed 3D pinwheels under an applied current57.Scale bar is 1 µm. Kirigami has found application in various fields beyond art, including in science, engineering, and technology. For example, kirigami has been used in the design of stretchable electronics, where the kirigami patterns allow the electronics to stretch and deform without breaking, making them suitable for wearable devices, sensors56, soft robotics54, and optoelectronic devices57 (Figure 2.5). Kirigami has also been applied in the design of deployable structures, where the material can be folded and compacted for transportation and then expanded and deployed for use, such as in space 18 exploration. Moreover, kirigami has been utilized in the design of morphable materials, where the structures can change their shape or functionality in response to external stimuli, such as in shape-shifting and self-folding structures58–60. These applications highlight the versatility and potential of kirigami in various fields and its ability to create materials and structures with unique mechanical properties. With the increasing demand for wearable devices and flexible electronics, the need for materials that can stretch and deform without losing their mechanical integrity has grown exponentially. Kirigami-based mechanical metamaterials offer a unique solution to this challenge by providing stretchability and flexibility through their designed patterns of cuts and folds. The deformation of a kirigami sheet has been shown to be reversible and programmable according to the target configuration, without compromising functionality22,32,50,61,62. For example, kirigami structure can be used to create stretchable sensors that can be integrated into wearable devices to monitor physiological signals, such as heart rate, temperature, or motion, for healthcare applications. The design of kirigami patterns with target mechanical properties requires a deep understanding of the underlying mechanical principles and the ability to predict and control their mechanical behavior. 2.1.2.2 Kirigami structural design and mechanical properties The planar structured kirigami is comprised of scalable subunits called kirigami units63. For example, a kirigami pattern with linear cuts in one direction have subunits that may be defined by the dimensions of the cut size (C), lateral gap (Gl), transversal gap (Gt), and thickness (t) (see Figure 2.6)56. The planar metamaterial has two stages of deformation in tension: in-plane and out-of-plane deformation. The 2D configuration of kirigami is able to achieve a complex structure due to the freedom of deformation added by introducing hierarchical cuts in thin sheets64. The 3D 19 deformed configuration resulting from uniaxial or biaxial tension is caused by the rotation of the kirigami units at the transversal gaps or hinges (Figure 2.6). Therefore, the symmetry and size of the geometric properties of a kirigami pattern control the resultant 3D configuration induced by tension. The critical load for rotation/buckling of the hinges depends on the thickness of the sheet, and the size and number of transversal and lateral gaps56,65. Research has shown that an increase in any of these parameters will change of critical buckling force of the hinges 66,67. Figure 2.6 Kirigami unit consist of geometric parameters that have effect on mechanical deformation of the structure upon a tensile load. In the design of kirigami patterns, two main configurations (hierarchal and motif) can be used64. Hierarchal patterns are defined as repeating patterns of kirigami units on a smaller scale within the unit. As the level of hierarchy increases, the kirigami structure’s flexibility will be improved. In fact, an increase in the level of hierarchy increases the number of rotational hinges which affect the 3D configuration after applying a tensile load. However, as the level of hierarchy of the structure increases, the out-of-plane rotation of units at the higher level of hierarchy is limited to smaller angles64. In contrast to hierarchy, there is a motif of cuts can be designed. Using different motifs will increase the freedom of the hinge’s rotation. For example, using two motifs in a higher level of hierarchy increases the out-of-plane rotation angle of the smallest units and the final strain of the structure64. For simplicity in the following section, I discuss the stiffness of a simple 20 kirigami-based metamaterial, with a linear cut pattern and only one level of hierarchy with one cut motif. Kirigami units can be considered as an array of arranged beams defined by the kirigami pattern. Kirigami structures are also known as stiffness tunable metamaterials that exhibit low elastic modulus, which nonlinearly changes with the strain (Figure 2.7)53,54,56,68,69. The Poisson ratio is not excluded from this nonlinear behavior of stiffness, and changes as the kirigami structure is stretched. The Poisson ratio of this periodically patterned metamaterial is negative in the out-of-plane direction and zero/positive in the plane of the tensile load52. The stiffness of this kirigami structure can be estimated by assuming the end condition of the kirigami units as either fixed-fixed or pinned-pinned70. Figure 2.7 The four stages of deformation of a simple kirigami sheet under uniaxial tension. a) The elastic modulus of the structure changes with increasing strain56. b) An elastomer kirigami structure with linear cuts shows three elastic deformation stages51. The stiffness of kirigami structure depends on the geometry of the kirigami unit and can be designed/programed using beam theory. However, the approach to estimate the stiffness is In-plane deformation Out-plane rotation Out-plane deformation a b 21 different for each stage of deformation because the kirigami unit loading conditions change as the whole metamaterial structure is strained. The in-plane stiffness can be estimated based on the assumption that all units are connected beams (Figure 2.8). Figure 2.8 The Deflection of kirigami structure due to uniaxial tension. a) The tensile loading condition on a kirigami unit; b) Two possible boundary conditions of end points for the rectangular beam (kirigami unit). The top image shows Pinned-pinned condition, with zero moment at the end points, and at bottom the fixed-fixed boundary condition, with moment at the end points of 𝑴𝑴𝐀𝐀 and 𝑴𝑴𝐁𝐁; c) A linear kirigami pattern mimics springs in series and parallel during tensile strain. A homogenous kirigami pattern with 10 lateral rows and 5 transversal columns is depicted. The deflection for one kirigami unit agrees with the theory valid for a cantilever beam. Equation 2.1 shows the deflection of a kirigami unit (also see Figure 2.8a). 𝐸𝐸𝐼𝐼𝑑𝑑2𝛿𝛿𝑑𝑑𝑥𝑥2= 𝑀𝑀(𝑥𝑥) 2.1 Here 𝐸𝐸 is elastic modulus of the sheet material, 𝐼𝐼 is area moment of inertia of the unit’s cross-section, 𝑀𝑀(𝑥𝑥) is the resultant moment along the x-axis (Figure 2.8a) and 𝛿𝛿 is the deflection of the kirigami unit along the x-axis. The second moment of inertia for the cross-section of the kirigami unit depends on if the deformation stage is in place or out of plane, as shown in Equations 2.2 and 2.3. a b c 22 𝐼𝐼𝐶𝐶𝑖𝑖−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 = 𝑡𝑡𝐺𝐺𝑠𝑠312 2.2 𝐼𝐼𝑜𝑜𝑜𝑜𝐵𝐵−𝑜𝑜𝑠𝑠−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 = 𝑡𝑡3𝐺𝐺𝑠𝑠12 2.3 The moment of a kirigami unit depends on the boundary conditions at its endpoints and can differ for fixed-fixed and pinned-pinned assumptions22 (Figure 2.8b). By applying the assumed boundary conditions, the deflection of the beam (Equation 2.1) can be solved as 𝛿𝛿~𝛼𝛼 𝑝𝑝(𝐶𝐶+𝐺𝐺𝑡𝑡)3𝐸𝐸𝐸𝐸, where P is the tensile load on kirigami unit and α is a constant value that depends on the boundary condition. This indicates that the deformation of the kirigami unit depends on its geometric parameters (t, C, Gt and Gl). Furthermore, using Hooke's law and the deflection of the kirigami unit, the stiffness can be calculated as, 𝑘𝑘 = 𝑃𝑃𝛿𝛿 2.4 The stiffness of the kirigami unit during in-plane deformation is denoted by k. The stiffness can be calculated using Equations 2.2, 2.4, and the deflection of a unit during the in-plane deformation stage: 𝑘𝑘in−plane ≈𝐸𝐸𝐺𝐺𝑠𝑠3𝑡𝑡(𝐶𝐶 + 𝐺𝐺𝐵𝐵)3 2.5 For the out-of-plane stiffness of the kirigami unit, a rotation of 90º at the units is considered. Using equations 2.3, 2.4, and deflection of the unit gives: 𝑘𝑘out−of−plane ≈𝐸𝐸𝑡𝑡3𝐺𝐺𝑠𝑠(𝐶𝐶 + 𝐺𝐺𝐵𝐵)3 2.6 A linear kirigami pattern with linear cuts consists of connected columns and rows of kirigami units that behave like connected springs in series and parallel, following Hooke's law (Figure 2.8c). Therefore, the total stiffness for in-plane and out-of-plane deformation (𝑘𝑘in−plane and 23 𝑘𝑘out−of−plane) can be calculated using equations 2.5 and 2.6 by applying Hooke's law and assuming uniform deformation in all units. 𝐾𝐾𝐶𝐶𝑖𝑖−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 ≈𝑁𝑁𝐵𝐵𝑁𝑁𝑠𝑠𝑘𝑘𝐶𝐶𝑖𝑖−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 2.7 𝐾𝐾𝑜𝑜𝑜𝑜𝐵𝐵−𝑜𝑜𝑠𝑠−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 ≈𝑁𝑁𝐵𝐵𝑁𝑁𝑠𝑠𝑘𝑘𝑜𝑜𝑜𝑜𝐵𝐵−𝑜𝑜𝑠𝑠−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 2.8 Here 𝑁𝑁𝐵𝐵 and 𝑁𝑁𝑠𝑠 represent the total number of transverse rows and lateral columns in the structure, respectively. These above-mentioned equations can hold for more complicated motifs such as triangular, circular, or square shapes. In summary, various kirigami patterns can be designed and fabricated by simply tuning the unit's geometric properties and the number of connected units in the structure. 2.2 Principles of wettability and adhesion 2.2.1 Wettability A solid-solid bonded interface with undesired adhesion is called fouling71. The adhesion of most solid foulants to a surface invariably involves the precursory step of a liquid coming into contact with the solid surface. Accordingly, before studying the adhesion of solid foulants, it is important to understand the adhesion of liquids to solid surfaces. When a liquid droplet comes into contact with a surface, it can either spread over the surface completely, or form a sessile droplet. The contact angle of the sessile droplet depends on the chemical composition of the liquid and the surface, as well as the topography of the surface. The balance of solid-air, solid-liquid, and liquid-air surface energies/tensions at the triple-phase contact line determines the equilibrium state of the system (Figure 2.9). Thomas Young's force balance equation for a liquid droplet on a surface, presented as Equation 2.9, calculates the Young’s contact angle, θY. 24 cos 𝜃𝜃𝑌𝑌 = 𝛾𝛾𝐶𝐶𝑠𝑠 − 𝛾𝛾𝐶𝐶𝑠𝑠𝛾𝛾𝑠𝑠𝑠𝑠 2.9 Figure 2.9 (a) Liquid droplet on flat, homogeneous, rigid surface and Young's contact angle definition. 𝜸𝜸𝒍𝒍𝒍𝒍 , 𝜸𝜸𝒔𝒔𝒍𝒍 and 𝜸𝜸𝒔𝒔𝒍𝒍 are the surface tension of the liquid, the solid surface energy, and the solid-liquid interfacial tension. (b) Liquid droplet on rough surface in Wenzel wetting state where liquid completely penetrates surface asperities. The relationship between the experimentally observed apparent contact angle (θ) and the Young contact angle (𝜃𝜃𝑌𝑌) varies depending on the surface characteristics. On ideal and smooth, chemically heterogeneous surfaces with 100% vapor, θ can approximate 𝜃𝜃𝑌𝑌. However, on rough surfaces, contact angles are irrelevant for Young's equation. It is important to consider that the measured apparent contact angle on a surface can vary by up to 20º between different points on the surface72. When the volume of a liquid droplet increases, the contact line moves and the droplet advances on the surface and adopts an advancing contact angle (𝜃𝜃adv). Conversely, when the droplet's volume decreases due to careful withdrawal of the liquid, the contact angle decreases, and the contact line moves at a receding contact angle (𝜃𝜃rec). These dynamic contact angles represent constant values that can characterize the wettability of non-ideal surfaces that do not adopt 𝜃𝜃𝑌𝑌. Vapor Solid Liquid 𝛾𝛾𝑠𝑠𝑠𝑠 𝛾𝛾𝐶𝐶𝑠𝑠 𝛾𝛾𝐶𝐶𝑠𝑠 Rough surface 𝛾𝛾𝐶𝐶𝑠𝑠 a b 𝜃𝜃 𝜃𝜃𝑤𝑤 25 Young’s theory assumes a perfectly smooth surface and one that is chemically homogeneous. Later, Wenzel revised Young’s equation for rough surfaces using a defined surface roughness, r, which is the ratio of actual to projected contact line. The apparent contact angle on a rough surface, θw, may be calculated using Equation 2.10. Wenzel’s equation indicates that roughness of surface magnifies the surface’s wettability (either hydrophilic or hydrophobic). cos 𝜃𝜃𝑊𝑊 = 𝑟𝑟 × cos 𝜃𝜃𝑌𝑌 2.10 A surface can also have heterogeneous chemistry and be composed of different materials with varying surface energies. Cassie and Baxter recognized this and modified the analysis of surface wetting by examining the energy required to create a contact line of a liquid on such surfaces. The apparent contact angle on a chemically heterogeneous surface is calculated as a weighted sum of the contributions from each material, cos 𝜃𝜃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶−𝐵𝐵𝐶𝐶𝐵𝐵𝐵𝐵𝐶𝐶𝐵𝐵 = � 𝑓𝑓𝐶𝐶 × 𝑟𝑟𝐶𝐶𝑖𝑖𝐶𝐶=1× cos 𝜃𝜃𝑌𝑌,𝐶𝐶 2.11 Here 𝑓𝑓𝐶𝐶 is the fraction of material i, 𝑟𝑟𝐶𝐶 is the Wenzel roughness of material i, and cos 𝜃𝜃𝑌𝑌,𝐶𝐶 is the Young’s contact angle material i. In these different liquid wetting cases, the droplet adhesion can vary significantly. However, for the sake of simplicity, we will focus on discussing droplet adhesion in relation to Young's contact angle (θ). When a liquid droplet is placed on a chemically homogeneous, inert, perfectly flat, and rigid surface (and therefore adopts Young’s contact angle), it exhibits a thermodynamic work of adhesion, Wa. This work can be determined by calculating the energy required to break the bond between the liquid/solid and create two new interfaces (solid/vapor and liquid/vapor). 26 𝑊𝑊𝐶𝐶 = 𝛾𝛾𝐶𝐶𝑠𝑠 + 𝛾𝛾𝑠𝑠𝑠𝑠 − 𝛾𝛾𝐶𝐶𝑠𝑠 2.12 Here, 𝛾𝛾𝐶𝐶𝑠𝑠 is interfacial energy between solid and liquid, 𝛾𝛾𝑠𝑠𝑠𝑠 is surface tension of liquid and 𝛾𝛾𝐶𝐶𝑠𝑠 is the solid’s surface energy. The work of adhesion can therefore be calculated by combining Equations 2.9 and 2.12 as, 𝑊𝑊𝐶𝐶 ≈ 𝛾𝛾𝑠𝑠𝑠𝑠(1 + cos 𝜃𝜃𝑌𝑌) 2.13 2.2.2 Adhesion Adhesion between two surfaces involves different attraction forces such as intermolecular, electrostatic, chemical, and/or mechanical interlocking. Hard foulants include a range of solids such as ice, biofoulants, wax, inorganic deposition, etc. The undesirable adhesion causes tremendous issues in many industries, for example, the deposition of wax in oil transfer pipelines, icing of power cables, and the colonization of substrata by fouling organisms (barnacles) on ships. There is an active area of research investigating antifouling surfaces to prevent the deposition of these problematic adhesives71,73,82–84,74–81. However, each one of the hard foulants has different chemistry, physical properties, and a unique deposition process. Noteworthy, the adhesives exist in different scales, from dozens of nanometers to many meters. Therefore, every individual foulant requires a specific approach to decrease the adhesion strength or prevent their deposition. Understanding the adhesion mechanism of a solid foulant on surfaces is essential for developing antifouling coatings and materials. The removal of a solid foulant from a substrate requires enough stress at the interface to overcome the interfacial bonding between foulant and surface. Figure 2.10 shows different types of cracks that can occur in the form of mode I (opening), mode II (shear or sliding), and mode III (tearing) de-bonding. For homogenous materials with isotropic fracture 27 behavior, mode I is the dominant mode. Mode III is less common, but it can occur in some materials. Figure 2.10 Different modes of fracture at the interface of two different materials. If a crack is initiated and propagates at the foulant-solid interface, then the event can be considered as purely adhesive failure (Figure 2.11). On the other hand, if the failure occurs within the bulk of the foulant or the solid substrate, the mode of failure is called cohesive failure (Figure 2.11). The mode of failure is determined by comparing the adhesion strength of the foulant-solid bond, τfs, to the cohesive strengths of the foulant, τf, and solid, τS. Note that here shear (mode II) is considered, but similar definitions for mode I (tensile) and mode III (torsional) adhesive or cohesive fracture exist. Mixed-mode fracture, though the most common scenario, is not discussed in detail in this thesis for simplicity. N Mode I Mode II Mode III 28 Figure 2.11 The possible fracture propagation results for mode II fracture. During cohesive fracture, the crack kinks into either the foulant or solid substrate, rather than propagating along the interface. However, films, coatings, and layered materials have interfaces that can significantly affect fracture behavior. The dimensions of a two-layered interface determine how interface fracture happens. In the case of a short-length interface, debonding primarily occurs when applied stress at interface overcomes the interfacial strength (i.e. the interfacial strength controls the fracture), leading to spontaneous debonding of the entire interface upon fracture. The adhesion strength of the foulant-solid bond is then defined as the peak force required to dislodge the solid foulant (FAdhesion), divided by the cross-sectional area of the dislodged foulant (AInterface = W×L). Notably, the adhesion strength of the interface is constant as the length of the interface increases in the strength control regime. 𝜏𝜏fs = 𝐹𝐹Adheison/𝐴𝐴Interface 2.14 29 In larger bi-layer interfaces, crack initiation and propagation can take less energy than the spontaneous debonding of the entire interface observed in strength-controlled interfacial fracture. The initiated crack will arrest if its stored strain energy falls short of the energy required for infinite crack propagation. However, the energy stored in cracks of a critical length is sufficient to keep growing until whole interface is de-bonded, regardless of the interface size. The cohesive length, which is defined as 𝜉𝜉 = 𝐸𝐸∗Γ/𝜏𝜏2, helps determine if the interfacial fracture is primarily governed by interfacial toughness or strength. Here, 𝐸𝐸∗ represents the effective elastic modulus of the foulant and Γ represents the interfacial toughness. In the toughness-controlled regime, the adhesion is characterized by the critical force, as described in Equation 2.16. 85 The critical force (𝐹𝐹𝑐𝑐) is the force required to dislodge a foulant with a finite thickness (t) from an infinitely thick solid substrate. 𝐹𝐹𝑐𝑐 = √2𝐸𝐸∗Γ𝑡𝑡 2.15 2.3 Antifouling surfaces Surfaces with low adhesion of solid foulants are mainly designed using one of three simple passive (no energy input) strategies71,86: chemical functionalization, tuning the mechanical properties of the surface, and/or texturing of the surfaces. On the other hand, active methods (energy input) are also used to remove foulants from a surface. 2.3.1 Chemical functionalization The chemistry of surfaces controls the type of possible bond between the foulant and the surface. Substrates with low surface energy provide weak bonds at the interface87–90. In order to facilitate the removal of hard foulants from the interface, one approach is to minimize the interfacial energy. Lower surface energy decreases the work of adhesion with the foulant before solidifying at the 30 surface (recall Equations 2.14 and 2.15). For example, Meuler et al. conducted a study in which they investigated the relationship between the adhesion strength of ice on smooth, rigid surfaces and the practical work of adhesion of these surfaces with liquid water91. Their findings revealed a high correlation between these two factors. The adhesion strength of ice on the surfaces they evaluated showed a better correlation with the receding contact angle of water rather than other wettability parameters. 𝑊𝑊𝐶𝐶 ≈ 𝛾𝛾𝐶𝐶𝑠𝑠 (1 + cos 𝜃𝜃rec) 2.16 These findings of Meuler et al. and other research studies have shown that the use of low surface energy materials, such as fluorinated, modified polyethylene, and polydimethylsiloxane (PDMS)-based coatings, results in reduced foulant adhesion strength92,93. This can be attributed to the limited van der Waals bonding that occurs at the interface between the surfaces and the foulant. However, despite efforts to modify surface chemistry, these modifications have a limited impact on reducing adhesion strength at interface because the wettability of a surface is limited to a narrow range. For example, increasing the receding contact angle from 0° to 120° only decreases Wa by four times in Equation 2.14 above. 2.3.2 Tuning mechanical properties To decrease the adhesion of a foulant to a surface, an alternative approach is to modify the physical properties of the surface. As mentioned earlier, the dimension of the foulant plays a role in determining the dominant fracture mechanism at the interface. In cases where the strength-controlled regime applies, fabricating a coating to reduce the adhesion strength is a suitable approach. Kendall demonstrated that the maximum normal stress required to cause adhesive fracture at the interface between a hard foulant and a soft material is proportional to its elastic 31 modulus and inversely proportional to its thickness94. Subsequently, Chaudhury et al. conducted further research that validated similar relationships between the shear adhesion strength of a rigid foulant on a thin, soft elastomer95. They found that the elastic modulus of the elastomers, in both normal and shear loading at the interface, significantly influenced the adhesion strength of the foulant71,85,86 as, 𝜏𝜏~�𝐸𝐸𝑊𝑊𝐶𝐶𝑡𝑡 2.17 Here, 𝜏𝜏, 𝐸𝐸 and 𝑡𝑡 are adhesion strength of the foulant, the elastic modulus of the soft material, and the substrate's thickness, respectively. Equation 2.17 shows that the adhesion strength of hard foulants on an elastic surface is influenced by the elastic modulus, which, unlike the receding contact angle, can be adjusted over a very wide range. Multiple works showed that by applying an elastomer coating, the adhesion strength of hard foulants can be effectively reduced. However, it is important to consider the trade-off associated with reducing the elastic modulus, as it can lead to decreased surface robustness in harsh conditions74,95,96. The above strategy for lowering the interfacial strength of a foulant is only valid for shorter interfaces. However, when dealing with larger interfaces, as discussed above the interfacial fracture is governed by interfacial toughness. Therefore, minimizing the interfacial toughness with a foulant is the correct design strategy. Interfacial toughness represents the energy dissipated during the creation of a unit area of a new crack or de-bonded surface. Reducing the interfacial toughness decreases the interface’s capacity to store strain energy. Assuming linear elastic fracture mechanics, the following equation can be used to design the toughness of a foulant/surface interface85. 32 Γ~ 𝜏𝜏2𝑡𝑡2𝐺𝐺 2.18 Equation 2.18 indicates that a low interfacial toughness (LIT) coating can be realized through various methods, including decreasing the inherent shear strength of the interface (𝜏𝜏), reducing the coating thickness (t), or increasing the shear modulus of coating (G). Researchers have successfully developed surfaces with low interfacial toughness with ice by applying thin coatings of plasticized PDMS, polytetrafluoroethylene (PTFE), quasicrystalline coatings and polyvinyl chloride (PVC)85,97. These surfaces effectively shed meter-sized ice under their own weight, resulting in significantly reduced apparent ice-adhesion values85. 2.3.3 Textured surfaces Introducing texture to a smooth surface is one approach to change its wettability and therefore foulant adhesion98–100. Some critical applications of this method are inspired by nature, e.g. superhydrophobic surfaces (SHS) inspired by the Lotus leaf101 or lubricated surfaces inspired by the Pitcher plant102. SHS surfaces, characterized by their unique texture and material chemistry, exhibit exceptionally high receding angles of water. As explained earlier, higher receding angles result in a lower work of adhesion. Moreover, textured surface of SHS have less contact area with a liquid (before solidifying to a solid foulant) and, as a result, require less force to remove foulants from the surface. In terms of ice as a foulant, SHS have demonstrated good laboratory scale icing delay and easier removal of supercooled water droplets when gently deposited on surface, due to their low thermal conductivity, minimal surface/droplet contact area and entrapped air pocket within their texture103–105. However, the icing delay of SHS is typically measured on the scale of minutes. Moreover, the air trapping effect is not observed when SHS are exposed to humid air or 33 when water droplets impinge at high speeds99. Consequently, ice formation can initiate from the troughs of SHSs and progressively grow, ultimately penetrating within the textured surfaces and causing mechanical interlocking once the liquid is frozen. Another type of antifouling coating uses surface texturing combined with lubricant infusion into the porosity. The lubricant infused surface replaces the solid-foulant interface with liquid-foulant interface that results in extremely low or no interfacial adhesion strength. For example, Zhao et al. fabricated silicon-oil infused icephobic coatings that demonstrated a low shear ice adhesion strength for cylindrical ice at -10 °C.106 Similarly, Liu et al. presented fluorinated lubricant PDMS films for significant ice adhesion reduction and delaying icing107. However, for such surfaces the lubricant loss reduces the useful lifetime of the coating108. 2.3.4 Active methods All the antifouling methods mentioned above are considered as passive methods that utilize coatings with specific surface properties to lessen adhesion, with the goal that the foulant can be removed by its own weight, wind, or other aerodynamic/environmental forces. Because natural forces may not always ensure foulant removal, active methods, including mechanical or ultrasound techniques, heat or electrical energy application, and chemical dissolution or bond weakening, are employed to effectively reduce the interfacial strength or fully eliminate foulants from surfaces. For example, passive methods alone are often insufficient for reliable ice removal. Therefore, active measures are necessary to effectively address ice accretion and its associated problems in many industrial settings. To remove the solid-solid bond between ice and a surface, active methods melt ice at the surface, allowing the liquid water to be easily removed. Electrical devices have been widely employed for active de-icing on a variety of surfaces109–111 and utilize joule heating to raise 34 the temperature of the accreted ice above 0 °C, facilitating its removal through a phase change to liquid water112–115. Proper thermal/electrical conductivity is required to maximize the de-icing efficiency while minimizing energy consumption112,116,117. Graphene-based heaters109,118, hot air pumping119, conductive polymer-based heaters120–122 and most commonly metallic heating systems123–126 have all been used to provide sufficient heat to melt the interfacial ice. However, all previous active foulant removing techniques that involve heating have required the entire interface to be raised above the melting temperature of foulant. A related active de-icing method involves using chemicals such as inorganic salts, organic compounds, and alcohols to remove accreted ice on aircraft wings, pavements, trains, and rail switches127,128. These chemicals function by lowering the freezing point of water, effectively preventing it from freezing and forming ice, or causing ice to melt. In general, active methods consume considerable energy to remove foulants. 2.3.5 Hybrid methods Hybrid fouling removal methods combine multiple approaches to enhance the effectiveness of fouling mitigation and/or removal. These methods leverage the advantages of different fouling removal strategies to achieve improved fouling control with the goal of expending less energy. In these methods, passive techniques are initially employed to minimize fouling and create a fouling-resistant surface. This can include the use of surface coatings as explained above. However, if fouling still occurs despite these preventive measures, active methods are utilized to remove the accumulated foulants. For example, hybrid de-icing methods that combine active de-icing and SHS coatings have recently been explored. Cheng et al. fabricated a SHS coating using magnetic particles for hybrid de-icing and showed that increasing the temperature above 0 °C allowed the coating to exhibit excellent ice/water removal129. Additionally, Gao et al. demonstrated the hybrid 35 use of a SHS coating and electrical heating for wind turbine de-icing12. They showed significant energy savings (90%) when de-icing the whole turbine blade by only coating the leading edge with their SHS coating and electrical heaters. Hybrid de-icing methods can also employ lubricant infusion. Jamil et al. used silicone lubrication on a candle soot coating as a natural light absorber130. In their work, conductive iron nano particles served as a heat dissipator, eventually melting the iced interface. Chapter 3: Suspended kirigami surfaces for multi-foulant adhesion reduction High foulant adhesion remains a critical issue in a wide range of industries, such as ice accretion on aircraft, bio-foulants on ships, wax build-up within pipelines, and scale formation in water remediation. Previous anti-fouling surfaces have only shown promise for reducing the adhesion of a single foulant system; a multi-foulant anti-fouling technology remains elusive. In this chapter, we introduce a mechanical metamaterial-based approach to develop anti-fouling surfaces applicable to a wide range of fouling substances. The approach of employing mechanical metamaterial structures to reduce the adhesion strength of a foulant represents a new direction that has not been previously explored in anti-fouling surface studies. The Suspended Kirigami Inverted Nil-adhesion Surfaces, or SKINS, show significantly reduced adhesion of ice, different waxes, dried mud, pressure-sensitive adhesive tape, and a marine foulant simulant. SKINS mimic the wrinkling of hard films adhered to soft substrates. Foulant adhesion can be minimized by this wrinkling, which may be controlled by tuning the kirigami motif, sheet material, and foulant dimensions. SKINS reduce adhesion mechanically and were found to be independent of surface energy, enabling their fabrication from commonplace, hydrophilic polymers like cellulose acetate. Optimized SKINS exhibited extremely low foulant adhesion, for example ice adhesion strengths less than 5 kPa (a >250-fold reduction from aluminum substates) and were found to maintain their performance on curved surfaces like transmission cables. The low foulant adhesion persisted over 30 repeated foulant deposition and removal cycles, demonstrating the durability of SKINS. Overall, SKINS offers a previously unexplored potential of kirigami structures to achieving low foulant adhesion that is highly tunable in both geometry and material selection, is applicable to many different fouling substances, and maintains ultra-low foulant adhesion even on complex substrates over large, fouled interfaces. 37 3.1 Introduction The strong adhesion between hard foulants, such as barnacles, ice, clathrates, and waxes plagues many industries worldwide1–4. The development of permanent coatings that minimize foulant adhesion is an active area of research2,5–7. Strategies for lowering foulant adhesion include minimizing surface energy8, lubrication9,10, enabling interfacial cavitation11, or promoting crack initiation and propagation12. For example, Jiang et al. reduced the fouling of oil pipelines by wax using organogel coatings that exhibited paraffin wax shear adhesion strengths as low as 0.5 kPa13. Smith et al. have shown clathrate hydrate shear adhesion strengths as low as 100 kPa on coatings with low surface energy3. Researchers have also investigated the anti-fouling properties of hydrophobic carbosilane-based coatings using epoxy as pseudobarnacles. Epoxy adhesion strengths on silicone-coated glass using pull-off tests have been decreased tenfold14. The antifouling performance of soft surfaces has also been investigated. Soft substrates introduce a deformation incompatibility at the interface, which has been used to lower the adhesion strength of hard foulants. For example, Li et al. and Jamil et al. have both reported a low adhesion strength of ice on silicones fabricated with subsurface structuring15,16. Although promising for single foulant systems, coatings utilizing the above strategies to lower the adhesion of one foulant system have not demonstrated reduced adhesion for other foulants. This is significant because many engineering surfaces are commonly fouled by multiple substances during operation, such as boat hulls (ice and biofoulants), solar cells (snow and dust), automotive exteriors (mud and ice) or pipelines (waxes and clathrates). And yet, a low-adhesion material generalizable to many different foulants remains elusive. Here we demonstrate that kirigami sheets 38 with their underside rendered superhydrophobic (inverted), and when suspended above a substrate, substantially reduce the adhesion of many different solid foulants, including ice, waxes, dried mud, bio-foulant simulants (pseudobarnacles) like epoxy, and acrylic adhesive tape. We show the novel application of kirigami structures as anti-fouling surfaces, a property of the metamaterial that has never been demonstrated. The rational design of the geometry and material of the Suspended Kirigami Inverted Nil-adhesion Surfaces, or SKINS, allows for robust and scalable, low foulant adhesion. Note that different industries will each have their own definition of a nil-adhesion surface, i.e. one in which the adhesive bond of a foulant is sufficiently low to be practically useful. For example, hard biofoulants have been observed to release from properly designed antifouling marine paints above speeds of 12 knots17, and ice has been shown to dislodge from some ice-phobic materials during a strong breeze, when the ice adhesion strength is below ~ 30 kPa18. Kirigami, the ancient Japanese art of paper cutting, is defined by a pattern of cuts within a thin sheet, enabling dynamically tunable geometric structures19–21. Under tension, kirigami patterns transform from a planar sheet into a three-dimensional structure when the regions of the sheet in between the cuts, called kirigami hinges, buckle22–24. We hypothesized that this out-of-plane deformation would de-bond a solid adherent and that each cut could serve as a crack initiation site. However, rigidly adhering the kirigami sheet to an underlying substrate, the traditional configuration of a regular coating, would prevent hinge buckling and deformation. To allow for this degree of freedom while still conformally covering the substrate, we sparsely suspended kirigami sheets a distance h above the underlying surface (Figure 3.1). 39 Figure 3.1 Schematic of the five individual aspects of Suspended Kirigami Inverted Nil-adhesion Surfaces, or SKINS. To prevent leakage of the liquid precursors into the kirigami cuts, one side of our kirigami sheets was sprayed with a superhydrophobic coating (see Experimental Section). The kirigami sheet was then inverted (superhydrophobic side facing the substrate, uncoated side adhered to the foulant) before bonding the sheet to the surface only around its perimeter, leaving the sheet interior suspended (Figure 3.1). Foulant adhesion to SKINS was then studied by varying the design of the kirigami motif, the sheet material, and the dimensions and type of foulant. 3.2 Experimental Section 3.2.1 Materials The adhesion strength of many hard foulants on SKINS fabricated from different plastics was evaluated. The adhesives and plastic sheets were purchased from McMaster Carr unless specified 40 otherwise. The hard foulants were ice (frozen deionized water at -15°C), paraffin wax beads (Catalog No. 1085K91), carnauba wax (Catalog No. 7259T22), epoxy resin (Catalog No. 75045A65), mud (Air Dry Clay, Crayola), and acrylic pressure-sensitive tape (Catalog No. 1790N1). The plastics were ultra-high-molecular-weight polyethylene in four thicknesses, t = 0.101 mm, 0.127 mm, 0.0254 mm, and 0.0516 mm (UHMW-PE, Catalog No. 1441T14, 85655K11, 85655K13 and 85655K13), and t = 0.0127 mm sheets of polycarbonate (PC, Catalog No. 85655K13), polyester (PET, Catalog No. 8567K52), polytetrafluoroethylene (PTFE, Catalog No. 8569K38), Acetal (Catalog No. 5742T218) and cellulose acetate (CA, OHP Projector Film, thickness 0.127 mm, Uinkit, UK). 3.2.2 Methods 3.2.2.1 Fabrication of kirigami sheets To fabricate kirigami sheets with various patterns, a craft cutter machine (Brother ScanNCut SDX125E Electronic DIY Cutting machine) was used. Commercially available polymeric sheets were used for the material of the kirigami samples: cellulose acetate (CA), polycarbonate (PC), polyester (PET), Acetal, ultra-high molecular weight polyethylene (UHMW-PE) and polytetrafluoroethylene (PTFE) (Figure 3.2a). The prepared samples may be described by their cut size (C), their lateral gap (Gl), their transversal gap (Gt), and the thickness of the sheet (t) (Figure 3.2a, Table 3.1). We also defined the kirigami index, κ = Gl/Gt, as a non-dimensional representation of the flexural rigidity of the kirigami units. Note that the blade edge of the craft cutter is ~ 0.1 mm, and accordingly the range of C, Gl, and Gt investigated were limited to sizes physically fabricable using this mechanical cutting method. 41 Figure 3.2 Schematic of SKINS fabrication. a) The kirigami patterns with its geometric parameters are cut into thin sheets using craft cutter. b) The underside of the kirigami sheet is coated with a commercial superhydrophobic spray paint. c) The underside-coated kirigami sheet is adhered to a substrate using double-sided tape to confine only the edges of the sheet. Base material Size of cut, C (mm) Thickness, t (mm) Transversal gap, Gt (mm) Lateral gap, Gl (mm) Kirigami index () CA 15 0.127 0.5 7 14 5 10 3 6 2 4 1.5 3 0.75 7 9.34 3 4 2 2.67 1.5 2 42 1 1.33 1.5 7 4.66 5 3.34 3 2 2 1.34 1.5 1 3 7 2.34 3 1 2 0.67 1.5 0.5 1 0.34 2 7 3.5 3 5 2 1 1.5 0.75 UHMW-PE 10 0.127 2 2 1 20 0.127 2 2 1 15 0.1 2 2 1 0.254 2 2 1 0.516 2 2 1 Polyester 15 0.127 2 2 1 PTFE 15 0.127 2 2 1 Acetal 15 0.127 2 2 1 Polycarbonate (PC) 15 0.127 2 2 1 Table 3.1 Geometric characteristics of kirigami patterns on various substrates. 3.2.2.2 Preparing the superhydrophobic side of SKINS Leakage of the liquid foulant before solidifying could affect the adhesion measurements (Figure 3.3). To help reduce any possible leakage, the underside of the designed kirigami sheets were coated with a commercial superhydrophobic spray paint containing fluorinated particle (Neverwet, from Rustoleum, sprayed per manufacturer's instructions, see Figure 3.2b). The kirigami sheet 43 with this superhydrophobic underside was then suspended on the stage of the adhesion measurement setup using strips of double-sided tape around the perimeter (Figure 3.2c). Figure 3.3 The effect of modifying the underside of SKINS with a superhydrophobic (SH) coating; a) Adhesion strength of ice or wax on CA SKINS with and without SH coating on the underside. b) Ice adhesion strength on SKINS fabricated from both hydrophilic and hydrophobic polymers, with and without the SH coating. Each experiment was repeated at least 4 times. 3.2.2.3 Preparing bonded foulants on the surface of the SKINS For ice, deionized water was poured into a 3D-printed square mold (1 cm × 1 cm × 0.5 cm) on SKINS and frozen at -15˚C using a custom Peltier system described elsewhere11,28. For dried mud, the square molds were filled and were allowed to dry at ambient temperature (21˚C) for 24 hours. For paraffin and carnauba wax, pellets were heated to 100 ˚C and then poured into the mold sitting on the CA SKINS at room temperature, where they solidified as they cooled to room temperature. For pressure-sensitive tape, transparent acrylic cubes were adhered to two layers of acrylic pressure-sensitive tapes in the configuration: acrylic cube  quick stick hard duty (3M 9629PC) double-sided tape  light-duty (Office tape) double-sided tape  SKINS (Figure 3.1). A weight was held on the acrylic cube on SKINS for 1 min. The weight was varied according to the length of cubes (weight increased 100g/cm) to maintain constant pressure on the tape layers. Finally, for 44 the epoxy foulant, the epoxy resin and epoxy hardener with a weight ratio of 2:3 were mixed until homogenous, per manufacturer instructions, and then poured into the molds sitting on the UHMW-PE SKINS. The epoxy was then allowed to cure for 24 hours. To avoid edge effects and the specifics of the kirigami orientation underneath the foulant (for example, a foulant adhered solely to a kirigami hinge), millimetric kirigami gaps (Gl and Gt) were selected such that many kirigami units existed underneath the 1 cm2 foulant interfacial area. Larger Gl and Gt could be investigated, but this would require larger foulant areas. 3.2.2.4 Preparing the silicon coated CA sheet For curved SKINS evaluation the suspension was replaced with a silicone elastomer underneath the kirigami sheet to minimize leakage caused by hydrostatic pressure. To fabricate the elastomer layer for curved SKINS, a commercial silicone (Ecoflex™ 00-30 from Smooth-On Inc.) was mixed in a 1:1 base:crosslinker weight ratio, following manufacturer instructions. The prepared mixture was degassed in a vacuum chamber for 3 mins. Next, CA sheets were coated with a 0.5 mm layer of the ECO-Flex using a film applicator (TMAXCN). A doctor blade was pushed at a constant speed (500 µm/s) to evenly apply the elastomer layer on the surface. Finally, the plastic sheet coated using the film applicator was heated to 100 ºC for 10 minutes to cure the elastomer layer. 3.2.2.5 Adhesion strength measurements for planar SKINS The shear adhesion strength of different foulants was measured using a custom push-off method described elsewhere28 (Figure 3.4a). A force gauge (NEXTECH, DFS-series) was connected to a motorized linear stage moving at 50 µm/s. When the force gauge probe impacted the foulant, the detachment force was measured with 0.01 N accuracy (DFS 50 N). For all experiments, the height of the probe above the interface was 1 mm. The shear adhesion of various foulants was evaluated 45 on the planar SKINS (ice, paraffin wax, carnauba wax, epoxy adhesive, mud, and acrylic pressure-sensitive tape). The shear adhesion strength measurements were conducted at -15˚C for ice, and at room temperature (21˚C) for all other adhesives. After each shear adhesion measurement, the top side of the sheet was cleaned with ethanol and isopropyl alcohol (VWR International), using a Kim wipe (KimTech). Figure 3.4. a) Shear adhesion strength measurement custom-built setup. b) Semi-cylindrical adhesion tests: a container holds the semi-cylindrical surface, and two gaskets prevent the water from leaking. The optimized SKINS is wrapped around the acrylic semi-cylinder, and then the container is filled with water and placed inside a freezer for 24 hours. A circular probe was used to uniformly end-load the acrylic semi-cylinder. 3.2.2.6 Adhesion strength measurements for curved SKINS For the non-planar experiments, the silicone-coated sheets were cut with the craft cutter and then coated with Neverwet on the ECO-Flex side. This process prevented the leakage of water before freezing. The prepared CA SKINS was then wrapped around acrylic semi-cylinders with radii of 2.54 cm and various lengths, from 3 to 30 cm. The wrapped samples were then placed in 46 a container and sealed with two gaskets (Figure 3.4b). The container was then filled with water and placed in a -15˚C freezer for 24 h to freeze completely. For adhesion strength measurements, a force gauge probe equipped with a circular head was used to impact the acrylic semi-cylinders, similar to the foulant adhesion testing on planar surfaces. The adhesion strength of the curved surfaces was measured by recording the detachment force and interfacial area. For the powerline cable experiment, the cable was wrapped with the same CA SKINS described above for the curved surface testing. The SKINS-covered, 9-cm-long powerline cable was placed in the measurement container and gaskets according to the cable’s outer diameter were used. 3.2.2.7 Mechanical characterizations of SKINS The kirigami sheet's mechanical properties were measured in uniaxial tension using an instrument described elsewhere24. One end of each kirigami sheet was rigidly clamped, while the other end was connected to a force gauge mounted on a step motor with a motion controller. The kirigami sheets' cuts were aligned perpendicular to the tensile load. The moving probe applied a constant strain rate of 0.0625 %/s to the kirigami sheets until the strain reached 12%. The load cell accuracy was 0.1 N. The elastic moduli of CA kirigami patterns (II-VI) were measured using this tensile testing at least five times to gather variance. 3.2.2.8 Monitoring the SKINS deformation using photoelasticity CA's photoelasticity was used to monitor SKINS deformation during interfacial separation. Two polarizer sheets (with an angle of 90º with respect to each other) were positioned above a white light source. Then, the suspended kirigami CA sheet was constrained along its perimeter on a transparent stage using double-sided tape and was placed between the two polarizers. Crossed polarizers are unable to transmit light unless the linear direction of the transmitted light changes 47 between the two polarized sheets. To achieve this, pressure-sensitive tape was selected as the appropriate hard foulant due to its transparency. For adhesion strength measurements of pressure-sensitive tapes, the tape was adhered to transparent acrylic cubes (Figure 3.4a). The acrylic foulants were fabricated with a square cross-section of 1 cm side length and various lengths (0.5 cm to 10 cm). Then the transmitted light was then recorded while applying a shear load to release these cubes adhered with pressure-sensitive tape to the CA SKINS (Figure 3.5). Figure 3.5 Photoelasticity visualization of pressure-sensitive tape adhesion to SKINS when varying the dimensions of the foulant (Methods). The scale bar is 1 cm. 3.2.2.9 Contact angle measurements Contact angles of water droplets were measured using a Model 260 Contact Angle Goniometer (Rame-hart Instrument Co., Succasunna, NJ). 15 µL water droplets were deposited on the various base materials. Multiple measurements of advancing and receding contact angles were carried out by slowly adding and removing water from the droplet using a micro-syringe until the contact line began to move (Table 3.2). 48 Base Material Advancing contact angle (𝜃𝜃𝐶𝐶𝑎𝑎𝑠𝑠) Receding contact angle (𝜃𝜃𝐵𝐵𝐶𝐶𝑐𝑐) Surface energy (mJ/m2) 77,131–133 Cellulose acetate (CA) 79° ± 3° 31° ± 4° 49.2 Acetal 96° ± 3° 56° ± 4° 40 Polyester (PET) 94° ± 7° 62° ± 6° 39 Ultra-high molecular weight polyethylene (UHMW-PE) 105° ± 6° 71° ± 8° 32.5 Polycarbonate (PC) 100° ± 3° 73° ± 4° 43.3 Polytetrafluoroethylene (PTFE) 115° ± 3° 90° ± 7° 17 Table 3.2. The advancing and receding water contact angles on the various base materials. 49 3.3 Results and Discussion Initially, kirigami sheets were fabricated from cellulose acetate (CA) and ice was used as the fouling solid. The shear ice adhesion strength at -15℃ was measured following previous techniques1,28–30 on four illustrative surfaces: (1) a smooth CA sheet completely adhered to the cold plate, (2) a kirigami-patterned CA sheet completed adhered to the cold plate, (3) a suspended, smooth CA sheet, and (4) SKINS (Figure 3.6). The ice adhesion strength of smooth, fully confined CA was 450 ± 90 kPa, in line with previous reports of the bond between ice and hydrophilic polymers28,31. Adding the kirigami motif (an optimized pattern, discussed later) had a statistically insignificant effect of the observed ice adhesion strength, which indicated that kirigami by itself does not reduce foulant adhesion. However, when the smooth sheet of CA was suspended above the substrate the ice adhesion strength decreased to 101 ± 27 kPa. Figure 3.6 The ice adhesion strength of CA when fully constrained, with a kirigami motif, suspended, and as SKINS. Each experiment was repeated at least 4 times. 50 This was due to the increase in deformational freedom, and further studies with foulants at ambient temperature (discussed below) indicated that this was not a thermal effect. Moreover, when the kirigami motif was added to the suspended CA, forming SKINS, an ultra-low ice adhesion strength of 30 ± 5 kPa was observed, i.e. an order of magnitude decrease from the base material without any chemical modification. Similar results were observed for carnauba wax (Figure 3.7), indicating that the mechanism by which SKINS were able to reduce foulant adhesion was general. Figure 3.7 The adhesion strength of carnauba wax on cellulose acetate when fully constrained, with a kirigami motif, suspended (no kirigami motif), and as SKINS. Each experiment was repeated at least 4 times. We exploited the birefringence of CA to visualize foulant delamination in situ and to understand the fracture mechanism by which SKINS lower foulant adhesion (Figure 3.8). The foulant was changed from ice to an acrylic block adhered using transparent, double-sided tape such that polarized light could be transmitted through the SKINS and foulant (Methods, Figure 3.4a). During interfacial separation the wavelength of polarized light transmitted through the kirigami sheet and foulant was altered by the stress and deformation of the SKINS. 51 Figure 3.8 The visual observation of pressure-sensitive tape detachment from smooth, suspended CA and five different CA SKINS. From II to VI the stiffness of the sheet decreases, as does the wrinkle wavelength, as the strain at delamination increases. The birefringence allowed us to observe that the interface was wrinkling, described by a characteristic wavelength, 𝜆𝜆, and amplitude, A (Figure 3.9). Wrinkling is commonly observed during the compression of a bi-material interface when a large elastic mismatch exists between the two materials32–35. Prior work on the wrinkling of rigid, thin films adhered to compliant substrates has shown that λ∝(Ef /Es)α, where Ef and Es are the elastic moduli of the film and elastomer substrate, respectively, and 𝛼𝛼 is an empirical coefficient36,37. We studied the wrinkling of several SKINS configurations (Figure 3.9a) and found that α = 4.42 for the tape/SKINS interface which shows a higher number compared to previous studies (α = 2) 36,37. This difference could be caused by using the kirigami structure on a rigid polymer sheet, rather than the soft, thin elastomers that have been used previously. Note that there have been many excellent studies exploiting wrinkling for various engineering properties such as superhydrophobicity38 or microfluidic sieves39. However, in general the surface wrinkles are permanently engineered into the surface structure, often utilizing the oxygen plasma modification of strained polydimethylsiloxane. In contrast, SKINS are planar surfaces that only wrinkle upon shearing an adhered foulant, and upon foulant removal the planar orientation of SKINS is restored. 𝜀𝜀𝑑𝑑 = 3.4%𝜏𝜏tape = 160 kPa 𝜀𝜀𝑑𝑑 = 2.1%𝜏𝜏tape = 35 kPa 𝜀𝜀𝑑𝑑 = 5.1%𝜏𝜏tape = 21 kPa 𝜀𝜀𝑑𝑑 = 7.6%𝜏𝜏tape = 19 kPa 𝜀𝜀𝑑𝑑 = 2%𝜏𝜏tape = 17 kPa 𝜀𝜀𝑑𝑑 = 9.5%𝜏𝜏tape = 16 kPaI II III IV V VI1 cm 1 cm 1 cm 1 cm 1 cm 1 cm52 Figure 3.9 a) Logarithmic relationship between kirigami sheet (film) elastic modulus and the wavelength of wrinkling for CA SKINS during the detachment of pressure-sensitive adhesive tape. b) Schematic of the wrinkling instability observed for SKINS. The strain at which a wrinkled film delaminates from a compliant substrate is εd ∝ (t Ef)-1, where t is the thickness of the film37,40. Accordingly, by altering the geometry of the kirigami motif within SKINS to vary its modulus, both the strain at delamination (i.e. detachment of the fouling solid) and the wrinkling patterns were highly tunable (Figure 3.8). Keeping the cut size constant (C = 1.5 cm), we always observed a minimum in foulant adhesion near, regardless of the foulant type or physical size of Gt or Gl (Figure 3.3a and 3.11). For SKINS exhibiting kirigami motifs near κ ≈ 1, ice adhesion decreased as the cut size increased (Figure 3.10b), i.e. as the modulus of SKINS reduced. 𝒍𝒍𝒑𝒓𝒓𝒐𝒃𝒆𝒆 Foulantxzy𝝀𝜜xz𝛼𝛼 = 0.21a b𝛼𝛼 4.42 53 Figure 3.10 Kirigami geometry alters the adhesion strength of hard foulants. a) Ice adhesion strength on CA SKINS versus kirigami index, κ, at constant cut size C = 1.5 cm. The minimum ice adhesion strength was always κ ≈ 1. b) Ice adhesion strength of UHMW-PE SKINS utilizing three different cut sizes. Note that SKINS exhibiting the lowest modulus were not advantageous, as more strain energy was necessary to deform the sheet (folding of the entire kirigami sheet34) before delamination could occur (Figure 3.6, 3.11 and 3.12). For example, SKINS with pattern VI in Figure 3.8 (lowest modulus, see Figure 3.12) exhibited a shear adhesion strength with adhesive tape of τ = 16 kPa, but at εd = 9.5%. Conversely, SKINS with pattern V exhibited τ =17 kPa but at εd = 2%. Regardless of the strain at delamination, optimized SKINS decreased foulant adhesion to CA adhesion by at least an order of magnitude. a b54 Figure 3.11 Hard foulant adhesion strength on SKINS as a function of the kirigami index, which exhibited a minimum around κ ≈ 1: a) ice adhesion strength on UHMW-PE SKINS, and b) pressure-sensitive tape adhesion strength on CA SKINS. Figure 3.12 Mechanical response of CA SKINS with changing kirigami pattern geometry. a) The kirigami patterns showed two distinguishable elastic moduli during the tensile test. These two linear regions indicated in-plane and out-of-plane deformation of the kirigami units upon increasing strain. b) The CA kirigami patterns with higher elastic moduli absorbed more strain energy during deformation, for both in-plane and out-of-plane deformation stages. Strain energy was calculated by numerically integrating the stress-strain data in (a) using MATLAB. The reduced adhesion was general for SKINS fabricated from other plastics (Figure 3.14a-b), such as polycarbonate (PC), polyester, polyoxymethylene (Acetal), ultra-high molecular weight 55 polyethylene (UHMW-PE), and polytetrafluoroethylene (PTFE). For all polymers, using ice as the foulant, both the base material and the material with an optimized motif of kirigami cuts displayed high adhesion when fully attached to the substrate (Figure 3.14d). Suspending the material reduced the ice adhesion somewhat, about 2× for PC and over 5× for Acetal and UHMW-PE (Figure 3.13). Figure 3.13 The sheet thickness of SKINS was another parameter that affected the adhesion strength of hard foulants. Increasing the thickness of the optimized UHMW-PE SKINS decreased the elasticity of the kirigami, resulting in an increased ice adhesion strength. Each experiment was repeated at least 4 times. SKINS enhanced this adhesion reduction, with UHMW-PE and PTFE exhibiting ice adhesion strengths of 21 ± 4 kPa and 24 ± 9 kPa, respectively. However, a meaningful comparison is not to the sheet material but with respect to the protected substrate. The ice adhesion strength of bare aluminium is around 1,200 kPa41, and therefore UHMW-PE SKINS exhibited a roughly 60-fold reduction in adhesion compared to an unprotected aluminium surface. 56 Figure 3.14 a) Ice adhesion strength versus wettability for both suspended smooth sheets and SKINS. b) Ice adhesion strength of six different materials, comparing the fully constrained, smooth material, the fully constrained material with the kirigami pattern, the smooth and suspended material, and SKINS. Each experiment was repeated at least 4 times. Low surface energy polymers like PTFE or UHMW-PE should exhibit lower adhesion with water-based adhesives like ice or mud. Meuler et al. showed that the ice adhesion strength of smooth, hard polymer surfaces is proportional to the practical work of adhesion, Wa ∝ 1+cos θrec42. This was observed for both the base polymer materials and the same smooth sheets when suspended (Figure 3.14a-b) 43–46. However, for SKINS all ice adhesion strengths were statistically equivalent. The low foulant adhesion was controlled solely by the mechanical deformation induced by the SKINS design, and not surface chemistry. Similar results have been observed for the adhesion of ice to smooth, thick elastomers47. This is advantageous because SKINS can therefore be fabricated by sustainable, inexpensive, and commonplace polymeric materials. In addition, substrate bond strength is enhanced for SKINS fabricated from high surface energy polymers. The adhesion of other foulants was also explored (Figure 3.15a-b), including paraffin and carnauba waxes, dried mud, and reactive polymer resins like epoxy, which has been previously used as a bio-foulant simulant48. For all foulants the observed adhesion strength of SKINS was substantially a b PTFE 57 lower than the foulant adhesion on the suspended material, which in turn was an order of magnitude lower than the adhesion to the underlying aluminum substrate. The exact adhesive bond strength depended on the chemical and physical nature of the foulant. For example, epoxy forms strong chemical bonds with the surface and we reached our load cell capacity before being able to detach epoxy from bare aluminum. Similarly, epoxy adhered strongly to SKINS fabricated from high surface energy polymers like CA but was easily removed from UHMW-PE SKINS. The choice of SKINS material will therefore also depend on the types of foulants required to be repelled in the chosen engineering application. Figure 3.15 a) Adhesion strength of six different hard foulants: paraffin wax, pressure-sensitive adhesive tape, carnauba wax, ice, dried mud, and epoxy. Whereas the epoxy adhesive was adhered on UHMW-PE SKINS, and all other foulants were evaluated using CA SKINS. b) Optical top-down image of the six hard foulants adhered to SKINS. The interfacial area was 1 cm2. The first study (Figure 3.2a) investigated how the presence of the SH underside altered the observed adhesion, using ice and carnauba wax and representative foulants. Because ice froze much more slowly than wax, the SH spray reduced leakage only for the ice/SKINS system, as evidenced by an increased adhesion strength for SKINS without the SH coating. Leakage was also a b 58 visually apparent during some ice adhesion tests without the SH coating. However, for carnauba wax the SH spray was not needed (statistically equivalent adhesion strengths with/without the SH coating). In the second set of experiments (Figure 3.2b), ice was frozen on SKINS fabricated from CA or UHMW-PE. The SH underside lowered the observed ice adhesion strength more when the plastic sheet was hydrophilic, where leakage was more pronounced. Overall, while the SH underside of SKINS was not always needed, it did broaden the generalizable applicability of SKINS to a wider range plastics for fabrication, and reduced the adhesion further for a larger number of fouling substances. To characterize the durability of SKINS, we measured the adhesion strength of ice and carnauba wax on the optimized CA SKINS over repeated cycles of adhesion tests. Figure 4 demonstrates that SKINS were able to maintain low foulant adhesion strength over the 30 cycles of foulant deposition and removal for both investigated foulants (Figure 3.16b, d). The average adhesion strengths of ice and carnauba wax on the optimized CA SKINS were 72 ± 27 kPa and 23 ± 9 kPa, respectively. An independent student’s t-test indicated that the different in adhesion strength of the first adhesion cycle and 30th cycle was statistically insignificant, for both ice and carnauba wax. Recall that the wrinkling of the kirigami surface of SKINS results in the low foulant adhesion strength. This wrinkling could be affected if the mechanical properties of the kirigami substrate was altered after the 30 adhesion cycles due to plastic deformation. To verify that SKINS remained in the elastic regime during foulant removal, stress-strain testing was also performed before and after the 30 repeated cycles of adhesion (Figure 3-16a, c). The mechanical response of CA SKINS showed good consistency before and after repeated adhesion tests. Moreover, as the mechanical properties of SKINS depend on the substrate material, even higher durability requirements could be met simply by selection of a different plastic. 59 Figure 3.16 The durability of optimized CA SKINS. The mechanical properties of CA SKINS before and after 30 cycles of a) ice and b) carnauba wax adhesion tests. The insets show stretched SKINS before and after the 30 adhesion cycles. The adhesion strength of c) ice and d) carnauba wax on the optimized CA SKINS over 30 cycles of adhesion. Each cycle for adhesion measurement includes at least 4 repeats. The interfacial area of commonly fouled surfaces is typically many square meters, such as the length of overhead transmission lines or the exterior hull of a marine vehicle. As such, a useful anti-fouling surface should maintain its low foulant adhesion for larger foulants. In addition, Golovin et al. recently showed that the design of low-ice-adhesion coatings differs depending on the interfacial length; low adhesion is not guaranteed over varying length scale28. Accordingly, we studied the adhesion of ice and pressure sensitive adhesive to SKINS over different length scales 60 by independently varying the dimensions of the foulant along and perpendicular to the direction of the applied load (Figure 3-17a-b, 3-18). Figure 3.17 Scalability of SKINS on flat and curved surfaces. a) The force required to detach increasing lengths of adhered ice from both suspended CA and CA SKINS. b) The impact of ice width and length on the observed ice adhesion strength of the optimized CA SKINS. Each experiment was repeated at least 4 times. The force necessary to detach ice from suspended CA displayed the characteristic linear and plateau regions of interfacial fracture indicative of strength-controlled and toughness-controlled de-bonding28 (Figure 3-17a). However, SKINS displayed a linear increase in the interfacial fracture force with increasing length of ice, Lice, and was substantially lower at least up to Lice = 190 mm. Whereas the ice adhesion strength of SKINS was constant with increasing ice length, a substantial decrease was observed when increasing the width of ice, Wice. This was expected because strain energy can be dissipated through increased wrinkling but only along the length of the foulant. Accordingly, we observed that the number of wrinkles increased with increasing length of foulant but was constant as foulant width increased (Figure 3.5). Similar trends were observed with tape adhesion to SKINS (Figure 3-18b). a b 61 Figure 3.18 Hard foulants’ adhesion strength on SKINS as a function of foulant size. The increase in foulant width resulted in a larger decrease of the measured adhesion strength. a) SKINS ice adhesion strength decreased with an increase in the length and width of the foulant on the optimized UHMW-PE SKINS, and b) similarly for the pressure-sensitive adhesive foulant adhered to the optimized CA SKINS. SKINS were also able to reduce foulant adhesion over curved substrates which would be beneficial to many engineering surfaces such as airfoils, pipelines, or cordage. When adhered to a semi-circular acrylic pipe, SKINS was still able to lower the adhesion of ice, even below 5 kPa when the length of the piping was Lice = 14 cm (Figure 3-19a, 3-2c). Simple flexure was sufficient to de-bond a 30-cm length of accreted ice from SKINS protecting a curved surface, whereas the ice cohesively fractured without SKINS (Figure 3-19b). Flexure would be an attractive means of de-icing curved structures like powerlines. Accordingly, even when SKINS were used to protect a 9-cm length of actual overhead transmission line, the observed ice adhesion strength was 4.5 kPa (Figure 3-20, materials). This is the first demonstration of τ < 10 kPa ice adhesion strengths on a curved surface, and 300 times lower than the ice adhesion strength of bare, flat aluminum. Overall, the highly tunable geometry, independence of surface chemistry, and ultra-low foulant adhesion for a wide range of fouling substances makes SKINS a promising anti-fouling technology. a a b 62 Figure 3.19 a) Observed ice adhesion strength of SKINS wrapped around a semi-cylindrical acrylic substrate (Methods). b) Ice adhered to a semi-cylinder coated with suspended CA (left) and CA SKINS (right) before and after flexing, highlighting the ease at which ice is removed from SKINS. Mechanical metamaterials as general antifouling surfaces exhibit both exciting engineering properties as well as several unsolved challenges. The facile preparation of SKINS results in low foulant adhesion strengths that are independent of surface energy. This is attractive as SKINS may therefore be fabricated by readily available commodity plastics. The properties are also highly tunable through the geometric properties of the chosen kirigami motif. However, while foulant adhesion to SKINS is low, an external load is still required to de-bond adhered foulants. The environmental conditions will therefore determine if SKINS is a passive or active antifouling solution for a given industrial application. Moreover, for submerged applications such as marine fouling, the hydrostatic pressure may reduce the efficacy of SKINS through liquid penetration or deflection of the suspended kirigami sheet until it touches the protected substrate (and can no longer wrinkle). Use of a soft elastomer replacing the suspension, such as what was demonstrated in Figures 3-19 and 3-20, would appear to mitigate this issue. Further, while SKINS demonstrates scalability (Figure 3-19a), applications requiring a large planar area to be protected may require additional research, as fouling would occur directly above the suspension areas. Figure 3-6 a b 63 suggests that when the entirety of the sheet is confined, the observed foulant adhesion is very high. However, it is currently unknown how the foulant adhesion would change if it were adhered to both the confined and suspended areas of SKINS. Figure 3.20 CA SKINS protecting a 9-cm length of an overhead transmission cable, with an extremely low observed ice adhesion strength of 4.5 kPa. 3.4 Conclusions In conclusion, SKINS exhibit 3D deformation that enables mixed-mode fracture propagating at the interface. The 3D deformation causes wrinkling with a wavelength controlled by the geometric properties of the kirigami sheet. SKINS were able to decrease the adhesion strength of several different foulants regardless of the surface energy of the plastic sheet used for fabrication. SKINS also exhibited durability against repeated cycles of foulant adhesion and removal, and this was found to be independent of foulant type. The flexible and planar SKINS can also be applied to large, curved surfaces such as a powerline cables, and exhibited substantially lowered ice adhesion, demonstrating their practical utility. However, as this is the first demonstration of reduced adhesion using kirigami surfaces, many more fundamental scientific questions remain. For 64 example, here linear cuts were used for our kirigami motifs, but it is unknown how other types of kirigami structures (fractal-cuts, closed-loops, etc.) could change or even improve the antifouling performance of SKINS. Moreover, while we showed that SKINS decreased the adhesion strength of a pseudo-biofoulant, a realistic anti-biofouling application would require the complete submersion of SKINS under water. It would be worthwhile to investigate how fluid surrounding SKINS affects the antifouling performance. Alternatively, replacing the space beneath SKINS with a soft rubber, such as what was demonstrated in Figure 5f, might alleviate any issues associated with submersion altogether. Other future applications of SKINS include the de-icing of solar panels in cold and harsh conditions, as long as the base plastic sheet is selected from optically transparent polymers, such as CA, PC, or acrylic. All in all, we believe SKINS to be a platform technology representing a purely mechanical approach for decreasing the undesired adhesion of various fouling substances. 65 Chapter 4: Smart low interfacial toughness coatings for on-demand de-icing without melting Ice accretion causes problems in vital industries and has been addressed over the last decades with either passive or active de-icing systems. Considering that the latent heat of melting ice (334 J/g) is about 160⨉ greater than ice’s specific heat capacity (2.09 J/g °C), a hybrid de-icing system that could avoid melting would provide substantial energy efficiency benefits. This chapter presents a smart, hybrid (passive and active) de-icing system through the unique combination of a low interfacial toughness (LIT) coating, mini printed circuit board heaters, and an ice detecting metamaterial microwave sensor. The novelty of this work lies in the exploration of temperature-dependent interfacial properties of LIT coating and ice, a subject that has not been studied previously. The coating’s interfacial toughness with ice is found to be temperature dependent and can be modulated using embedded heaters. For example, the coating’s interfacial toughness at -30 °C decreases from 5.7 to 0.9 J/m2 when the heaters hold just 10% of the surface at -10 °C. The synergistic combination of the LIT material and periodic heaters results in a 100⨉ greater de-icing power density than a full-coverage heater system. The hybrid de-icing system also shows excellent durability towards repeated icing/de-icing, mechanical abrasion, outdoor exposure, and chemical contamination. Scalability is demonstrated using large (~ 1 m) iced interfaces. Further, a microwave sensor is designed to precisely detect the presence or absence of water or ice on the surface while operating beneath the LIT coating. Using a shift of 0.138 GHz in the resonant frequency and -2.76 dB in the resonant amplitude, the noncontact embedded sensor successfully monitors the status of the LIT coating wirelessly throughout the entire icing/de-icing process. 66 Overall, smart LIT materials offer a paradigm shift in de-icing that can efficiently render a surface ice-free without the need for energetically expensive interface melting. 4.1 Introduction Materials exhibiting low interfacial toughness (LIT) with ice represent a paradigm shift in how the adhesion between ice and a surface may be reduced, especially large (>cm) iced interfaces85,97. LIT materials minimize the strain energy necessary to propagate an interfacial crack between the ice and surface, enabling size-independent de-icing, i.e. requiring a constant applied force for ice removal irrespective of the size of the iced interface. To-date, various LIT materials have been reported, including polymers such as polypropylene, PTFE, and ultra-high molecular polyethylene (UHMW-PE)85, as well as aluminum-based quasicrystalline coatings97. Zeng et.al introduced a LIT coating comprised of porous PDMS that exhibited lower interfacial toughness and hydrophobicity with increasing porosity134. Dhyani et.al fabricated transparent LIT PDMS and polyvinylchloride (PVC) coatings for photovoltaic applications, simultaneously demonstrating both a low interfacial toughness and ice adhesion strength135. Yu et al. fabricated robust LIT coatings based on PTFE particle assemblies, where the interfacial toughness was maintained after repeated icing and de-icing cycles136. And yet, to-date LIT materials have only been used as passive de-icing coatings. In this work, we develop hybrid LIT de-icing coatings based on UHMW-PE. Whereas all previous hybrid de-icing technologies have required energy-intensive ice melting, incorporating LIT materials enables mechanical de-icing that circumvents the melting step. The mechanical properties of both the LIT coating and ice determine the toughness and strength of their adhesive interface. Accordingly, the influence of the elastic modulus on ice adhesion strength and interfacial 67 toughness is first measured at varying temperatures (-5 °C to -60 °C) for both the LIT material and ice. Next, we study the effect of thermal loading on the interfacial toughness using miniature printed circuit board resistive heaters. A comprehensive study using several lengths of ice is carried out to optimize the voltage required to increase the surface temperature to -5 °C, where the lowest interfacial toughness with ice was observed. The effect of the supplied heat on the coating’s interfacial toughness with ice is investigated by applying the optimal voltage. The coatings are further rendered ‘smart’ through the inclusion of an embedded microwave resonator sensor, enabling on-demand de-icing where the active system can be shut off immediately once the sensor detects that the surface has been de-iced. The microwave sensor consists of a split ring resonator and transmission lines and operates by exploiting the large difference in dielectric properties between water and ice, as previously demonstrated25,137,138. At the applied optimal voltage, the sensor's response to the presence and absence of ice is also recorded. 4.2 Experimental Section 4.2.1 Materials LIT coating The LIT film was ultra-high molecular weight polyethylene (UHMW-PE) with an adhesive backing and a thickness of 0.127 mm (McMaster Carr, Catalog No. 1441T11), used as received. Microwave sensor and heater The planar microwave sensor and the mini PCB resistive heaters were fabricated on Rogers RT/Duroid® 5880 Laminates, with permittivity of 2.2, loss tangent of 0.0009, dielectric thickness of 0.79 mm, and copper cladding thickness of 35 µm (Rogers Corporation, Ltd.). 68 4.2.2 Methods 4.2.2.1 Characterization of LIT coating's mechanical properties The storage and loss moduli of the UHMW-PE were measured using a Dynamic Mechanical Analyzer (TA Instruments) at an applied frequency of 1 Hz using a temperature sweep from -60 °C to +30 °C. The strain was not constant during the temperature sweep as the instrument was setup in auto strain adjustment mode. However, the strain variation measured was negligible (0.05% to 0.03%) and within the linear viscoelastic range. 4.2.2.2 Preparing the LIT coating For experiments where the heater was not involved, the UHMW-PE film was adhered uniformly to an aluminum (Al) sheet with a thickness of .254 mm (McMaster Carr, Catalog No. 9708K58) using the adhesive backing. For experiments involving the heater, the UHMW-PE film was adhered directly to the fabricated device (heat and sensor), again using the adhesive backing. The topography of the UHMW-PE surfaces on the Al or the heater/sensor was measured using a LEXT™ OLS5100 3D Laser Scanning Microscope (Figure 4.1). 69 Figure 4.1 Optical and topography images using a laser microscope. a UHMW-PE sheet (Root-mean-squared roughness, Sq = 1.53 µm); b Mini PCB heater covered by the UHMW-PE material (Sq = 1.51 µm); c The microwave sensor covered by the UHMW-PE coating (Sq = 1.50 µm). The scale bar for all images is 5 mm. 4.2.2.3 Ice adhesion measurement without heater The force necessary to de-ice the LIT surface was measured using a custom push-off method described elsewhere85. Ice cubes were formed on the surface of the UHMW-PE film using 3D-printed polylactic acid (PLA) molds of various lengths (Lice = 5 – 200 mm). First, the molds were filled with deionized water at room temperature. Next, the temperature of the surface was decreased to the target temperature using the Peltier stage, and the water was allowed sufficient time to fully freeze (minimum 1 hour). Once frozen, a moving probe with a motorized linear stage was connected to a force gauge (NEXTECH, DFS500). The force gauge probe with cross section 5 mm µm 275.3 70 of 5 mm ⨉ 10 mm impacted the ice’s mold at a constant speed of 100 µm/s, and the detachment force was measured with 0.1 N accuracy. The ice adhesion measurements were conducted at varying temperatures (-40 °C to -5 °C). The temperature of the ice and surface of the coating was monitored using a BK Precision 725 thermocouple with an accuracy of ±0.7 °C. After each measurement, the coating was cleaned with isopropyl alcohol (VWR International) using a Kim wipe (KimTech). The ice adhesion strength (τice) and interfacial toughness with ice (Γ) are important parameters measured to fully characterize the interface between a surface and ice85. In the strength-controlled regime of fracture, the force to dislodge ice (𝐹𝐹ice) is used to measure τice using the interfacial area, A, or τice = Fice / A. In the toughness-controlled regime of fracture that occurs for longer interfaces, this force plateaus at some critical value, Fc. One can calculate the toughness of the ice/coating interface using the measured Fc value, the modulus of ice, E, and the ice thickness, Hice, as Fc = 85. The interfacial length where the fracture transitions from strength- to toughness-mediated fracture is commonly referred to as the critical length, Lc. All of these parameters may directly depend on temperature, or indirectly due to temperature-dependent material properties. Accordingly, the ice adhesion strength and interfacial toughness of UHMW-PE with ice were measured from -5 °C to -40 °C and using ice lengths from 5 to 200 mm (Supplementary Figure S2, S3). The calculation of τice, Γ, and Lc from the Fice versus length measurements was performed following the procedure outlined in Golovin et al.85 For some experiments the measurement of Fice for longer lengths of ice served as a substitute for directly measuring Γ, and for such cases we assume Fice = Fc. 4.2.2.4 Sensor and heater design and fabrication A planar microstrip microwave sensor was designed and characterized to detect the presence or absence of ice and water on the surface. A planar microstrip sensor consists of traces of copper 71 that resonate at a specific frequency when arranged in a particular configuration, known as the resonant frequency. We recently utilized the large difference in the dielectric properties between water and ice for sensitive and accurate water, frost, and ice detection via planar microstrip resonators25. The microwave split ring resonator (SRR) sensor was designed in Ansys High-Frequency Structure Simulator (HFSS, see Figure 4.2). The sensor operated at a resonant frequency of 2 GHz, which was selected due to the difference in dielectric properties of water and ice at this frequency. Further, the sensing structure was optimized to exhibit a sharp bandpass response. The resonant frequency of a microstrip line is governed by the length of the SRR calculated using equation (4.1): 𝑙𝑙 = 𝑐𝑐2∗𝑠𝑠𝑟𝑟𝑟𝑟𝑟𝑟√𝜀𝜀𝑟𝑟 4.1 Here c is the speed of light (3 × 1011 mm/s), 𝑓𝑓𝐵𝐵𝐶𝐶𝐶𝐶 is the resonant frequency (2 GHz), and 𝜀𝜀𝐵𝐵 = 2.2 is the relative permittivity of the microstrip line. The calculated length of the SRR at 2 GHz was 50.7 mm. However, since the capacitances between the feedline, the SRR, and the split ring gap influence the resonant frequency, the length of the resonator was optimized in HFSS to 61.6 mm to achieve the desired resonant frequency. The dimensions of the final sensor design are shown in Figure 4.2. Mini PCB heaters are simple traces of copper arranged in a small, confined space with a resistance chosen to output the desired amount of heat. A resistive heater with a resistance of 5.9 Ω was additionally patterned on the substrate to provide the active de-icing capabilities. The heaters were placed at a distance of 3 cm and 12 cm from the sensitive region of the sensor (Figure 4.2). The sensor and the heater were fabricated following standard PCB fabrication methodologies.139 72 Figure 4.2 Dimension and spacing of heaters and sensor. a) SRR sensor and resistive heaters modeled in HFSS. b) Optical image of fabricated SRR sensor and resistive heaters. 4.2.2.5 Smart, active de-icing using the LIT coating The sensor response was monitored with a Keysight Technologies N9918A Vector Network Analyzer (VNA). The VNA measures the S-parameters of the microwave sensor over a wide range of frequencies. Similar to the ice adhesion testing on the bare LIT material, deionized water was poured in a 3D-printed mold on the surface of the LIT material and frozen at -25 °C (Figure 4.3). The temperature of the ice frozen directly above the heater was also measured using a thermocouple placed within the ice. To do so, the mold was half-filled with water, the thermocouple was inserted, and the water was allowed to freeze. Once the thermocouple was secured in the frozen ice and the temperature stabilized, the heater was engaged to raise the surface temperature locally around the heater from -25 °C to -5 °C. Similar to the testing procedure above, the de-icing performance was measured while engaging the heater and detecting the ice via the sensor. We define the de-icing time (tD) as the amount of time required for the heaters to increase the surface temperature to a target temperature, either -5 °C or -10 °C in this work. Note, however, that without the application of the external mechanical load, simply raising the temperature to -5°C does not de-ice the surface. Once at the target temperature, the force gauge probe impacted the adhered ice within the mold, and the detachment force was measured. The temperature of the ice 0.5 mm above the heater was also measured as -25 °C throughout the experiment, confirming Feed li SRR Resistive heaters 23 cm 8 cm 3 cm 12 cm a b 73 that the heating was indeed localized to the surface. While finding the correct operating voltage, the force probe was not engaged, and the ice filled mold was left intact. Figure 4.3 The experimental setup consisted of a VNA, power supply, cold Peltier stage, temperature data logger, PCB heaters, and the split ring resonator sensor. 4.2.2.6 Durability and Scalability Analyses Cyclic icing/deicing The LIT coating was applied on the surface of the heater panel and icing/de-icing cycles were performed using different lengths of ice (60 – 150 mm). The de-icing force was again measured at -20 ºC using a force gauge with accuracy of 0.1 N. At least 5 repeats were performed for each ice length. Following this, 13 additional icing/de-icing cycles were performed on this same sample using 150 mm lengths of ice. Finally, the 5 repeats of the 60, 80, and 100 mm pieces of ice were cyclically iced/de-iced, for a total of 43 repeated measurements on the same LIT/heater sample. Lateral shear f 3D printed mold Heaters SRR Peltier stage Temperature data logger N9918A vector network analyzer Power supply 74 Outdoor exposure The hybrid de-icing system was placed outdoors for 3 weeks in Toronto, ON, Canada, which included daily thermal fluctuations and one heavy rainstorm event on May 21, 2022 (see the topography changes in figure 4.4e). Afterwards, the de-icing force was measured using several lengths of ice at -20 °C. Surface contamination The LIT coating was contaminated following the work of Zarasvand et al.140, by pipetting acetone onto its surface and allowing it to evaporate (Figure 4.4b). The de-icing force for various ice lengths was then measured at -20 ºC, with at least 5 repeats for each length. Mechanical Abrasion The LIT coating was abraded using 800-grit silicon carbide electrocoated sandpaper (Alibaba Group, China). A power sander (RYOBI 1/3 Corded Sheet Sander, China) was then used to constantly abrade the material at 12,000 rpm for 15 min. The roughness and topography of the coating before and after abrasion were measured using a LEXT™ OLS5100 3D Laser Scanning Microscope (Figure 4.4c). After abrasion, the de-icing force of the coating was measured at -20 ºC for various lengths of ice (Figure 4.4). 75 Figure 4.4 Optical and topography images after durability tests. a) UHMW-PE sheet before exposure (Sq = 1.53 ± 0.05 µm) and after b) Chemical contamination by acetone (Sq = 1.92 ± 0.08 µm); c) mechanical abrasion (Sq = 3.4 ± 0.3 µm); d) Cyclic icing/de-icing (Sq = 1.4 ± 0.2 µm); and e) outdoor exposure (Sq = 1.5 ± 0.3 µm). The parameter of Sq is the Root Mean Square Roughness. (Top images are optical and bottom images are topography heightmaps). All scale bars are 300 µm. Large-scale de-icing using multiple heaters To evaluate the scalability of our hybrid de-icing system, a scaled-up version was designed (see supporting information, Figure 4.5a). All large-scale de-icing tests were run in a walk-in freezer (Climate Lab, KITE, at the University Health Network, Toronto, Canada) where the average temperature of the room during our 3 days of testing was -18 ± 1 ºC with a RH of 75 ± 5%. Using the same heater fabrication method as above, four identical heater panels with dimensions of 24 mm ⨉ 80 mm (W ⨉ L) were prepared. The panels were then installed on an Al sheet attached to wooden supports. The surface of the 4 panels was coated with one single UHMW-PE film measuring 80 mm by 960 mm (Figure 4.5b). Each heater was connected to a separate power supply set at the optimized voltage. To form a large piece of ice a silicone rubber mold with prescribed internal dimensions of 920 mm ⨉ 20 mm ⨉ 20 mm (L ⨉ W ⨉ H) was prepared and placed on the a b c d e µm 65.3 0 76 LIT coating. Next, the rubber mold was filled with deionized water and left to fully freeze. Once fully frozen, the rubber mold was removed and a 3D printed guard was placed around the front end of the ice, such that the force probe tip did not contact the ice directly. The de-icing force was then recorded using the same moving stage and force gauge as above. These tests were repeated at least 5 times. For an ice length of 500 mm the de-icing process was recorded on video, enabling us to monitor the crack propagation front in real time. Space limitations within the freezer prevented recording the larger, 920-mm-long ice detachment process, but the results were visually similar. Figure 4.5 De-icing force measurements in walk-in freezer at -20°C. a) The ice adhesion setup involving the UHMW-PE coated substrate, heaters, ice, and mechanical detachment instrumentation; b) Top view of LIT-coated substrate decorated with four heaters with dimensions of 2.54 cm ⨉ 8 cm. 77 4.3 Results and discussion 4.3.1 Effect of temperature on mechanical properties of coating, ice and their interface Before altering the temperature using an active de-icing system, the mechanical properties of the ice and LIT coating were first investigated to understand how they are affected by temperature, either directly or indirectly (for example, due to a change in elastic modulus). Both the interfacial toughness and ice adhesion strength depend on the mechanical properties of the coating and ice. The effect of temperature on the dynamic elastic modulus of polycrystalline ice has been previously measured to follow,141 𝐸𝐸 = [.104(1+1.07 × 10−3 T+1.87×10−6 T 2)]-1 ± 1% 4.2 Here E is the elastic modulus of ice in GPa and T is the temperature in ℃. Based on this equation, the modulus of ice decreases ~ 5% as the temperature increases from -40 °C to -5 °C. The elastic modulus of the coating was investigated using Dynamic Mechanical Analysis (Methods). The loss modulus was statistically constant between 25 °C and -60 °C (Figure 4.7a), whereas the storage modulus increased 22% when the temperature decreased from -5 °C to -40 °C. Accordingly, neither the ice nor the coating’s mechanical properties varied substantially over the thermal range investigated here and did not significantly affect the observed interfacial toughness and ice adhesion strength values discussed below. 78 Figure 4.6 The effect of temperature on the mechanical properties of the coating and its interface with ice. a) Storage and loss moduli of UHMW-PE between 25 °C and -60 °C. b) The toughness of the ice-UHMW-PE interface between -40 °C and -5 °C. c) The ice adhesion strength of UHMW-PE between -40 °C and -5 °C. Values for strength and toughness were taken from the linear and plateau regions of ice length versus removal force plots (Methods) as described previously85. Dashed lines are selected based on the best poly-fit on average value of error bars. The interfacial toughness and ice adhesion strength were clearly affected by temperature (Figure 4.7b-c). As can be seen in Figure 4.8, the asymptotic force required to remove large-scale ice increased from Fc = 145 N/cm at -5 °C, to Fc = 237 N/cm at -30 °C. This corresponds to an increasing interfacial toughness with ice from Γ = 2.2 ± 0.5 J/m2 to Γ = 5.6 ± 1.2 J/m2 over the same thermal range. However, at -40 °C cohesive detachment was observed where a portion of the ice remained on the surface after partial interfacial fracture. While the critical ice removal force decreased to 207 N/cm (statistically different from -30 °C and -20 °C, based on a Student’s t-test), with cohesive fracture this is no longer a true interfacial property. As discussed above, ice is at most 5% stiffer at the lower temperatures investigated here. This should have decreased the interfacial toughness, which was instead 2.2 times larger at -30 °C than at -5 °C. The ice adhesion strength was similarly 1.8 times greater (Figure 4.7b-c). Moreover, As the temperature was lowered, more force was required for ice removal. At -40 °C cohesive fracture was observed for 79 the majority of the measurements in the strength-controlled fracture regime, though adhesive fracture was observed when toughness controlled the interfacial de-bonding. Accordingly, the increases in interfacial strength and toughness cannot be explained as an indirect effect due to the change in mechanical properties of the interface. Instead, it would appear that both properties increase with decreasing temperature, at least for the UHMW-PE investigated here. This increase has been previously reported, but for only the ice adhesion strength, by several groups142–147. Figure 4.7 Effect of temperature on the interfacial properties between ice and the UHMW-PE material. a) Schematic of ice adhesion measurement on the LIT-coated Al sheet. b-f) The shear force per width required to remove the ice at different temperatures as a function of ice length: b, -5 °C, c, -10 °C, d, -20 °C, e, -30 °C, f, -40 °C. 4.3.2 Optimization of the operating voltage for joule heating With the thermal properties of the ice/LIT coating well-characterized, active de-icing was then studied using the mini PCB heaters (see experimental section). Initially, the heater was operated at 10 V, and a 20 mm long portion of ice above the LIT-coated heater (equal to the size of the 80 heaters, Figure 4.8) was melted. When trying to dislodge this partially melted ice, cohesive fracture and ice shattering occurred (Figure 4.8b). Accordingly, completely melting the ice adhered directly above the heater was actually detrimental to the performance of the LIT coating by causing cohesive fracture. Figure 4.8 Effect of melting ice at LIT-coated heater on de-bonding force. a) The force per width necessary to detach the ice from the LIT-coated heater for four different removal scenarios: (1) with the heater turned off at T = -20 °C; (2) with the heater turned off at T = -5 °C; (3) with the heater turned on such that TH = -5 °C although the remainder of the surface was at T = -20 °C; and (4) allowing the heater to melt the ice directly above it, also at T = -20 °C. b) Optical images of the ice when the heater was allowed to melt the ice directly above it. The top image shows the ice before applying the shear force. The middle image shows the shattered ice after cohesive failure. The bottom image shows the extent of the ice shattering with the 3D-printed mold removed. Given the results above, the operating voltage was then optimized so as to raise the interfacial temperature above the heater, but keep it < 0 °C. The Peltier stage held the overall system 81 temperature at T = -25 °C while the mini PCB heater was used to increase the temperature of the interface between a 150 mm long piece of ice and the UHMW-PE (Figure 4.10). To optimize the voltage for efficient thermal control, 0.5 V was initially supplied to the heater with a gradual increment of 0.5 V until the desired surface temperature was achieved after 30 seconds with the heaters on. Three temperature probes were installed to monitor the changes in the ice and interface temperatures (Figure 4.9 and 4.10). The first probe measured the temperature at the surface of the heater/LIT coating (TH). Two other temperature probes both measured the temperature inside the ice adhered to the LIT coating (Tice), either directly above the heater or 5 cm away along the length direction (Figure 4.10b). Figure 4.9 De-icing force measurements. Schematic setup for monitoring the ice and coating temperature during de-icing. At the target temperature of the LIT coating above the heater, TH, the force probe impacts the ice in shear in order to remove it from the surface. The parameters T and TH represent the overall system temperature and the temperature at the surface of the heater/LIT coating, respectively. To confirm that the heating was localized, first Tice directly above the heater and 5 cm away were compared (Figure 4.11a). Tice 50 mm away from the heater remained relatively constant for all input voltages tested, increasing at most 4 °C using 4.4 V. Tice directly above the heater increased with increasing voltages above 1.5 V, and at 4.4 V reached the desired temperature of -5 °C within 30 seconds of heating. For a piece of ice measuring 150 ⨉ 10 ⨉ 5 mm, 4.4 V over 30 seconds 82 increased both Tice and TH from -25 °C to -5 °C (Figure 4.1`a-b). The rate of change in Tice and TH were statistically equivalent at 5.0 ± 0.2 °C/V and 4.9 ± 0.2 °C/V, respectively. Decreasing the length of the ice from 150 mm to 20 or 60 mm lengths of ice did not affect these results (Figure 4.11c-d), and accordingly 4.4 V was used as the de-icing voltage for the remainder of this work. Figure 4.10 Operational power supply voltage optimization at T = -25 °C. a Ice temperature, Tice, as a function of mini PCB heater voltage, directly above the heater and 50 mm away along the length direction. The temperature of the interface above the heater (TH) and the temperature of the ice above the LIT material and heater (Tice), monitored for various lengths of ice: a and b 150 mm, c 60 mm, and d 20 mm. The target temperature of TH = Tice = -5 °C was reached after 30 seconds at 4.4 V for all three lengths of ice. 83 4.3.3 Using heaters to control LIT properties The coating’s interfacial toughness with ice, Γ, is temperature dependent (Figures 4.7 and 4.8), and the mini PCB heaters can be used to control the temperature of the interface (Figure 4.10). Accordingly, we investigated if the mini heaters could lower the interfacial toughness, using the 4.4 V optimized above. The force per width necessary for ice removal, Fice, with the heater operating at 4.4 V was measured for lengths of ice greater than 50 mm, well within the toughness-controlled regime of fracture. Two experiments were performed, one at T = -20 °C with the heater set to TH = -5 °C, and the other at T = -30 °C and TH = -10 °C. For both experiments, the Fice values corresponded much more closely with the values recorded when the entire system was held at TH rather than T (Figure 4.12a-b). For example, for an iced interface length of 150 mm, Fice = 290 ± 50 N/cm at -20 °C and Fice = 172 ± 15 N/cm at -5 °C (Figure 4.8). For T = -20 °C and the heater set to TH = -5 °C, the de-icing force was Fice = 157 ± 30 N/cm, statistically equivalent to the T = -5 °C value. Similar results were observed for other lengths of ice, as well as when using T = -30 °C and TH = -10 °C (Figure 4.11b). Accordingly, the mini PCB heaters could modulate the de-icing force and decreased the interfacial toughness from Γ = 4.8 J/m2 to 1.3 J/m2, and from Γ = 5.7 J/m2 to 0.9 J/m2 (Figure 4.11c), for the experiments shown in Figure 5a and 5b, respectively. This active de-icing system therefore has the capability to turn a high-toughness interface into a LIT interface on-demand, facilitating large-scale ice removal without melting. 84 Figure 4.11 Using the mini PCB heater to reduce the de-icing force, Fice, by locally increasing the temperature from T to TH, for different lengths of ice. a T = -20 °C increased to TH = -5 °C. b T = -30 °C increased to TH = -10 °C. c The effective reduction in interfacial toughness by locally increasing the temperature from T to TH using the mini PCB heaters. As toughness-mediated fracture is an energy release process, it is likely that increasing the temperature locally compensated for the energy release required for fracture propagation at the interface. As this thermal difference increases, a lower external load is required to propagate fracture. This is in good agreement with the results in Figure 4.12, where the interfacial toughness was reduced more when a larger thermal shift was initiated using the mini PCB heaters. Recall that, for toughness-mediated interfacial fracture, 𝐹𝐹𝐶𝐶𝑐𝑐𝐶𝐶 = �𝛤𝛤𝐸𝐸𝐻𝐻ice , where Hice is the thickness of the ice.148 Using the heater to locally increase the interfacial temperature from T = -30 °C to TH = -10 °C, the de-icing force was measured for ice thicknesses between Hice = 5 – 20 mm (Figure 6a). Here, a representative ice length of Lice = 105 mm was used, well within the toughness-controlled fracture regime (Figure 4.8). The square root dependence between ice thickness and the measured de-icing force was maintained (Figure 4.12b), indicating that the mechanics of fracture were not substantially altered when using the PCB heaters even though the temperature at the interface was nonuniform. Thermal imaging provided additional evidence of this nonuniformity (Figure 4.12a), further corroborating that the heating was localized (Figure 4.8) and that the temperature of the ice nonadjacent to the heater maintained the ambient temperature. 85 Figure 4.12 Active LIT de-icing geometry optimization. a) Infrared images demonstrating that locally TH was achieved regardless of the ice thickness. b) The square root dependence of the de-icing force with the thickness of the ice. c Measured de-icing time for various lengths of ice at T = -30 °C. Given that the entire interface does not need to be heated to improve the LIT properties, an important design parameter is then the required number of heaters and their size and spacing along the interface. We investigated what length of heater, LH, was necessary to decrease the interfacial toughness for various ice lengths, Lice. Again 4.4 V was applied over 30 seconds, and the force required to dislodge the ice was recorded at T = -30 °C. As the portion of ice above the heater (LH/Lice) decreased, the required de-icing force increased (Figure 4.13a). However, this was most significant for LH/Lice ≤ 10%, and beyond this the decrease in Fice was minimal. Accordingly, heaters occupying only 10% of the total interfacial area are sufficient for effective de-icing. As 86 expected, when the length of the ice above the heater was equal to the size of the heater (LIC/LH = 100%), a maximum decrease in the de-icing force was observed (Figure 4.13b). However, to minimize power consumption, smaller heaters would be desirable. A statistically insignificant increase in the measured force of ice detachment was observed when the percentage of the heater covered was reduced to 25%. Accordingly, small heaters placed sparsely will still result in good LIT properties while minimizing power consumption. For our laboratory conditions, installing 15 mm long heaters every 135 mm was optimal in order to maximize de-icing while minimizing the power consumption. Figure 4.13The de-icing force of the LIT material as a function of the percentage of: a) the heater length over the length of ice (LH/Lice), and b) the length of ice above the heater over the total heater length (LIC/LH). The heat flux required for modulating the interfacial temperature would be 𝑄𝑄= 𝑈𝑈2𝑅𝑅-1𝑡𝑡𝐷𝐷, where Q is the heater’s electrical energy consumption, U is the supply voltage, R is the electrical resistance of the heater, and tD is the de-icing time.111,149,150 For our de-icing experiments the operating voltage (U = 4.4 V) and heater resistance (R = 5.9 Ω) were constant, resulting in 𝑄𝑄 = 3.28 𝑡𝑡𝐷𝐷. The tD for each length of ice was measured and statistically analyzed (t-test), which revealed that for interfacial lengths between 50 mm and 150 mm, the de-icing time remained constant (Figure a b 87 4.12c). These results further support that the generated heat was localized and only increased the temperature of the length of ice directly above the heater. According to the recorded average de-icing time (88 ± 9 s), the consumed electrical energy was measured as 289 J. Note that the additional mechanical power needed to fracture the interface (ΓA ≈ 1 mJ) is low and would arise naturally from environmental forces such as wind, drag, or centripetal acceleration (in the case of wind turbines). The areal power density is a commonly used metric to compare the efficacy of de-icing systems. Previously studies investigating aircraft de-icing have required 10-25 kW/m2 to achieve ice-free surfaces, with heaters covering the entire iced area151–153. For our designed de-icing system the heaters cover only 10% of the surface, which decreases the power consumption by an order of magnitude. Moreover, the heater's resistivity increases with the length of the printed copper on the substrate as, R= l/σa (σ = 5.8⨉108 S/cm, l =2.5 m, and a =1.08⨉10 -8 m2). Using the 4.4 V de-icing supply voltage, the power density of our heater is W = 𝑈𝑈2𝑅𝑅-1A-1 = 2 kW/m2 (A is the area covered by the heater). Accordingly, not only do the heaters cover just 10% of the total area, but their resistivity is also 10⨉ lower, overall resulting in heaters with a power density 100⨉ higher than the same heaters fully covering the surface. However, this is still an underestimate in the total efficiency gains as our heaters raise 10% of the interface to a subzero temperature, which would not de-ice even for complete heater coverage as it would need to bring the surface to at least 0 °C, and typically much higher.151–153 88 4.3.4 Durability and scalability In order for our designed hybrid de-icing system to find real-life usage, its performance must be consistent, durable, and scalable. In terms of consistency, the system was exposed to 43 repeated cycles of icing/de-icing. Initially, the ice detachment force for different ice lengths (L > Lc) was measured while the heater locally raised the interfacial temperature from -20 °C to -5 °C (Figure 4.14a). The critical detachment force for this first set was 131 ± 21 N, corresponding to an interfacial toughness with ice of Γ = 1.5 ± 0.4 J/m2. Additional icing/de-icing cycles were then conducted using 150 mm lengths of ice, followed by a repeat of the initial characterization. After these 43 icing/de-icing cycles, the average de-icing force was statistically equivalent (p-value: 0.22) to its initial value. The surface roughness was also unaffected (Figure 4.4), indicating that the process of icing and de-icing did not damage the surface. Figure 4.14 Durability and scalability of hybrid de-icing system. a) Cyclic icing/de-icing tests for ice lengths of 60 – 150 mm (L > Lc). Fice values before and after the icing/de-icing cycles are statistically equivalent (p-value: 0.22). b) The de-icing force or interfacial toughness required to remove various lengths of ice after mechanical abrasion, chemical contamination, and outdoor exposure for three weeks. 89 The de-icing force for various ice lengths in the toughness-controlled regime of fracture was also recorded after mechanical abrasion, chemical contamination, and outdoor exposure for three weeks (Figure 4.14b, see Figure 4.15 for full datasets). The UHMW-PE coating maintained its low ice detachment force for both outdoor exposure and surface contamination (p-value > 0.22), demonstrating the environmental durability of the hybrid de-icing system. Only the harsh abrasion increased the de-icing force statistically significantly (p-value : 0.002; Figure 4.14b). This was due to the increase in roughness of the LIT material, from Sq = 1.55 µm to 3.39 µm, and this was statistically significant (Figure 4.4). As interfacial toughness represents a strain energy per unit surface area, the commiserate increase in toughness with roughness was expected. Note, though, that the increase in interfacial toughness observed while using the heaters to modulate the local interfacial temperature of the abraded UHMW-PE (3.4 ± 0.9 J/m2) was still substantially less than that of the unabraded UHMW-PE film without heaters (6.1 ± 1.2 J/m2, see Figure 4.6). Accordingly, the hybrid de-icing system can compensate for any mechanical damage by using the heaters to achieve the required toughness value for a given application. Figure 4.15 De-icing force versus ice length of LIT material after durability characterization. a) After abrasion with 800 grit sandpaper for 15 minutes; b) After contamination with acetone residue; c) After exposure to the outdoors for 3 weeks in Toronto, ON, Canada. The parameters Fice and TH represent the force to dislodge ice and the temperature at the surface of the heater/LIT coating, respectively. 90 One advantage of de-icing using LIT materials is their scalability, as the de-icing force is constant for large lengths of ice85,97. To determine if our hybrid de-icing strategy was also scalable, we fabricated a larger-scale system measuring a full meter in length, and utilizing multiple heaters spaced periodically such that only 10% of the surface area was heated (in-line with the results from Figure 4.13a-b). The entire setup was then placed inside a walk-in freezer held at -20 °C and iced using a 2-cm-wide piece of ice (Figure 4.6). The de-icing force was measured for a 920 mm length of ice when the heaters modulated the local interfacial temperature to TH = -5 °C. Figure 4.15a shows the de-icing force as a function of ice length for our hybrid de-icing system with heaters underneath only 10% of the total iced area. The de-icing force necessary to dislodge the large-scale ice was statistically equivalent to the values observed for the smaller-scale testing (p-value: 0.08), confirming the scalability of the developed system. Scattering of the light between the LIT material and ice during interfacial separation also allowed us to monitor the crack propagation in real time (Figure 4.16b). After ~ 6 seconds the stored strain energy within the interface was released, and the surface was cleanly de-iced with no adhered residue remaining. 91 Figure 4.16 Scaled up de-icing. a) De-icing force for multi-heater hybrid de-icing system, up to a length of 920 mm. The inset shows the accreted and de-iced surfaces. b) Movie stills depicting the interfacial crack propagation and adhesive fracture underneath ice with a length of 500 mm and a width of 2 cm. All tests in a-b were conducted with 2.54 cm wide heaters locally raising the temperature -20 °C to -5 °C. 4.3.5 Smart active de-icing using a microwave resonator sensor Another consideration in the design of an efficient hybrid LIT de-icing system would be when to utilize the heaters and for how long. A ‘smart’ system could be envisaged through the addition of an ice sensor which could provide environmental information indicating when to switch the heaters on and off. We fabricated such a smart LIT de-icing system using an embedded microwave sensor (Methods), based on our previous work85. Such sensors operate by detecting shifts in their resonant amplitude and/or frequency, and were first optimized using simulation (Figure 4.17a). Once optimized computationally and fabricated experimentally, the effect of the LIT material on the sensor’s response was investigated. After depositing the UHMW-PE on the sensor, the resonant frequency shifted by 97 MHz, and the resonant amplitude shifted by 1.18 dB, resulting in a resonant peak at 1.908 GHz and -14.73 dB, as shown in Figure 4.16b. This shift in resonance was expected as the LIT material has a dielectric constant of 2 at 2 GHz (Figure 4.18a) and a small loss 92 factor, which shifted the resonant frequency without reducing the resonant amplitude significantly. A sample under test with a permittivity of 2.2 (to simulate ice), loss tangent of 0.0009, and dimensions of 5 ⨉ 5 ⨉ 0.79 mm was introduced in the vicinity of the sensor. The sensor's simulated and measured responses have a resonant frequency, resonant amplitude, and quality factor of 2.005 GHz, - 12.95 dB and 205, and 2.182 GHz -15.92 dB, and 167, respectively. The interaction between the material and the sensor's electromagnetic fields caused a shift in the resonant frequency and resonant amplitude by 30 MHz and -2.8 dB, respectively. The simulation results closely follow with the measured response of the sensor, with the slight difference between the two attributed to errors in the fabrication process and soldering of the Sub-Miniature A (SMAs), neither of which were considered in the simulation. The parameter, S21 shows the measurements of power transmitted from port 1 to port 2. Figure 4.17 a) Simulated and measured transmission coefficient (S21) for the bare sensor and in the presence of a material under test (MUT). b) The smart hybrid LIT de-icing system’s sensor response to water, adhered ice, detached ice, and the bare sensor. 1.8 1.9 2 2.1 2.2Frequency (GHz)-60-45-30-150S21 (dB)Simulated-bareMeasured-bareSimulated-MUTMeasured-MUTa b b 93 With the sensor optimized, it was then used to successfully monitor the entire icing and de-icing process through characteristic changes in the sensor’s resonant frequency (Figure 4.17 and 4-19). Initially, the sensor detected the unfrozen water that was pipetted into the 3D-printed ice mold, through the complete disappearance of the resonant profile (Figure 4.18b). This detection was possible due to the high permittivity (90) and loss factor (0.3) of liquid water. As the water froze, the resonance profile was recovered due to the much smaller permittivity and loss factor of ice, 3.2 and 0.001, respectively. A shift from the baseline (bare sensor) of 0.138 GHz in the resonant frequency and -2.76 dB in the resonant amplitude was observed as the water froze above the sensor. Additional water was then added to achieve the desired thickness of ice, causing the effective dielectric constant of the sensor’s environment to change, and resulting in a drop of 0.034 GHz in the resonant frequency and -4.26 dB in the resonant amplitude. This two-step ice formation can be observed in the sensor's measured response (Figure 4.19b) and demonstrates that the sensor can detect complex ice compositions including mixtures of water and ice (a common form of precipitate). 94 Figure 4.18 Effective relative permittivity of UHMW-PE measured using a N1501A dielectric probe kit. The 𝜺𝜺𝒓𝒓,𝒆𝒆𝒆𝒆𝒆𝒆′ of LIT coating changes by increasing resonant frequency. b) The S21 spectrum response of the sensor with and without the LIT material covering the SRR. After the water was completely frozen on the coated sensor, as verified by the non-changing resonant profile, the heater was engaged (again 4.4 V for 30 seconds) to obtain a local temperature of TH = -5 ℃. Next, a shear force was applied to the ice block until it detached from the sensor at Fice. Due to the horizontal configuration of our icing setup (see Figure 4.9), even after interfacial fracture the detached ice sits on the surface unadhered, with a small gap in between the ice and LIT coating remaining due to the intrinsic roughness of the UHMW-PE and the imperfect fracture surface. The difference between adhered and unadhered ice was also detectable, as evidenced by the sudden change in the sensor's measured resonant amplitude (-10.61 dB to -14.43 dB) and frequency (1.734 GHz to 1.872 GHz). In more realistic de-icing scenarios environmental forces such as gravity, wind shear, drag, etc. would completely remove the ice from the surface, and even more precise detection would be possible as the sensor would return to its baseline profile. Finally, the system was thawed, and the sensor’s resonant profile began to disappear, indicating the presence of liquid water on the surface (Figure 4.19a-b). The spectrums of the bare resonator, 1.5 2 2.5Frequency (GHz)22.022.042.062.08r,effUHMW-PE1.6 1.8 2 2.2 2.4Frequency (GHz)-50-40-30-20-10S21 (dB)Uncoated sensorCoated sensora b 95 water, ice, and detached ice were all distinct and distinguishable. Overall, the smart hybrid LIT system was able to monitor and detect ice formation and de-icing in real time, even with the microwave sensor embedded underneath the LIT coating, i.e. contactless detection where direct contact with the precipitate was not necessary. Figure 4.19 a) Resonant sensor’s response Recorded resonant amplitude and b) resonant frequency vs. time depicting the water being frozen to the LIT coating covering the sensor, heating the surface locally from T to TH, detaching the ice at TH with a shear force, and then thawing the system. While the energy efficiency of a de-icing system is highly specific to the application and environmental conditions, here our smart hybrid LIT de-icing system reduces energy consumption in four synergistic ways. First, the use of LIT materials enables mechanical de-icing and therefore eliminates the de-icing step requiring the largest energy input: the phase change of solid ice to liquid water. Second, the mechanism of LIT materials allows for our resistive heaters to be sparsely placed on the surface, requiring only ~10% of total areal coverage to be effective. Third, the meandering traces of copper used to construct the heaters display an order of magnitude improvement in resistance over current heating systems, leading to faster de-icing at smaller Water added to sensor Water freezing to ice Ice adhered to the surface Ice detached from the surface Ice thawed 0 11.5 23 34.5 46Time (min)-35-27.5-20-12.5-5Resonant amplitude (dB)-22-16.5-11-5.50Temperature(° C)Resonant amplitude TemperatureHeater on and shear force applied 0 11.5 23 34.5 46Time (min)1.51.71.92.12.3Resonant frequency (GHz)-22-16.5-11-5.50Temperature(° C)Resonant frequency TemperatureHeater on and shear f li d a b Water added Water added Temperature (℃) Temperature (℃) 96 applied voltages. And finally, the inclusion of an ice sensor enables the smart system to only be activated when ice is truly adhered to the surface, and also allows the system to be turned off immediately following de-icing. Given the fatal consequences of aircraft icing, and the strong push towards renewables such as wind power, our smart hybrid LIT de-icing system may find immediate usage in multiple ice-prone sectors worldwide, especially considering its energy efficiency, scalability, and durability. 4.4 Conclusion In this chapter, we explored a hybrid de-icing system utilizing LIT coatings, where thermal modulation substantially improved the de-icing performance without melting the interface. The interfacial toughness between ice and UHMW-PE was found to be 2.2 times larger at -30 °C than at -5 °C. Accordingly, resistive heaters were patterned periodically underneath the LIT material and were optimized to locally raise the temperature to a warmer but still sub-zero value. Heaters underneath only ~10% of the total ice length were effective at reducing the interfacial toughness, as if the entire surface were held at this temperature even though 90% of the surface wasn’t heated. The hybrid LIT de-icing system was also rendered smart through the addition of a microwave resonator sensor. The sensor operated at a resonant frequency, a resonant amplitude, and a quality factor of 2.005 GHz, -12.95 dB, and 205, respectively, taking advantage of the significant differences in dielectric properties between ice and water at the resonant frequency. This planar, contactless device embedded underneath the LIT material was able to monitor and detect ice formation and removal in real-time. Considering the larger environmental forces experienced in ice-prone applications such as flight or wind energy, the results presented here may find immediate usage in improving the efficiency of real-world de-icing systems. 97 Chapter 5: Kirigami enabled microwave resonator arrays for wireless, flexible, passive strain sensing Wireless and highly sensitive flexible strain sensors would have widespread application across a number of different fields. Here, the novel combination of two different metamaterials, one mechanical and one electronic, is demonstrated for its potential as such a sensor. This chapter demonstrates how the integration of electrical and mechanical metamaterials leads to a wireless highly sensitive strain sensor, an area that had not been investigated prior to this research. An array of split ring resonators (SRRs) were mounted on a bespoke kirigami sheet. The hybrid kirigami structure was designed specifically for the resonator array, both in terms of its physical dimensions and elastic response. Mechanical tests in concert with finite element modelling confirmed that the hybrid kirigami structure, containing two disparate kirigami motifs, exhibited a high range of strain and out-of-plane rotation without plastic deformation. The microwave sensing was designed to monitor variations in the S11 response of the resonators as a function of out-of-plane kirigami hinge rotation. The mounted array of SRRs on the hybrid kirigami sheet could wirelessly detect changes in strain with high sensitivity (>30 MHz shift in resonant frequency; >30 dB shift in resonant amplitude) over a large range of strain, from 0.6% to 21.3%. 5.1 Introduction Flexible strain sensors have broad applicability, from electronic skins to structural health monitoring systems to wearable devices36,154,155. Flexible strain sensors are usually designed using nanoparticles with magnetic properties, along with nanowires, carbon nanotubes, or graphene-based materials156. However, strain sensors capable of simultaneous high sensitivity, high 98 stretchability, and wireless operation remain elusive. Mechanical metamaterials are a promising class of candidate materials for strain sensors because their unique mechanical properties originate from structural geometry rather than material composition22. These properties can include negative Poisson ratio, negative compressibility, vanishing shear modulus, and extremely low elastic modulus5,6,23,69,157,158. The stretchability, negative Poisson ratio, and ease of fabrication make kirigami structures, one type of mechanical metamaterial, particularly suited for developing flexible, wireless strain sensors. Previous electronic strain sensor designs have focused on frequency selective surfaces, Split Ring Resonators (SRRs), or conductive polymer composites.18,20,159,160 These sensors utilized electromagnetic waves as a means of monitoring strain. Using electromagnetic waves for sensing, i.e. microwave sensing, has recently gained interest due to its low cost and high precision.42,161,162 Microwave sensors have demonstrated their utility in a wide range of applications, including the detection and monitoring of solid materials, gases, liquids, and even biomolecules.162,163 Microwave sensors may be either wired or wireless. Wired microwave sensors have been used for applications that requires high sensitivity38,163–165, whereas wireless monitoring is advantageous when non-contact and non-invasive measurement is required.41,163 This current study focused on demonstrating a highly sensitive, wireless strain sensor by combining the high sensitivity of an array of wireless microwave resonator sensors with the tunable, large strain capability of kirigami sheets. 99 5.2 Experimental Section 5.2.1 Materials and Methods Kirigami patterns were fabricated from transparent cellulose acetate sheets (OHP Projector Film, thickness 0.127 mm, Uinkit, UK) with an elastic modulus of 2.4 GPa and a total sheet size of 19 cm × 15 cm (Figure 5.1 and Table 5.1). Three kirigami patterns (A, B, and AB) were cut using a craft cutter (Silver Bullet Professional, USA). The mechanical response of the patterns with constant lateral gaps (Gl = 1 cm) was evaluated using horizontal uniaxial strain-stress tests and a custom-made setup (Figure 5.1c). The tensile apparatus included a force gauge mounted on a step motor with a motion controller. The speed was set to a constant rate of 50 µm/s. The probe-end of the force gauge was connected to grips that held the kirigami sheet. The other end of the kirigami sheet was rigidly clamped. Strain and the out-of-plane deformation of the kirigami sheet were measured optically. 100 Figure 5.1 Design of kirigami patterns. a) Schematic of the three kirigami patterns. b) Geometric properties of heterogenous Pattern AB with two different kirigami motifs (Pattern A and B). See Table 1 for the exact dimensions of each parameter in Pattern AB. The scale bar is 1 cm. c) Schematic of tensile testing apparatus. (d) A torn kirigami sheet (Pattern B) at deformation stage IV. The stress on some hinges went beyond the yield point at a strain of ΔL/L = 16%. The sheet was viewed through cross polarizers such that photoelasticity aided in visualization of the deformation process. The lateral distance between cuts for Pattern B was 𝑮𝑮𝐥𝐥 = 1 cm. 101 Pattern A Pattern B Pattern AB Homogenous Homogenous Heterogeneous Transverse gap (mm) GtA = 15 GtB = 7 15 7 Cut size (mm) CA = 26 CB = 34 26 34 Number of lateral columns NlA = 5 NlB = 4 5 4 Number of transverse rows NtA = 14 NtB = 14 7 7 Predicted stiffness using Equation (2), Kpredicted (N/cm) Stage I Stage II Stage III 8.69 1.11 2.79 Table 5.1 The geometric characteristics of the three kirigami patterns (also see Figure 5.1b). For all three patterns, Gl = 1 cm and the sheet thickness was t ≈ 127 µm. The experimental sensor was fabricated using an array of 16 SRRs on a Rogers Ultralam 3850 substrate and a chemical etching printed circuit board fabrication method.166 The dielectric permittivity and loss tangent of the Ultralam 3850 substrate were 2.90 and 0.003, respectively. The substrate had a dielectric thickness of 25 μm and a copper layer thickness of 9 μm. The resonators were attached to the cellulose acetate kirigami sheet using double-sided tape (Figure 5.2b). A standard Horn antenna was used as the transceiver antenna to communicate with the resonator array (Figure 5.2a). A Keysight N9918A Vector Network Analyzer (VNA) was utilized to measure the reflection power, S11, as the response of the sensor. The IF bandwidth, output power, and the number of data points of the VNA were set to 1 kHz, 0 dBm, and 1601, respectively. The VNA was calibrated for a frequency range of 2.2 GHz to 2.9 GHz. The kirigami sheet was positioned on a foam structure and the Horn antenna was placed between absorbers 15 cm from each side (Figure 5.2c). 102 Figure 5.2 Sensor array design. a) Schematic of the simulation setup for this work. b) Optical image of the kirigami structure mounted with the split ring resonators at a strain of ΔL/L = 20%. c) Optical image of the experimental setup. The antenna was positioned vertically above the kirigami sheet at distances of 21 cm and 27 cm in order to validate the wireless function of the sensor for measuring strain. The resonator array was placed directly underneath the horn antenna, aligned with the E-field direction of the antenna. The kirigami sheet was fixed on one side by clamping it to the foam and the other side was stretched in 1 mm increments. The strain measurements were performed for 30 consecutive strain points (ΔL = 30 mm). For certain strain values, the S11 response was recorded in time intervals of 30 seconds to investigate both the relaxation of the kirigami sheet, as well as the consistency of the S11 response of sensor. 78 mm 124 mm 90 mm 215 cm 100 mm 12 mm 7.5 mm 17 mm 40 mm Antenna 16-resonator array Absorber a c b 103 5.3 Simulations Electronic Simulations were carried out in ANSYS High Frequency Structure Simulator (HFSS) to investigate the response of the sensor as a function of rotation angle and therefore strain. The convergence threshold was set to 0.008 and a wave port was used for the excitation of the antenna. The antenna was implemented using the PEC boundary condition and for resonators the finite conductivity boundary condition used (Figure 5.3). The sensor structure was positioned inside a vacuum box with radiation boundary condition for field simulations of the antenna. The response of the antenna was simulated for different angles which were attributed to the various strain values. Changing the angle of the SRRs affected the S11 response of the sensor. The strain sensor was comprised of 16 ground defected split ring resonators, a kirigami sheet with Pattern AB (Table 5.1), and a transmitter antenna (Figure 5.2). The same sensor structure was investigated in both the simulations and experiments. Figure 5.3 (a) Perfect boundary condition on the antenna. (b) Vacuum box with radiation boundary condition. (c) Ring resonators with finite conductivity boundary condition. 104 Finite Element Simulations (FES) were conducted to validate the measured rotation angle of the kirigami hinges for the three experimentally fabricated patterns (Figure 5.4). The numerical responses of Patterns A, B, and AB under uniaxial displacement were obtained using Abaqus finite element software (version 6.14). In these simulations, the angle of the three-dimensional deformation of each cut was estimated based on the corresponding longitudinal strain within the sheet. The kirigami sheet was made of cellulose acetate with density, modulus of elasticity, and Poisson ratio of 1,300 kg/m3, 2.4 GPa, and 0.38, respectively. The kirigami structures were modelled as deformable solid sheets matching the actual size of the sheet and kirigami motifs used in the experiments (Table 5.1). Dynamic implicit was used as a solver, considering nonlinearity effects. Matching the actual experimental conditions, one end of the sheet was clamped and a uniform uniaxial strain of ΔL/L = 20% was applied on the other end of the sheet. For pattern A, B, and AB, 1569, 1368, and 1446, elements of type C3D20R were generated, respectively, consisting of 20-node quadratic elements with reduced integration. The average rotation angle of the three-dimensional deformation for the three patterns was estimated based on stretching strains using an interval of 0.006. 105 Figure 5.4 Finite element simulations. Comparison of the angle of 3D deformation, θ, versus uniaxial strain obtained experimentally and numerically for kirigami a) Pattern A, b) Pattern B, and c) Pattern AB. The inset shows the out-of-plane deformation of the kirigami sheet subjected to a maximum strain of ΔL/L = 20%. d) Rotation of Pattern AB kirigami hinges for ΔL/L = 3.3%, 8%, and 14%, showing excellent agreement with the numerical simulations. 106 5.4 Results and Discussion Considering the frequency response of available horn antenna, planar split ring resonators (SRRs) were designed to operate at 2 – 3 GHz. This selected frequency range imposed certain dimensional restraints, and therefore SRRs measuring 17 mm x 12 mm were fabricated and also simulated using High Frequency Structure Simulator (HFSS). To accommodate this size of SRRs, cellulose acetate kirigami sheets were fabricated such that the lateral and transverse distances between cuts, Gl and Gt, respectively, matched the physical size of the resonator (Figure 5.1a, Table 5.1). This kirigami motif is designated Pattern A. Finite element simulation (FES) of this sheet under uniaxial tension test confirmed that the dominant out-of-plane rotation of the kirigami structure occurred at the hinges (Figure 5.4), so the resonators were mounted on the hinges of Pattern A. The out-of-plane stiffness of a kirigami sheet can be designed according to,51,68,167. 𝐾𝐾𝑜𝑜𝑜𝑜𝐵𝐵−𝑜𝑜𝑠𝑠−𝑝𝑝𝑠𝑠𝐶𝐶𝑖𝑖𝐶𝐶 ≈ 𝑁𝑁t𝐸𝐸ℎ3𝐺𝐺l[𝑁𝑁l(𝐶𝐶 − 𝐺𝐺t)3]−1 5.1 where Nl is the number of lateral rows of cuts of cut length C, E is the elastic modulus of the sheet of thickness h, and Nt is the number of transverse rows of cuts (Figure 5.1a, b). Accordingly, the stress needed to deform the kirigami sheet was readily tunable by altering the modulus and thickness of the sheet material, without altering the kirigami geometry. Note that Equation (5.1) is valid only when the kirigami hinges have rotated completely from horizontal to vertical, whereas here the focus was hinges at rotation angles, θ, less than 90°. Regardless, Equation (5.1) offers a simple design rule for altering the stress-strain response of a kirigami sheet. Under uniaxial tension, our kirigami sheets exhibited four distinct stages of deformation as a function of strain, ΔL/L: (I) in-plane deformation, (II) out-of-plane rotation, (III) out-of-plane deformation, and (IV) plastic deformation (Figure 5.5a). The out-of-plane rotation was critical for 107 the microwave resonator sensor’s mode of operation (detecting ΔL/L via θ). At some critical strain, out-of-plane rotation becomes more energetically favorable than in-plane bending, given by ΔL/L ≈ t/Gl ≈ 1.3% for all three kirigami patterns.51,65 Although the physical dimensions of Pattern A could accommodate the microwave resonator sensors, Pattern A ultimately exhibited limited out-of-plane rotation as a function of strain (Figure 5.5b). Figure 5.5 Mechanical properties of kirigami patterns. a) The four stages of deformation of a kirigami sheet under uniaxial tension. b) Mechanical response of the three kirigami patterns subjected to uniaxial tension. A second pattern (Pattern B) was then designed to exhibit large out-of-plane rotation and deformation. This was achieved by decreasing the transverse gap size and increasing the cut length (Table 5.1). Pattern B exhibited out-of-plane rotation up to ΔL/L ≈ 24%, and out-of-plane deformation until a strain of ≈ 36% (Figure 5.5b). However, the size of the hinges made Pattern B unfeasible to mount the sensors as the kirigami cuts extended into the split ring resonator area. Hinge size and out-of-plane rotation were found to be coupled, making this issue intractable for all homogenous kirigami patterns, i.e. patterns with a single set of C, Gl, and Gt values. a b 108 5.4.1 Derivation of heterogenous pattern stiffness To decouple the out-of-plane rotation from the hinge size a heterogenous pattern (Pattern AB) was developed that incorporated both cut sizes and transverse gap lengths from Pattern A and Pattern B (Figure 5.1a-b). The out-of-plane stiffness of a kirigami sheet exhibiting heterogeneous cut motifs may be estimated assuming each pattern acts as a series of connected springs. The stiffness of a kirigami sheet with a homogenous pattern of kirigami motifs is Equation (5.2), 𝐾𝐾𝐴𝐴𝐵𝐵 = 𝐾𝐾𝐴𝐴𝐾𝐾𝐵𝐵𝐾𝐾𝐵𝐵𝜙𝜙𝐴𝐴2 +𝐾𝐾𝐴𝐴(1−𝜙𝜙𝐴𝐴)2 5.2 where we recall that the sheet has elastic modulus E and thickness t. Each cut has a length C and there are gaps between cuts that are spaced Gl in the lateral direction and Gt in the transverse direction (see Figure5.1a-b). For the entire sheet there must be a finite number of kirigami motifs, denoted Nl in the lateral direction and Nt in the transverse direction. The sheet acts as a spring following Hooke's law. For a sheet comprised of two different kirigami motifs of stiffness KA and KB, the stiffness of the heterogenous pattern, KAB, will act as a series of connected springs, following, 1𝐾𝐾𝐴𝐴𝐵𝐵= 1𝐾𝐾𝐴𝐴′ + 1𝐾𝐾𝐵𝐵′ 5.3 Here the 𝐾𝐾𝐴𝐴′ and 𝐾𝐾𝐵𝐵′ terms arise because the cuts in each individual motif in the heterogeneous pattern are no longer spaced Gl, and there are not Nt rows for each pattern. To account for this, we introduce the fraction of Pattern A, ϕA, within the overall Pattern AB. Doing so yields, 𝐺𝐺𝑠𝑠′ = 𝐺𝐺𝑠𝑠𝜙𝜙𝐴𝐴 and 𝑁𝑁𝐵𝐵′ = 𝜙𝜙𝐴𝐴𝑁𝑁𝐵𝐵 5.4 for Pattern A, and similarly, 109 𝐺𝐺𝑠𝑠′ = 𝐺𝐺𝑠𝑠(1 − 𝜙𝜙𝐴𝐴) and 𝑁𝑁𝐵𝐵′ = (1 − 𝜙𝜙𝐴𝐴)𝑁𝑁𝐵𝐵 5.5 for Pattern B. Combining Equation (5.4) and (5.5) in (5.2) for both patterns yields, 𝐾𝐾𝐴𝐴′ = 𝐾𝐾𝐴𝐴𝜙𝜙𝐴𝐴2 and 𝐾𝐾𝐵𝐵′ = 𝐾𝐾𝐵𝐵(1 − 𝜙𝜙𝐴𝐴)2 5.6 Therefore, 𝐾𝐾𝐴𝐴𝐵𝐵 = 𝐾𝐾𝐴𝐴𝐾𝐾𝐵𝐵𝐾𝐾𝐵𝐵𝜙𝜙𝐴𝐴2 + 𝐾𝐾𝐴𝐴(1 − 𝜙𝜙𝐴𝐴)2 5.7 This is Equation (5.3) from the main manuscript. One can readily see that, for example, as 𝜙𝜙𝐴𝐴 →0, KAB reduces to KB, as it should. For a heterogeneous pattern with equal parts of Pattern A and Pattern B, such as Pattern AB in this work, Equation (5.7) further reduces to 𝐾𝐾𝐴𝐴𝐵𝐵 = 4𝐾𝐾𝐴𝐴𝐾𝐾𝐵𝐵𝐾𝐾𝐵𝐵 + 𝐾𝐾𝐴𝐴 5.8 where the heterogeneous pattern is comprised of Pattern A and Pattern B with individual stiffnesses KA and KB, respectively. Thus, the stiffness of the heterogeneous pattern could be readily designed by adjusting ϕA using two kirigami motifs of well-characterized stiffness. Accordingly, the stiffness of Pattern AB during in-plane deformation, out-of-plane rotation, and out-of-plane deformation experimentally matched the values predicted by Equation (5.2), within experimental uncertainty (Figure 5.6a). Moreover, the mechanical response of Pattern AB matched Pattern A at small strains (ΔL/L < 8%), and matched Pattern B at larger deformation (8%<ΔL/L <24%) (Figure 5.6b). This allowed Pattern AB to rotate and deform over a wide range of strains inaccessible to either Pattern A or Pattern B individually, making it ideal for accommodating the SRRs while still exhibiting large strain. The designed structure exhibited a maximum strain of 110 21.3% for repeatable sensing. As prior strain sensors have demonstrated a wide range of stretchability, 2% – 800%168,169, the stretchability of Pattern AB categorized this particular kirigami resonator array as a low-stretchability sensor. The tailorable design of kirigami patterns through Equation (5.1) allows for a controllable maximum strain that does not exceed the elastic limit of the material. For example, increasing Nl by 53% in Pattern AB resulted in a 55% larger maximum strain (Figure 5.7). Figure 5.6 a) Experimental and predicted stiffnesses of three kirigami patterns. b) Kirigami hinge rotation angle as a function of strain for the three kirigami patterns studied in this work. a b 111 Figure 5.7 Alteration in the mechanical response of kirigami Pattern AB when increasing the number of lateral columns, Nl, from 15 to 23. The microwave resonators were able to detect the change in strain via the rotation angle, θ, of the kirigami hinges. A highly-sensitive kirigami strain sensor should exhibit a small degree of rotation over a wide range of strain for improved resolution. Pattern AB also exhibited the lowest angle of rotation per strain (θ[ΔL/L]-1 = 3.2°/%) compared to Pattern A (3.9°/%) or Pattern B (3.4°/%), at least for ΔL/L < 8%. All 3 patterns exhibited less rotation at larger strains, but Pattern AB (1.69°/%) rotated less than Pattern B (1.74°/%). Pattern A may have rotated the least for ΔL/L > 8% (1.51°/%) but began to plastically deform and fracture above ΔL/L > 18%, making it unfeasible as a large-strain sensor (Figure 5.1d,5.5b). Excellent correlation was also observed between experimental and numerical results for all three kirigami patterns (Figures 5.4), but Pattern AB was selected as the optimal kirigami pattern for the development of the microwave resonator strain sensors. The rotation angle is the critical parameter for the strain sensor’s performance with dynamic loadings. Irreversible strain could result from high mechanical hysteresis during repeated 112 loading/unloading cycles. Over one hundred cycles of loading/unloading, low hysteresis was observed and θ was not altered, another reason Pattern AB was selected (Figure 5.8). Figure 5.8 a) Mechanical response of kirigami Pattern AB subjected to uniaxial tension for 100 cycles of stretching and releasing to ΔL/L ≈ 18%. b) Mechanical hysteresis of kirigami Pattern AB during the first stretch-release cycle. c) Hysteresis in the rotation angle of the resonators at the one hundredth stretch-release cycle. d) Comparison of the rotation angle between the first and one hundredth stretch cycle of Pattern AB. When a SRR was attached to the kirigami sheet, a weak signal was observed when the sheet was strained. Instead, an array of 16 SRRs was considered in order to increase the resonant amplitude due to increased wave absorption. The resonator array was placed in the radiative near field of the a b c d 113 antenna such that mutual coupling between the antenna and resonators would occur, which enabled monitoring the alterations of the resonators. The radiative near field of an antenna is defined by,170 0.62 × �𝐷𝐷3𝜆𝜆′≤ Radiative near field region ≤ 2𝐷𝐷2𝜆𝜆′ 5.9 Here D is the largest dimension of the antenna and 𝜆𝜆′ is the wavelength of the antenna at the operation frequency. The E-field impinging on the sensor should be parallel to the resonator gaps. Any misalignment between the polarization of the impinging electric field and the sensor leads to signal degradation and zero Polarization Loss Factor (PLF). Utilizing a robust mechanical apparatus to fix the position of the antenna, pre-calibrating the sensor, and postprocessing the measured results will enhance the sensor’s response. ANSYS High Frequency Structure Simulator (HFSS) simulations were conducted to understand how the angle of the resonators, caused by the kirigami hinge rotation, would affect the frequency response (Figure 5.9a). The simulations demonstrated that, as the resonator gap was aligned with the E-field direction, the electric field was concentrated in the resonator gap, which enhanced the sensing profile of the sensor. The sensors operate by detecting variations in the electromagnetic field around the SRRs which will vary as the kirigami sheet is strained and the hinges rotate As the hinges rotate beyond some critical angle, θcr ≈ 18° according to the HFSS simulations, the resonators begin to deflect the wave away. Accordingly, beyond θcr the sensor array senses fewer signals. Another reason Pattern AB was chosen as the optimal kirigami design for these experiments was its lowest rotation per strain that allowed for higher sensitivity at lower strains. Note, however, that kirigami structures capable of larger or smaller strain may be designed (Figure 5.6) and by changing the number and size of the resonators in the array, stronger reflected signals 114 are possible. In other words, SRR arrays mounted to kirigami sheets are a general new concept that can be optimized for a wide variety of different applications. Figure 5.9 Sensor array design. a) HFSS results for the SRR array rotated to different angles. b) The S11 response and c) phase response of the antenna for different amounts of strain in the kirigami sheet, over a frequency span of 2.75 GHz to 2.85 GHz. d) The S11 response of the antenna without the kirigami sheet. Experimentally, a 16 SRR array was fabricated on the Pattern AB kirigami sheet, which was placed 21.5 cm below an antenna horn (Figure 5.2a). The bare antenna was designed to exhibit two resonant frequencies between 2.2 GHz to 2.9 GHz (Figure 5.9d). The resonant frequencies were 2.47 GHz and 2.72 GHz. A third resonant frequency was observed in the S11 response of the sensor when the resonator array was mounted on kirigami Pattern AB. This third resonant frequency was 2.2 2.34 2.48 2.62 2.76 2.9Frequency (GHz)-60-48-36-24-120S11(dB)0%0.6%1.2%10%21.3%ΔL/L(%)2.2 2.34 2.48 2.62 2.76 2.9Frequency (GHz)-180°-90°0°90°180°Phase 0%0.6%1.2%10%21.3%ΔL/L (%)2.2 2.34 2.48 2.62 2.76 2.9Frequency (GHz)-70-58-46-34-22-10S11(dB) = 0° = 12° = 14° = 18° = 20° = 24° = 28°L/La 2.75 2.8 2.85 -180-90090180 2.75 2.8 2.85 -60-35-10c b e 2.2 2.34 2.48 2.62 2.76 2.9Frequency (GHz)-45-36-27-18-90S11(dB)-180°-90°0°90°180°Phase115 at 2.79 GHz, exhibited an S11 amplitude of -30.47 dB, and resembled the resonant frequency of the SRR array, which matched the resonant frequency of the individual resonators. In accordance with the HFSS simulations, the frequency response of the sensor array was found to depend on the strain of the kirigami sheet (Figure 5.9b). Both the S11 amplitude and phase depended on the strain via the rotation of the kirigami hinges. The phase data helped clarify the sensor’s mode of operation (Figure 5.9c). An array of SRRs enhances the resonant amplitude at the resonant frequency and this was due to the larger overall wave absorption of the array structure compared to a single resonator, which increased the sensitivity of the sensor. With increasing strain up to a maximum of 21.3%, the S11 parameter of the antenna at 2.79 GHz decreased (Figure 5.9b). For ΔL/L = 0.6% and 1.2%, the S11 values were -37.53 dB and -54.13 dB, respectively. Maximum wave absorption was observed for the SRR array structure at ΔL/L = 1.2% due to the decrease in the S11 parameter. This indicated that a very small portion of the wave was reflected to the antenna, which decreased the resonant frequency matching. There was a -23.66 dB difference between the S11 value at 0% and 1.2% strain. Further stretching the kirigami sheet increased the antenna’s S11 amplitude. The amplitude of the S11 parameter was -43.59 dB and -39 dB for ΔL/L = 1.8% and 2.4%, respectively. A minor shift in resonant frequency was observed as the strain increased but the shift was negligible compared to the S11 amplitude changes. The nearfield criterion of the antenna was also satisfied by moving the antenna to a vertical distance of 27 cm above the kirigami sheet (Figure 5.10). A resonant frequency of 2.76 GHz was observed for the array of resonators at this increased distance. Altering the distance between the antenna and kirigami array supported the notion that the system works for a variety of distances, as long as the resonator array remains within the near field of the antenna. 116 Figure 5.10 Kirigami microwave sensor results. a) The S11 response and b) the phase response of the antenna for a kirigami sheet distance of 27 cm, for various percentages of strain. Inserts have the same labels of axis as main graphs. Further experiments utilizing the kirigami resonator array demonstrated that the S11 response of the sensor did not change over time (Figure 5.11a). The kirigami sheet maintained its level of strain, and consequent angle of hinges, for several minutes without a relaxation of the stress (Figure 5.6a). This was important for the long-term usage of the sensor, such as in structural health monitoring applications. As long as the plastic deformation of the kirigami sheet was avoided (Figure 5.2b), the sensor array could also be repeatedly stretched and relaxed without altering the microwave response. Overall, tracking both the resonant amplitude and resonant frequency allowed the kirigami sensor array to operate as a precise and accurate wireless strain sensor over a ΔL/L ≤ 10% range of strain (Figure 5.11b). The sensitivity or gauge factor (GF) of a strain sensor is the relative change of the electrical response versus strain response. Strain in this work was monitored by either the microwave amplitude or frequency shift. In Figure 5.11b, the amplitude varied from -20 dB to -57 dB over a strain of ≈ 21.3%, and based on the following equation, the GF of the strain sensor was: 117 𝐺𝐺𝐹𝐹 = ∆𝐴𝐴 𝐴𝐴0�∆𝐿𝐿/𝐿𝐿 = 15.51 5.10 This is sufficient for the vast majority of metallic and ceramic components, and other kirigami patterns could be fabricated to allow for larger values of strain by using alternate heterogeneous kirigami patterns designed via Equation (5.6). Figure 5.11 a) The S11 response of the antenna at ΔL/L = 0.6% at 5 time points over 120 seconds. b) The change in resonant frequency and resonant amplitude as a function of kirigami sheet strain for an antenna distance of 21.5 cm. 5.5 Conclusions In summary, we demonstrated the precise, wireless strain sensing capability of the designed sensor using microwaves and the tunable, large-strain capability of kirigami sheets. A heterogenous kirigami pattern provided the optimal platform to mount the array of split-ring resonators (SRRs) of specific size on the hinges of the kirigami sheet. Uniaxial tensile tests revealed that the heterogenous kirigami pattern exhibited ideal mechanical properties for strain sensing applications. Finite element simulations confirmed that the structure containing two distinct kirigami motifs exhibited a large range of strain and out-of-plane rotation of the hinges without 0 1 2 3 4 5 6 7 8 9 10Strain, L/L(%)-60-50-40-30-20Resonant Amplitude (dB)2.792.82.812.822.83Resonant Frequency (GHz)2.2 2.34 2.48 2.62 2.76 2.9Frequency (GHz)-30-25-20-15-10-5S11(dB)0 sec30 sec60 sec90 sec120 secb a 118 plastic deformation. The microwave sensing tests monitored variations in the S11 response of the resonators as a function of the array's rotation on the kirigami hinges. The array of SRRs mounted on the heterogenous kirigami sheet wirelessly detected changes in strain with high sensitivity over a broad range of strain (0.6% to 21.3%). The combination of these two metamaterials shows promise for applications where non-contact and precise strain measurements are required, such as in structural health monitoring and biomedicine. 119 Chapter 6: Flexible self-folding kirigami structures triggered by humidity and its application in sensing This chapter conducts a study on the fabrication and mechanical behavior of self-folding kirigami structures. Various materials including bare Nylon 6-6 of different thicknesses and coated Nylon 6-6 using silicone elastomer, hydrophilic fumed silica, and carbonyl iron powder were utilized in the preparation of silicone-coated nylon sheets. Uniaxial strain-stress tests were employed to examine the mechanical response of the fabricated kirigami structures. Furthermore, infrared and UV/Vis spectroscopies were implemented to scrutinize the thermal evolution of these silicone-coated sheets. A bespoke microwave humidity sensor, fabricated using a PDMS elastomer-coated nylon surface, was devised to monitor the deformation of kirigami patterns resulting from humidity alterations. The custom microwave testing setup, inclusive of a waveguide and vector network analyzer, facilitated this process. The findings revealed that the geometry of the kirigami units significantly influenced the deformation in response to a humidity gradient. Kirigami structures with extended cut patterns exhibited out-of-plane rotation as the humidity increased, primarily due to the swelling of the kirigami hinges. Conversely, the kirigami structures with linear cuts, despite their high stretchability, demonstrated no discernible out-of-plane deformation with an increase in humidity. In essence, this study contributes novel insights into the fabrication, mechanical characteristics, and humidity-induced deformation behavior of self-folding kirigami structures. This opens avenues for potential applications in humidity sensing and actuation devices. 6.1 Introduction Self-folding materials have found use in the development of advanced functional materials, including robotic materials, thin film materials, mechanical and optical metamaterials, electronic 120 devices, antennas, space structures, and biomaterials49,58–60,171–175. The ability to self-fold offers numerous advantages such as reduced manufacturing costs and the ability to combine the favorable properties of 2D and 3D structures. In particular, the use of shape-shifting materials that enable the formation of complex 3D geometries from initially flat materials, such as kirigami, is promising for the fabrication of surface-functionalized lattice structures. There are various triggers that can be used to initiate the self-deformation of kirigami structures, including temperature, moisture/humidity, light, and chemical reactions. The choice of trigger depends on the specific application and the material properties of the kirigami structure. Moisture has a significant impact on the physical and mechanical properties of many polymers, particularly those that can form hydrogen bonds. In polymers such as nylon, moisture can decrease the glass-transition temperature, increase water permeability in the homopolymer but decrease it in copolymers, and accelerate stress-aging characteristics176–179. The amide groups along the nylon chains form hydrogen bonds with water molecules, causing them to become attracted to the polymer matrix and diffuse into it, leading to swelling. The semi-crystalline nature of nylon causes the amorphous regions of the polymer to swell more than the crystalline regions, resulting in deformation or warping of the material180. In this chapter, thin films of nylon have been used for the purpose of self-deforming kirigami structure that are triggered by a relative humidity change. Two potential applications of the fabricated flexible self-folding kirigami structures are introduced. One of the investigated applications is in apparel, via humidity and body temperature regulation. The second application involves using the kirigami structure to detect humidity changes in the environment in combination with a microwave technique, similar to approach demonstrated in prior chapters. 121 6.2 Experimental section 6.2.1 Material Nylon 6-6 with thicknesses of 0.1, 0.127, and 0.256 mm were purchased from McMaster Carr and were used for kirigami fabrication. A commercial silicone (Ecoflex™ 00-30 from Smooth-On Inc.) was also used. The hydrophilic fumed silica used was AEROSIL from EVONIK, with an average particle size of 50 nm. The carbonyl iron powder utilized in the experiment was purchased from Sigma Aldrich. 6.2.2 Methods 6.2.2.1 Preparing the silicone coated nylon 6-6 sheet To control the bending direction of the nylon 6-6 sheet, one side was spin-coated with silicone elastomer. The elastomer layer was fabricated by mixing a 1:1 base to crosslinker weight ratio, following the instructions provided by the manufacturer. Iron and fumed silica particles were also added to the silicone mixture at a ratio of 5 wt% to study thermal effect of these particles. After mixing, the prepared mixtures with and without particles were degassed in a vacuum chamber to remove any trapped air. Next, the nylon sheets were coated with the uncured elastomer using a spin coater at various spinning speeds (500 – 2000 rpm). This allowed for the production of different thicknesses of the elastomer layer on the nylon sheet. Finally, the nylon sheet, coated with the elastomer using the spin-coater, was left at room temperature for 4 hours to allow the elastomer layer to cure. 6.2.2.2 Fabrication of kirigami sheets To fabricate kirigami sheets with various patterns, a craft cutter machine (Brother ScanNCut SDX125E Electronic DIY Cutting machine) was used to cut both bare nylon and the elastomer-coated nylon sheets. The various kirigami patterns investigated are listed in Table 6.1 and can be 122 seen in Figure 6.1a. Note that the blade edge of the craft cutter is ~ 0.1 mm, and accordingly the range of cut sizes investigated was limited to sizes physically fabricable using this mechanical cutting method. 123 Geometric parameters Transverse gap, Gt (mm) Lateral gap, Gl (mm) Projected cut, C' (mm) Cut Angle, 𝜔𝜔 (°) Ratio of actual cut line, C, to C' Material Pattern A, linear 2 10 15 0 1 Bare Nylon Pattern B, square 2 10 15 90 1.67 Bare Nylon Pattern C, triangle 2 10 15 120 1.34 Bare Nylon Pattern D, circle 2 10 15 N/A 1.57 Bare Nylon Pattern E, rectangle 2 10 15 90 2 Bare Nylon Pattern F, rectangle 2 10 15 90 2.34 Bare Nylon Pattern G, square 2 10 15 90 1.67 100 µm thick silicone on Nylon Pattern H, square 2 10 15 90 1.67 300 µm thick silicone on Nylon Pattern I, square 2 10 15 90 1.67 500 µm thick silicone on Nylon Table 6.1 The geometric characteristic of kirigami patterns used in this chapter. 124 6.2.2.3 Mechanical characteristics of fabricated kirigami The mechanical response of each fabricated kirigami sheet was evaluated using uniaxial strain-stress tests. A Mark-10 ESM303 was used that included a force gauge with accuracy of 0.01N, operated at a constant stretching rate of 50 µm/s. Grips attached to the probe-end of the force gauge held the kirigami sheet, while the other end of the sheet was rigidly clamped. Kirigami sheets with a width of 4 cm and length of 10 cm were used for tensile tests. 6.2.2.4 Thermal evolution of silicone-coated kirigami sheet Fourier Transform Infrared Spectroscopy (FTIR) To assess changes in the infrared spectrum of the sheets, we conducted FTIR analysis on the PDMS-coated kirigami sheets that we prepared. We investigated the effects of particles and the PDMS elastomer on the transmission/absorption of mid-IR wavelengths, which range from 3µm to 8µm and correspond to 1250 cm-1 to 3300 cm-1 in the IR spectrum. To this end, we used PDMS-coated nylon sheets with and without iron or nano silica particles, varying in thickness from 50µm to 450µm. Ultraviolet–visible spectroscopy (UV/Vis) A Spectrum 3+ (Perkin Elmer) UV/Vis Spectrophotometer was used to measure the transmission and reflection of the prepared samples over a wavelength range of 200 nm to 1100 nm. Thermal imaging The prepared samples were placed above a hot plate set at 40 °C, separated by a thermally insulating layer comprised of a bubble wrap sheet covered by aluminum foil. A thermal camera (HIKMICRO, China) was used to observe the change in sheet surface temperature over time. 125 6.2.2.5 Effect of humidity on kirigami deformation The effect of a change in humidity on the kirigami structure was studied using a custom build set-up (Figure 6.1b). An ultrasonic humidifier (LEVOIT) was used to generate cold mist at a constant rate. The produced stream of cold mist was transported via a hose to an acrylic chamber measuring 25 cm × 25 cm × 25 cm. The chamber was equipped with a humidity controller that turned the humidifier on/off (Inkbird Humidity Controller IHC200) and had a measuring range of 5% to 99% and an accuracy of ±3% RH. The kirigami sheet was positioned and secured to the chamber's outlet using foam tape while ensuring no leaks were present on the sides of the sheet. Replacing the nylon kirigami sheet with one fabricated using cellulose acetate confirmed that the flow rate of the cold mist caused negligible deformation of the kirigami (Figure 6.1c). Figure 6.1 Design of kirigami patterns a) The different kirigami units in this work, with similar projected cuts (the dashed lines), Gt and Gl. From top to bottom, pattern A, B, C and D. b) The schematic of custom-built humidity chamber. c) A comparison of nylon (swells) and cellulose acetate (does not swell) kirigami sheets when exposed to a humidity change. The effect of flow on the deformation of kirigami units was negligible. The scale bars are 5 mm. High relative humidity Humid air flow from humidifier Low relative humidity Humidity controller Closed chamber Deformed self-folding Kirigami b a Pattern A Nylon Kirigami at 80% RH CA Kirigami at 80% RH c Pattern B Pattern C Pattern D 126 6.2.2.6 The microwave humidity sensor fabrication A layer of PDMS elastomer, 100 µm thick, was coated onto a Nylon surface. Following this, a 1.5cm×2cm kirigami unit was fabricated as previously outlined. To track the microwave response of the deforming kirigami unit due to humidity changes, a 0.5cm×0.5cm copper sheet was attached to the PDMS-coated side. Custom microwave testing setup was used with a microwave waveguide probe, sealed chamber, humidity controller, humidity sensor, and a vector network analyzer (Figure 6.2). The kirigami structure was positioned 30 mm below the probe's radiating aperture in the sealed chamber. Deformations of self-folding structure were observed from outside of chamber. The |S11| (dB) parameters were recorded by a Keysight N5222B Vector Network Analyzer (VNA), and data was acquired every 8 seconds via a custom LABVIEW program. The VNA was set to an output power of -5 dBm, an IF bandwidth of 1 kHz, and 2001 sweep points. A 5V fan was also used ensured air circulation in the chamber, while humidity was controlled by introducing nitrogen and water vapor through two mass flow controllers. Humidity changes were measured every 5 seconds using a BK Precision 725 humidity sensor, with resolutions of ±0.7 ºC and ±2.5% RH. 127 Figure 6.2 The experimental setup consisting of a microwave waveguide probe and Kirigami structure for studying humidity-based structural variations. 6.3 Results and Discussion 6.3.1 The effect of kirigami unit geometry on deformation by humidity gradient A nylon 6-6 sheet with a thickness of 50 µm was initially used to create the kirigami sheets. A study was conducted on the out-of-plane self-folding of different kirigami patterns, all with similar C’, Gt, and Gl (as shown in Figure 6.1a). Experimentally, the humidity within the a closed chamber was altered using a constant stream of cold mist (see the experimental section). Bare nylon kirigami patterns (A, B, C and D) were installed blocking the only outlet of this chamber, and the out-of-plane deformation was monitored as the humidity was increased (Figure 6.2a). Despite the high stretchability of the kirigami pattern with linear cuts (studies in previous chapters), no obvious out-of-plane deformation during the humidity increase was observed (Figure 6.2a). However, the kirigami units of all other patterns showed out-of-plane rotation as the humidity increased (Figure 6.2). Overall, the connectivity of kirigami units by itself didn’t promote the self-folding deformation. The swollen kirigami hinges triggered the rotation of the entire unit. Then the entire 128 asymmetric kirigami unit of patterns B, C, and D underwent a curling motion while the humidity increased. Turning the humidifier off, the deformed nylon surface eventually returned to its undeformed configuration as the water evaporated and the relative humidity decreased. Figure 6.3 The 3D deformation of various nylon kirigami structures. a) Image of deformed kirigami patterns at ∆RH ~30%. All images are side views of the deformed kirigami sheet except for Pattern A which is a top view of the undeformed sheet. The scale bar is 1 cm. b) The measured angle of deformation of various kirigami units for differences in relative humidity from 30% to 80%. According to these findings, Patterns B, C, and D caused out-of-plane deformation, which can be attributed to the asymmetric shape of the swollen layer (as shown in Figure 6.2a). The structure of nylon swells on the higher humidity side, creating unequal stress on the two sides of the kirigami sheet. In response, the kirigami units curl until the stress is alleviated. The out-of-plane deformation of the kirigami sheets at various RH was studied by measuring the 3D rotation angle of the kirigami units. The 3D rotational angle rate for pattern B changed to 1.05 °/%RH, which was lower than the rates measured for patterns C and D (1.8°/%RH and 1.61°/%RH, respectively). The larger rate of hinge rotation with humidity observed in patterns C and D could be attributed to Pattern A Pattern B Pattern C Pattern D a b 129 their asymmetric shape in relation to the transverse gap direction. The specific geometry of these patterns promotes this type of deformation, as compared to pattern B. Above a 40% humidity difference, pattern C of the kirigami unit exhibited complete curling without any further rotation at the hinges (Figure 6.3). Figure 6.4 The curling of kirigami structure with triangular extended cuts (Pattern C). The scale bar is 1 cm. The rotation angle during the change in relative humidity (∆RH%) was studied to investigate the effect of the length of the extended cuts (C') in kirigami patterns B, E, and F, as shown in Figure 6.4a. Patterns B and E exhibited statistically similar rates of change in rotational angle, with rates of 1.05°/%RH and 1.01°/%RH, respectively (p-value: 0.53). However, Pattern F demonstrated a slightly higher rate of rotational angle (1.30°/%RH) when the humidity increased beyond ∆RH% > 30%. Overall, the findings from Figures 6.2 and 6.4 suggest that the asymmetric shape of the kirigami units has a greater impact on the out-of-plane rotation angle than the dimensions of the extended cut within the pattern. However, the larger extended cuts in Patterns E and F created a larger opening, allowing for increased transfer of humid air between the two sides of the kirigami structure (Figure 6.4b). ∆RH = 42% ∆RH = 48% ∆RH = 50% ∆RH = 45% 130 Figure 6.5 3D deformation of nylon kirigami structures with different lengths of extended cuts. a) The measured angle of deformation of various kirigami units for relative humidity changes from 30% to 80%. b) Schematic of deformed kirigami structure due to the humidity gradient on two sides of it. As the extended cut is longer the opening after deformation of the unit is larger 6.3.2 The effect of geometric design of kirigami structure on mechanical properties Apart from the humidity-responsive folding properties of the kirigami patterns discussed above, the designed kirigami patterns also differed in their mechanical response during uniaxial tension. The linear pattern of cuts had the lowest stiffness, and the kirigami structure fractured at a strain of 39% (Figure 6.5a). The kirigami units incorporating a higher cut angle (Pattern C, 120°) and shorter cut size fractured at a lower strain (≈ 20%). Similar results have been shown in previous studies when the corner of cuts had sharp angles181. The two other kirigami patterns investigated were found to have similar values of strain at fracture (approximately 24%), but the kirigami pattern with curved cuts exhibited a higher stiffness during the out-of-plane deformation stages, compared to the sheet with the square kirigami pattern. a 𝜃𝜃° 𝐶𝐶′ High RH% Low RH% Rotation Kirigami Unit b 131 Figure 6.6 The effect of the kirigami unit’s geometry on its tensile response. a) Tensile tests results for the four different nylon kirigami structures, using an elongation rate of 50 µm/s. b) Visual observation of the torn kirigami sheets after tensile testing. The scale bar is 2 cm. We next investigated the impact of various geometric parameters on the stiffness and maximum strain capacity of kirigami structures. When the cuts size was increased (Patterns E and F), the transition from out-of-plane deformation to out-of-plane rotation ceased. We further examined the effect of two different kirigami patterns, E and F, on the ultimate strain of the sheet. Interestingly, we found that both patterns had an ultimate strain comparable with Pattern B, but the energy required for deformation was significantly less for pattern B. This finding indicates that Patterns E and F was more compliant than Pattern B in tension. In addition to the geometric kirigami parameters, we also investigated the role of the thickness of the base material (nylon 6-6) on the mechanical properties of the kirigami structure. Decreasing the thickness of the sheet by 16% led to an increase in the strain at which plastic deformation a b 132 occurred. At lower strain values (less than 10%) during in-plane deformation, the two structures showed similar stiffness. However, we also observed that the kirigami structure with thinner substrate had a lower stiffness, as was predicted by equation (2.1). Figure 6.7 a) The effect of extended cut size on mechanical response of patterns B, E and F. b) The effect of nylon sheet thickness on the stiffness and ultimate strain of kirigami pattern B. 6.3.3 Thermal and mechanical properties of PDMS elastomer coated kirigami structure The uncoated nylon sheet is susceptible to swelling from both sides when exposed to humid air or water droplets, which makes it challenging to control the direction of deformation. However, applying a coating to one side of the sheet allows one to control the deformation direction, making it suitable for creating humidity sensitive kirigami structures. Here, this was investigated by depositing a thin layer of PDMS elastomer on one side of the kirigami sheet, and then monitoring the surface. Figure 6.7 shows thermal images of a coated and uncoated nylon sheet during 30 sec atop a 40ºC hotplate. The results indicate that the PDMS layer with low thermal conductivity and high emissivity of mid-IR results in lower temperature at the surface. As depicted in Figure 6.7, the application of a thin layer of this PDMS coating (only 50 µm) on the nylon surface leads to a a b 133 reduced surface temperature, ranging from 23ºC to 30ºC. This was lower than the bare nylon surface where the surface temperature ranged from 32ºC to 34ºC. The use of a thicker PDMS coating resulted in even better insulation properties, as expected. For instance, on the surface of the nylon sheet coated with 450 µm thick PDMS layer, the temperature ranged between 17ºC and 30ºC. 134 Figure 6.8 The effect of various thickness of PDMS elastomer (50 µm to 450 µm) on the surface temperature of nylon with thickness of 50 µm, over 30 seconds of observation. a) Bare surface of the hotplate. The surface temperature of sample at b) 0 sec, and c) 30 sec of contact. The scale bar is 1 cm. a b c 34.2 13 6 34.2 13.6 41.2 13 3 40.2 15 8 33.7 15 8 34.7 18 2 39.6 15.0 32.7 19.8 35.0 16.3 39.0 15.8 33.8 18.7 35.1 14.2 Bare Nylon Htotal = 100 µm Htotal = 300 µm Htotal = 500 µm 135 The mechanical properties of the multilayer sheet is another important parameter that directly affects the kirigami structure’s stiffness and its final deformed shape at different levels of strain and humidity changes. The effective elastic modulus may be found using, EEffective=EPDMS×(APDMS/Atotal) + ENylon×(1-APDMS/Atotal) 6.1 where EPDMS is elastic modulus of the PDMS utilized (125 kPa), ENylon is elastic modulus of nylon (2.7×106 kPa) and APDMS and Atotal are the cross-sectional areas of the PDMS and total multilayer sheet, respectively. As the width of the PDMS and nylon the same APDMS/Atotal= HPDMS/Htotal. Therefore, the thicker PDMS coating resulted in a lower elastic modulus of the coated nylon sheets because the cross-section of the multilayer material (nylon + PDMS) became more dominated by the mechanical properties of the PDMS. Accordingly, the coated kirigami sheets exhibited less stiffness during uniaxial tension, followed by plastic deformation of the hinges and finally tearing of the kirigami structure (Figure 6.8a). Figure 6.9 The effect of the PDMS coating on the mechanical properties of the nylon kirigami Pattern B. a) The comparison of mechanical response during tensile test of kirigami pattern B fabricated using coated and non-coated nylon sheets with thickness of 50 µm. b) The out-of-plane deformation of the coated nylon kirigami Pattern B sheets for different thicknesses of the PDMS layer. 136 The 3D deformation of coated kirigami Pattern B also exhibited a higher degree of rotation of the kirigami units with increasing humidity (Figure 6.8b). This again indicated that a nonuniform stress was created at the cross-section of the kirigami unit, particularly in the hinges where the main rotation occurs. This was due to the layer of PDMS elastomer which does not swell with water. However, kirigami structures with thicker PDMS layers (250 µm and 450 µm) showed a lower rate of rotation angle with changes in humidity (Figure 6.8b). This could be attributed to the higher stress required to bend or deform the thicker elastomer, which limits the rotation angle of the kirigami structure. It was also observed that at low humidity changes (∆RH < 20%), the kirigami structure with 450 µm thick elastomer coating showed less out-of-plane rotation at the hinges (See Figure 6.8b). 6.3.4 Self-deforming kirigami structures This section focuses on investigating two applications of fabricated kirigami structures. Due to the humidity sensitivity and thermal isolation of the PDMS-coated nylon kirigami structure, the first application demonstrates using the technology as thermal and humidity regulating substrates. The second application involves using the kirigami structure to detect humidity changes in the environment in combination with a microwave sensing technique. 6.3.4.1 Flexible thermal and humidity regulating substrate Here particle additives were used to improve thermal isolation and increase the IR reflectivity of the PDMS elastomer layer. The surface temperature of the nylon sheets coated with a mixture of particles and PDMS elastomer (at a weight ratio of 5 to 95) was also investigated. The particles investigated were either carbonyl iron or fumed silica, which have low thermal conductivity (less than 2.2 Wm−1K−1). These particles also exhibit adsorption peaks in the mid-IR spectrum (1300 137 cm-1 to 3300 cm-1), indicating low transmissivity of these wavelengths. Figure 6.9 presents the thermal images of flat nylon sheets (no kirigami patterns) coated with the silicone elastomer containing 5wt% of these particles. The results indicate that the particles affect the surface temperature after 30 seconds of heat transfer. Figure 6.9b shows nylon coated with a mixture of PDMS elastomer and carbonyl iron particles, which exhibited a surface temperature similar to that of previously discussed results of the PDMS elastomer coating (from 19ºC to 32ºC). However, in the case of silica particles, the surface temperature was lower (ranging from 22ºC to 28ºC). Due to these improved insulative properties, the nylon coated with silica particles and PDMS elastomer was therefore used as the base substrate for kirigami fabrication. Figure 6.10 The effect of non-conductive particles dispersed in the PDMS elastomer layer (450 µm) on the surface temperature of nylon with thickness of 50 µm after 30 sec of contact. The coating on nylon is PDMS elastomer a) without particles b) with 5wt% carbonyl iron particles and c) 5 wt% fumed silica particles. The scale bar is 1 cm. FTIR analysis revealed that the PDMS coatings with and without nano silica particles had similar transmission in the 650 cm-1 to 4000 cm-1 range (Figure 6.10). The majority of the adsorption occurred at wavelengths less than 1400 cm-1. However, the addition of carbonyl iron particles to the PDMS mixture increased the transmission of IR wavelengths from 650 cm-1 to 1400 cm-1, 35.1 14.2 a 36.5 13.6 b 35.0 17.5 c 138 which was consistent with the higher surface temperature observed through thermal imaging compared to the PDMS-coated nylon and PDMS-fumed silica-coated nylon surfaces (Figure 6.9b). Figure 6.11 The FTIR spectra for PDMS, PDMS elastomer layer with 5 % fumed silica and 5% carbonyl iron over the range of 650 cm-1 to 4000 cm-1. In addition, ultraviolet–visible (UV/Vis) spectroscopy was conducted to investigate the transparency and reflection of shorter wavelengths (ranging from 200 nm to 1100 nm) for coated and non-coated nylon surfaces (Figure 6.11). The effect of the PDMS coating without particles was negligible compared to the bare nylon surface. However, the PDMS layer resulted in slightly more transmission and reflection of wavelengths less than 700 nm. Both particles reduced the transmission and reflection of UV and IR wavelengths to less than 60% of bare nylon and PDMS-coated nylon. The carbonyl iron particles showed the least transmission and reflection of almost 60%, which indicated that the coating absorbed most of the wavelengths. The PDMS coating 139 containing 5% fumed silica particles demonstrated efficient absorption and reflection, which was consistent with the thermal imaging results. Figure 6.12 a) The reflection and transmission of wavelengths from 200 nm to 1100 nm from different surfaces, including bare nylon, nylon coated with PDMS, nylon coated with PDMS/carbonyl iron, and nylon coated with PDMS/fumed silica. (a) Transmission and (b) Reflection. We also investigated the influence of humidity on the surface temperature of the nylon kirigami surface coated with the PDMS elastomer mixed with fumed silica particles (Figure 6.12a). As the humidity increased on the uncoated side, the entire structure deformed, and the kirigami units underwent out-of-plane rotation around the hinges, allowing vapor to pass through the deformed surface. Thermal imaging also revealed that upon deformation of the structure, thermal convection was possible, resulting in a decrease in the surface temperature (Figure 6.12b). The combination of a PDMS elastomer coating with silica particles and a self-folding kirigami structure enables effective thermal regulation. The coating prevents the passage of MID-IR and UV light until humidity increases, at which point the opening of hinges in the kirigami structure promotes convection and reduces temperature. Overall, this self-folding kirigami has the potential for applications in personal thermal regulation and other scenarios requiring passive control of humidity and temperature. UV IR a b 140 Figure 6.13 The effect of humidity change and temperature on the self-folding behavior of the nylon coated kirigami. a) The kirigami units deform out-of-plane as relative humidity changes. b) The IR images show the drop in surface temperature as the kirigami units deformed. The temperature of the surface underneath was 40ºC. The scale bar is 5 mm. All images are taken in 45º from the substrate. 6.3.4.2 The microwave humidity sensing using self-deforming kirigami In this section to study the effects of humidity on a designed kirigami unit, a copper sheet was attached to a PDMS elastomer-coated Nylon kirigami unit and placed 30 mm below a microwave waveguide probe (Figure 6.2). As the humidity within the chamber increased up to 80%, a drop in the measuring frequency from 16.97 to 16.91 GHz was observed (Figure 6.14a). This change was significant only when the relative humidity (RH) rose above 60%. However, the frequency remained consistent when RH ranged from 0% to 60%. Meanwhile, the amplitude showed a different pattern, with the read-on amplitude significantly increasing by more than +23 dB as the RH in the chamber rose from 0% to 80%. The maximum amplitude was recorded when RH changed from 30% to 80%, whereas no significant change in amplitude was seen at lower RH levels due to minimal structural changes. This suggests that change of wave reflection back to the waveguide was minimal when RH varied from 0% to 30%. The experiment was repeated twice to ensure consistency, as shown in Figure 6.14, validating the effectiveness of our fabricated a b ∆RH =10% ∆RH =15% ∆RH =35% ∆RH =45% a 141 microwave humidity detector. It is important to mention that the design flexibility of the kirigami unit offers the possibilities for adjusting the working range of the humidity sensor in future research. Figure 6.14 The S11 response of the antenna. a) and b) display the change in resonant frequency and amplitude, respectively, in response to humidity changes within the closed chamber housing the kirigami unit. These changes are depicted for a microwave waveguide distance of 30 mm from the self-folding kirigami unit. 6.4 Summary This chapter introduces a new combination of metamaterials known as self-folding kirigami structures. These structures have the potential to revolutionize the development of functional materials with numerous applications, personal thermal regulators, humidity detectors, electronic devices, and more. The process for fabricating these structures involved using nylon 6-6 and a silicone elastomer coating, which triggered self-deformation based on changes in relative humidity. By optimizing the coating and kirigami unit design, we have successfully demonstrated two potential applications for flexible self-folding kirigami. First, humidity and temperature regulation. Second, humidity sensing, as the structural changes resulting from changes in relative humidity can be detected and quantified, resulting in a sensitive and responsive humidity sensor. Resonant Amplitude (dB) Resonant Frequency (GHz) a b 142 This technology opens up more possibilities for the development of advanced sensing and actuation technologies that can be integrated into a wide range of devices. 143 Chapter 7: Conclusion and future work In this thesis, I investigated the engineering potential of metamaterials for interfacial tuning across a diverse range of applications, including antifouling, de-icing, strain sensing, and humidity control and detection. Section 7.1 offers a comprehensive summary of the research conducted and highlights the novelty of each research works in the preceding chapters, while Section 7.2 delineates the recommended avenues for future research, which are derived from the findings discussed here. 7.1 Summary of main findings In Chapter 3, I conducted a study demonstrating, for the first time, the novel use of a mechanical metamaterial, kirigami, for antifouling purposes. I created a fabricated surface structure called SKINS (Suspended Kirigami Inverted Nil-adhesion Surfaces), which exhibits 3D deformation that enables mixed-mode fracture propagation at the interface. The 3D deformation causes wrinkling with a wavelength controlled by the geometric properties of the kirigami sheet. The experiments showed that SKINS were able to significantly decrease the adhesion strength of several different foulants regardless of the surface energy of the plastic sheet used for fabrication. Furthermore, SKINS exhibited durability against repeated cycles of foulant adhesion and removal and demonstrated practical utility for large, curved surfaces such as powerline cables. However, the SKINS presented need to be improved in various aspects to suit practical applications. For instance, the perforated surface of the SKINS may lead to reduced durability and the infiltration of undesired contaminants, ultimately affecting their performance as an antifouling technology. Additionally, the current method of installing SKINS on surfaces relies on double-sided tapes, which may not be suitable for all types of surfaces. Furthermore, it is important to note that the 144 impact of dynamic forces, such as the flow of foulants in the liquid phase, on the surface has not been thoroughly studied. In Chapter 4, I explored the use of electrical metamaterials for de-icing purposes. I developed a new hybrid de-icing system utilizing coatings that exhibit low interfacial toughness (LIT) with ice, where thermal modulation substantially improved the de-icing performance without melting the interface. I discovered that temperature exerts a significant influence on the interfacial properties of at least one LIT coating (ultra-high molecular weight polyethylene, UHMW-PE) and ice—a crucial finding that had remained unexplored until now. The interfacial toughness between ice and UHMW-PE was 2.2 times larger at -30°C than at -5°C. Accordingly, resistive heaters were patterned periodically underneath the LIT material and were optimized to locally raise the temperature to a warmer but still sub-zero value. Heaters underneath only ~10% of the total ice length were effective at reducing the interfacial toughness, as if the entire surface were held at this temperature even though 90% of the surface wasn't heated. This remarkable finding leads to a breakthrough in understanding the effect of temperature on interfacial toughness between ice and LIT coatings. Before my work, hybrid de-icing systems had always required melting of the ice to be effective. To enhance the effectiveness of this hybrid LIT de-icing system, I used a microwave resonator (electrical metamaterial) sensor for icing/de-icing detection. The sensor operated at a resonant frequency, a resonant amplitude, and a quality factor of 2.005 GHz, -12.95 dB, and 205, respectively, taking advantage of the significant differences in dielectric properties between ice and water at the resonant frequency. This planar, contactless metamaterial embedded underneath the LIT material was able to monitor and detect ice formation and removal in real-time. 145 Considering the larger environmental forces experienced in ice-prone applications such as flight or wind energy, the results presented here may find immediate usage in improving the efficiency of real-world de-icing systems. Considering all the points mentioned about the de-icing technology developed in Chapter 4, it is evident that the selected LIT coating, despite displaying insufficient durability, may not be a viable long-term solution for highly abrasive conditions. Furthermore, the type of ice utilized in this chapter is limited to laboratory-created glaze ice, and the effects of ice with varying properties have not been addressed. Additionally, the sensor employed for ice detection is only capable of sensing ice in one specific location, whereas for practical applications, it would need to be designed to detect ice in multiple locations on the surface. In Chapter 5, I discussed the combination of mechanical and electrical metamaterials used as a strain sensor. For this application, by utilizing the large-strain capability of kirigami sheets and the precise, wireless strain sensing capabilities of split-ring resonators (SRRs), a heterogeneous kirigami pattern was developed to mount an array of SRRs on the hinges of the kirigami sheet. The resulting structure exhibited a large range of strain and out-of-plane rotation without plastic deformation, and the array of SRRs was able to wirelessly detect changes in strain with high sensitivity over a broad range of strain. This combination of materials has potential applications in structural health monitoring and biomedicine where non-contact and precise strain measurements are required. I demonstrated the precise, wireless strain sensing capability of the designed sensor using microwaves and the tunable, large-strain capability of kirigami sheets. A heterogeneous kirigami pattern provided the optimal platform to mount the array of split-ring resonators (SRRs) of specific size on the hinges of the kirigami sheet. Uniaxial tensile tests revealed that the 146 heterogeneous kirigami pattern exhibited ideal mechanical properties for strain sensing applications. Finite element simulations also confirmed that the structure containing two distinct kirigami motifs exhibited a large range of strain and out-of-plane rotation of the hinges without plastic deformation. This innovative achievement introduces a new approach to strain sensing and opens up possibilities for diverse applications in fields such as robotics, wearable technology, and structural monitoring. My work is the first to specifically couple the design of kirigami and SRRs. Taking into account all the aspects mentioned regarding the fabricated sensor, it has demonstrated positive results in several areas. However, there are still areas that require improvement. For instance, while the sensor has demonstrated good performance, there are limitations that need to be addressed. These limitations include the sensor's sensitivity to environmental changes and its ability to detect strain in only one direction. To overcome these challenges, a more versatile and flexible kirigami design is necessary to enable strain detection in multiple directions. Additionally, the distance between the antenna and the SRR needs to be carefully controlled to ensure accurate and reliable results. Thus, further advancements are needed to enhance the mechanical and electrical aspects of the sensor's design. In Chapter 6, I developed and fabricated a combination of metamaterials using a flexible self-folding kirigami structure and microwave sensing techniques for humidity control and detection. The designed patterns were both humidity-triggered and stretchable up to 50%. The 3D deformation of various kirigami units during humidity changes was investigated, with self-folding patterns achieved out-of-plane deformation angles of up to 70º. The optimized kirigami pattern was applied to a nylon sheet coated with a polydimethylsiloxane (PDMS) elastomer for thermal control or cooling purposes. The effect of PDMS elastomer layer thickness on kirigami 147 deformation was studied, and it was found that incorporating silica nanoparticles in the coating improved the thermal insulation properties. By utilizing a copper coating on the surface of the self-folding kirigami structure, I also successfully detected humidity changes in real-time using a wireless technique. This innovative combination of materials and structures offers a new route for humidity control and detection. Despite the overall results, it is important to acknowledge areas that may be in need of improvement. The sensing results indicate that the deformation of the kirigami unit with a change in humidity from 0 to 50% leads to an insignificant alteration in the microwave response. Therefore, there is a need to improve the mechanical aspect of the metamaterial design. Furthermore, further investigation of the kirigami structure with multiple units is required. The presence of hysteresis in the humidity response due to variations in the absorption rate and desorption rate of Nylon also needs to be minimized. Improvements in these areas are necessary to enhance the overall performance and reliability of the design. In conclusion, the development of metamaterials, specifically mechanical and electrical metamaterials, has shown significant potential for a range of interfacial applications. The use of kirigami sheets as a mechanical metamaterial has led to the development of SKINS, which have demonstrated a significant reduction in adhesion strength against various foulants and have the potential for use in antifouling applications on large, curved surfaces such as powerline cables. Meanwhile, the use of electrical metamaterials has led to the development of a hybrid LIT de-icing system that improves de-icing performance without melting the interface, as well as a wireless strain sensing system that utilizes the large-strain capability of kirigami sheets and the precise sensing capabilities of SRRs. These advances in metamaterials research have the potential to 148 significantly impact various industries, leading to more efficient and effective solutions for real-world problems occurring at interfaces. 7.2 Future work recommendations Chapter 3 (Antifouling): Future work could focus on further optimizing the SKINS design and materials for increased antifouling performance and durability. Investigations into the effectiveness of SKINS in various environmental conditions, such as extreme temperatures or exposure to saltwater, would be beneficial. Moreover, many fundamental scientific questions remain regarding the antifouling performance of SKINS. While we demonstrated the effectiveness of linear cuts for our kirigami motifs, it is unknown how other types of kirigami structures, such as fractal-cuts, closed-loops, or the ones investigated in Chapter 6 could improve the antifouling performance of SKINS. Additionally, I propose conducting further in-depth studies to enhance our understanding of the wrinkling deformation that occurs on the surface of SKINS during the removal of foulants. By delving deeper into this phenomenon, we can gain valuable insights into the underlying mixed mode fracture mechanics and factors influencing the wrinkling process. Furthermore, our experiments focused on the reduction of adhesion strength for pseudo-biofoulants, and it remains to be seen how SKINS perform when fully submerged in water and exposed to real marine organisms. Additionally, exploring the use of a coating on SKINS could help minimize the environmental impacts such as flow of a liquid on SKINS and their performance as a antifouling solution. Moreover, for practical applications such as de-icing of power cables there is demand for optimizing the SKINS installation. Chapter 4 (Hybrid de-icing): Future research could involve the optimization of the hybrid LIT de-icing system for various applications, such as aviation, wind turbines, or transportation 149 infrastructure. The development of more energy-efficient and cost-effective heating elements, as well as the investigation of new materials or coatings with enhanced thermal properties, could further improve the system's performance. Moreover, the integration of the de-icing system with predictive models or real-time weather data could help in automating and optimizing the de-icing process, potentially saving energy and reducing costs. Furthermore, it should be noted that the findings in this chapter were limited to a single type of LIT coating, namely UHMW-PE. Thus, I recommend conducting a comprehensive investigation to explore the effect of temperature on other LIT coating materials, both polymeric and recently reported metallic ones97. By studying a broader range of LIT coatings, we can gain a more comprehensive understanding of how temperature influences their performance and interfacial properties. Moreover, it is essential to emphasize that this study exclusively relied on a localized heating system to manipulate the interfacial temperature. Consequently, the impact of a temperature gradient at the interface or variations in heating rate on the ice adhesion remains an unanswered question, requiring further investigation. It is also suggested to investigate the wireless methods of sensing for detection of icing/de-icing for applications at remote locations such as Arctic environments. This would enable efficient and reliable monitoring of icing conditions without the need for extensive manual intervention or on-site equipment. Chapter 5 (Strain Sensing): Future work in this area could involve the miniaturization and integration of the strain sensor into wearable devices or other electronics for biomechanical monitoring, healthcare applications, or robotics. The development of more robust and flexible SRR designs could improve the durability and range of the strain sensor, while advancements in wireless communication and data processing techniques could enhance the sensor's real-time performance. 150 Additionally, investigating the use of other metamaterials or smart materials in the strain sensor design could lead to new sensing capabilities or improved sensitivity. Although the kirigami resonator array provides many advantages over alternate strain sensor designs, several aspects could still be improved upon. First, the kirigami resonator array would be considered a low stretchability strain sensor as a maximum strain was ≈ 21%. Modification of the geometric parameters of kirigami structure would be necessary to design a high stretchability sensor. Further, only cellulose acetate was used to fabricate the kirigami structures. The modulus and thickness of the kirigami base material also affect the maximum strain and 3D deformation, and other materials could provide enhanced performance. Second, the kirigami structure must be within the near field of the antenna to remain coupled, meaning wireless, but local, interrogation is required. However, other works indicate that distance-independent sensing is possible. Third, if the antenna was off-angle, the noise in the received signal would increase until sensing was no longer feasible. However, this effect can be minimized by an initial calibration process, which is necessary for any high-sensitivity sensor technology. Misalignment can also be avoided using a mechanically robust platform attached to the interrogator antenna to ensure alignment. Chapter 6 (Humidity Control and Detection): Future research could focus on further optimizing the self-folding kirigami structure and SRRs for enhanced humidity control and detection capabilities. Developing new materials or coatings with improved hygroscopic properties or exploring alternative designs for the self-folding kirigami structure could lead to improved performance in various humidity conditions. Additionally, integrating the humidity control and detection system into building automation systems, greenhouses, or other environments requiring precise humidity control could lead to energy savings and improved environmental control. 151 Moreover, using the self-folding kirigami for energy harvesting in humid conditions could open new avenues for sustainable energy generation. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Graduate
Golovin, Kevin
Uhl, Alexander R.
Metamaterial development for mechanical interfacial tuning and sensing applications
Text