Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Xia, Yakang
Doctor of Philosophy - PhD
University of British Columbia
Pool boiling is a promising solution for meeting the heat dissipation requirements in high-heat-flux applications such as supercomputers, nuclear reactors, and laser generators. Surface characteristics greatly influence boiling heat transfer, which makes it crucial to understand their impact for enhanced heat transfer surface design. However, certain aspects of pool boiling, especially bubble behaviors and heat transfer with wettability contrast remain unclear. This thesis investigates the influence of surface wettability, textures, and orientation on pool boiling heat transfer using water as the working fluid. The research includes five steps. Firstly, thin-film heaters are fabricated to enable precise temperature measurements with an infrared camera. Microstructures are created on the boiling surface using laser ablation. Together with silanization grafting, different surfaces ranging from super-hydrophilic to super-hydrophobic are produced. Secondly, experimental studies are conducted focusing on observing injected air bubble formation and motion behaviors to understand the effects of surface wettability. Results demonstrate that surface wettability significantly influences bubble size and departure frequency, and bubbles spread to the super-hydrophobic region due to wettability contrast. Thirdly, pool boiling tests on different wetting surfaces show differences in bubble formation and departure. Nucleate boiling curves indicate that the heat transfer coefficient and critical heat flux generally increase as surface wettability transitions from super-hydrophobic to super-hydrophilic. The quantitative assessment of surface effects using surface-fluid factor confirms higher values for super-hydrophobic surfaces. For hydrophilic surfaces, the effect remains relatively constant over the nucleate boiling range, while hydrophobic surfaces show dependency on heat flux. These findings are consistent with boiling tests on a biphilic surface comprising super-hydrophobic and super-hydrophilic regions. Fourthly, a biphilic surface is designed to enable vapor bubbles to move on the surface, resulting in enhanced heat transfer compared to a super-hydrophilic surface. The driving force responsible for the directional spontaneous bubble movement on the surface is analyzed. Furthermore, investigating the influence of surface inclination angle reveals better heat transfer performance in vertical pool boiling. This study not only elucidates the influence of surface wettability on bubble behaviors and heat transfer but also provides valuable insights for understanding and optimizing the surface design to enhance boiling heat transfer.
INFLUENCE OF SURFACE CHARACTERISTICS ON POOL BOILING HEAT TRANSFER by Yakang Xia M.Sc., Khalifa University of Science and Technology, UAE, 2016 B.Eng., China University of Petroleum (Beijing), China, 2014 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) September 2023 © Yakang Xia, 2023 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Influence of surface characteristics on pool boiling heat transfer submitted by Yakang Xia in partial fulfillment of the requirements of the degree of Doctor of Philosophy Examining Committee Dr. Sunny Ri Li, School of Engineering Supervisor Dr. Sina Kheirkhah, School of Engineering Supervisory Committee Member Dr. Vladan Prodanovic, University of British Columbia-Vancouver Supervisory Committee Member Dr. John Frostad, University of British Columbia-Vancouver University Examiner Dr. Hongwei Wu, University of Hertfordshire External Examiner iii Abstract Pool boiling is a promising solution for meeting the heat dissipation requirements in high-heat-flux applications such as supercomputers, nuclear reactors, and laser generators. Surface characteristics greatly influence boiling heat transfer, which makes it crucial to understand their impact for enhanced heat transfer surface design. However, certain aspects of pool boiling, especially bubble behaviors and heat transfer with wettability contrast remain unclear. This thesis investigates the influence of surface wettability, textures, and orientation on pool boiling heat transfer using water as the working fluid. The research includes five steps. Firstly, thin-film heaters are fabricated to ensure uniform heat flux and enable precise temperature measurements with an infrared camera. Microstructures are created on the boiling surface using laser ablation. Together with silanization grafting by chemical vapor deposition, different wetting surfaces ranging from super-hydrophilic to super-hydrophobic are produced. Secondly, experimental studies are conducted focusing on observing injected air bubble formation and motion behaviors to understand the effects of surface wettability. Results demonstrate that surface wettability significantly influences bubble size and departure frequency, and bubbles spread to the super-hydrophobic region due to wettability contrast. Thirdly, pool boiling tests on different wetting surfaces show differences in vapor bubble formation and departure. Nucleate boiling curves indicate that the heat transfer coefficient and critical heat flux generally increase as surface wettability transitions from super-hydrophobic to super-hydrophilic. The quantitative assessment of surface effects using the surface-fluid factor confirms higher values for super-hydrophobic surfaces. For hydrophilic surfaces, the effect remains relatively constant over the nucleate boiling range, while hydrophobic surfaces show dependency on heat flux. These findings are consistent with boiling tests on a biphilic surface comprising both super-hydrophobic iv and super-hydrophilic regions. Fourthly, a biphilic surface is designed to enable vapor bubbles to move on the surface, resulting in enhanced heat transfer compared to a super-hydrophilic surface. The driving force responsible for the directional spontaneous bubble movement on the surface is analyzed. Furthermore, investigating the influence of surface inclination angle reveals better heat transfer performance in vertical pool boiling. This study not only elucidates the influence of surface wettability on bubble behaviors and heat transfer but also provides valuable insights for understanding and optimizing the surface design to enhance boiling heat transfer. v Lay Summary This thesis explores how surface wettability, textures, and orientation affect heat transfer in pool boiling. By creating different wetting surfaces using a simple method and observing bubble behaviors, this study finds that surface wettability significantly affects bubble size and departure frequency, and the bubble subject to wettability contrast spreads towards super-hydrophobic regions. Pool boiling tests reveal that the heat transfer coefficient and critical heat flux generally increase as surface wettability transitions from super-hydrophobic to super-hydrophilic. A biphilic surface design enhances heat transfer compared to a super-hydrophilic surface, and the study analyzes the driving force behind directional bubble movement. Pool boiling on vertical surfaces shows improved heat transfer performance. These findings deepen our understanding of surface wettability's influence on bubble behavior and heat transfer, providing insights for optimizing the surface design to enhance boiling heat transfer. vi Preface The research work in this Ph.D. dissertation is conducted under the supervision of Prof. Sunny Ri Li. in the Thermal Management & Multi-phase Flows Lab (TMMFL) and the Applied Micro and Nanosystems Facility in the School of Engineering at the University of British Columbia (Okanagan campus). Parts of this dissertation have been published in several journals, conference proceedings, and presentations. Publication List Refereed Journal Articles [J1] Xia, Y. and Li, R., 2023. Effects of surface wettability and inclination angle on bubble behaviors and pool boiling heat transfer. International Journal of Heat and Mass Transfer (In preparation). [J2] Xia, Y., Li R., Gao, X. and Li, R., 2023. Pool Boiling on a Biphilic Surface Where Bubbles Can Move Horizontally on the Surface. International Journal of Heat and Mass Transfer，215, p.124540. doi.org/10.1016/j.ijheatmasstransfer.2023.124540 [J3] Xia, Y., Gao, X. and Li, R., 2022. Surface effects on Sub-cooled pool boiling for smooth and laser-ablated silicon surfaces. International Journal of Heat and Mass Transfer, 194, p.123113. doi.org/10.1016/j.ijheatmasstransfer.2022.123113. [J4] Xia, Y., Gao, X. and Li, R., 2021. Influence of surface wettability on bubble formation and motion. Langmuir, 37(49), pp.14483-14490. doi.org/10.1021/acs.langmuir.1c02444 [J5] Xia, Y., Gao, X. and Li, R., 2020. Management of surface cooling non-uniformity in spray cooling. Applied Thermal Engineering, 180, p.115819. doi.org/10.1016/j.applthermaleng.2020.115819. [J6] Gao, X., Xia, Y. and Li, R., 2021. Transient boiling of a droplet stream quenching vii microstructured surfaces. International Journal of Heat and Mass Transfer, 164, p.120580. doi.org/10.1016/j.ijheatmasstransfer.2020.120580. Refereed Conference Proceedings [C1] Y. Xia, R. Li. (2023). “EFFECTS OF SURFACE WETTABILITY AND INCLINATION ON BUBBLE BEHAVIORS AND POOL BOILING HEAT TRANSFER”. 17th International Conference of Heat Transfer, August, 14-18, 2023, Cape Town, South Africa. [C2] Y. Xia, R. Li. (2022). “Bubble Manipulation by Surface Wettability Pattern and Its Application in Pool Boiling”. 7th Thermal and Fluids Engineering Conference (TFEC 2022), May, 16-18, 2022， Las Vegas, NV, USA. [C3] Y. Xia, R. Li. (2021). “THERMAL-FLUIDIC BEHAVIORS OF BUBBLES IN POOL BOILING ON MICRO-TEXTURED SURFACE”. 6th International Workshop on Heat/Mass Transfer Advances for Energy Conservation and Pollution Control, August 13-16, 2021, Harbin, China. [C4] Y. Xia, R. Li. (2021) “INFLUENCE OF SURFACE WETTABILITY ON BUBBLE BEHAVIORS IN POOL BOILING”. 5-6th Thermal and Fluids Engineering Conference (TFEC), 2021, 26–28 May, Virtual. [C5] Xia, Y., Li, R. Thermal Management of the Hotspot on the Spray Cooled Surface Using Jet Impingement". In CSME 2019 Congress of CSME and CFDSC, London, Ontario, Canada viii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ................................................................................................................................ v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables ................................................................................................................................ xi List of Figures .............................................................................................................................. xii Nomenclature ........................................................................................................................... xviii Acknowledgments ....................................................................................................................... xx Dedication ................................................................................................................................... xxi Chapter 1 Introduction................................................................................................................. 1 1.1 Background ........................................................................................................................... 1 1.2 Research objectives ............................................................................................................... 3 1.3 Structure of this dissertation .................................................................................................. 5 Chapter 2 Literature Review ....................................................................................................... 6 2.1 Pool boiling heat transfer ...................................................................................................... 6 2.2 Enhanced pool boiling heat transfer by surface modification ............................................. 11 2.2.1 Influence of textured surfaces on pool boiling heat transfer ........................................ 11 2.2.2 Influence of surface wettability on pool boiling heat transfer ...................................... 15 2.2.3 Influence of boiling surface inclination angle on pool boiling heat transfer ................ 21 Chapter 3 Influence of Surface Wettability on Bubble Formation and Motion ................... 25 3.1 Background ......................................................................................................................... 25 3.2 Experimental methodology ................................................................................................. 27 3.3 Air bubbles on SHO and SHI surfaces ................................................................................ 33 3.4 Air bubble spreading on the biphilic surface ...................................................................... 36 3.5 Vapor bubble behaviors on the biphilic surface in pool boiling ......................................... 39 3.6 Summary ............................................................................................................................. 42 ix Chapter 4 Surface Effects on Sub-cooled Pool Boiling for Smooth and Laser-ablated Silicon Surfaces ........................................................................................................................... 43 4.1 Background ......................................................................................................................... 43 4.2 Experimental methodology ................................................................................................. 43 4.2.1 Preparation of boiling substrates .................................................................................. 44 4.2.2 Data analysis ................................................................................................................. 50 4.3 Results and Discussions ...................................................................................................... 53 4.3.1 Boiling on hydrophilic surfaces with varied laser scan spacings ................................. 53 4.3.2 Boiling on SHI and SHO surfaces ................................................................................ 55 4.3.3 Evaluation of surface effects ........................................................................................ 58 4.3.4 Boiling on biphilic surface ........................................................................................... 61 4.4 Summary ............................................................................................................................. 66 Chapter 5 Pool Boiling on a Biphilic Surface Where Bubbles Can Move Horizontally on the Surface ......................................................................................................................................... 67 5.1 Background ......................................................................................................................... 67 5.2 Experimental methodology ................................................................................................. 68 5.2.1 Surface Fabrication and Characterization ..................................................................... 68 5.2.2 Experimental Setups and Procedures ........................................................................... 71 5.2.3 Heat transfer data analysis ............................................................................................ 72 5.3 Results and Discussions ...................................................................................................... 74 5.3.1 Air bubble moving on the wedge-shaped SHO track ................................................... 74 5.3.2 Pool boiling on the biphilic surface .............................................................................. 78 5.4 Summary ............................................................................................................................. 84 Chapter 6 Effects of surface wettability and Inclination on bubble behaviors and pool boiling heat transfer .................................................................................................................... 86 6.1 Background ......................................................................................................................... 86 6.2 Experiments ......................................................................................................................... 87 6.2.1. Experimental setup ...................................................................................................... 87 6.2.2. Surface processing and characterization ...................................................................... 88 x 6.2.3 Data analysis ................................................................................................................. 92 6.3 Results and Discussions ...................................................................................................... 92 6.3.1 The influence of surface wettability ............................................................................. 92 6.3.2 The influence of surface inclined angle ........................................................................ 97 6.4 Summary ........................................................................................................................... 107 Chapter 7 Conclusions and Future Work .............................................................................. 108 7.1 Conclusion ......................................................................................................................... 108 7.2 Further work ...................................................................................................................... 110 Bibliography .............................................................................................................................. 112 Appendix .................................................................................................................................... 117 Appendix A ............................................................................................................................. 117 Appendix B ............................................................................................................................. 119 Appendix C ............................................................................................................................. 124 Appendix D ............................................................................................................................. 126 xi List of Tables Table 4.1 The tested surfaces are measured for the apparent contact angle (CA) and contact angle hysteresis (CAH). .......................................................................................................................... 50 Table 4.2 Critical heat fluxes and the corresponding excess temperatures, (∆Te�����)CHFfor all the tested surfaces. .............................................................................................................................. 59 Table 6.1 The contact angles of different wetting surfaces .......................................................... 92 xii List of Figures Figure 2.1 Typical boiling curve for water at atmospheric pressure with a corresponding saturation temperature of 100°C [15] .............................................................................................................. 7 Figure 2.2 Schematic of the bubble on hydrophilic and hydrophobic surfaces: (a) hydrophilic; (b) hydrophobic .................................................................................................................................... 8 Figure 2.3. Examples of textured surfaces in pool boiling studies. (a) micro pin-fin [19], (b) pillar [21], (c) tube [21], (d) nanowire [23] ............................................................................................ 13 Figure 2.4 Examples of textured surfaces by laser ablation in pool boiling. (a) copper surface [24], (b) aluminum surface [26], (c) stainless steel surface [10], (d) silicon surface [5] ...................... 15 Figure 2.5 Classification of wetting based on static contact angle θ. (a) Hydrophilic surface (10°<θ<90°), (b) Hydrophobic surface (150°>θ>90°), (c) Superhydrophilic surface (θ<10°) (d) Superhydrophobic surface (θ>150°) ............................................................................................. 16 Figure 2.6 (a) Wenzel wetting state, (b) Cassie-Baxter wetting state ........................................... 16 Figure 2.7 Examples of biphilic surfaces in pool boiling. (a) Hexagon spot on the silicon surface [31], (b) circular spot on the aluminum surface [26], (c) Square spot on the silicon surface [36], (d) circular post on the silicon surface [20] ........................................................................................ 20 Figure 3.1 Schematic of the experimental setups (a) air bubble observation; (b) pool boiling. ... 29 Figure 3.2 Laser patterned AlN surfaces and surface wettability (a) SEM of the orifice and surface textures; (b) 3D profile of the surfaces on the two sides of the orifice; (c) contact angles on the AlN surface, and the phenomena when the surface is in air and water. ....................................... 31 Figure 3.3 SEM-EDS elemental mapping of the AlN surface: (a) EDS map of element distribution on the SHI region; (b) EDS map of element distribution on the SHO region (c) Weight percentage of the elements on SHI region; (d) Weight percentage of the elements on SHO region. ............. 33 Figure 3.4 Air bubble cycle when air is injected through the 500 µm orifice on the AlN surface at the air flow rate of 10 mL/min (a) on SHI surface; (b) on SHO surface; (c) bubble departure diameter; (d) bubble departure frequency. .................................................................................... 35 xiii Figure 3.5 Air bubble spreading on the biphilic AlN surface. (a)Spreading on the bare surface without air film; (b) Spreading on SHO surface with an ultrathin air layer (air-infused case). (t = 0 ms refers to the moment when the air bubble starts appearing from the orifice); (c) schematic of the bubble leading and trailing contact angles and curvatures; (d) variation of bubble leading edge position and contact angles on both sides ..................................................................................... 38 Figure 3.6 Process of small air bubble coalescence with existing air cap on the surface and coalescence air bubble departure. (t = 0 ms refers to the moment when the small new air bubble starts appearing from the hole) ..................................................................................................... 39 Figure 3.7 Characterization of the silicon wafer surface after laser ablation (a) SEM; (b) 3D confocal image .............................................................................................................................. 40 Figure 3.8 Vapor bubble behaviors in pool boiling. (a) a1-schematic of the surface wettability pattern, a2-heater configuration, a3-schematic of the pool boiling; (b) b1-bubble starts from SHO region at 𝑇𝑇� = 98℃, b2-isolated bubble on SHI region and merged bubble on SHO region at 𝑇𝑇� =110℃ b3-merged bubble on SHI region and vapor film on SHO region at 𝑇𝑇� = 117℃; (c) bubble spreading merging processes from SHI regions on two sides towards the SHO region in the center at 𝑇𝑇� = 125℃ . ............................................................................................................................... 41 Figure 4.1 Experimental setup for pool boiling. (a) schematic, (b) photo. ................................... 45 Figure 4.2 Schematic of the boiling substrates: (a) Boiling and heating surfaces of the boiling substrate; (b) Bottom view of the heating surface. The squares with a × a represent two IR image pixels and a =284 μm. .................................................................................................................. 46 Figure 4.3 (a) Confocal images of the laser-ablated surfaces with two scan spacings w=25 μm and 50 μm; (b) SEM images of the surfaces; (c) Surface roughness: Sq (root-mean-square roughness) and 𝑅𝑅𝑊𝑊 (Wenzel roughness). ........................................................................................................ 47 Figure 4.4 (a) Apparent contact angles of water droplets on the surfaces: CA = 52° for the HI surface; CA = 105° for the HO surface; and CA = 163° for the SHO surface. (b) Advancing and receding contact angles for the SHO surface. Formation and departure of air bubbles on (c) SHI surface and (d) SHO surface. ........................................................................................................ 49 xiv Figure 4.5 Boiling heat transfer characteristics on surfaces HI, SHI-w50 and SHI-w25. (a) Nucleate boiling curves (data points with arrows are CHFs); (b) global heat transfer coefficient........................................................................................................................................................ 54 Figure 4.6 Bubble behaviors on different surfaces at varied global heat fluxes (for each image 𝑞𝑞′′����and 𝑇𝑇� are provided). For each test, the image showing the largest vapor accumulation on the surface is selected. (a) The HI surface; (b) the SHI-w25 surface. ................................................ 55 Figure 4.7 Temporal evolution of a single vapor bubble on (a) SHO-w25 (𝑞𝑞′′���� = 7.2 W/cm2); (b) SHI -w25 (𝑞𝑞′′���� = 5.7 W/cm2). ...................................................................................................... 57 Figure 4.8 Heat transfer in nucleate boiling on two hydrophilic surfaces (SHI-w25, HI) and two hydrophobic surfaces (HO, SHO-w25). (a) Nucleate boiling curves; (b) global heat transfer coefficient. .................................................................................................................................... 58 Figure 4.9 (a) Surface-fluid factor for the five tested surfaces; (b) Comparison of the surface-fluid factors of Water-silicon on the five wetting surfaces to those in the Rohsenow correlation.. ...... 61 Figure 4.10 Bubble behaviors on the biphilic surface at different global heat fluxes. (a) q′′��� =16.4 W/cm2, (b) 𝑞𝑞′′���� = 23.8 W/cm2, (c) 𝑞𝑞′′���� = 44.6 W/cm2. .................................................... 63 Figure 4.11 Temperature distribution of the biphilic surface at three global heat fluxes: (a) q′′��� =16.4 W/cm2, (b) 𝑞𝑞′′���� = 23.8 W/cm2, (c) 𝑞𝑞′′���� = 44.6 W/cm2 . ................................................... 63 Figure 4.12 (a) Local surface temperature along the centerlines (dash lines in Figure 4.11); (b) Temperature gradient along the centerlines. ................................................................................. 65 Figure 4.13 Local heat transfer coefficient along the centerlines calculated based on the data shown in Figure 4.12. ............................................................................................................................... 65 Figure 5.1 (a). Dimensions of the SHI and SHO regions on the biphilic surface. (b) Steps of fabricating the biphilic surface. (c) Contact angles of different regions: c1-SHI; c2-HO; c3-SHO. (d) SEM of the SHI and SHO regions: d1-SHI; d2 SHO. (e) Confocal images of SHI and SHO regions: e1-SHI; e2-SHO. ............................................................................................................. 70 Figure 5.2 (a) Air bubble experimental study setup. (b) Pool boiling experimental setup. (c) Thin-film heater assembly: c1- top view; c2-bottom view. ................................................................... 73 xv Figure 5.3 Schematic of the boiling substrates indicating heat analysis: (a) Boiling and heating surfaces of the boiling substrate. (b) Bottom view of the heating surface. The squares with 𝑎𝑎 × 𝑎𝑎 represent two IR image pixels and 𝑎𝑎 =266 𝜇𝜇𝜇𝜇. ................................................................. 73 Figure 5.4 (a) Sequential images showing air bubbles moving on the wedge-shaped SHO track in the wedge-diverging direction. (b) Positions of the leading and training edges of the bubble versus time. .............................................................................................................................................. 75 Figure 5.5 Force analysis of a gas bubble on the wedge-shaped SHO track. The wedge angle is 𝜂𝜂. The angles 𝜃𝜃1 and 𝜃𝜃2 are not but can be approximated as local contact angles, because the two-track borderlines are not perpendicular to the baseline of the cross-section, but 𝜂𝜂 is small. ........ 77 Figure 5.6 Pool boiling heat transfer results on the SHI and biphilic surface. (a) Boiling curves of water. (b) Heat transfer coefficient versus heat flux. .................................................................... 79 Figure 5.7 Comparison of bubble behaviors on the SHI and biphilic surfaces at various global surface heat fluxes. (a) SHI surface. (b) Biphilic surface. ............................................................ 80 Figure 5.8 (a) Onset of nucleate boiling at the tips of the SHO wedge-shaped tracks (𝑞𝑞′′���� =11 W/cm2) (b) Vapor bubble moving on the wedge-shaped SHO track in pool boiling at 𝑞𝑞′′���� =141 W/cm2. (c) Location of trailing edge versus time for the 4 bubbles. ................................... 82 Figure 5.9. Comparison of the development of a bubble and the temperature distribution on the SHI surface and the biphilic surface for 𝑞𝑞′′���� = 176 W/cm2. (a) Bubble development on the SHI surface. (b) Bubble development on the biphilic surface. ............................................................ 83 Figure 6.1 Schematic of the experimental setups. (a) Air bubble study. (b) Pool boiling ............ 87 Figure 6.2. Schematic of the fabrication of surfaces used in this study ........................................ 89 Figure 6.3 Surface characterization. (a) SEM image, (b) 3D confocal image .............................. 90 Figure 6.4 Chemical reaction for surface hydrophobization. (a) PFOS coating, (b) PDMS brush....................................................................................................................................................... 91 Figure 6.5. Apparent contact angles of the different wetting surfaces. ........................................ 92 xvi Figure 6.6. Bubble formation behaviors at the air injection flow rate of 1mL/min on different wetting surfaces at α=0°. (a) HI surface. (b) SHI surface. (c) HO-PFOS surface. (d) HO-PDMS surface. (e) SHO surface. (The moment the air bubble starts emerging is set as t=0) .................. 94 Figure 6.7. Formation and departure of vapor bubbles from (a) SHI surface (heat flux q′′��� =13.3W/cm2). (b) HI surface (𝑞𝑞′′���� = 9.3W/cm2). (c) SHO surface (𝑞𝑞′′���� = 16.3W/cm2). ........... 95 Figure 6.8 Formation and departure of vapor bubbles from (a) HO-PDMS surface (heat flux q′′��� =10.5W/cm2). (b) HO-PFOS surface (𝑞𝑞′′���� = 10.5W/cm2) ........................................................... 96 Figure 6.9 Temperature distributions and bubble behaviors on different horizontally positioned wetting surfaces at a heat flux of around 25W/cm2. (a)HI surface. 𝑇𝑇� = 117℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/𝑐𝑐𝜇𝜇2. (b) SHI surface. 𝑇𝑇� = 113℃, 𝑞𝑞′′���� = 30𝑊𝑊/𝑐𝑐𝜇𝜇2 . (c) HO-PDMS surface. 𝑇𝑇� = 117.5℃, 𝑞𝑞′′���� =24𝑊𝑊/𝑐𝑐𝜇𝜇2 . (d) HO-PFOS surface. 𝑇𝑇� = 115℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/𝑐𝑐𝜇𝜇2 ............................................ 97 Figure 6.10. Growth and departure of injected air bubbles from the SHI surface. (a) α =30°. (b) α =60°. (c) α =90°. ........................................................................................................................... 98 Figure 6.11. Growth and departure of injected air bubbles from the HI surface. (a) α =30°. (b) α =60°. (c) α =90°. ........................................................................................................................... 99 Figure 6.12. Growth and departure of injected air bubbles from the HO-PDMS surface. ......... 101 Figure 6.13. Growth and departure of injected air bubbles from the HO-PFOS surface. (a) α =30°. (b) α =60°. (c) α =90°.................................................................................................................. 102 Figure 6.14. Bubble displacement versus time on surfaces at different inclined angles. (a) HO-PDMS. (b) HO-PFOS. ................................................................................................................ 102 Figure 6.15 Formation and departure of vapor bubbles from the vertical surface (a) SHI surface (heat flux 𝑞𝑞′′���� = 13.4𝑊𝑊/𝑐𝑐𝜇𝜇2). (b) HI surface (𝑞𝑞′′���� = 16.4𝑊𝑊/𝑐𝑐𝜇𝜇2) .......................................... 103 Figure 6.16 Formation and departure of vapor bubbles from the vertical surface (a) HO-PDMS surface (heat flux 𝑞𝑞′′���� = 10.5𝑊𝑊/𝑐𝑐𝜇𝜇2). (b) HO-PFOS surface (𝑞𝑞′′���� = 10.5𝑊𝑊/𝑐𝑐𝜇𝜇2) ................. 104 Figure 6.17 Temperature distributions and bubble behaviors on different vertically positioned wetting surfaces at a heat flux of around 25W/cm2. (a)HI surface. 𝑇𝑇� = 114℃, 𝑞𝑞′′���� = 25.7𝑊𝑊/𝑐𝑐𝜇𝜇2. (b) SHI surface. 𝑇𝑇� = 110℃, 𝑞𝑞′′���� = 30𝑊𝑊/𝑐𝑐𝜇𝜇2. (c) HO-PDMS surface. 𝑇𝑇� = 117℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/xvii 𝑐𝑐𝜇𝜇2 . (d) HO-PFOS surface. 𝑇𝑇� = 112℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/𝑐𝑐𝜇𝜇2. (e) SHO surface. 𝑇𝑇� = 129℃, 𝑞𝑞′′���� =24𝑊𝑊/𝑐𝑐𝜇𝜇2. .................................................................................................................................. 105 Figure 6.18 Comparison of pool boiling curves on different wetting surfaces at two inclination angles. (a) α =0°, (b) α =90°, (c) α =0° and α =90° .................................................................... 106 Figure 7.1 Schematic of the fabrication of biphilic boiling surfaces o be used in future study . 111 xviii Nomenclature 𝐴𝐴 heating area a actual side length of image pixel b silicon substrate thickness Fx total force in the 𝑥𝑥 direction on the moving gas bubble hair natural air-free convection heat transfer coefficient h global heat transfer coefficient ∆ℎ uncertainty of heat transfer coefficient ℎ(𝑥𝑥, 𝑦𝑦) height of interface at (x,y) ℎ𝑝𝑝+ dimensionless pit depth H height of the gas bubble interface in the central plane N number of pixels within the heating area 𝐴𝐴 𝑛𝑛�⃗ unit normal vector for the interface 𝑝𝑝𝐺𝐺,𝑠𝑠 pressure of the gas flow at the gas-liquid interface 𝑝𝑝𝐿𝐿,𝑠𝑠 pressure of the liquid flow at the gas-liquid interface 𝑝𝑝𝜎𝜎 capillary pressure 𝑝𝑝𝑔𝑔 partial pressure of the trapped air 𝑞𝑞′′���� global heat flux ∆𝑞𝑞′′ uncertainty of heat flux 𝑞𝑞 boiling heat transfer rate 𝑞𝑞𝑖𝑖𝑖𝑖𝑖𝑖 in-plane heat conduction heat loss at the periphery of the heater 𝑞𝑞𝑎𝑎𝑖𝑖𝑎𝑎 heat loss caused by the natural air convection 𝑞𝑞𝑎𝑎𝑎𝑎𝑟𝑟 heat loss by radiation 𝑟𝑟 radius of visible bubbles Ra the average roughness 𝑆𝑆(𝑥𝑥, 𝑦𝑦, 𝑧𝑧) liquid-gas interface at position (x, y, z) Sm he spacing between surface peaks 𝑇𝑇 time-averaged temperature distribution 𝑇𝑇� global surface-average temperature 𝑇𝑇𝑎𝑎𝑖𝑖𝑎𝑎 ambient air temperature xix 𝑇𝑇𝑠𝑠𝑠𝑠𝑎𝑎 surrounding temperature 𝑇𝑇𝑂𝑂𝑂𝑂𝑂𝑂 onset of nucleate boiling temperature 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠 saturation temperature 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠,𝑎𝑎𝑠𝑠𝑎𝑎 water saturation temperature at atmospheric pressure ∆𝑇𝑇𝑒𝑒����� global surface excess temperature 𝛿𝛿𝑇𝑇 temperature difference between the two adjacent pixels across the border. ∆𝑇𝑇� uncertainty of average surface temperature W width of bubble base Greek symbols σ Stefan-Boltzmann constant γ surface tension 𝜃𝜃 apparent contact angle 𝜌𝜌𝐿𝐿 liquid density α surface inclination angle η wedge diverging angle Abbreviations CA contact angle CAH contact angle hysteresis CHF critical heat flux EDS Energy-dispersive X-ray spectroscopy HO hydrophobic HTC heat transfer coefficient MEMS Micro-electromechanical Systems ONB onset of nucleate boiling PID proportional-integral-derivative SHO super-hydrophobic SHI super-hydrophilic xx Acknowledgments I am deeply grateful to my supervisor Prof. Sunny Ri Li for his supervision, support, patience and guidance during my Ph.D. study. Without his invaluable help, I could not complete my Ph.D. study and research. I would also like to thank Dr. Sina Kheirkhah and Dr. Vladan Prodanovic for being on my advisory committee. I appreciate their time and constructive comments over the past few years at the University of British Columbia. Thanks also go to Prof. Hongwei Wu for being my external examiner, Prof. John Frostad for being the university examiner. Then, I want to thank my groupmates, Dr. Xuan Gao, for his teaching and training of lab facilities operation, as well as his assistance, encouragement, and sharing his expertise and viewpoints with me. Thanks also go to my colleagues Dr. Anshuman Dey, Ruoyao Li, Mojitaba, and Yadi Cao for their assistance in the forms of advice, encouragement and discussion. Special thanks go to Mr. Kai Xiang Oh from Dantec Dynamics Company, for very helpful training on Stereo- and μ-PIV systems and solving many technical problems during my practical operation. Many thanks to my dear friends, Dr. Xiaoxiao Zhao, Dr. Jintao Han, Dr. Jianyu Niu, Dr. Xiao Chen, Dr. Fangyu Gai, Dr. Xinjie Fan, Dr. Wentan Li, Dr. Yijia Shao and Dr. Erkang Qiu for their insightful comments and suggestions on my Ph.D. study. I would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for the funding support. Last but not least, I would like to thank my family for their support and care over all these years of education. I would like to acknowledge my dear girlfriend Hui Luo for her invaluable support and encouragement. xxi Dedication To my family for their love and support! 1 Chapter 1 Introduction This chapter provides the background, motivation, objective, as well as the structure of this dissertation. 1.1 Background With the miniaturization and more feeding power of high-performance applications such as microelectronics, electric cars, and nuclear reactors [1, 2], cooling methods such as air and single-phase liquid cooling are not able to handle high heat fluxes beyond 100 W/cm2 [3, 4]. Pool boiling with the advantages of simple hardware and the capability of transmitting large amounts of heat with small temperature differences is considered a promising cooling method and has been employed in different industries involving electronic cooling and power generation systems. [5, 6]. In pool boiling, the heat transfer coefficient (HTC) and critical heat flux (CHF) are two important parameters. The HTC quantifies the heat transfer capacity for a given difference between the surface temperature and the fluid’s saturation temperature or determines the required temperature difference for a given heat transfer rate. CHF represents the limit of the operation system, and once heat flux goes beyond the CHF, the temperature will experience an abrupt increase due to the formation of vapor films, which may cause burnout of the device. Therefore, it is of great importance to increase HTC and extend CHF. Both passive methods such as surface modification [5-10] and active techniques using either electric, magnetic or acoustic fields [11-13] have been investigated to explore how to enhance pool boiling heat transfer performances. Compared with active methods, passive methods are more feasible in applications since they do not require extra energy, and the hardware is simpler. Among the passive methods, surface modification is the most widely used for heat transfer enhancement. 2 One type of surface modification is surface texturing, and the textured surfaces usually have uniform surface chemistry, hydrophilic or hydrophobic. Another type of surface modification is wettability patterning, and there is rising interest in biphilic surfaces, on which hydrophobic and hydrophilic spots distribute in some pattern. Although much work has been carried out on surface modification for pool boiling, there are several limitations for most of the previous studies: 1) Little attention has been paid to bubbles generated close to or on biphilic borders, where the bubble behavior could be significantly affected by the local wettability contrast. And there is a lack of knowledge about local boiling heat transfer at the borders between hydrophilic areas and hydrophobic areas. This is particularly important for a biphilic surface composed of many (super)hydrophilic and (super)hydrophobic regions. 2) Few studies have covered the large spectrum of wettability ranging from super-hydrophilicity to super-hydrophobicity. 3) The discussion of surface effects mainly relied on the measured boiling curves and observed bubble behaviors but lacked quantitative assessment and comparison. 4) In pool boiling on a horizontal flat surface, bubble motion relative to the surface is mainly in the vertical direction. Could bubbles move horizontally on the boiling surface, and how would the boiling heat transfer be affected? 5) The existing studies about the influence of surface wettability on pool boiling are limited mainly to horizontal upward-facing surfaces, and the coupled influence of surface wettability and the surface inclination angle is rarely studied. Therefore, a systematic study of the effects of surface characteristics is necessary to identify the physics of bubble dynamics and heat transfer, to optimize the boiling surface design. The 3 objective of this study is to systematically investigate the effects of surface texture, wettability and orientation on bubble dynamics and heat transfer in pool boiling, aiming to provide more insight into the physics of pool boiling. 1.2 Research objectives The overall objective of this study is to conduct a systematic study of the effects of surface characteristics on bubble dynamics and heat transfer in pool boiling. More specifically, the research is divided into six tasks: 1). Surface texturing and wettability modification. 2). Thin film resistive heater fabrication. 3). Influence of surface wettability on bubble formation and motion. 4). Surface effects on sub-cooled pool boiling for smooth and laser-ablated silicon surfaces. 5). Pool boiling on a biphilic surface where bubbles can move horizontally on the surface. 6). Effects of surface wettability and inclination angle on bubble behaviors and pool boiling heat transfer. • To get the textured functional surfaces for pool boiling, a nanosecond laser (Oxford Lasers-A-355 micromachining system) is used in this study to create surface textures, which has excellent advantages including high-speed operation, low cost, and accurate local treatment. Surface hydrophobization is obtained by silanization grafting. The surface textures are observed by the scanning electron microscope (SEM, Mira3 XMU, Tescan, Brno, Czech Republic) and the laser confocal scanning microscope (LEXT OLS5000, Olympus Corporation, Tokyo, Japan). The wettability is evaluated by measuring the contact angles using a goniometer (Model 90, Rame ́-hart Instrument Co., Succasunna, NJ). • To get more accurate and detailed temperature information on the boiling surface, instead of using several thermocouples, a thin film resistive heater which enables the measurement of temperature distribution using an infrared camera is designed and fabricated on the back 4 side of the boiling surface using the sputter coating system (Armstrong Engineering Corp.). A 35 nm thick layer of chrome is coated onto the surface, followed by a 60 nm thick layer of gold. The chrome layer here is to enhance the adhesion of the gold layer, and the gold layer is the heating element. Copper foils are connected to the gold layer by lead soldering. • To study the influence of surface wettability on bubble formation, and the bubble motion behaviors close to or on biphilic borders, isothermal air bubble formation from a submerged orifice on different wetting surfaces is observed using a high-speed camera, followed by the vapor bubble behaviors on the boiling surface in pool boiling. The bubble movement is analyzed • To investigate the effects of surface wettability on pool boiling, various wetting surfaces from super-hydrophilic to super-hydrophobic are fabricated and tested in terms of the bubble behaviors and heat transfer performances using the high-speed camera and the infrared camera. With the bubble dynamics and the temperature information, it is expected to get a basic understanding of the local convection, vapor removal, and heat transfer processes around isolated growing vapor bubbles. Some interaction behaviors between bubbles on well-characterized surfaces can also be investigated. The effects of surface characteristics can be well understood in comparison with the observation of adiabatic gas bubble dynamics. • To investigate the influence of horizontally moving bubbles on the surface, a biphilic surface consisting of both low and high-wettability regions is designed and fabricated to enable bubbles to transport on the surface. The heat transfer performances on this biphilic surface are compared with the super-hydrophilic surface. 5 • To explore the influence of surface wettability and surface inclination angle on the bubble behaviors and heat transfer in pool boiling, different wetting surfaces are fabricated and tested on surfaces positioned from 0° (horizontally upward) to 90° (vertical). 1.3 Structure of this dissertation This dissertation has 7 chapters. Chapter 1 introduces the research background, objective and motivation, as well as the structure of this dissertation. Chapter 2 provides a literature review of the influence of surface characteristics on pool boiling. Chapter 3 presents the experimental study of the influence of surface wettability on bubble formation and motion. Chapter 4 reports the experimental study of the surface wettability effects on sub-cooled pool boiling for smooth and laser-ablated silicon surfaces. Chapter 5 presents the experimental study of pool boiling on a biphilic surface where bubbles can move horizontally on the surface. Chapter 6 discussed the effects of surface wettability and inclination angle on bubble behaviors and pool boiling heat transfer. Chapter 7 summarizes the outcomes from the research in this dissertation and proposes further work about the influence of surface characteristics on pool boiling study. 6 Chapter 2 Literature Review Pool boiling can handle high heat flux dissipation with simple hardware. In this chapter, a literature review of the pool boiling studies related to the work in this dissertation is provided. Pool boiling regimes are first introduced followed by a review of previous research articles on pool boiling enhancement, which focus on the influences of surface texture, surface wettability and orientation on the heat transfer performance. 2.1 Pool boiling heat transfer Nukiyama [14] reported the first typical water boiling curve for a horizontal electrically heated nichrome wire immersed in a pool of distilled water. The test was conducted in a temperature-controlled environment at atmospheric pressure with a corresponding saturation temperature of 100°C. As shown in Figure 2.1, with the increase of wall superheat temperature (the difference between the wall temperature and the saturation temperature), four regimes were identified, which include free convection boiling (Region 1), nucleate boiling (Regions 2 and 3), transition boiling (Region 4) and film boiling (Region 5). In the free convection regime at a small superheat (<5℃), no bubble is observed on the surface but a thermal boundary layer and the heat from the surface is transferred to liquid by natural convection. As the surface superheat becomes larger than 5℃, the steam bubbles start to initiate from certain locations where there are imperfections such as cavities. This superheat is called the onset of nucleate boiling (ONB). The nucleate boiling regime consists of two different regimes, which are the individual bubble regime and the slugs and columns regime. Isolated bubbles generate and detach in the individual bubble regime, while bubbles merge and coalesce in the second regime before leaving the boiling surface as slugs and columns. Nucleate boiling concludes at the CHF, beyond which is the transition 7 boiling regime. It is unstable and heat flux decreases with the increasing superheat until it reaches the minimum heat flux Leidenfrost point. At this moment, the surface is covered by superheated vapors, which indicates the beginning of the film boiling. Heat transfer in this regime mainly depends on radiation and heat conduction. Different from the temperature-controlled mode, in the heat flux-controlled mode, after reaching the CHF, it immediately enters the film boiling regime and leads to the failure of the heater. From the perspective of efficient cooling, nucleate boiling is desirable because it has relatively high heat transfer coefficients. The ONB and the CHF are also very important since they represent the start and end of nucleate boiling. Early onset of nucleate boiling and large CHF are preferable for efficient heat transfer. The enhanced pool boiling study focuses on the nucleate boiling regime. Figure 2.1 Typical boiling curve for water at atmospheric pressure with a corresponding saturation temperature of 100°C [15] 8 The onset of nucleate boiling Early ONB means easier initiation of nucleate boiling, which normally refers to a low excess superheat. Considering a vapor bubble including the possibility of non-condensable gas to be in equilibrium, based on the Laplace-Young equation and Clausius-Clapeyron equation, the superheat corresponding to the bubble with radius r can be derived as 2g satsat gl lvR T TT T T pp h rγ ∆ = − = −   (2.1) where Tsat is saturation temperature, T is the surface temperature, Rg is the gas constant 8.314 J / mol·K, γ is the surface tension, and hlv is latent heat, pl is the pressure in the liquid, and r is the radius of the vapor bubble. It shows that the presence of non-condensable gas reduces the superheat required to generate bubbles. Without non-condensable gas, Eq. (2.1) can be simplified as 2 satsatv lvTT T Th rγρ∆ = − = (2.2) Figure 2.2 Schematic of the bubble on hydrophilic and hydrophobic surfaces: (a) hydrophilic; (b) hydrophobic 9 For a small vapor bubble to appear on the boiling surface with the volume V as shown in Figure 2.2, the radius depends on the surface wettability and can be expressed as 3 33(2 3cos cos )Vrπ θ θ=+ − (2.3) Therefore, Eq. (2.2) can be rearranged as 332 (2 3cos cos )3satv lvTTh Vγ π θ θρ+ −∆ = (2.4) where 𝜌𝜌𝑣𝑣 is the vapor density, V is the volume of the vapor bubble, and θ is the surface contact angle. Heat Transfer Coefficient and Critical heat flux The rate of heat transfer and CHF strongly depend on the nature of nucleation and the type and condition of the heated surface. Rohsenow [16] proposed the first and most widely used correlation for nucleate boiling 31/2,'',( )Prp l eL vs l lv ns f lv lc Tgq hC hρ ρµγ ∆ − =         (2.5) Here 𝑞𝑞𝑠𝑠′′ is the heat flux, µ𝑙𝑙 is the dynamic viscosity of the liquid, ℎ𝑙𝑙𝑣𝑣 is the enthalpy of vaporization, g is the gravitational acceleration, 𝜌𝜌𝑙𝑙 is liquid density, 𝜌𝜌𝑣𝑣 is vapor density, γ is surface tension, 𝑐𝑐𝑝𝑝,𝑙𝑙 is the specific heat of the liquid, Δ𝑇𝑇𝑒𝑒 = 𝑇𝑇 − 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠 is excess temperature, 𝑃𝑃𝑟𝑟𝑙𝑙 is liquid Prandtl number, 𝐶𝐶𝑠𝑠,𝑓𝑓 is the surface fluid factor and n is the experimental constant equal to 1 for water and 1.7 for other fluids. However, this correlation applies only to clean and relatively smooth surfaces. 10 The critical heat flux correlation based on a hydrodynamic stability analysis was proposed by Zuber [17] 1/4''2( )L vc lv vvgq Kh γ ρ ρρρ −=    (2.6) Here 𝑞𝑞𝑖𝑖′′ is the critical heat flux, K is a constant that depends on heating surface geometry, ℎ𝑙𝑙𝑣𝑣 is the enthalpy of vaporization, g is the gravitational acceleration, 𝜌𝜌𝑙𝑙 is liquid density, and 𝜌𝜌𝑣𝑣 is vapor density. Although it can give reasonable prediction to the value of CHF for flat surfaces, the effects of the surface characteristics were not taken into account, such as wettability, roughness as well as the orientation of the boiling surface. Kandlikar [18] addressed the combined effects of contact angle and surface orientation, and a correlation was proposed. [ ]1/21/4'' 0.5 1 cos 2( ) (1 cos )cos ( )16 4recedingc lv v receding L vq h gθ πρ θ α γ ρ ρπ+  = + + −   (2.7) Here 𝑞𝑞𝑖𝑖′′ is the critical heat flux, ℎ𝑓𝑓𝑔𝑔 is the enthalpy of vaporization, 𝜌𝜌𝑣𝑣 is vapor density, g is the gravitational acceleration, 𝜌𝜌𝑙𝑙 is liquid density, θreceding is the dynamic receding contact angle of the liquid-vapor interface with the solid heater surface, γ is surface tension and α is the surface inclination angle. However, this model is tested only for the hydrophilic cases. Some other correlations have also been derived by adding different parameters based on the classical correlations mentioned above. For example, Liang et al. [84] devised a new correlation for heat transfer on the micro-pit surfaces by incorporating the dimensionless pit depth (ℎ𝑝𝑝+) to the Rohsenow’s correlation in in Eq. (2.5). 0.1020.33''0.089( ), '' 0.025,10.312 Pr ( )( )php l e nss f l slv l lv L v pc T qC qh h g hγµ ρ ρ++ ∆=  −  (2.8) 11 Kim et al. [85] incorporated the surface roughness parameters into the CHF correlation by Kandlikar [18] in Eq. (2.7) and provided a new correlation. [ ]1/21/4'' 0.5 1 cos 2 351.2cos0.811 ( ) (1 cos ) ( ) ( )16 4 1 cosreceding ac lv v receding L vmRq h gSθ π θρ θ γ ρ ρπ θ+  = + + + − + (2.9) where Sm is the spacing between surface peaks, and Ra is the average roughness. From these correlations, it is obvious that surface characteristics such as surface roughness, wettability, as well as inclination angle, play a great role in heat transfer performances. Fundamental knowledge of the influence of surface properties on pool boiling is the starting point for surface design and has been investigated in many studies. The following section summarizes the enhanced pool boiling heat transfer studies by surface modification, mainly focusing on wettability and surface orientation. 2.2 Enhanced pool boiling heat transfer by surface modification Surface modification is the most widely used method to enhance pool boiling heat transfer. One type of surface modification is surface texturing, and the textured surfaces usually have uniform surface chemistry, hydrophilic or hydrophobic. Another type of surface modification is wettability patterning. The review will focus on the influence of textured surfaces, surface wettability, and surface inclination angles. 2.2.1 Influence of textured surfaces on pool boiling heat transfer With the development of surface processing technology, various surface patterns in micro/nano scale as shown in Figure 2.3 have been fabricated and investigated to enhance pool boiling heat transfer, such as micro-pin-fin [19], pillar [21], micro/nano tube [21, 22], nanowire [23]. Cao et al. [19] compared the heat transfer on the micro-pin-fin surface with the smooth 12 surface and reported that the heat transfer coefficient and the critical heat flux were increased by more than 200 and 65–83% on the micro-pin-fin surfaces. The reasons exist in the fact that on the micro-pin-fin surface, the bubble departure diameter is smaller while the departure frequency is larger, and the surface improves the liquid rewetting ability. Song et al. [21] investigated the heat transfer on the micro tube surfaces, and demonstrated simultaneous CHF and HTC enhancements of up to 62 and 244%, respectively. The enhancement of HTC is attributed to active bubble nucleation from micro tube cavities, while CHF enhancement is a result of microlayer evaporation around micro tube sidewalls. Moreover, higher CHF values were achieved by separating the micro tube clusters by 2 mm from each other. Fan et al. [22] tested the nanotube-coated titanium substrate and found the nanotubes are effective in enhancing both the heat transfer coefficient and CHF, and the heat transfer coefficient is increased by around 65% over the plain surface. The improvements of the nanotube surfaces could be attributed to the modified bubble dynamics and effective rewetting of the dry spots, related to the faster liquid replenishment due to capillary wicking of the tube-like structure. Shim et al. [23] reported that the CHF increased by 178% as a result of the increased volumetric wicking rate due to the existence of nanowire compared with the plain surface. 13 Figure 2.3. Examples of textured surfaces in pool boiling studies. (a) micro pin-fin [19], (b) pillar [21], (c) tube [21], (d) nanowire [23] However, these textures require complicated fabrication procedures involving various facilities such as Micro-Electro-Mechanical Systems (MEMS) Fabrication. Recently, a single-step surface modification method, laser ablation has been used to modify surfaces in boiling-involved studies. This method is applicable for both metal [24-27] and non-metal materials [5, 28-31] with the advantages of high-speed operation, low cost, and high accuracy. As shown in Figure 2.4, the surfaces by laser processing are different from the micro-pin fin surfaces by etching having smooth walls, and there are some porous textures formed on the laser-ablated regions. Kruse et al. [24] conducted pool boiling tests on copper surfaces processed by the femtosecond laser. As shown in Figure 2.4a, the laser processing creates a self-organized microstructured surface, which was reported to reduce the heat transfer performance. This is because the microstructured oxide layer formed from laser processing has a low thermal conductivity, which acts as an insulator. Može et al. [26] studied the pattern geometry optimization on superbiphilic aluminum surfaces fabricated 14 by laser ablation for enhanced pool boiling heat transfer. A strong wettability contrast was reported to be important for high heat transfer performance on the wettability-patterned surfaces. Može et al. [27] continued the pool boiling study on laser-engineered microcavity surfaces, and both the SHI and SHO surfaces with laser-engineered microcavities provide enhanced pool boiling heat transfer. The fast, low-cost, fabrication process by laser processing was said to have great potential for advanced thermal management applications. Among the non-metal surfaces used, silicon wafers are mostly used [5, 6, 29], since it is a widely used substrate in many applications such as microelectronics. Liu et al. [5] experimentally studied the pool boiling of FC-72 on laser-ablated Si wafer surfaces. Ablation was done by using the laser beam to scan the surface in straight lines, and the surface roughness changes with the scan spacing. As compared to the smooth surface, laser ablation with small scan spacings of 30 µm and 70µm was found to increase HTC but show limited effects on CHF. The surfaces ablated with scan spacings larger than 100 µm showed significant enhancement in both CHF and HTC. Serdyukov et al. [29] conducted pool boiling experiments on fully laser-ablated silicon wafer surfaces, for which the laser scan spacing is small enough to have the entire surface covered by laser-ablated textures. Early onset of nucleate boiling and smaller bubble departure diameters were observed for these surfaces. The heat transfer coefficient increased by 49.5% over the non-polished surface of the silicon wafer and 234% over the polished surface. The reported enhanced heat transfer on textured surfaces was attributed to the increase of surface area, more nucleation sites, improved surface wettability and capillary-assisted suction of the porous structure. However, contradictory conclusions exist. For example, Može et al. [27] found the fully laser-ablated surfaces showed limited enhancement. These findings related to the influence of laser scan spacing are not consistent, and more tests are required. 15 Figure 2.4 Examples of textured surfaces by laser ablation in pool boiling. (a) copper surface [24], (b) aluminum surface [26], (c) stainless steel surface [10], (d) silicon surface [5] 2.2.2 Influence of surface wettability on pool boiling heat transfer Pool boiling is characterized by cyclic bubble formation, grow-up, and departure, the heat transfer performance is closely related to bubble dynamics. Boiling surface wettability, which affects the bubble-surface interaction, was reported to have a significant effect on heat transfer. Surface wettability is generally characterized by the equilibrium contact angle (CA) of a sessile droplet on the surface in the air (θ in Figure 2.5), which is defined as the angle between a liquid and gas/vapor interface on a solid surface, taken through the liquid at the point where all three phases meet, and indicates how well the fluid contacts the solid. It depends on the force balance 16 of the surface tension of each solid-fluid-vapor phase. The surface is considered hydrophilic if 10°<θ<90°, and hydrophobic if 150°>θ>90°. The surface is termed SHO when the contact angle is larger than 150°, and SHI if θ<10°. For droplets resting on rough surfaces, two limiting states are usually considered. If the liquid completely penetrates the rough grooves and stays in contact with the microstructured surfaces, it belongs to the Wenzel wetting state (see Figure 2.6a). In contrast, if the liquid droplet is suspended on the top of surface microstructures, it is the Cassie-Baxter wetting state (see Figure 2.6b). The SHI surface in air (CA<10°) becomes superaerophobic under water, and vice versa for the SHO surface (CA>150°) [32, 33]. Figure 2.5 Classification of wetting based on static contact angle θ. (a) Hydrophilic surface (10°<θ<90°), (b) Hydrophobic surface (150°>θ>90°), (c) Superhydrophilic surface (θ<10°) (d) Superhydrophobic surface (θ>150°) Figure 2.6 (a) Wenzel wetting state, (b) Cassie-Baxter wetting state The influence of surface wettability on pool boiling heat transfer has been studied by lots of scholars. Phantom et al. [34] studied the effects of surface wettability on pool boiling heat transfer and nucleation mechanisms using the stainless steel substrate. The surface wettability is 17 varied by nanocoating with different materials, of which the contact angle ranges from 20° to 100°. Results indicated that the vapor bubble has a larger departure diameter and lower departure frequency on the hydrophilic surface with a greater wettability. The hydrophobic surface enables bubble initiation at a lower superheat, but it is difficult for the bubble to detach from the hydrophobic surface, which would merge with the surrounding bubbles and form a large bubble. The surface with a water contact angle of 0° or 90° was reported to have the best heat transfer coefficient. Betz et al. [31] studied the nucleation site density and heat transfer coefficient in pool boiling as a function of superheat on the hydrophilic, hydrophobic, SHI and SHO surfaces. Bubbles on the SHI surface nucleate at a surface superheat about three times lower than the hydrophilic surface. Hydrophobic surfaces nucleate at values of superheat about one order of magnitude lower than hydrophilic surfaces, and superhydrophobic surfaces nucleate at superheats another order of magnitude lower than hydrophobic surfaces. The hydrophobic surface features large HTC at low superheat, but this value stays constant at larger superheat because the large contact diameter of the bubbles limits the maximum number of active nucleation sites. The hydrophilic surface provides a higher HTC than the hydrophobic surface, albeit at a higher superheat. The SHI surfaces exhibit the highest measured HTC of all surfaces with spatially uniform wettability. The enhancement of HTC was indeed attributed to an increase in available nucleation sites. The vast majority of prior studies have reported that hydrophobic surfaces promote early ONB (onset of nucleate boiling), although bubbles tend to stay on hydrophobic surfaces, which leads to lower CHF (critical heat flux). On the contrary, hydrophilic surfaces delay the CHF, but they need larger superheat to initiate nucleate boiling [20, 31, 34, 35]. To utilize the advantages of both hydrophobic and hydrophilic surfaces, the use of a mixed hydrophobic-hydrophilic surface, as the surface patterns shown in Figure 2.7 (often termed as 18 heterogeneous wetting surface or biphilic surface) was proposed to combine both the early ONB on hydrophobic and the high CHF on hydrophilic surfaces [20, 26, 27, 31, 35-38]. On biphilic surfaces consisting of hydrophilic and hydrophobic areas, the hydrophilic areas are for achieving high CHF, while the hydrophobic areas are for the early onset of nucleate boiling. Betz et al. [37] experimentally studied pool boiling on smooth biphilic silicon surfaces and concluded that having the hydrophobic hexagonal spots on the hydrophilic surface significantly improved the CHF and HTC by 65% and 100%, respectively. Hydrophilic networks indeed are shown to efficiently prevent the formation of an insulating vapor layer. Jo et al. [35] conducted similar pool boiling tests on heterogeneous wetting surfaces with different hydrophobic spot sizes, spacing distances and the number of spots. As compared to a wholly hydrophilic surface, these biphilic surfaces showed improved boiling heat transfer with insignificant change of CHF. The improved heat transfer was attributed to the early onset of nucleate boiling (ONB), continuous bubble generation without waiting time, and shorter bubble growth time due to bubble coalescence. Chen and Qiu [38] tested a biphilic surface that was fabricated by adding several hydrophobic islands on a hydrophilic surface. The biphilic surface shows enhancement of both HTC and CHF. The HTC enhancement is attributed to the increased nucleation sites, decreased bubble departure size, residual vapor after departure, and more bubble interactions, while the enhanced CHF is due to the moderated Helmholtz instabilities, which prevents the formation of an insulating vapor layer. Regarding biphilic textured surfaces, Betz et al. [31] studied boiling heat transfer on SHI, SHO, and superbiphilic surfaces made on silicon wafers through O2 plasma treatment and Teflon coating. Biphilic surfaces have higher performance in pool boiling than surfaces with spatially uniform wettability. Due to the complexity of boiling heat transfer, the enhancement mechanism is still not completely understood. Jo et al. [20] found that, as compared to hydrophilic textured surfaces, the 19 biphilic textured surfaces significantly improved the heat transfer coefficient but showed limited effects on the CHF. Motezakker et al. [39] varied the ratio of the hydrophobic area to the hydrophilic area and found the optimal ratio to be ~38%. Može et al. [26] studied the optimal surface pattern of low and high wettability regions for enhanced boiling heat transfer on aluminum superbiphilic surfaces. The best heat transfer performance is achieved on the superbiphilic surface having 0.5 mm diameter SHO spots with a spot pitch of 1 mm on the SHI background, and the corresponding superhydrophobic area fraction is around 23%. The enhanced heat transfer was attributed to the fact that the optimal wetting pattern provides a high density of potentially active nucleation sites but still allows for the development of the thermal boundary layer thus not inhibiting the activation of neighboring spots. Lim et al. [36] controlled the bubble departure diameter by varying the size and pitch of the hydrophobic pattern on the biphilic silicon surface. The biphilic surface with the square spot having a side length of 2 mm, a pitch of 4mm and the corresponding area ratio of 25% enhanced both CHF and HTC as 14.5% and 34.1% over the bare heater surface, respectively. 20 Figure 2.7 Examples of biphilic surfaces in pool boiling. (a) Hexagon spot on the silicon surface [31], (b) circular spot on the aluminum surface [26], (c) Square spot on the silicon surface [36], (d) circular post on the silicon surface [20] The heat transfer enhancement on biphilic surfaces is attributed to the increased nucleation site density, early onset of nucleate boiling on (super)hydrophobic surface under low heat flux conditions, and delayed CHF by the (super)hydrophilic background at high heat flux conditions [31, 35]. Besides, the separation of vapor and liquid pathways are also mentioned to be responsible for the enhanced heat transfer [20, 36]. Hence, the heat transfer analysis on biphilic surfaces is mainly related to the distinguishable bubble behaviors in different (super)hydrophilic and (super)hydrophobic regions. Although much work has been carried out on surface modification for pool boiling, there are several limitations for most of the previous studies. Firstly, few studies have covered the large spectrum of wettability ranging from super-hydrophilicity to super-hydrophobicity. Second, the discussion of surface effects mainly relied on the measured boiling curves and observed bubble behaviors, but did not consider quantitative assessment and 21 comparison. Third, most studies on biphilic surfaces focused on global heat transfer, and there is a lack of knowledge about local boiling heat transfer at the borders between hydrophilic areas and hydrophobic areas. A fundamental understanding of the bubble dynamics on biphilic surfaces is necessary for manipulating the bubble movement and optimizing the biphilic surface design. In addition, the complicated and chaotic boiling process limits the observation of bubble behaviors. 2.2.3 Influence of boiling surface inclination angle on pool boiling heat transfer Surface inclination angle is another factor which influences the pool boiling heat transfer, since bubble dynamics is directly influenced by the buoyancy force, which is closely related to the surface orientation. However, pool boiling studies about the influence of surface wettability are conducted mainly on horizontally upward-facing surfaces, and the effect of surface inclination has not yet been considered comprehensively. In real applications, the boiling surface may be positioned at different angles. The change in buoyancy force due to the surface inclination may lead to different bubble dynamics. Therefore, the bubble behaviors on surfaces at different inclination angles are of significance to get a better understanding of the heat transfer mechanisms and provide guidance for the surface design. In the dissertation, 0° refers to the horizontal up-facing position, 90° represents the vertical position, and 180° stands for the horizontal down-facing position. Some experiments have been conducted to investigate its effect on pool boiling. Chang and You [40] experimentally studied the effects of heater orientation on pool boiling on microporous surfaces in saturated FC-72 compared with plain surface. On the plain surface, higher inclination angle (between 0° and 90°) provided better heat transfer in the nucleate boiling regime. In the case of 90°, inactive nucleation sites were activated by the rising bubbles from active sites below them on the surface. However, as heat flux increased further, this effect disappeared. The nucleate 22 boiling curves on micro-porous surface for all orientations are almost identical. Maximum CHF is achieved at horizontal upward surface, and the CHF decreases with inclination angle. El-Genk and Suszko [41] obtained a different conclusion that the heat transfer coefficient decreases gradually with the increased inclination angle in the saturated nucleate boiling experiments using PF-5060 dielectric liquid on inclined rough copper surfaces. El-Genk and Suszko [42] did another test using PF-5060 on dimpled copper surfaces. The heat transfer coefficient was found to increase with the increasing inclination angle in the discrete bubbles region at low heat flux (<5 W/cm2) due to the induced mixing by sliding bubbles in the boundary layer along the heated surface. Similar conclusions were also reported by Jung and Kim [43], Mei et al. [44] and Kwark et al. [45] using water as the working fluid. However, Jun et al. [46] found the heat transfer coefficient showed almost the same wall superheat values for all orientations throughout all heat flux ranges on microporous copper surfaces in saturated water and there is no clear distinction of the heat transfer coefficient for various orientation angles even at low heat fluxes. This is attributed to the fact that the numerous active nucleation sites on the microporous surface start slugs and columns at much lower heat fluxes compared to the plain surfaces. Tanjung and Jo [47] used a narrow thin film heater to observe the surface orientation effects on bubble behaviors in pool boiling, which were categorized into three regions: upward facing (0° < α < 90°), vertical and near downward facing (90° < α < 165°), and downward facing (165° < α < 180°). In the upward-facing region, the vapor was generated and detached easily in the vertical direction of the heated surface. According to the vapor behaviors at various inclination angles, the CHF decreased with the increase of the surface orientation from horizontal facing upward to downward. No detailed reason is given for this phenomenon. Tran et al. [48] investigated the effects of surface inclination and type of surface roughness on the nucleate boiling heat transfer performance of HFE-7200 dielectric fluid. Surface 23 of higher roughness benefits heat transfer, among which the sandblasted rough surfaces is superior to those by polished and machined surfaces. At the low wall superheats, the nucleate boiling HTC of the 90° inclined surface is higher than that of the horizontal one (0°); however, when the wall superheat is increased near to 30°C, the nucleate boiling HTCs of the two inclinations converge together. Dadjoo et al. [49] investigated the effects of heater orientation (0° to 90°) and surface roughness on pool boiling of SiO2/water nanofluid over a copper flat plate heater. Results indicated that in nanofluid boiling, increasing the inclination angle of the heater surface increases the CHF and decreases the heat transfer coefficient, while in boiling DI water, both CHF and heat transfer coefficient decreases with the angle of the heater surface. It was found that deposition of nanoparticles and bubble movements have important effects on nanofluid boiling over inclined surface. Chuang et al. [50] investigated the effects of heating orientations from 0° to 90° on the characteristics of boiling heat transfer and bubble dynamics. A copper block heated by the cartridge heater is used as the boiling surface and the temperature is measured by the embedded thermocouples. The HTC increases noticeably as the inclination angle increases at low heat fluxes, and the enhancement becomes less at high heat fluxes. This can be explained by the heat transfer mechanism difference at different heat fluxes. At low heat fluxes, both the single-phase convection and nucleate boiling heat transfer are responsible for the heat transfer, and the increasing inclination angle enhances the convection because of the higher buoyancy force, and the violent stirring action of isolated bubbles also benefits bubble removal and enhances nucleate boiling. At high heat fluxes, nucleate boiling dominates, and the heating surface characteristics, instead of the bubble movement determines the heat transfer performance. Liao et al. [51] studied the effects of surface orientation on the CHF in water pool boiling using three smooth hydrophilic surfaces with the apparent contact angles of 0°, 30° and 55°. The boiling surface is heated by a cartridge heater 24 and the temperature is acquired using three thermocouples. The surface orientation varies from horizontal up-facing 0° to down-facing horizontal 180°. The HTC is reported to decrease slightly and the CHF has no significant change as the orientation increases from 0° to 90°. Results also indicated that the effects of orientation and contact angle on CHF are not independent. The literature review indicates that inconsistent observations and contradictory conclusions exist in the influence of surface orientation on pool boiling heat transfer, and the CHF results have a large discrepancy. The effects of surface inclination angle on these surfaces are necessary to get a better understanding of the heat transfer mechanisms and provide guidance for the surface design. In addition, the influence of the surface inclination angle of different wetting surfaces has not received much attention. Overall, it can be seen that although available studies show the importance of surface characteristics on boiling heat transfer, there is still a lack of a systematic parametric study about the independent and coupled effects of surface texture, wettability and orientation on heat transfer in pool boiling. The impacts of the surface characteristics were not clearly elucidated. Inconsistent observations and contradictory conclusions exist. In order to gain deeper insight into the physics of pool boiling, more systematic studies about the effects of surface characteristics need to be conducted. 25 Chapter 3 Influence of Surface Wettability on Bubble Formation and Motion This chapter presents an experimental study about the influence of surface wettability on bubble formation and motion. Since the boiling process is complicated and chaotic, an air bubble formed from an orifice submerged in water by air injection is used to evaluate the effects of surface wettability on bubble dynamics. Together with the vapor bubble behaviors in pool boiling, the influence of surface wettability is investigated. This study is designed to assess the hypothesis that air bubbles could be used to simulate vapor bubbles in nucleate boiling to evaluate the influence of surface wettability to the bubble behaviors. The content of this chapter has been published in Langmuir [52]. 3.1 Background In the studies of pool boiling on biphilic surfaces consisting of both (super)hydrophilic and (super)hydrophobic regions [20, 26, 27, 31, 35, 36], general bubble behaviors on low and high wetting surfaces are observed, and heat transfer performances are evaluated by comparing the heat transfer coefficients and CHF values based on the boiling curves. The heat transfer enhancement on biphilic surfaces is attributed to the increased nucleation site density, early onset of nucleate boiling the (super)hydrophobic surfaces under low heat flux conditions, and delayed CHF by the (super)hydrophilic background at high heat flux conditions [31, 35]. Besides, the separation of vapor and liquid pathways are also mentioned to be responsible for the enhanced heat transfer [20, 36]. Hence, the heat transfer analysis on biphiliic surfaces is mainly related to the distinguishable bubble behaviors in different (super)hydrophilic and (super)hydrophobic regions. However, little attention has been paid to bubbles generated close to or on biphilic borders, where the bubble 26 behavior could be significantly affected by the local wettability contrast. This is particularly important for a biphilic surface composed of many (super)hydrophilic and (super)hydrophobic regions. Therefore, a fundamental understanding of the bubble dynamics on biphilic surfaces is necessary for manipulating the bubble movement and optimizing the biphilic surface design. However, the complicated and chaotic boiling process limits the observation of bubble behaviors. Therefore, in this study, air flow is injected from a submerged orifice to evaluate the influence of surface wettability on bubble formation and spreading behaviors subject to wettability difference, which excludes the disturbance from heating and provides better visualization of the bubble behaviors. Air bubble manipulation in a submerged state has been investigated both using active [53-55] and passive methods [56-67] related to surface-wetting properties. For example, bubble movement on the superwetted surface covered by special lubricant (ferrofluid or paraffin) under the stimulus of magnetic field or light is investigated for the bubble intentional control [53, 54]. In addition, some surfaces doped with magnetic-particles are used to manipulate bubble motion under the stimulus of magnetic field [55]. Most of passive methods are based on surface wettability patterns such as wedge-shaped channel [56-58], constant-width channel [59], Janus surface [60-62], cone [63, 64], wettability gradient surfaces [65, 66] and dual rails [67]. The surfaces were mainly fabricated by chemical etching [55, 62, 64] and laser milling [57, 59-61, 63, 66, 67]. The pump-less air bubble spreading is driven by the force arising from the asymmetric channel geometry or wettability gradient. These studies provide some insight into the influence of surface wettability on air bubble’s transportation in aqueous medium. However, in these studies [56-59, 63, 64, 66, 67], air was injected onto the surface to form air bubbles, which is different from the vapor bubble generation in boiling. 27 In the present work, air bubble dynamics are experimentally studied using a unique methodology so that the observation of air bubbles helps understand the effects of surface wettability on vapor bubble dynamics. SHI and SHO surfaces are obtained by fast, flexible and accurate laser ablation [25] and silanization grafting. Air is injected through a hole located at the center of homogeneous SHO and SHI surfaces, respectively, followed by the study when air is injected through the hole right on the biphilic border between the SHO and SHI areas. The focus is put on the behavior of bubbles generated on the biphilic border. The understanding would be helpful for establishing design guidelines for surface enhancement to achieve enhanced heat transfer performances. 3.2 Experimental methodology Figure 3.1 shows the two experimental setups used in this study, of which Figure 3.1a is for the isothermal air bubble dynamics study, while the setup shown in Figure 3.1b is for pool boiling. As shown in Figure 3.1a, the air is injected into a cubic transparent glass chamber (50×50×50 mm3) by a syringe pump (GenieTouch™, US) at different air flow rates through the orifice drilled through the substrate. In Figure 3.1b, a silicon wafer with a thin film heater on its back side is used to generate vapor bubbles. Deionized water inside the transparent water chamber (250×200×200 mm3) is boiled for 1 hour to minimize the amount of non-condensable gas entrapment before conducting tests. Then the deionized water is maintained at the set temperature by an auxiliary heater of 1000W and a proportional-integral-derivative (PID) controlling system. Bubble behaviors are captured by a high-speed camera (Phantom M310) with a Nikon manual focus lens (Micro-Nikkor 105 mm f/2.8). A LED camera video light matrix (YONGNUO 600L) is used as the illumination light source. Images containing bubble statistics are processed using ImageJ. The temperature distribution is captured by the IR camera (FLIR SC660) at the frame rate of 120 fps. 28 All the air bubble tests are conducted at room temperature and atmospheric pressure under constant gas flow rates. The pool boiling chamber is open to the ambient atmosphere through a slit at the upper corner to ensure the boiling tests are performed under atmospheric pressure. The Aluminum nitride (AlN) sheet with dimensions of 30 mm×22 mm×0.385 mm (Ningxia ASCENDUS, China) is used as the substrate for air bubble study, because it is suitable for drilling through holes for air injection, while for the following pool boiling test, silicon wafer (TOPSiL GlobalWafers Co., Ltd.) is chosen as the substrate. It is first cleaned using DI water in an ultrasonic cleaner for 30 min and dried before the laser ablation. A nanosecond laser (Oxford Lasers-A-355 micromachining system), which has a wavelength of 355 nm with a pulse length of 15 ns is used to ablate the surface at the power of 8.5 W with the attenuation of 10%, pulse frequency of 20 kHz, scan speed of 10 mm/s with an equal spacing of 25 µm on the left side and 100 µm at the right side. An orifice of 500 µm in diameter is drilled at the center of the substrate at the scan speed of 5 mm/s at the power of 8.5W with an attenuation of 40% for the first time and 25% for the second time. After laser ablation, the substrate is cleaned with ethanol and deionized water via ultrasonication. The silicon wafer is scanned by the laser using the same settings except that the laser power attenuation is set at 5%. Both micro/nanostructures and low surface energy are required to achieve superhydrophobicity. The AlN surface itself is hydrophilic with a contact angle of 70° and becomes SHI with the surface textures by laser ablation. Silanization grafting on the laser-ablated SHI surface is used to get the SHO surface. The SHI surface is firstly processed by O2 plasma to saturate the surfaces with reactive hydrophilic hydroxyl moieties. Then it is immersed into a solution made of 1 mL of (TRIDECAFLUORO-1, 1, 2, 2-TETRAHYDROOCTYL) TRIETHOXYSILANE (Gelest Inc.) and 99 mL methanol (≥99.8%, Certified ACS, Fisher 29 Chemical™) for 30 min at room temperature, after which it is baked at 120°C for 2h on a hotplate (Cole Parmer 03405-20 StableTemp Digital Hot Plate), and the entire surface becomes SHO. To make a biphilic surface, part of the surface needs to be changed to SHI. This is done by using the selective laser ablation method and applying another ablation process [68] to the area targeted for SHI at a spacing of 10 µm and the power of 8.5W with an attenuation of 5%. The second ablation process changes the surface chemistry to hydrophilic by removing silanization grafting on the surface. As a result, the laser-ablated surface becomes SHI. A thin film heater is fabricated on the back side of the silicon wafer substrate using a sputter coating system (Armstrong Engineering Corp.). A 35 nm thick layer of chrome is coated onto the surface, followed by a 60 nm thick layer of gold. The chrome layer here is to enhance the adhesion of the gold layer, and the gold layer is the heating element. Copper foils are connected to the gold layer by lead soldering. The heating area of the heater is 10 mm×15 mm, and the resistance is around 1 Ω. Figure 3.1 Schematic of the experimental setups (a) air bubble observation; (b) pool boiling. 30 Micro/nanostructures on the AlN sample surface from laser processing are observed by a scanning electron microscope (SEM, Mira3 XMU, Tescan, Brno, Czech Republic). The surface roughness is measured using a laser confocal scanning microscope (LEXT OLS5000, Olympus Corporation, Tokyo, Japan). Contact angles are measured using the sessile droplet technique by a goniometer (Model 90, Rame ́-hart Instrument Co., Succasunna, NJ). Five measurements are conducted at different positions on each sample surface and the average is reported. As the SEM images shown in Figure 3.2a, the orifice with a diameter of 500 µm is drilled at the center of the AlN substrate. After laser ablation, microgrooves are formed at the laser beam scanning lines and some hierarchical rough patterns are formed from the redeposited debris. The structures on the two sides of the orifice are different. The left side is covered with irregular micro cavities and nodules, while the right side does not have as many nodules. This is because after the surface has been treated to be SHO, the right side is milled again by the laser beam to become SHI. The difference is more obvious in the 3D confocal images in Figure 3.2b. The left side is rougher than the right side, and the average roughness values of the left and right sides are 8.694 µm and 2.907 µm, respectively. The biphilic surface as shown in Figure 3.2-c1 is SHO on the left side with an equilibrium contact angle of 162° ±0.5° and it has a quite small contact angle hysteresis with a roll angle less than 1°. Surface pinning is negligible on the surface. The right side is SHI including the periphery of the orifice, of which the contact angle is around zero. In the air, a droplet spreads out completely when it is dipped on the SHI region, and a thin water film is visible as shown in Figure 3.2-c2, while when the surface is submerged in water, a thin air mirror-like reflective layer is observed on the SHO region (see Figure 3.2-c3). The SHO region behaves super-aerophilic, while the SHI region becomes super-aerophobic in water. Figure 3.3 shows the SEM-EDS elemental mapping of the AlN surface, which includes the EDS maps and weight percentages of 31 element distributions on the SHI and SHO regions. The elements of F, Si and C on the AlN surface confirm the existence of the coating from silanization grafting, and the surface after the second surface only contains Al, N and O, which indicates that the silanization grafting has been removed. Figure 3.2 Laser patterned AlN surfaces and surface wettability (a) SEM of the orifice and surface textures; (b) 3D profile of the surfaces on the two sides of the orifice; (c) contact angles on the AlN surface, and the phenomena when the surface is in air and water. 32 33 Figure 3.3 SEM-EDS elemental mapping of the AlN surface: (a) EDS map of element distribution on the SHI region; (b) EDS map of element distribution on the SHO region (c) Weight percentage of the elements on SHI region; (d) Weight percentage of the elements on SHO region. 3.3 Air bubbles on SHO and SHI surfaces The dynamics of a bubble on solid surfaces are significantly affected by surface wettability. To investigate the bubble behaviors on surfaces with wettability contrast, the formation and departure of air bubbles on the SHI and SHO surfaces are firstly visualized by the high-speed 34 camera at 5000 fps, respectively. The moment when the air bubble starts emerging from the orifice is set as t = 0 ms. As shown in Figure 3.4a on the SHI surface, as the supplied air pressure overcomes the capillary pressure at the orifice, the bubble emerges as a nearly spherical segment (see Figure 3.4a at 4 ms) due to the surface tension. At t = 8 ms, a bubble tip bulge is observed, which is caused by the impingement of gas flow coming out from the orifice with the bubble. The impact decays with the bubble growth and expansion, and at t = 12 ms, the bubble bulge disappears. As the bubble continues to grow, the bubble contact line shrinks and is only connected to the 0.5 mm orifice by a neck, which thins and eventually ruptures at the point of bubble detachment. For the next bubble to emerge, it again needs to overcome the capillary pressure at the orifice. On the SHO surface, as shown in Figure 3.4b, the bubble behaviors are different from those on SHI surface. When submerged in water the SHO surface is covered by a thin layer of air, so there is a large base for the bubble formation on SHO surface (20 mm × 20 mm). Air coming out from the orifice merges into the existing air layer on the surface, and as more air is injected, a convex air bulge is formed (see image at t = 0.48 s). Since the base of the bubble is bounded on the surface, the hydrostatic forces result in a force holding the bubble to the SHO surface. Figure 3.4b shows the development of the bubble profile. With the continuous air flowing into the bubble, the bubble height increases. As a result, hydrostatic pressure along the bubble height becomes significant and causes the lower part of the bubble to shrink, which is visible from profile 4 in Figure 3.4b. Further shrinking results in capillary pressure-driven necking (see profile 5 in Figure 3.4b). After the bubble detaches from the surface, a small portion of the air bubble is left on the surface, and it serves as a nucleus for the next bubble’s initiation. Therefore immediately after the previous bubble departure, a new air bubble is initiated on the SHO surface without waiting. 35 Figure 3.4c and Figure 3.4d compare the bubble departure diameter and frequency at different air injection flow rates ranging from 10 mL/min to 50 mL/min on SHI and SHO surfaces. The departure diameter is derived from the area of the bubble identified from the images, and each average diameter is calculated from 50 air bubbles. The departure frequency is obtained by counting the number of bubbles detaching from the orifice within a period of time using high-speed camera images. Although the increased air flow rate gives rise to increases in both the departure size and frequency for both surfaces, there is a significant difference between the two surfaces. The departure diameter on the SHO surface is 9.6 mm, and 4.4 mm on SHI surface at the flow rate of 10 mL/min. The bubble departure frequency from SHI surface is about 10 times that from SHO surface. This can explain why vapor bubbles in pool boiling on SHI surface can detach from the surface easier than that on SHO surface. Figure 3.4 Air bubble cycle when air is injected through the 500 µm orifice on the AlN surface at the air flow rate of 10 mL/min (a) on SHI surface; (b) on SHO surface; (c) bubble departure diameter; (d) bubble departure frequency. 36 3.4 Air bubble spreading on the biphilic surface To further investigate the influence of surface wettability on bubble dynamics, the air is injected from the orifice located at the boundary of the SHO and SHI regions (15 mm×7.5 mm for both domains) at 10 mL/min on the AlN substrate. Figure 3.5 displays the time-lapse images of bubble generation and spreading on the biphilic surface, and the moment when the air bubble starts emerging from the orifice is set as t = 0 ms. As the biphilic surface is submerged in water, a thin air layer is formed on the SHO region (Figure 3.5b). To observe how the surface wettability influences air bubble movement clearly, the air film on the SHO region is first removed by an injection syringe (Figure 3.5a). Since the orifice is drilled after surface hydrophobization, the orifice and its periphery are SHI due to the microstructure and laser ablation. Therefore, as shown in Figure 3.5a, at the beginning stage (t < 7 ms), the air bubble behaves in the same manner as the air bubble on SHI surface shown in Figure 3.4a, which is symmetric. With the continuous air injection, the contact line advances outwards as the bubble expands, and upon contacting the SHO region on the left side, it displays drastically different dynamic contact line behaviors on the two sides. The leading edge starts spreading towards the left direction (t=10 ms) under the driving force caused by the surface wettability contrast. While spreading, the bubble is growing with continuous air injection, and it experiences significant deformation. As shown in Figure 3.5c, in the viewed plane, the local curvature at the leading edge is − 1 𝑅𝑅𝐴𝐴⁄ , and 1 𝑅𝑅𝑂𝑂⁄ for the trailing edge. Here we have defined the surface normal vector toward the inside air as positive. To fully describe the local surface curvature at each edge, we also need to consider the orthogonal curvature, which is the curvature in the normal plane that is orthogonal to the viewed plane. Nevertheless, since the difference of curvature between the two edges in the viewed plane is the maximum, the difference of capillary pressure between the two 37 edges can be approximately expressed as𝛥𝛥𝑝𝑝~𝜎𝜎(1/𝑅𝑅𝐴𝐴 + 1/𝑅𝑅𝑂𝑂). It is this pressure difference that causes the bubble to expand toward the left on the surface. Ahead of the gas bubble leading edge is a thin precursor film caused by dewetting of liquid on the textured SHO surface, and it moves faster than the air bulk. At t = 18 ms, the tip of the precursor (bright region) has arrived at the left edge of the SHO domain, and as shown in Figure 3.5b, the air bubble spreads over the entire SHO area at t = 45 ms. After that, the air inside the bubble moves back and forth and the local curvatures at the two edges keep changing until the oscillation is damped out. The oscillation is accompanied by the shift between local maximum capillary pressure and dynamic pressure at the two edges. Figure 3.5d presents the temporal variation of air bubble leading and trailing contact angles, as well as the leading edge position. Here the contact angle refers to the angle between the liquid-vapor interface and the liquid-solid interface. The leading contact angle is the angle measured from the leading edge of the bubble just as it moves, and the receding contact angle is the angle of the trailing edge. Contact angles on both sides change from about 25° to 150°. However, the trailing edge changes gradually while the leading edge changes abruptly due to the wettability contrast. When the SHO domain is covered by a thin layer of air, as shown in Figure 3.5b, the air bubble spreading behaviors are similar to those without air film. A small air bubble emerges from the orifice and grows up, and the moment when the bubble base reaches the SHO region, the bubble was elongated and transported towards the SHO side. Figure 3.5 shows the formation of the air cap covering the entire SHO area. After that, the continuous injection of airflow periodically causes the air cap to protrude at the injection orifice, and the protrusion then merges with the entire air cap. This process can be seen from Figure 3.6, where t-0 is set when the third protrusion becomes visible. The merging process causes the air cap 38 to oscillate between the orifice and the far side of the SHO region. The vertical height of the air cap continues to grow, and the hydrostatic pressure difference becomes large enough to cause the lower part of the air cap to shrink. The capillary pressure comes into play to further drive the necking process, which eventually ends with a breakup. Figure 3.5 Air bubble spreading on the biphilic AlN surface. (a)Spreading on the bare surface without air film; (b) Spreading on SHO surface with an ultrathin air layer (air-infused case). (t = 0 ms refers to the moment when the air bubble starts appearing from the orifice); (c) schematic of the bubble leading and trailing contact angles and curvatures; (d) variation of bubble leading edge position and contact angles on both sides 39 Figure 3.6 Process of small air bubble coalescence with existing air cap on the surface and coalescence air bubble departure. (t = 0 ms refers to the moment when the small new air bubble starts appearing from the hole) 3.5 Vapor bubble behaviors on the biphilic surface in pool boiling Air bubble behaviors on the surface with wettability contrast have been observed, and the influence of surface wettability on vapor bubble dynamics is investigated in pool boiling at different surface temperatures. A single-side polished silicon wafer is used as the substrate because it has a smooth surface which is better and easier than the AlN for heater fabrication and soldering. On the polished side, the wettability patterned surface was prepared by laser ablation at the same laser settings as those for the AlN substrate except that the laser attenuation is set as 5% and silanization. As the SEM and 3D confocal images shown in Figure 3.7, similar surface textures (microgrooves and ridges) are generated on the silicon wafer surface. 40 Figure 3.7 Characterization of the silicon wafer surface after laser ablation (a) SEM; (b) 3D confocal image As shown in Figure 3.8-a1, the black regions are SHI while the green region is SHO. The three rectangular regions have the same dimensions of 5 mm ×10 mm. At the center location of the SHO region, there is one circular SHI area (1.5 mm in diameter), which is designed to see how the vapor bubble from the SHI region spreads radially towards the surrounding SHO region. Figure 3.8-a2 shows the thin film heater fabricated on the other side of the silicon wafer. The heating area as shown in Figure 3.8-a3 is just beneath the SHI and SHO regions. The pool boiling tests were conducted at a subcooled condition of 90℃ after the DI water was degassed for 1 hour. When the heater is powered on, uniform heat flux is provided on the surface, and due to the low surface energy, boiling starts firstly on the SHO region as the image and schematic shown in Figure 3.8-b1 at 𝑇𝑇� = 98℃. Here 𝑇𝑇� is the average surface temperature of the heater area. As the temperature increases, the small bubbles in the SHO area merge into a thin vapor layer, which hinders local nucleate boiling. But at the same time, isolated bubbles appear on the SHI region (see Figure 3.8-b2). Continuously increasing the surface temperature to 𝑇𝑇� = 117℃ results in the 41 coalescence of bubbles on the SHI region, as shown in Figure 3.8-b3. At the SHI-SHO borders, the vapor tends to move into the SHO region. This causes vapor in the SHI regions to move to the region. As shown in Figure 3.8c, bubbles in the SHO region grow faster than the other regions, and vapor bubbles depart mainly from the SHO region. As a result, the liquid phase close to the solid surface flows toward the central region. This is different from boiling on a surface with homogeneous wettability, where liquid close to the surface lacks horizontal flow. Figure 3.8 Vapor bubble behaviors in pool boiling. (a) a1-schematic of the surface wettability pattern, a2-heater configuration, a3-schematic of the pool boiling; (b) b1-bubble starts from SHO region at 𝑇𝑇� = 98℃, b2-isolated bubble on SHI region and merged bubble on SHO region at 𝑇𝑇� =110℃, b3-merged bubble on SHI region and vapor film on SHO region at 𝑇𝑇� = 117℃; (c) bubble spreading merging processes from SHI regions on two sides towards the SHO region in the center at 𝑇𝑇� = 125℃. 42 3.6 Summary In summary, the SHI, SHO and biphilic surfaces are fabricated by laser ablation and silanization grafting. The influence of surface wettability on the dynamics of air bubbles and vapor bubbles in pool boiling is investigated. The results indicate that surface wettability plays a significant role in bubble formation, growth and departure. On the biphilic surface, due to the difference in the wettability between the SHI and SHO regions, the bubble spreads towards the SHO region under the driving force from the Laplace pressure difference. In addition, the air bubble behaviors on the biphilic surface are compared with the vapor bubble behaviors on a wettability-patterned surface in pool boiling. Vapor bubbles generated on the SHI region spread to the SHO region when they contact the SHO border, which induces horizontal motion of the vapor and surrounding water. This study provides useful information for the design of biphilic surfaces to optimize heat transfer performances. 43 Chapter 4 Surface Effects on Sub-cooled Pool Boiling for Smooth and Laser-ablated Silicon Surfaces This chapter presents an experimental study on different wetting smooth and laser-ablated silicon surfaces. Bubble behaviors and boiling curves, as well as the temperature distribution, have been compared. The content of this chapter has been published in the International Journal of Heat and Mass Transfer [69]. 4.1 Background As mentioned in the literature review in Chapter 2, although much work has been carried out on surface modification for pool boiling, more research work is still required to cover a large spectrum of wettability, and quantify the influence of surface wettability on pool boiling heat transfer. In addition, the local heat transfer at the border between the hydrophilic and hydrophobic regions on the mixed wetting surfaces has rarely been studied. This study is designed to gain more insight into the influence of surface wettability on the pool boiling heat transfer from super-hydrophilic to super-hydrophobic surfaces as well as biphilic surfaces. Smooth and laser-ablated surfaces with wettability ranging from super-hydrophilic to super-hydrophobic are tested. The effect on pool boiling is quantitatively assessed and compared using the surface-fluid factor. A special laser-ablated biphilic surface is fabricated to have half the surface being super-hydrophilic and the other half super-hydrophobic. This surface allows to study the local bubble dynamics and local heat transfer at the location of abrupt change of surface wettability. 4.2 Experimental methodology Experiments are conducted using the setup shown in Figure 4.1. The water chamber for pool boiling is made of transparent polycarbonate, and its dimensions are 200 mm×250 mm×200 mm. 44 The boiling substrate is made from a silicon wafer. The upper surface is the boiling surface, while the lower surface with a film heater is the heating surface. The preparation of the boiling substrate is described later in this section. The film heater is powered by a programmable DC power supply. Deionized (DI) water is used as the working fluid, and it is pre-boiled to minimize the amount of noncondensable gas entrapment before conducting experiments. An auxiliary heater of 1000 W and a proportional-integral-derivative (PID) controlling system are used to maintain the water at a constant temperature of 90 ℃. With the chamber open to the ambient atmosphere, tests are conducted under atmospheric pressure. Vapor bubble behaviors on the boiling surface and temperature on the heating surface are captured simultaneously by a high-speed camera (Phantom M310) at 10,000 fps (frames per second) and an infrared camera (FLIR SC600) at 120 fps. 4.2.1 Preparation of boiling substrates The boiling substrate is schematically shown in Figure 4.2. In the present work, boiling substrates with dimensions of 30 mm × 22 mm are cut from silicon wafers that have a polished side and an unpolished side. The silicon wafers have a thickness 𝑏𝑏 = 385 µ𝜇𝜇 , and thermal conductivity 𝑘𝑘 = 130 W m ∙ K⁄ . One reason is that silicon is a very popular base substrate in microelectronic devices where immersion cooling can be applied [29]. The other reason is silicon surfaces can maintain stable surface properties such as wettability. Additionally, using a thin substrate allows for the use of an IR camera for high spatial resolution of surface temperature measurement. The polished side is used as the boiling surface, while the unpolished side is used as the heating surface, where a thin film heater with area 𝐴𝐴 = 12 𝜇𝜇𝜇𝜇 × 12 𝜇𝜇𝜇𝜇 is fabricated using sputter coating technique. 45 Figure 4.1 Experimental setup for pool boiling. (a) schematic, (b) photo. A few steps are taken to develop a thin film resistive heater on the unpolished side of the substrate. The silicon wafer pieces are cleaned with acetone and IPA (Isopropyl Alcohol) respectively for 10 minutes using an ultrasonic cleaner and then dried by a hotplate in the cleanroom. An area of 12 mm×12 mm at the center of the unpolished surface of the substrate is sputter-coated with a 35 nm thick layer of chrome, followed by a 70 nm thick layer of gold. The chrome layer is for enhancing the adhesion of the gold layer to the silicon surface, and the gold layer is the heating element with electrical resistance around 1 Ω. Copper foils are soldered to the gold layer. For the IR camera, the heater is painted black with a known emissivity of 0.9. 46 Figure 4.2 Schematic of the boiling substrates: (a) Boiling and heating surfaces of the boiling substrate; (b) Bottom view of the heating surface. The squares with a × a represent two IR image pixels and a =284 μm. Within an area of 20 mm × 20 mm centered on the polished side of the substrate, laser ablation is applied to form micro/nanostructures by using an A-355 laser micromachining system (Oxford Lasers). The laser has a wavelength of 355 nm with a pulse length of 15 ns, and the maximum power of the laser beam is 8.5 W. The substrate is fixed on a transition stage, and the laser beam is set to scan at 10 mm/s in both 𝑥𝑥 and 𝑦𝑦 directions at a constant spacing. For the present work, two spacings w =25 µm and 50 µm are used in laser ablation to make two textured surfaces (denoted as w25 and w50). The surface processing is conducted in the air under atmospheric conditions at a pulse frequency of 20 kHz and a power attenuation of 5%. Due to oxidation, the laser-processed region has a high hydrophilicity. The surfaces after laser ablation are observed by a scanning electron microscope (SEM) and a 3D laser confocal microscope (LEXT OLS5000). As shown in Figure 4.3b, microgrooves are generated along the laser-scanning pathways, and on the two sides of the microgrooves, some mounds are formed. Figure 4.3a shows the 3D confocal images of the surfaces, which provide the root-mean-square roughness and the Wenzel roughness shown in Figure 4.3c. 47 Figure 4.3 (a) Confocal images of the laser-ablated surfaces with two scan spacings w=25 μm and 50 μm; (b) SEM images of the surfaces; (c) Surface roughness: 𝑆𝑆𝑞𝑞 (root-mean-square roughness) and 𝑅𝑅𝑊𝑊 (Wenzel roughness). Table 4.1 shows five surfaces that are prepared and tested in the present work. The contact angles of each surface are tested at three different locations using a goniometer (model 90, Ramé-hart Instrument Co., Succasunna, NJ) and the average is reported with a maximum uncertainty of ±1.5°. Without any surface modification, the boiling surface, i.e. the smooth side of the silicon wafer, is hydrophilic with the apparent contact angle CA = 52o (see Figure 4.4a), and this surface is named HI. After laser ablation with either 𝑤𝑤 = 25 𝜇𝜇𝜇𝜇 or 50 𝜇𝜇𝜇𝜇, the surfaces become SHI, as the surfaces can be completely wetted by a water droplet (CA ~ 0o). These two surfaces are named SHI-w25 and SHI-w50. In the present work, hydrophobicity is achieved by applying silane grafting. Here is a brief description of the silanization process. First, the substrate is cleaned in an 48 ultrasonic cleaner using DI water, acetone, and then methanol, with 10 minutes for each. Then, the boiling surface is plasma treated. Following the plasma treatment, the substrate is put in a solution of (TRIDECAFLUORO-1, 1, 2, 2-TETRAHYDROOCTYL) TRIETHOXYSILANE and methanol for 6 hours, and the volume fraction of TRIETHOXYSILANE is 1%. Finally, the substrate is kept on a hotplate at 150 ℃ for 12 hours. For the substrates without laser ablation, applying this silane grafting changes the smooth boiling surface from hydrophilic to hydrophobic (CA = 105° as shown in Figure 4.4a), and the surface is named HO. Applying silane grafting to the laser-ablated surface with 𝑤𝑤 = 25 𝜇𝜇𝜇𝜇 makes the surface super-hydrophobic [70] (CA = 163° as shown in Figure 4.4a), and the surface is named SHO-w25. The contact angle hysteresis (CAH) is measured by using the needle method as shown in Figure 4.4b to determine the advancing and receding contact angles, and the difference between the two angles is CAH. CAH is measured for the HI, HO, and SHO surfaces, and the values are shown in Table 4.1. For SHO-w25, CAH = 3.2°, which shows low hysteresis. Although in the present work the wetting regime cannot be observed during contact angle measurement, this small hysteresis possibly indicates Cassie wetting regime. In addition to measuring the static profile of water droplets on the surfaces in room air, the surfaces are compared by observing the dynamic behaviors of air bubbles on the surfaces immersed in water. Figure 4.4c and Figure 4.4d show air bubbles on two surfaces, SHI-w25 and SHO-w25. The two substrates without film heaters are made specifically for the air bubble test. At the center of each substrate, a 500 µm-diameter through hole is fabricated using the laser micromachining system. A continuous air flow is injected through the hole and into the water pool at room temperature. On the SHI surface (see Figure 4.4c), the air bubble shows almost no spreading on the surface and detaches from the surface at a small size. On the SHO surface (see Figure 4.4d), injected air first spreads as a gas blanket, and the center height keeps growing. Eventually, 49 capillarity-driven necking and buoyancy forces come into play and result in bubble departure. As shown in Figure 4.4d, a thin layer of air remains on the SHO surface, which maintains the Cassie wetting regime. Figure 4.4 (a) Apparent contact angles of water droplets on the surfaces: CA = 52° for the HI surface; CA = 105° for the HO surface; and CA = 163° for the SHO surface. (b) Advancing and receding contact angles for the SHO surface. Formation and departure of air bubbles on (c) SHI surface and (d) SHO surface. 50 Table 4.1 The tested surfaces are measured for the apparent contact angle (CA) and contact angle hysteresis (CAH). Surface Chemistry Smooth Laser-ablated w =25 µm Laser-ablated w =50 µm Hydrophilic HI(Hydrophilic) CA = 52° CAH = 18° SHI-w25 (Super-hydrophilic) CA ≈ 0° SHI-w50 CA ≈ 0° Hydrophobic HO(Hydrophobic) CA = 105° CAH = 18.4° SHO-w25 (Super-hydrophobic) CA = 163° CAH = 3.2° ---- 4.2.2 Data analysis For a given surface, experiments are conducted at varied heater powers denoted by 𝑞𝑞𝑔𝑔𝑒𝑒𝑖𝑖. For each power, when boiling reaches a steady state, the IR camera is used to take thermal images of the heating surface of the substrate, and the high-speed camera is used to simultaneously capture the bubble dynamics on the boiling surface of the substrate. For each test, 500 consecutive IR images (in total ~ 4 seconds) are averaged to obtain a time-averaged temperature distribution, and the temperature is denoted by 𝑇𝑇. This temperature is considered as the temperature of the boiling surface. This approximation is based on the evaluation of the Biot number 𝐵𝐵𝐵𝐵 = ℎ�𝑏𝑏 𝑘𝑘⁄ , where ℎ� is the average heat transfer coefficient of boiling and will be defined later. For the present work, ℎ� is mostly of the order of magnitude of 104 𝑊𝑊 𝜇𝜇2 ∙ ℃⁄ , which gives Bi < 0.1. The boiling heat transfer is evaluated based on the global heat transfer in the heating area 𝐴𝐴. Hence, the global surface temperature is the surface-averaged temperature obtained from the time-averaged IR image by using 1T TN= ∑ (4.1) 51 Here 𝑁𝑁 is the total number of pixels in the heating area in the IR images. For 𝐴𝐴 =12 𝜇𝜇𝜇𝜇 × 12 𝜇𝜇𝜇𝜇, 𝑁𝑁~500. The global heat transfer coefficient can be calculated from sat eq A qhT T T′′= =− ∆ (4.2) Here, 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠 = 100℃ is the saturation temperature of water, and ∆𝑇𝑇𝑒𝑒����� = 𝑇𝑇� − 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠 is the global surface excess temperature. The global heat flux 𝑞𝑞′′���� = 𝑞𝑞 𝐴𝐴⁄ is based on the boiling heat transfer rate 𝑞𝑞 that takes place within the heating area 𝐴𝐴. As schematically shown in Figure 4.2, applying energy balance to the control volume 𝐴𝐴 × 𝑏𝑏, the boiling heat transfer rate can be determined from gen inc air radq q q q q= − − − (4.3) It includes three heat losses. The first heat loss 𝑞𝑞𝑖𝑖𝑖𝑖𝑖𝑖 is the in-plane heat conduction at the periphery of the heater [71, 72]. As shown in Figure 4.2b, the local in-plane heat conduction can be evaluated based on the temperature difference, 𝛿𝛿𝑇𝑇 , between the two adjacent pixels across the border. Summing up the local in-plane conduction along the heater’s periphery gives incTq abk bk Taδ δ= =∑ ∑ (4.4) The heat loss 𝑞𝑞𝑎𝑎𝑖𝑖𝑎𝑎 is the heat transfer from the heating surface to the ambient air by natural convection, which is given by ( )air air airq h A T T= − (4.5) where 𝑇𝑇𝑎𝑎𝑖𝑖𝑎𝑎 = 20 ℃ is the ambient air temperature. Based on the correlation of free air-cooling [73], hair ≈ 2.5 W/m2·K is used. The heat loss 𝑞𝑞𝑎𝑎𝑎𝑎𝑟𝑟 is the radiation heat transfer from the heating surface to the surrounding, which is 52 ( )4 4=rad surq A T Tεσ − (4.6) Here σ is the Stefan-Boltzmann constant, 𝑇𝑇𝑠𝑠𝑠𝑠𝑎𝑎 = 20℃ is the surrounding temperature, and ε =0.9 is the emissivity. The uncertainty of the average temperature is evaluated by Eq. (4.7), which combines the standard deviation of the 500 consecutive temperature values and the systematic uncertainty of the IR camera measurement. The uncertainties of heat flux and heat transfer coefficient in this study are calculated with Eq.s (4.8) and (4.9) using the uncertainty propagation method proposed by Coleman and Steele [74]. 2 2sys TT u u∆ = + (4.7) 1/22 2 2 22 2 2 2gen air inc radq q q qqgen air inc radq q q qu u u u uq q q q′′        ′′ ′′ ′′ ′′∂ ∂ ∂ ∂ = + + +         ∂ ∂ ∂ ∂        (4.8) 1/22 22 2Th qh hu u uTq ′′    ∂ ∂ = +    ′′ ∂∂     (4.9) where 𝑢𝑢𝑠𝑠𝑠𝑠𝑠𝑠 = 0.5℃ is the systematic uncertainty of IR camera, 1/2211 ( )1NT iTiu S T TN = = = − − ∑ is the random error of average temperature, 𝑢𝑢ℎ� is the uncertainty of global heat transfer coefficient, 𝑢𝑢𝑞𝑞′′���� is the uncertainty of global heat flux, 𝑢𝑢𝑞𝑞𝑎𝑎𝑎𝑎𝑎𝑎 is the uncertainty of heat loss by air convection, 𝑢𝑢𝑞𝑞𝑎𝑎𝑖𝑖𝑖𝑖 is the uncertainty of heat loss by in-plane conduction, and 𝑢𝑢𝑞𝑞𝑎𝑎𝑎𝑎𝑟𝑟 is the uncertainty of heat loss by radiation. Accounting for all these random 53 and systematic uncertainties, the maximum uncertainties in average surface temperature, heat flux, and heat transfer coefficient are 1.2℃, 0.2 𝑊𝑊/𝑐𝑐𝜇𝜇2, and 0.27 𝑊𝑊/𝑐𝑐𝜇𝜇2·𝐾𝐾, respectively. 4.3 Results and Discussions 4.3.1 Boiling on hydrophilic surfaces with varied laser scan spacings This section focuses on the effect of laser scan spacing on boiling heat transfer. Three hydrophilic surfaces including HI, SHI-w25, and SHI-w50 are tested. Repeatability tests are conducted on both smooth and textured surfaces (HI and SHI-w25) to evaluate the measurement repeatability, and a reproducibility test is conducted on another newly fabricated SHI-w25 surface. Results indicate that the measurements can be repeated and reproduced with negligible difference. For each type of surface, boiling tests are conducted for varied heater powers 𝑞𝑞𝑔𝑔𝑒𝑒𝑖𝑖 , which is changed from low to high until the CHF is reached. The CHF is characterized by a sharp temperature rise, followed by the failure of the heater. For each test, the global heat flux 𝑞𝑞′′���� and the surface temperature 𝑇𝑇� are obtained using Eqs. (4.1-4.6). The data are plotted as the nucleate boiling curve for each tested surface. The nucleate boiling curves for the three hydrophilic surfaces are shown in Figure 4.5a. The boiling curve of the SHI-w50 surface is above that of the HI surface, and the boiling curve of the SHI-w25 surface is further above that of SHI-w50 surface. Generally, differences between the three curves are relatively small for low heat fluxes and become more noticeable for large heat fluxes. Laser ablation significantly increases the CHF. For the HI surface, CHF is 97 W/cm2, while CHFs are 210 W/cm2 and 218 W/cm2 for the SHI-w50 and SHI-w25 surfaces, respectively. The global heat transfer coefficient is calculated and presented in Figure 4.5b, which clearly shows that laser ablation enhances heat transfer in nucleate boiling. The smaller the scan spacing, the more 54 heat transfer enhancement. Nevertheless, it should be noticed that for the two SHI surfaces, the scan spacing shows a less significant effect for the heat fluxes close to the CHFs. Figure 4.5 Boiling heat transfer characteristics on surfaces HI, SHI-w50 and SHI-w25. (a) Nucleate boiling curves (data points with arrows are CHFs); (b) global heat transfer coefficient. Figure 4.6 presents the bubble behaviors at varied heat fluxes on the HI and SHI-w25 surfaces. There are a few noticeable differences between the two surfaces. First, the onset of nucleate boiling starts at a relatively low heat flux on the SHI-w25 surface than on the HI surface. The bubble shapes on the HI surface are different from those on the SHI surfaces due to the wettability difference, and bubbles on the SHI surface depart more frequently. The pre-departure vapor bubbles on the SHI surface at its CHF = 218 W/cm2 are apparently bigger than the HI surface with CHF = 99 W/cm2. The strong wetting and wicking effects of the textured surfaces promote surface rewetting, and therefore, the SHI-w25 surface has the largest CHF among the three surfaces. 55 Figure 4.6 Bubble behaviors on different surfaces at varied global heat fluxes (for each image, 𝑞𝑞′′����and T� are provided). For each test, the image showing the largest vapor accumulation on the surface is selected. (a) The HI surface; (b) the SHI-w25 surface. 4.3.2 Boiling on SHI and SHO surfaces To study the effect of surface wettability, we focus on two surfaces SHI-w25 and SHO-w25, which have the largest contrast of surface wettability in the present work. First, the dynamic behaviors of bubbles formed on the two surfaces are compared in Figure 4.7. Relatively low heat fluxes, 5.7 W/cm2 for the SHI surface and 7.2 W/cm2 for the SHO surface, are used so that single bubbles are generated on the surfaces for most of the time. Figure 4.7 shows bubble formation and departure from the two surfaces, and the time points at the early stage of the formation are chosen 56 as the initial time points. Similar to the observation from Figure 4.4d, a vapor blanket forms and covers the SHO surface, and the blanket keeps rising from its center until a large bubble departs. After the vapor bubble departure, the surface is still covered by a layer of vapor film, from which the next bubble starts its formation in the Cassie wetting regime. On the SHI-w25 surface as shown in Figure 4.7b, the bubble remains spherical during formation, growth and departure, which is mainly because the bubble does not spread on the hydrophilic surface. Clearly, bubbles formed on the SHO surface are larger and depart less frequently. For the SHO surface (Figure 4.7a), the bubble departure period is around 11.3 s, and the bubble is about 7 mm in diameter. For the SHI surface (Figure 4.7b), the departure period and diameter are around 62 ms and 4.4 mm, respectively. Figure 4.7 shows significant differences in vapor bubble dynamics between the SHI surface and the SHO surface. Considering a small volume of vapor, due to different apparent contact angles, its shape on the SHO surface features the wide spread and low height, while it has a high profile with a small spread on the SHI surface. As seen from Figure 4.7b, even at an early stage, the shape of the bubble on the SHI surface is favorable for buoyancy and capillary necking, which are the driving forces for bubble departure. However, as seen from Figure 4.7a, vapor on the SHO surface has to accumulate such that the bubble height is sufficient for the hydrostatic and capillary pressures to cause deformation toward a slender shape, which allows capillary necking and then buoyancy to come into play. 57 Figure 4.7 Temporal evolution of a single vapor bubble on (a) SHO-w25 (𝑞𝑞′′���� =7.2 W/cm2); (b) SHI -w25 (𝑞𝑞′′���� = 5.7 W/cm2). Different bubble behaviors lead to different heat transfer characteristics. This can be seen in Figure 4.8, which compares these two surfaces and another two surfaces, HI and HO, at varied global heat fluxes. Figure 4.8a shows significant differences in boiling between the hydrophilic and hydrophobic surfaces. Boiling on the two hydrophilic surfaces, HI and SHI-w25, features narrow surface temperature ranges but large heat flux ranges, while boiling on the two hydrophobic surfaces, HO and SHO-w25, shows large surface temperature ranges but narrow heat flux ranges. For example, maintaining the surface temperature at 200 ℃ can only be possible for hydrophobic surfaces. The two hydrophilic surfaces show higher CHFs (218 W/ cm2 for SHI-w25 and 97 W/ cm2 for HI) than the two hydrophobic surfaces (48 W/cm2 for HO, and 37 W/cm2 for SHO-w25). In other words, CHF decreases from super-hydrophilicity to super-hydrophobicity. The global heat transfer coefficient is calculated and plotted versus the global heat flux in Figure 4.8b. Overall, the two hydrophilic surfaces show more effective heat transfer than the two hydrophobic surfaces. However, for low heat fluxes, the hydrophobic surfaces show slightly higher heat transfer coefficients. This agrees with the observation that the hydrophobic surfaces show earlier onset of nucleate boiling than the hydrophilic surfaces. For the hydrophilic surfaces, the heat transfer coefficient increases with increasing the heat flux. However, for the hydrophobic 58 surfaces, the heat transfer coefficient decreases with increasing the heat flux. The higher the heat flux, the more vapor is generated. In the nucleate boiling regime, a large vapor flow enhances two-phase flow on the hydrophilic surfaces but inhibits heat transfer on the hydrophobic surfaces, because bubbles on the hydrophobic surfaces tend to spread and coalesce and do not depart easily. Figure 4.8 Heat transfer in nucleate boiling on two hydrophilic surfaces (SHI-w25, HI) and two hydrophobic surfaces (HO, SHO-w25). (a) Nucleate boiling curves; (b) global heat transfer coefficient. 4.3.3 Evaluation of surface effects Figure 4.5 and Figure 4.8 have shown the effects of laser scanning spacing and surface wettability on boiling heat transfer. First, the tested surfaces have different CHFs. As a summary, the CHF and corresponding excess temperatures are shown in Table 4.2. Second, different surfaces have different nucleate boiling curves. To quantitively show the surface effects, the surface-fluid factor in the Rohsenow correlation [16] for nucleate boiling is used for the discussion below. 59 Table 4.2 Critical heat fluxes and the corresponding excess temperatures, (∆Te�����)CHF, for all the tested surfaces. Surface CHF (W/cm2) (∆𝑇𝑇𝑒𝑒�����)𝐶𝐶𝐶𝐶𝐶𝐶 (℃) SHI-w25 218 22 SHI-w50 210 23 HI 97 27 HO 48 154 SHO-w25 37 166 Based on the Rohsenow correlation, two non-dimensional parameters are defined and introduced here, which are Prpe nfgcY Th= ∆ (4.7) and 1 3 1/6fgqXh gσµ ρ ′′  =         (4.8) Here, µ, 𝜌𝜌, 𝑐𝑐𝑝𝑝, 𝑃𝑃𝑟𝑟, and σ are the dynamic viscosity, density, specific heat, Prandtl number, and surface tension coefficient of the liquid at 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠, respectively. Additionally, ℎ𝑓𝑓𝑔𝑔is the enthalpy of vaporization, and g is the gravitational acceleration. For water, the constant 𝑛𝑛 = 1. The surface-fluid factor can be defined as sfdYCdX =    (4.9) 60 which is the average of 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑⁄ over the entire nucleate boiling curve. Here 𝐶𝐶𝑠𝑠𝑓𝑓 is used to quantify the surface effect for nucleate boiling (from the onset to the CHF), while 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑⁄ quantifies the surface effect at a given boiling condition. Large values of 𝐶𝐶𝑠𝑠𝑓𝑓 and 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑⁄ mean hindrance to boiling. Based on Eqs. (4.7) and (4.8), the boiling tests on the five surfaces are plotted in Figure 4.9a. The data points for the three hydrophilic surfaces appear to be linear, while the data points for the two hydrophobic surfaces are very nonlinear. A simple and approximate way to get the surface-fluid factor defined in Eq. (4.9) is to apply a linear fitting to the data points, and the linear slope is taken as 𝐶𝐶𝑠𝑠𝑓𝑓. The value of 𝐶𝐶𝑠𝑠𝑓𝑓 increases for surfaces in the order of SHI, HI, HO, and SHO. In other words, 𝐶𝐶𝑠𝑠𝑓𝑓 increases with increasing hydrophobicity or decreasing hydrophilicity. A close look at the data points of the HO and SHO-w25 surfaces shows 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑⁄ increases with increasing 𝑑𝑑 and 𝑑𝑑. In other words, the hydrophobic surfaces show increasing hindrance to boiling with increasing the global heat flux and increasing the global surface temperature. Figure 4.9b compares the surface fluid factors of the Silicon-water in this study with those surface fluid factors in the Rohsenow correlation. The SHI surfaces have a similar value to the water-copper and water-platinum combinations. This is another way to interpret the trend observed in Figure 4.8b, where for the two hydrophobic surfaces the global heat transfer coefficient decreases with increasing the global heat flux. To facilitate the discussion here, from Eqs. (4.7) and (4.8), we can write ( )12 3 dYh qdX− ′′∝    (4.10) and 61 ( )32edYh TdX− ∝ ∆    (4.11) In Eqs. (4.10) and (4.11), for HO and SHO surfaces, the dependency of 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑⁄ on 𝑞𝑞′′���� and ∆𝑇𝑇𝑒𝑒����� affects how ℎ� changes with 𝑞𝑞′′���� and ∆𝑇𝑇𝑒𝑒�����. Figure 4.9 (a) Surface-fluid factor for the five tested surfaces; (b) Comparison of the surface-fluid factors of Water-silicon on the five wetting surfaces to those in the Rohsenow correlation. 4.3.4 Boiling on biphilic surface The results above have shown a significant dependency of bubble dynamics and boiling heat transfer on surface wettability. How would pool boiling be affected if a surface has regions with different wettability? In this section, a biphilic surface, 10 mm ×12 mm, consisting of two regions with a large contrast of wettability, SHI-w25 (CA ~ 0°) and SHO-w25 (CA ~ 163°), is tested. The two regions have the same dimensions of 5 mm ×12 mm. Three global heat fluxes are tested. Figure 4.10 shows bubble behaviors under the three tested fluxes. For the lowest heat flux 16.4 W/cm2 (see Figure 4.10a), small bubbles are visible in 62 the SHI region, but not in the SHO region. The black-white contrast between the SHI and SHO areas where no bubbles are formed indicates different wetting regimes. The SHO region is covered with a vapor film and therefore is the Cassie wetting regime, while the SHI region is the Wenzel wetting regime. For the intermediate heat flux 23.8 W/cm2, the departure of big bubbles takes place in the SHO region at a period ~1 second. For the highest heat flux 44.6 W/cm2, the departure of large bubbles still takes place in the SHO region, but the departure period reduces to ~60 ms. A close observation of Figure 4.10 shows that, at the border between the two regions where a step change of wettability exists, vapor moves from the SHI region to the SHO region, and the moved vapor joins the big bubbles leaving the SHO region. Based on the average of 500 consecutive IR images, the surface temperature distribution is presented in Figure 4.11. The SHO region is hotter than the SHI region. By averaging the temperature for the SHI and SHO regions separately, the area-averaged temperatures for the two regions are obtained and compared. For the lowest heat flux, the SHO region is 25 ℃ hotter than the SHI region, while the difference reaches 90℃ for the highest heat flux. 63 Figure 4.10 Bubble behaviors on the biphilic surface at different global heat fluxes. (a) 𝑞𝑞′′���=16.4 W/cm2, (b) 𝑞𝑞′′���=23.8 W/cm2, (c) 𝑞𝑞′′���=44.6 W/cm2. Figure 4.11 Temperature distribution of the biphilic surface at three global heat fluxes: (a) 𝑞𝑞′′���=16.4 W/cm2, (b) 𝑞𝑞′′���=23.8 W/cm2, (c) 𝑞𝑞′′���=44.6 W/cm2. 64 To gain insight into the local heat transfer, the temperature along the centerline (dashed lines in Figure 4.11) across the SHI and SHO regions is plotted in Figure 4.12a. Temperature distribution in the SHO region is significantly nonuniform, and the hot spot is located roughly at the center of the SHO region. The temperature gradient along the centerline of the surface can be calculated using 1 1 1 11 1 2ii i i ix i iT T T T Tx x x a+ − + −+ −∂ − −≈ =∂ − (4.12) where the subscript 𝐵𝐵 is used to number the data points shown in Figure 4.12a. The locus interval of the data points is the pixel side length of the IR images (see Figure 4.2b). The calculated temperature gradient is plotted in Figure 4.12b. At the SHO-SHI border, there is a local maximum temperature gradient for all three tested heat fluxes. Along the centerline, due to symmetry, the temperature gradient in the direction normal to the centerline can be approximated to zero. Hence, the local heat transfer coefficient can be calculated using ( )1 12ii ie xx xT T bh T q kx x a+ − ∂ ∂′′∆ = + −  ∂ ∂  (4.13) where the global heat flux has been used. The calculated local transfer coefficient for the three cases is plotted in Figure 4.13. Clearly, the SHO region shows a lower heat transfer coefficient than the SHI region. In the SHO region, lower heat fluxes show better heat transfer performance, while the opposite trend appears for the SHI region. The transition between the two trends occurs roughly at the border between the two regions. 65 Figure 4.12 (a) Local surface temperature along the centerlines (dash lines in Figure 4.11); (b) Temperature gradient along the centerlines. Figure 4.13 Local heat transfer coefficient along the centerlines calculated based on the data shown in Figure 4.12. 66 4.4 Summary In this study, four types of surface wettabilities, SHI, HI, HO, and SHO, are experimentally studied in subcooled boiling of water. A silicon wafer is used as the boiling substrate. The polished side of the substrate is used as the boiling surface. Laser ablation and silanization grafting are used to create surface texture and modify the surface chemistry. The unpolished side of the substrate is used as the heating surface, which is sputter coated to form a thin film heater. This allows to use the Infrared camera to measure surface temperature with high spatial resolution. Comparing the HI surface with the SHI surfaces, the surface texture affects bubble formation and departure, causing the heat transfer coefficient and CHF to increase. The comparison of results between SHI-w25 and SHI-w50 shows that laser ablation with smaller scan spacing results in a higher heat transfer coefficient and larger CHF. Significant differences in bubble behavior and heat transfer coefficient and CHF are found among the SHO, HO, HI, and SHI surfaces. For the SHI and HI surfaces, the heat transfer coefficient increases with the surface heat flux. However, the heat transfer coefficient decreases with increasing the heat flux for the SHO and HO surfaces. The trends are also revealed from the boiling tests on a biphilic surface, which has half the boiling surface being SHO and the other half being SHI. In this study, the surface-fluid factor is used to quantify the surface effect. The factor’s value increases for the surface wettability varied from SHI to SHO. For surfaces with hydrophobic surface chemistry, the surface effect changes with surface heat flux and surface temperature. For surfaces with hydrophilic surface chemistry, the surface effect remains relatively constant over the nucleate boiling regime. 67 Chapter 5 Pool Boiling on a Biphilic Surface Where Bubbles Can Move Horizontally on the Surface In this chapter, the effects of bubbles’ horizontal movement on the boiling surface have been investigated in terms of the pool boiling heat transfer. Bubble behaviors and boiling curves on a biphilic surface have been compared with a SHI surface. This study is designed to assess the hypothesis that the horizontal bubble movement on the surface benefit heat transfer. The content of this chapter has been published in the International Journal of Heat and Mass Transfer [83]. 5.1 Background Pool boiling is characterized by cyclic bubble formation, grow-up, and departure, and the heat transfer performance is closely related to bubble dynamics. A few passive techniques have been studied to enhance pool boiling heat transfer by facilitating bubble departure from the boiling surface. For example, the chimney-like shroud [75] and hollow conical structure [76] were added above the boiling surface to improve both liquid feeding and vapor release dynamics. Either using a self-propelling swing-like heater [77] or attaching a vibration material close to the heating surface [78] benefits heat transfer by taking advantage of the flow induced by bubble formation and release. In pool boiling on a horizontal flat surface, bubble motion relative to the surface is mainly in the vertical direction. There are two research questions: 1) Could bubbles move horizontally on the boiling surface? 2) Then how would the boiling heat transfer be affected? One answer to the first question is surface modification. However, biphilic surfaces dotted with hydrophobic spots cannot achieve significant on-surface motion of bubbles. Recent studies have shown that air bubbles underwater can spontaneously move on a horizontal surface where a hydrophobic track with expanding width is surrounded by a hydrophilic region [56, 58, 79-81]. This pump-less gas bubble 68 transportation underwater is driven by the Laplace pressure difference. Most of the research work is limited to unidirectional Janus membrane for separation [79] and gas transportation for chemical reaction and combustion [56, 57]. For the 2nd question, there is no answer as little work has been conducted to test this kind of biphilic surface in pool boiling. In the present work, a specially designed biphilic surface that enables the on-surface transportation of gas bubbles is fabricated and applied to pool boiling. The new surface is a super-biphilic surface on which two wedge-shaped SHO tracks are bounded by hydrophobic area and connected to a SHI region. The wedge-shaped SHO tracks are expected to be able to remove vapor generated in the SHI region. The focus is put on the effect of the on-surface transportation of vapor bubbles on the boiling heat transfer. 5.2 Experimental methodology 5.2.1 Surface Fabrication and Characterization Figure 5.1 presents the procedure of fabricating the biphilic surface. A P-type single-side polished silicon wafer (TOPSiL Global-Wafers Co., Ltd. Thickness 𝑏𝑏=385 µm) is used as the substrate. The polished side is used as the boiling surface, while the unpolished side is coated with a thin film heater to form the heating surface. Figure 5.1a shows the biphilic boiling surface, which consists of three wettabilities: SHI, SHO, and HO. The central square region is SHI, the two wedge-shaped tracks and the two rectangular areas are SHO, and the rest of the surface is HO. Hence each SHO track connects the square SHI region to a rectangular SHO region, and it is bounded by hydrophobic areas on its two sides. The SHO tracks are expected to transport gas bubbles from the SHI region to the SHO region. The two wedge-shaped tracks are symmetrical about the centerline of the substrate and have a wedge angle η= 14°. They are 8 mm long and the starting 1mm of the wedge track tip is in the SHI region. 69 Figure 5.1b shows the three steps of fabricating the biphilic surface, which include laser milling I (1st-time laser milling), silanization grafting, and laser milling II (2nd-time laser milling). Firstly, laser milling I is applied to the bare silicon surface (30 mm×22 mm) to create the two wedge-shaped tracks. The laser beam from a nanosecond laser micromachining system (A-355, Oxford Lasers) is used to scan the surface line-by-line at 10 mm/s in the horizontal direction at a constant spacing of 5 µm. The laser has a wavelength of 355 nm with a pulse length of 15 ns. Laser milling is conducted in the air under atmospheric conditions at a pulse frequency of 20 kHz and a 5% power attenuation of the 8.5 W laser power. In the process of laser milling, surface oxidation occurs simultaneously, resulting in morphologies with SHI properties. Then silanization grafting is applied in the following four sub-steps to turn two wedge-shaped tracks SHO. 1) The substrate with the wedge tracks is ultrasonically cleaned in deionized water (DI) for 15 min to remove the existing powder generated from laser milling. 2) The substrate is put in the O2 plasma to saturate the surface with reactive hydrophilic hydroxyl moieties. 3) The surface is hydrophobized by being immersed in a silanization solution for 5h at room temperature. The solution is made of 1 mL of (TRIDECAFLUORO-1, 1, 2, 2-TETRAHYDROOCTYL) TRIETHOXYSILANE (Gelest Inc.) and 99 mL methanol (≥99.8%, Certified ACS, Fisher Chemical™). 4) The substrate is baked at 150°C for 5h on a hotplate. After silanization grafting, the two wedge-shaped tracks are SHO and the rest of the surface is hydrophobic. To create the SHI square region, another laser milling with the same laser settings as laser milling I is applied to the center region (8 mm × 8 mm) but excluding the beginning 1 mm tip of the diverging wedge-shaped track. At the same location as the SHI square region but on the non-polished side of the substrate, a thin-film resistive heater is fabricated using sputter coating. A 35nm-thick layer of chrome is first deposited on the surface, and then a 70 nm layer of gold is added. Two thin copper foils are 70 connected to the gold layer by soldering, and the resistance is around 1 Ω. The heating area is 8 mm×8 mm, which is just beneath the SHI region on the boiling surface side. Figure 5.1 (a). Dimensions of the SHI and SHO regions on the biphilic surface. (b) Steps of fabricating the biphilic surface. (c) Contact angles of different regions: c1-SHI; c2-HO; c3-SHO. (d) SEM of the SHI and SHO regions: d1-SHI; d2 SHO. (e) Confocal images of SHI and SHO regions: e1-SHI; e2-SHO. 71 Surface wettability is evaluated using the static contact angle of DI water droplets of 10 µL measured by a Contact Angle Goniometer (Ramé Hart 90-U3-Pro) at room temperature in the air. Measurement is repeated at five different locations, and the average is taken. As shown in Figure 5.1c, the apparent contact angle of the SHO region is around 162° with a small contact angle hysteresis less than 2°, while the SHI region has an apparent contact angle of ~0°. The remaining region of the surface is hydrophobic with an apparent contact angle of 104°. The microstructure of the laser-ablated silicon surface is examined using a scanning electron microscope (SEM) and a laser scanning confocal microscope (OLS5000, Olympus Corporation, Tokyo, Japan). As shown in Figure 5.1d, laser ablation melts the top surface material and the re-deposited material forms a porous layer on both sides of the microgrooves. Since the spacing of laser scanning is small, the laser-ablated regions are fully covered by bumps and groove structures with randomly distributed multiscale cavities. The roughness (root mean square height) of the textured surface as shown in Figure 5.1e is around 3.5 µm for both the SHI and SHO regions. 5.2.2 Experimental Setups and Procedures Since it is difficult to study a single bubble in pool boiling, the functional biphilic surface is first evaluated by observing the movement of air bubbles on the surface. This would help analyze and understand the vapor bubble dynamics in pool boiling. Therefore, two experimental setups as shown in Figure 5.2 are used in this study, of which Figure 5.2a is for the isothermal air bubble study, while Figure 5.2b is the setup for pool boiling tests. DI water is used as the working fluid for both experiments. As shown in Figure 5.2a, the biphilic surface is put on the bottom wall of a water chamber with dimensions of 100 mm×80 mm×150 mm. A needle connected to a syringe pump (GenieTouch™, US) is used to introduce air bubbles at the minor end of the SHO track. Air injection is conducted at a flow rate of 1 mL/min, and the air bubble behaviors are captured by a 72 high-speed camera (Phantom M310) at 10,000 fps (frames per second). All the air bubble tests are conducted at room temperature and atmospheric pressure under constant air flow rates. The pool boiling experimental setup is schematically shown in Figure 5.2b. The water chamber 200 mm×250 mm×200 mm, is made of transparent polycarbonate. The water temperature is maintained at 90℃ for all the pool boiling tests by an auxiliary heater of 1000 W and a proportional-integral-derivative (PID) temperature-controlling system. The exploded view of the thin-film heater assembly is shown in Figure 5.2c. A programmable DC power supply is used to power the thin-film heater. Water in the chamber is pre-boiled to minimize the amount of entrapped non-condensable gas in the water before conducting tests. For each test, after the boiling reaches a steady state, vapor bubble behaviors on the boiling surface are recorded by the high-speed camera at 10,000 fps with a resolution of 0.06 mm/pixel, and temperature distribution on the heating surface is captured by an infrared camera (FLIR SC600) at 120 fps. The IR camera used in this study has an accuracy of 0.5 °C with a sensitivity of 0.05 °C. The IR camera is within the calibration period by the FLIR Company. For accurate temperature measurement, the heating layer is painted black and the emissivity of the heater surface is calibrated as 0.9 by comparing the IR camera reading with the thermal couples of high accuracy. The pool boiling chamber is open to the ambient through a slit at the upper corner to ensure the boiling tests are performed under atmospheric pressure. 5.2.3 Heat transfer data analysis The heat analysis on the surface as schematically shown in Figure 5.3 was conducted using the same method as that in Section 4.2.2, and the maximum uncertainties in average surface temperature, heat flux, and heat transfer coefficient are 1.5℃, 0.2 𝑊𝑊/𝑐𝑐𝜇𝜇2, and 0.3 𝑊𝑊/𝑐𝑐𝜇𝜇2·𝐾𝐾, respectively. 73 Figure 5.2 (a) Air bubble experimental study setup. (b) Pool boiling experimental setup. (c) Thin-film heater assembly: c1- top view; c2-bottom view. Figure 5.3 Schematic of the boiling substrates indicating heat analysis: (a) Boiling and heating surfaces of the boiling substrate. (b) Bottom view of the heating surface. The squares with 𝑎𝑎 × 𝑎𝑎 represent two IR image pixels and 𝑎𝑎 =266 𝜇𝜇𝜇𝜇. 74 5.3 Results and Discussions 5.3.1 Air bubble moving on the wedge-shaped SHO track The test methodology for injecting air bubbles onto the SHO track has been described in section 5.2.2 and shown in Figure 5.2a. Once the injected air bubble touches the SHO surface, it spreads on the SHO track. The spreading causes the bubble to detach from the injection needle, and then the needle starts injecting the next bubble. Figure 5.4a shows three consecutive bubbles (~1 mm in diameter) moving on the biphilic surface. After detaching from the injection needle, each air bubble appears as a bulge-like shape naturally moving to the right, along which direction the SHO track diverges. The bulge-like shape results from the fact that the gas bubble does not spread beyond the SHO-HO border to the HO region. - In other words, the difference in wettability across the SHO-HO border pins the triple line on the border and in the SHO region. The motion of the bubble is determined by tracking the loci of the leading and trailing edges relative to the tip of the wedge-shaped track. As shown in Figure 5.4b, both the trailing and leading edges move roughly at a constant velocity, ~0.8 m/s. 75 Figure 5.4 (a) Sequential images showing air bubbles moving on the wedge-shaped SHO track in the wedge-diverging direction. (b) Positions of the leading and training edges of the bubble versus time. To explain this natural motion, a gas bulge is schematically shown as two cross sections in Figure 5.5. The SHO track diverges along 𝑥𝑥 direction, and 𝑧𝑧 is normal to the substrate surface. The two cross sections are at two different 𝑥𝑥 locations. The following discussion focuses on force 76 analysis. The liquid-gas interface of the gas bubble can be expressed by 𝑆𝑆(𝑥𝑥, 𝑦𝑦, 𝑧𝑧) = ℎ(𝑥𝑥, 𝑦𝑦) −𝑧𝑧 = 0. The unit normal vector for the interface is 𝑛𝑛�⃗ = ∇�⃗ 𝑆𝑆 �∇�⃗ 𝑆𝑆�� , which points toward the gas phase. For simplicity, we consider both gas and liquid inviscid. Hence, the stress balance at the gas-liquid interface is ( ), , 0G s L sp n p n p nγ− + + =   (5.1) Here 𝑝𝑝𝐺𝐺,𝑠𝑠 and 𝑝𝑝𝐿𝐿,𝑠𝑠 are the pressure of the gas flow and liquid flow at the interface, and 𝑝𝑝𝛾𝛾 is the capillary pressure. The total force in the 𝑥𝑥 direction on the gas bubble is ( ), Gx G s GA V VpF x p ndA x p dV dVx∂ = ⋅ = ⋅ −∇ = − ∂ ∫ ∫ ∫   (5.2) where 𝑝𝑝𝐺𝐺 is the pressure inside the gas bubble. The minus sign of the gradient term is due to the inward direction of 𝑛𝑛�⃗ . As the hydrostatic pressure is a function of z only, ,G sG pppx x xγ∂∂∂ ≈ =∂ ∂ ∂ (5.3) Here Eq. (5.1) and 𝜕𝜕𝑝𝑝𝐿𝐿,𝑠𝑠 𝜕𝜕𝑥𝑥⁄ = 0 have been used. The capillary pressure can be approximately expressed by ( )222sin 2sinHp nW x Wγθ θγ γ γ ∂= −∇ ⋅ ≈ − ≈ ∂  (5.4) where 𝛾𝛾 is the surface tension, and 𝜃𝜃 is the apparent contact angle. 𝜕𝜕2𝐻𝐻 𝜕𝜕𝑥𝑥2⁄ is the curvature of the interface in the central plane of the gas bulge along the axial direction, and it is insignificant as compared to the curvature in the cross-section. From Eq. (5.4), we can further write 22cos 2sinp d dWx W dx W dxγ θ θ θγ∂  ≈ − ∂   (5.5) Substituting Eq. (5.5) into Eq. (5.3) and putting Eq. (5.3) into Eq. (5.2), we have 77 22sin 2cosxV VdW dF dV dVW dx W dxθ θ θγ γ   ≈ + −      ∫ ∫ (5.6) Figure 5.5 shows 𝜃𝜃2 > 𝜃𝜃1, which can be observed from the bubbles in the inset of Figure 5.5. Hence, there is a spatial gradient of the apparent contact angle, 𝑑𝑑𝜃𝜃 𝑑𝑑𝑥𝑥⁄ > 0. Since 𝜃𝜃 > 𝜋𝜋 2⁄ , cos 𝜃𝜃 < 0. In the direction x, the SHO region diverges, i.e. 𝑑𝑑𝑊𝑊 𝑑𝑑𝑥𝑥⁄ > 0. In the present work, 𝑑𝑑𝑊𝑊 𝑑𝑑𝑥𝑥⁄ = 2 tan(𝜂𝜂 2⁄ ). Based on the foregoing discussion, the two terms on the right-hand side are positive. This indicates that the capillarity force is in the direction that the SHO region diverges, which is the driving force that causes the gas bubble to move in the same direction. Equation (5.6) also indicates that a gas bubble with local equilibrium wetting around its edges, i.e. 𝑑𝑑𝜃𝜃 𝑑𝑑𝑥𝑥⁄ = 0, would not be able to remain stationary on the SHO track. Figure 5.5 Force analysis of a gas bubble on the wedge-shaped SHO track. The wedge angle is 𝜂𝜂. The angles 𝜃𝜃1 and 𝜃𝜃2 are not but can be approximated as local contact angles, because the two-track borderlines are not perpendicular to the baseline of the cross-section, but 𝜂𝜂 is small. 78 5.3.2 Pool boiling on the biphilic surface To investigate the effect of the wedge-shaped SHO tracks on pool boiling, pool boiling tests with varied heat fluxes are conducted on the biphilic surface in comparison with a SHI surface without the SHO tracks. Figure 5.6 compares the boiling curves of the biphilic surface with that of the SHI surface. The smooth surface without laser processing, which is hydrophilic (HI), is also plotted for comparison. The boiling curves are obtained by gradually increasing the heat flux until it reaches the CHF, which is characterized by a sharp increase in the surface temperature, followed by the failure of the thin-film heater. In Figure 5.6a, the boiling curves show the relation between the surface heat flux and the surface superheat for the two surfaces. The corresponding HTC is plotted as a function of surface heat flux in Figure 5.6b. As shown in Figure 5.6a, the SHI surface provides enhanced heat transfer compared with untreated surface because of the increased nucleation site density and strong wetting and wicking effect of the textured surface [69]. Comparison with the untreated surface is not analyzed here in detail, and the focus is to evaluate the influence of the biphilic surface pattern by comparing with the SHI surface. Compared with the SHI surface, enhancements are noticeable in both the heat transfer coefficient and CHF. For 𝑞𝑞′′���� < 150 𝑊𝑊/𝑐𝑐𝜇𝜇2, the average temperature is lower on the biphilic surface than that on the SHI surface, and correspondingly the HTC is larger. For 𝑞𝑞′′���� >150 𝑊𝑊/𝑐𝑐𝜇𝜇2, the average surface temperatures for both surfaces are larger than 115℃. For the SHI surface, further increase in heat flux leads to a slight decrease in the average surface temperature. This is possibly caused by the activation of smaller cavities as the nucleation sites, which takes away more heat from the surface, resulting in the temperature decreases. Similar phenomena were also reported by Liu et al. [30]. For the biphilic surface, further increase of heat flux causes the average temperature to increase. Hence, in Figure 5.6b, for high heat flux, the SHI surface shows 79 larger HTC values than the biphilic surface. However, the biphilic surface has a larger CHF than the SHI surface. The CHF achieved on the biphilic surface is 237 W/cm2 at a wall superheat of 24.9 ℃, while it is 207 W/cm2 at a wall superheat of 15.6 ℃ for the SHI surface. Hence, the biphilic surface extends the ranges of heat flux and surface temperature for the nucleate boiling regime. Figure 5.6 Pool boiling heat transfer results on the SHI and biphilic surface. (a) Boiling curves of water. (b) Heat transfer coefficient versus heat flux. To understand the mechanisms behind the differences of heat transfer performance between the biphilic surface and the SHI surface, bubble behaviors on these two surfaces at various heat fluxes are compared. As shown in Figure 5.7, for both surfaces the average temperature increases with increasing the heat flux. Similar bubble behaviors are observed in the central region on both surfaces. At low heat fluxes (𝑞𝑞′′���� < 30 W/𝑐𝑐𝜇𝜇2) isolated bubbles are generated at random locations on the boiling surface. With the increase of heat flux, more bubbles are generated and merge to form a large bubble, which continues to grow till departing vertically. The bubble departure diameter as shown in Figure 5.7c increases with increasing the heat flux for both surfaces, and 80 generally, the biphilic surface shows smaller departure sizes than the SHI surface at heat fluxes larger than 125W/cm2. Figure 5.7 Comparison of bubble behaviors on the SHI and biphilic surfaces at various global surface heat fluxes. (a) SHI surface. (b) Biphilic surface. In the present work, it is observed that the ONB on the biphilic surface occurs at a lower heat flux than the SHI surface. On the biphilic surface, early bubbles appear at the tips of the SHO 81 wedge-shaped tracks. This can be seen in Figure 5.8a and can be explained from two aspects. First, there could be entrapped air in the porous cavities in the SHO region. Although water has been degassed before testing, it is difficult to completely remove air from the micro-structured SHO region. According to Faghri, and Zhang [82], the bubble temperature for the onset of nucleate boiling 𝑇𝑇𝑂𝑂𝑂𝑂𝑂𝑂 can be written as 1 1 2gsat ONBpT T rγ   − ∝ −      (5.7) Here 𝑇𝑇𝑠𝑠𝑎𝑎𝑠𝑠 is the saturation temperature for the pressure of the liquid phase, 𝑟𝑟 is the radius of visible bubbles, and 𝑝𝑝𝑔𝑔 is the partial pressure of the trapped air. With trapped air, the onset of nucleate boiling occurs at a relatively low temperature. On the other hand, if there is no trapped air, 𝑝𝑝𝑔𝑔 = 0, and we assume the bubbles formed at the onset of nucleate boiling are spherical caps. Hence, Eq. (5.7) can be changed to 3 1/31 1 (2 3cos cos )sat ONBT Tθ θ − ∝ + −   (5.8) where 𝜃𝜃 is the apparent contact angle. For the surface with a larger apparent contact angle, the temperature for the initial vapor bubble growth is lower. As discussed above, the tip areas of the SHO tracks allow local ONB to take place at a low surface temperature. Bubbles generated on the SHO surface tend to spread over the surface [69]. Because of the wedge shape of the SHO track, these vapor bubbles can move away, allowing new vapor bubbles to form. This phenomenon is visible in Figure 5.8a. It seems vapor bubbles stop moving halfway to the SHO rectangles, and this is caused by vapor condensation. As the heat flux increases, boiling takes place in both the SHO and SHI areas of the boiling surface. As a result, bubbles generated close to the SHI-SHO border move into the SHO track if their contact lines 82 touch the SHI-SHO border. Figure 5.8b shows the transport of vapor bubbles for a relatively high heat flux 𝑞𝑞′′���� = 141𝑊𝑊/𝑐𝑐𝜇𝜇2. The motion of the vapor bubbles is similar to the observation of air bubbles shown in Figure 5.4a. the loci of the trailing edges of the 4 bubbles shown in Figure 5.8b are plotted in Figure 5.8c, which shows the bubbles move on the SHO at velocities ranging from 0.8 m/s to 1.2 m/s, which is similar to the air bubble movement velocity of 0.8 m/s as shown in Figure 5.4b. The discussion above helps explain the heat transfer enhancement by the biphilic surface for 𝑞𝑞′′���� < 150 𝑊𝑊/𝑐𝑐𝜇𝜇2 (see Figure 5.6), and the enhancement can be attributed to the early ONB and the horizontal removal of bubbles from the boiling region. Figure 5.8 (a) Onset of nucleate boiling at the tips of the SHO wedge-shaped tracks (𝑞𝑞′′���� = 11 W/𝑐𝑐𝜇𝜇2) (b) Vapor bubble moving on the wedge-shaped SHO track in pool boiling at 𝑞𝑞′′���� = 141 𝑊𝑊/𝑐𝑐𝜇𝜇2. (c) Location of trailing edge versus time for the 4 bubbles. 83 Clearly, although bubbles are difficult to detach from a regular region with super-hydrophobicity, the wedge-shaped SHO track causes bubbles to naturally move away from the boiling region. On the way of transportation, the horizontally moving bubbles condense due to the subcooled bulk water and stop moving. With the increase of the heat flux, more bubbles are generated from the minor tip of the wedge track, and at the same time, bubbles generated close to the SHI-SHO border move into the SHO track. Consequently, more bubbles leave the boiling surface by horizontal transportation, and the horizontal transportation causes different liquid flow dynamics from the vertical departure bubbles. In addition, the coalesced large bubble detaching from the center is smaller on the biphilic surface than that on the SHI surface. Figure 5.9. Comparison of the development of a bubble and the temperature distribution on the SHI surface and the biphilic surface for 𝑞𝑞′′���� = 176𝑊𝑊/𝑐𝑐𝜇𝜇2. (a) Bubble development on the SHI surface. (b) Bubble development on the biphilic surface. 84 Figure 5.9 compares the development of a bubble on the SHI surface and the biphilic surface for 𝑞𝑞′′���� = 176𝑊𝑊/𝑐𝑐𝜇𝜇2, respectively. The bubble behaviors are typical for heat fluxes larger than 150 W/cm2 in the present work. Generally, the growing bubble first shows the expansion of the contact line toward the edge of the SHI region. As the bubble height increases, the contact line retracts toward the center of the SHI surface, and necking takes place close to the surface until a detached bubble is formed. It is clear that during expansion, the contact area is larger on the biphilic surface, due to the existence of the SHO track. At the later stage of the detaching, there is vapor bubble transportation on the SHO. Comparing the bubble behaviors in Figure 5.9 and the heat transfer shown in Figure 5.6 for high heat fluxes, it might be reasonable to say that the large contact area causes the lower HTC for the biphilic surface, but the transport of vapor on the SHO track results in the increase of CHF. The horizontal vapor delays the transition from nucleate boiling to CHF. 5.4 Summary A biphilic surface combining a SHI region with two wedge-shaped SHO tracks is fabricated by laser ablation and silanization grafting. Air bubble movement on the wedge-shaped SHO track of the biphilic surface underwater was observed using a high-speed camera and the forces responsible for the pump-less movement were analyzed. The air bubble moves spontaneously at the speed of 0.8 m/s under the Laplace pressure difference due to the geometry gradient and wetting contrast. The influence of the biphilic surface on vapor bubble behaviors and heat transfer in pool boiling was studied experimentally by a high-speed camera and an infrared camera simultaneously. Early onset of nucleate boiling takes place on the tip of the SHO track within the heating area, and the vapor bubbles spontaneously move away along the SHO track with speeds ranging from 0.8 85 m/s to 1.2 m/s. The heat transfer performance on the biphilic surface is compared with that on a SHI surface, and results indicate that at 𝑞𝑞′′���� < 150𝑊𝑊/𝑐𝑐𝜇𝜇2, the average temperature is lower and the HTC is larger on the biphilic surface than that on the SHI surface. The enhancement can be explained by the early onset of nucleation on the SHO tip within the SHI area and improved bubble removal ability via horizontal spontaneous movement. For high fluxes 𝑞𝑞′′���� > 150𝑊𝑊/𝑐𝑐𝜇𝜇2 , the boiling surface is mostly covered by vapor. The SHO tracks allow vapor to spread to a larger extent, resulting in lower heat transfer coefficients than the SHI surface. However, the transport of vapor by the two SHO tracks delays the ending of the nucleate boiling regime to higher heat flux and surface temperature. 86 Chapter 6 Effects of surface wettability and Inclination on bubble behaviors and pool boiling heat transfer In this chapter, the influence of surface wettability and inclination on the bubble behavior and pool boiling has been studied. A conference paper based on the content in this chapter has been published as a conference paper at the 17th International Conference of Heat Transfer. 6.1 Background The existing studies about the influence of surface wettability on pool boiling are mainly conducted on horizontally upward-facing surfaces, and the effect of surface inclination has not yet been considered comprehensively. This study is designed to assess the hypothesis that the surface wettability’s effect on inclined surfaces could be different from the horizontal surfaces. This study aims to experimentally investigate the influence of surface wettability and inclination angle on bubble behaviors and heat transfer in pool boiling. In addition to vapor bubble observation, due to the complicated and chaotic boiling phenomena, and to exclude the disturbance from heating, non-condensable bubble formation from a submerged orifice by air injection is used to simulate the vapor bubbles in pool boiling. In this study, a study about the influence of surface wettability and inclination angle on bubble dynamics is conducted. Different wetting surfaces ranging from the super-hydrophilic to the super-hydrophobic surfaces are fabricated by a one-step fast and accurate method of laser ablation and chemical vapor deposition. The surfaces are put underwater and positioned horizontally upward at 0° and vertically at 90°. The bubble behaviors under different surface conditions are compared, and the influence of surface contact angle hysteresis is also investigated by comparing two hydrophobic surfaces with similar apparent contact angles, but different contact angle hysteresis. 87 6.2 Experiments 6.2.1. Experimental setup Two experimental setups are used in this study. The experimental setup used for the air bubble study is schematically shown in Figure 6.1a, which consists of the water chamber, the syringe pump (GenieTouchTM), a high-speed camera (Phantom M310) with a Nikon manual focus lens (Micro-Nikkor 105 mm f/2.8) and the different wetting surfaces. The water chamber is made of transparent polycarbonate, and the dimensions are 150 mm×150 mm×250 mm. The silicon surface with an orifice of 0.5 mm in diameter is fixed at the bottom of the water chamber, and a syringe pump is connected to it for air injection. The bubble formation and behaviors are captured by the high-speed camera at 5,000 fps (frames per second). The tests are conducted at room temperature and atmospheric pressure under constant air flow rates. Experiments are conducted from the case when the surface is positioned at α=0°, and then it is tilted to 30°, 60° and 90°. Figure 6.1 Schematic of the experimental setups. (a) Air bubble study. (b) Pool boiling 88 Pool boiling tests were conducted using the experimental setup shown in Figure 6.1b, which consists of a water chamber made of the same materials but with larger dimensions of 250 mm×200 mm× 200 mm. The water in the chamber is preheated by an auxiliary heater and maintained at 90℃ by the proportional-integral-derivative controlling system. A thin film heater (heating area12mm×12mm) is fabricated by sputter coating on the backside of the same silicon wafer for generating vapor bubbles. The heater is supplied by a DC power supply. After reaching the steady-state, the high-speed camera is used to capture the bubble behaviors at 5000fps, and an infrared camera (FLIR SC660) is used to record the temperature distribution on the heating surface at 120 fps. 6.2.2. Surface processing and characterization The P-type single-side polished silicon wafer with dimensions of 30 mm ×22 mm ×0.385 mm is used as the substrate, of which the unpolished side with a thin-film heater is the heating surface, while the polished side is the boiling surface. The thin-film heater is fabricated using the Angstrom Engineering Corp. Nexdep Deposition System. A 35 nm layer of chrome is coated firstly at the deposition rate of 1 Å per second, followed by a 70 nm layer of gold. Then two thin copper foils are connected to the two ends of the gold layer by soldering. To match the heating area, the region of interest (ROI) on the boiling surface has the same dimensions of 20 mm ×5 mm, and its resistance is around 4 Ω. Five different wetting surfaces are used in this study, which includes one hydrophilic surface (HI), two hydrophobic surfaces (HO) having similar contact angles but different contact angle hysteresis, a super-hydrophilic (SHI) surface, and a super-hydrophobic (SHO) surface. The silicon surface itself is hydrophilic and is used directly after being cut into the size of 30 mm ×22 mm. A 0.5mm orifice is drilled at the center of the surfaces for air bubble study by the nanosecond laser 89 from the A-355 laser micromachining system (Oxford Lasers), while the surfaces for pool boiling study have no orifice. To get the SHI surface, as shown in Figure 6.2, the HI surface is scanned by the nanosecond laser line-by-line at a spacing of 5 µm in the air under atmospheric conditions. The laser-processed region has dimensions of 20 mm ×5 mm. The laser has a wavelength of 355 nm with a pulse length of 15 ns and the maximum power of 8.5W. The surface processing is conducted at the pulse frequency of 20 kHz, and a power attenuation of 5 %. As the SEM and 3D confocal images shown in Figure 6.3, after laser ablation, porous textures like mounds are formed from the redeposited debris. The laser-processed region becomes SHI. To get the hydrophobic surfaces, silanization grafting by chemical vapor deposition is used. In this study, two chemicals are used to get two hydrophobic surfaces. One is 1, 3-dichlorotetramethyldisiloxane (denoted as PDMS), and the other is (tridecafluoro-1, 1, 2, 2-tetrahydrooctyl) trichlorosilane (denoted as PFOS). Figure 6.2. Schematic of the fabrication of surfaces used in this study 90 Figure 6.3 Surface characterization. (a) SEM image, (b) 3D confocal image The substrate is first ultrasonically cleaned in DI water for 15 min to remove the particles generated from laser ablation. After drying with compressed air, it is treated by the O2 plasma for 5 min to saturate the surface with reactive hydrophilic hydroxyl moieties. Then the substrate is put in a closed plastic container with 1, 3-dichlorotetramethyldisiloxane in a plastic cap. After 10 min, the substrate is washed with isopropyl alcohol for 15s to remove the unbounded chains and dried with compressed air. Finally, it is baked on a hotplate at 80℃ for 30 min. The coating of (tridecafluoro-1, 1, 2, 2-tetrahydrooctyl) trichlorosilane is similar but in a vacuum chamber with a pressure of -0.7 bar. Figure 6.4 shows the schematics of the chemical reaction, and given the successful formation of the monolayers, the surface will become hydrophobic. Surface coated with (tridecafluoro-1, 1, 2, 2-tetrahydrooctyl)trichlorosilane is denoted as HO-PFOS, and the surface with 1, 3-dichlorotetramethyldisiloxane is denoted as HO-PDMS. The SHO surface is fabricated from the SHI surface using the same method as the smooth surfaces. Here since the contact angles and contact angle hysteresis for SHO surfaces using the two chemicals are similar, the SHO surface is fabricated using the PDMS coating only. 91 Figure 6.4 Chemical reaction for surface hydrophobization. (a) PFOS coating, (b) PDMS brush As shown in Figure 6.5, the wettability of the surface is evaluated by contact angles measured using the Ramé Hart 90-U3-Pro Contact Angle Goniometer. The apparent contact angle is measured by putting a 10 µL DI water droplet on the surface at room temperature in the air. The advancing and receding contact angles are measured by adding and removing water droplet volume. The apparent contact angles (CA) and contact angle hysteresis (CAH) are listed in Table 1. The HI surface has an apparent CA=53°, the HO-PDMS has an apparent CA=108° with the CAH=5°, and the HO-PFOS has a similar CA=110°, but with a larger CAH=18°. The contact angle of the SHO surface is 165° and negligible CAH<1°, while the SHI surface has a contact angle of around 0°. 92 Figure 6.5. Apparent contact angles of the different wetting surfaces. (a) HI surface. (b) HO-PDMS. (c) HO-PFOS. (d) SHO surface Table 6.1 The contact angles of different wetting surfaces Surface Apparent CA CAH HI 53° --- HO-PDMS 108° 5° HO-PFOS 110° 18° SHO 165° <1° SHI ~0° --- 6.2.3 Data analysis The heat transfer data are obtained after pool boiling reaches the steady state and are processed using a similar method as that in Section 4.2.2. 6.3 Results and Discussions 6.3.1 The influence of surface wettability The influence of surface wettability on bubble formation is firstly investigated by comparing the air bubble behaviors as shown in Figure 6.6 on the five different wetting surfaces positioned horizontally facing upwards at α=0°. The air injection flow rate is 1mL/min, and the moment when the air bubble comes out from the orifice is set as t=0. As shown in Figure 6.6a on the hydrophilic 93 surface, the air bubble appears as a spherical segment as the supplied air pressure overcomes the capillary pressure at the orifice. As more air is injected, the air bubble keeps growing up until necking takes place, which breaks up as the bubble departs. The bubble formation and behaviors on the SHI surface as shown in Figure 6.6b are similar to the HI surface. Differences exist in the bubble departure size and the bubble growth time. The departure diameter in this study is derived from the area of the bubble identified from the images using ImageJ. The departure size from the HI surface is 3 mm while it is 2.4 mm on the SHI surface. On the HI surface, it takes 48.5ms for the bubble to grow and depart, and it is about twice that for the bubble on the SHI surface. Bubble behaviors on the hydrophobic surfaces as shown in Figure 6.6c and d are quite different from the hydrophilic surface. At the beginning for both hydrophobic surfaces, the bubble is in a semi-spherical shape. As more air is injected, the bubble grows up while the triple line expands until it occupies the maximum base area. The bubble then becomes elongated vertically accompanied by the shrinkage of the triple line. As the bubble goes to a high profile, the hydrostatic pressure difference causes the lower part of the bubble to start necking close to the orifice, which then makes the capillary pressure come into play. At the later stage of detaching, hydrostatic pressure on the bubble above the neck generates buoyancy force, which pulls the bubble upward and speeds up the detaching process. The difference between the two hydrophobic surfaces is the departure diameter, and the departure diameters are 5.9 mm and 5.5 mm, respectively from the HO-PDMS and HO-PFOS surfaces. In addition, it takes twice the time on the HO-PDMS surface for the bubble to grow and depart. This can be explained by the small contact angle hysteresis of the HO-PDMS surface, which induces relatively low resistance. As the image in Figure 6.6c at t=336 ms and the image in Figure 6.6d at t= 490 ms shown, with a small CAH, the bubble triple 94 line can expand further on the HO-PDMS surface, which means the bubble has a larger base area and therefore a larger departure diameter. On the SHO surface, as shown in Figure 6.6e, a thin air layer is first formed covering the whole SHO region. As more air is injected, the bubble grows but the bubble base is pinned at the boundary of the SHO region and cannot go beyond the SHO region. Then similar to the hydrophobic surface, a neck is formed, which becomes thinner and thinner and leads to the detachment of the bubble when rupture happens. The bubble departure diameter is 6.8 mm on the SHO surface, and it takes around 9s for the bubble to depart, which takes the longest time among the five cases. Figure 6.6. Bubble formation behaviors at the air injection flow rate of 1mL/min on different wetting surfaces at α=0°. (a) HI surface. (b) SHI surface. (c) HO-PFOS surface. (d) HO-PDMS surface. (e) SHO surface. (The moment the air bubble starts emerging is set as t=0) 95 The influence of surface wettability on the vapor bubble behaviors and heat transfer in pool boiling has also been studied on the HI, HO, SHI and SHO surfaces. As shown in Figure 6.7, the single vapor bubble behaviors on the HI, SHI and SHO surfaces are quite similar to the air bubble behaviors shown in Figure 6.6. The vapor bubble behaviors on the HO surfaces are shown in Figure 6.8. It is difficult to control the experimental conditions to enable only a single bubble on the hydrophobic surface, and several bubbles appear on the surface simultaneously. The vapor bubbles on both the HO-PDMS and HO-PFOS surfaces look similar to the air bubble on the corresponding surfaces. The vapor bubble on the HO-PDMS surface has a larger base than the bubble on the HO-PFOS surface, which is consistent with the air bubble observation in Figure 6.6. However, unlike air bubbles, it is difficult for a single vapor bubble to detach from the hydrophobic boiling surface, especially under subcooling conditions. The vapor bubbles on both hydrophobic surfaces tend to coalesce with the neighbor bubbles and then depart from the surface after reaching a certain volume. Figure 6.7. Formation and departure of vapor bubbles from (a) SHI surface (heat flux 𝑞𝑞′′���� = 13.3𝑊𝑊/𝑐𝑐𝜇𝜇2). (b) HI surface (𝑞𝑞′′���� = 9.3𝑊𝑊/𝑐𝑐𝜇𝜇2). (c) SHO surface (𝑞𝑞′′���� = 16.3𝑊𝑊/𝑐𝑐𝜇𝜇2). 96 Figure 6.8 Formation and departure of vapor bubbles from (a) HO-PDMS surface (heat flux 𝑞𝑞′′���� = 10.5𝑊𝑊/𝑐𝑐𝜇𝜇2). (b) HO-PFOS surface (𝑞𝑞′′���� = 10.5𝑊𝑊/𝑐𝑐𝜇𝜇2) To examine the influence of surface wettability on the surface temperature distribution, the temperature distributions on the different wetting surfaces are compared at similar heat fluxes of around 25 W/cm2. As shown in Figure 6.9, significant differences exist in the temperature distributions. The SHI surface has a relatively low and uniform temperature, while hotspots can be observed on the HO and SHO surfaces. This is because the vapor bubbles on the SHO and HO-PDMS surfaces tend to stay on the surface, and it is difficult for them to detach from the surface. The temperature on the HO-PFOS surface is lower and more uniform than on the HO-PDMS surface because the small vapor bubbles can detach from the surface more easily after coalescence. 97 Figure 6.9 Temperature distributions and bubble behaviors on different horizontally positioned wetting surfaces at a heat flux of around 25W/cm2. (a) HI surface. 𝑇𝑇� = 117℃, 𝑞𝑞′′���� =25.5𝑊𝑊/𝑐𝑐𝜇𝜇2. (b) SHI surface. 𝑇𝑇� = 113℃, 𝑞𝑞′′���� = 30𝑊𝑊/𝑐𝑐𝜇𝜇2. (c) HO-PDMS surface. 𝑇𝑇� =117.5℃, 𝑞𝑞′′���� = 24𝑊𝑊/𝑐𝑐𝜇𝜇2 . (d) HO-PFOS surface. 𝑇𝑇� = 115℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/𝑐𝑐𝜇𝜇2. (e) SHO surface. 𝑇𝑇� = 132℃, 𝑞𝑞′′���� = 24𝑊𝑊/𝑐𝑐𝜇𝜇2. 6.3.2 The influence of surface inclined angle The influence of surface inclination angle is firstly investigated by comparing the air bubble formation and behaviors at three different surface inclination angles of α=30°, 60° and 90° on four different wetting surfaces (SHI, HI, HO-PDMS and HO-PFOS). Here SHO surface is not considered, since bubble formation on the SHO surface depends much on the SHO area. Vapor bubbles on these surfaces in pool boiling are studied at α=90° only in comparison with those at α=0° since there is difficulty to position the setup at α=30° and 60° using the current experimental setup. The bubble behaviors on the inclined SHI and HI surfaces are similar as well. As shown in Figure 6.10 and Figure 6.11, at the beginning of the bubble growth stage, for the three inclination 98 angles of α=30°, 60° and 90°, the bubble behaves the same as that at α=0°, which remains symmetrical about the orifice axis. This is because the air injection momentum dominates the process. With the increase of the bubble size, the main difference between the horizontal surface and inclined surface appears that the bubble growth loses the symmetry about the orifice axis, and under the influence of buoyancy force, it tends to align in the vertical direction. In addition, after departure, it slides over the inclined surface for a short distance (see image at 26.5ms in Figure 6.10b, and 50ms in Figure 6.10c), and then deviates from the inclined surface while rising. This can be attributed to the hydrodynamics of the surrounding water caused by the asymmetry bubble on the inclined surface. The bubble departure diameter on the SHI surface remains 2.4 mm with the increasing inclination angle from α=0° to 90°. However, the departure diameter on the inclined HI surface is 2.3 mm, which is smaller than that on the horizontal surface. Figure 6.10. Growth and departure of injected air bubbles from the SHI surface. (a) α =30°. (b) α =60°. (c) α =90°. 99 Figure 6.11. Growth and departure of injected air bubbles from the HI surface. (a) α =30°. (b) α =60°. (c) α =90°. Different from the hydrophilic surface, on the hydrophobic surfaces as shown in Figure 6.12 and Figure 6.13, instead of detaching directly from the injection orifice, it slides along the inclined surface. At the beginning stage, the bubble remains symmetrical about the orifice axis, and as the bubble base expands beyond the orifice, the upper edge (leading edge) of the bubble first moves along the inclined surface under the driving of the injected air pressure. As the bubble grows up, the trailing edge travels upwards along the surface, and no more air will be injected into the bubble as it drifts out of the orifice. Then the bubble travels along the surface until it reaches the boundary of the substrate and pins to it. The bubble detaches from the surface after another bubble coalesces with it. The volume of the bubble on the hydrophobic surface is calculated from the volume of the 100 coalesced bubble by two bubbles using the same method as that for the bubble departure diameter. Results indicate that the bubble volume on the hydrophobic surfaces decreases with the increasing inclination angle, and the volume of the bubbles on the two hydrophobic surfaces at the same inclination angles are almost the same, which are around 36 mm3, 23 mm3, and 19 mm3 for α =30°, 60° and 90°, respectively. Although the bubble behaves similarly on the two hydrophobic surfaces, there are still some differences. First, the bubble shape is different. For example, on the HO-PDMS surface, the bubble at t=247ms in Figure 6.12a is not as tall as the bubble on the HO-PFOS surface at t=240ms as shown in Figure 6.13a. The second difference is the bubble behavior at the early stage on the vertical surface. On the HO-PDMS surface, the bubble bends downwards (see the bubble at t=12ms as shown in Figure 6.12c), while the bubble on the HO-PFOS surface at t=21ms in Figure 6.13c bends upwards. The reason is that on the HO-PDMS surface, the leading edge of the bubble starts going beyond the orifice as the bubble is still small, and the bubble is still symmetrical about the orifice axis. On the HO-PFOS surface, the leading edge starts moving after the bubble has been large enough to be aligned towards the vertical direction by the buoyancy force. To quantify the bubble sliding speed along the inclined HO surfaces, the displacement of the bubble as a function of time is acquired from the images by the high-speed camera. Here the displacement refers to the distance between the bubble trailing edge and the orifice. The moment when the trailing edge drifts out of the orifice is set as t=0. As shown in Figure 6.14a, the displacements of the bubbles on the HO-PDMS surface at the three inclined angles increase linearly with time, and with the increase of the inclination angle from 30° to 90°, the sliding speed firstly increases and then decreases. The bubble on the surface at α=60° has the largest velocity of 0.064 m/s. On the HO-PFOS surface, as shown in Figure 6.14b, the sliding speed follows the same 101 trend as that on the HO-PDMS surface. The largest sliding speed of 0.035m/s is on the surface at α=60° but it is only about half of that on the HO-PDMS surface at the same inclination angle. Figure 6.12. Growth and departure of injected air bubbles from the HO-PDMS surface. (a) α =30°. (b) α =60°. (c) α =90°. 102 Figure 6.13. Growth and departure of injected air bubbles from the HO-PFOS surface. (a) α =30°. (b) α =60°. (c) α =90° Figure 6.14. Bubble displacement versus time on surfaces at different inclined angles. (a) HO-PDMS. (b) HO-PFOS. 103 The vapor bubble behaviors on the SHI and HI surfaces at α =90° are shown in Figure 6.15. Similar to the air bubbles, on both surfaces a small cap is formed in the beginning, and as it grows up, the bubble moves away from the nucleation site under the buoyancy force and detaches from the vertical surface, which then disappears due to condensation. It takes around 30 ms for the bubble to form and detach from the surfaces under the current working conditions. Different from the vapor bubbles on SHI and HI surfaces at α =0°, as shown in Figure 6.7, the bubble slides along the surface for a distance, which causes vorticity, and the bubble deviates from the surface after the base of the bubble has no contact with the surface. Figure 6.15 Formation and departure of vapor bubbles from the vertical surface (a) SHI surface (heat flux 𝑞𝑞′′���� = 13.4𝑊𝑊/𝑐𝑐𝜇𝜇2). (b) HI surface (𝑞𝑞′′���� = 16.4𝑊𝑊/𝑐𝑐𝜇𝜇2) On the hydrophobic surfaces, as shown in Figure 6.16, the vapor bubbles behave similarly to the air bubbles as shown in Figure 6.12 and Figure 6.13. The bubble clings to the surface and slides along it, which merges with other bubbles in the way and finally leaves the surface as a large bubble. The difference between the bubbles on these two surfaces is that the merged bubble has a small base on the HO-PFOS surface than on the HO-PDMS surface. In addition, the vapor bubbles 104 on the vertical surfaces can leave the boiling surface more easily than the bubbles on the horizontally upward-facing surface. Figure 6.16 Formation and departure of vapor bubbles from the vertical surface (a) HO-PDMS surface (heat flux 𝑞𝑞′′���� = 10.5𝑊𝑊/𝑐𝑐𝜇𝜇2). (b) HO-PFOS surface (𝑞𝑞′′���� = 10.5𝑊𝑊/𝑐𝑐𝜇𝜇2) The temperature distributions and bubble behaviors at similar heat fluxes on the different vertically positioned wetting surfaces are presented in Figure 6.17. Similar to the case at α=0°, the SHI and HI surfaces have more uniform temperature distributions, while hotspots appear on the HO and SHO surfaces. Compared with the temperature distributions at α=0° as shown in Figure 6.9, the average temperature is lower at α=90° under similar heat fluxes. 105 Figure 6.17 Temperature distributions and bubble behaviors on different vertically positioned wetting surfaces at a heat flux of around 25W/cm2. (a) HI surface. 𝑇𝑇� = 114℃, 𝑞𝑞′′���� =25.7𝑊𝑊/𝑐𝑐𝜇𝜇2. (b) SHI surface. 𝑇𝑇� = 110℃, 𝑞𝑞′′���� = 30𝑊𝑊/𝑐𝑐𝜇𝜇2. (c) HO-PDMS surface. 𝑇𝑇� =117℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/𝑐𝑐𝜇𝜇2 . (d) HO-PFOS surface. 𝑇𝑇� = 112℃, 𝑞𝑞′′���� = 25.5𝑊𝑊/𝑐𝑐𝜇𝜇2. (e) SHO surface. 𝑇𝑇� = 129℃, 𝑞𝑞′′���� = 24𝑊𝑊/𝑐𝑐𝜇𝜇2. The boiling curves of the five different wetting surfaces are measured at both upward-facing horizontal (0°) and vertical (90°) positions. The heat flux provided to the heater is controlled to be less than 80 W/cm2 to make sure the heater is not damaged and can be used for all the tests. As shown in Figure 6.18, the boiling curve shifts left as the wettability increases from SHO to SHI 106 for both upward-facing horizontal and vertical positions. However, the HO surfaces have better heat transfer performances than the HI and SHI at lower heat fluxes, which could be explained by the early onset of nucleate boiling. The slope of the boiling curve of the surface HO-PFOS is larger than that of the HO-PDMS surface, and this is possibly caused by the larger base of the bubble and increased covering time of the boiling surface. Figure 6.18c compares the case of α =0° and α =90°, and it was observed that nucleate boiling heat transfer increases as the surface orientation rotates from upward-facing horizontal (0°) to vertical (90°). The vertical surface enables the bubble to detach from the surface more easily than at the horizontal position. Figure 6.18 Comparison of pool boiling curves on different wetting surfaces at two inclination angles. (a) α =0°, (b) α =90°, (c) α =0° and α =90° 107 6.4 Summary In this study, five different wetting surfaces are fabricated by laser ablation and silanization grafting via chemical vapor deposition, which include the hydrophilic, hydrophobic surfaces, SHI and SHO surfaces. The influences of surface wettability and inclination angle on bubble formation, growth and departure are investigated. The surface inclination is varied between the horizontal up-facing orientation (0°) and the vertical orientation (90°). Air bubble formation from a submerged orifice is used to study bubble behaviors. A thin-film resistive heater fabricated on the unpolished side of a silicon wafer substrate is used as the heating element. Air bubble behaviors are recorded by a high-speed camera, and compared with the vapor bubbles from pool boiling. In addition, the effects of surface contact angle hysteresis (CAH) on bubble dynamics are investigated by testing two hydrophobic surfaces with similar apparent contact angles but different CAH. The bubbles generated from the hydrophilic and SHI surfaces can depart from the orifice directly under the buoyancy force. While on the hydrophobic surface positioned at an inclination angle, the base of the bubble first expand to a large area, and then slides along the surface, which can only depart after merging with the adjacent bubble. The surface contact angle hysteresis not only influences the geometry of the bubble on the surface but also leads to different heat transfer performances. Results from the pool boiling indicate that the slope of the boiling curve increases as the wettability increases from SHO to SHI, except for the cases at lower heat fluxes. Pool boiling on the surface at a vertical position has better heat transfer performances. The findings from the study are useful and may provide some guidance for boiling surface design. 108 Chapter 7 Conclusions and Future Work 7.1 Conclusion In conclusion, this thesis focused on investigating the influence of surface wettability, textures, and orientation on pool boiling heat transfer. The results revealed that surface wettability plays a significant role in bubble formation, growth, and departure. The use of biphilic surfaces, consisting of both super-hydrophobic and super-hydrophilic regions, demonstrated the directional movement of bubbles towards the super-hydrophobic region under the influence of the Laplace pressure difference. The observation of air bubble movement on the biphilic surface provided valuable insights into the forces responsible for pump-less motion. Pool boiling tests on smooth and laser-ablated silicon surfaces with varying wettability were conducted and results indicated that the formation and departure of vapor bubbles were found to vary significantly with surface wettability. On the super-hydrophobic surface, a vapor blanket is firstly formed, which covers the surface and the blanket keeps rising from its center until a large bubble departs. After the vapor bubble departure, the surface is still covered by a layer of vapor film, from which the next bubble starts its formation in the Cassie wetting regime. The bubble remains spherical during formation, growth and departure. The bubble is larger on the super-hydrophobic surface and departs less frequently. Nucleate boiling curves showed that both the heat transfer coefficient and the critical heat flux increased with the surface wettability changing from super-hydrophobic to super-hydrophilic. The laser ablated surface with laser scan spacing of 25µm shows more enhancement than the surface with the spacing of 50 µm, and the scan spacing shows less significant effects for the heat fluxes close to the CHF. Additionally, the heat transfer coefficient showed different trends for hydrophilic and hydrophobic surfaces with increasing heat flux. The surface-fluid factor was used to quantitatively assess the effect of surface wettability, 109 with higher values observed as the wettability transitioned from super-hydrophilic to super-hydrophobic. The hydrophobic surfaces show increasing hindrance to boiling with increasing the global heat flux and increasing the global surface temperature. The impact of surface chemistry on hydrophobic and hydrophilic surfaces was found to be dependent on heat flux and temperature that Furthermore, the fabrication of a biphilic surface with super-hydrophilic and super-hydrophobic regions allowed for the examination of vapor bubble behaviors and heat transfer in pool boiling. The early onset of nucleate boiling on the super-hydrophobic region within the SHI area, along with the horizontal movement of vapor bubbles along the super-hydrophobic track, resulted in enhanced heat transfer performance at lower heat fluxes. However, at higher heat fluxes, the vapor-covered surface and spreading of vapor led to lower heat transfer coefficients compared to the super-hydrophilic surface. Nevertheless, the biphilic surface extended the nucleate boiling regime, increasing the critical heat flux. In addition, the study investigated the effects of surface wettability and inclination angle on bubble formation, growth, and departure using different wetting surfaces. The analysis included the influence of surface contact angle hysteresis on bubble dynamics. The experimental findings indicated that the slope of the boiling curve increased with increasing wettability from super-hydrophobic to super-hydrophilic, except at lower heat fluxes. The vapor bubbles have different shapes on these two hydrophobic surfaces due to the different contact angle hysteresis, and the bubbles moves more easily along the HO-PDMS surface, while it is easier for the bubble to detach from the HO-PFOS surface. The boiling curve on the HO-PFOS surface has a larger slope than that on the HO-PDMS surface. Moreover, pool boiling at a vertical position exhibited better heat transfer performance. 110 Overall, this study contributes to the understanding of the role of surface wettability in pool boiling heat transfer. The findings provide valuable insights for optimizing heat transfer performances through the design of biphilic surfaces and the selection of appropriate surface textures and wetting properties. 7.2 Further work Based on the current research status and to gain a deeper understanding of the influence of surface characteristics on pool boiling heat transfer, future research is suggested as the following: (1) A new experimental setup that can be positioned at various inclination angles will be fabricated, so the influence of boiling surface inclination angle can be investigated systematically. (2) More biphilic surface patterns will be designed and fabricated. Some examples are shown in Figure 7.1. 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Zhang, Fundamentals of Multiphase Heat Transfer and Flow. 2020: Springer. [83].Xia, Y., Li, R., Gao, X. and Li, R., 2023. Pool boiling on a biphilic surface where bubbles can move horizontally on the surface. International Journal of Heat and Mass Transfer, 215, p.124540. [84].Liang, G., Chen, Y., Yang, H., Li, D. and Shen, S., 2020. Nucleate boiling heat transfer and critical heat flux (CHF) from micro-pit surfaces. International Journal of Heat and Mass Transfer, 152, p.119510. [85].Kim, J., Jun, S., Laksnarain, R. and You, S.M., 2016. Effect of surface roughness on pool boiling heat transfer at a heated surface having moderate wettability. International Journal of Heat and Mass Transfer, 101, pp.992-1002. 117 Appendix Appendix A Appendix A describes the main process of microfabrication of thin-film heater used in the study of pool boiling in Chapters 3, 4, 5 and 6. Sputter Coating Process: The thin-film heater contains three layers including Cr layer as the adhesive layer, and Au as the heating layer. The silicon wafer is sputter-coated firstly with Cr layer followed by the Au layer. The detailed parameters of sputter coating are shown below. Metal: Cr Sputter system: Armstrong Engineering Corp. Nexdep Deposition System Power Source: Alternating Current Process gas: Argon Process gas pressure: 3.0 mTorr Gas flow rate: 8.06 sccm Deposition rate: 1 Å/s Final thickness: 35 nm Deposition time: 5 minutes Metal: Au Sputter system: Armstrong Engineering Corp. Nexdep Deposition System Power Source: Alternating Current Process gas: Argon Process gas pressure: 3.0 mTorr 118 Gas flow rate: 8.06 sccm Deposition rate: 1 Å/s Final thickness: 60nm, 70 nm Deposition time: Around 10 minutes 119 Appendix B G code used by the nanosecond laser for surface processing. G code #1 Milling rectangular area DVAR $THICK $ZDROP $REPS G91 ; Set to incremental mode G71 ; Set to metric mode $THICK = 0.385 $REPS = 10 $ZDROP = $THICK / $REPS REPEAT 2 ; Start of command block to be repeated. The number of squares per row is 2 BEAMON ; Turn the laser on REPEAT $REPS ; Start of command block that is to be processed ten times sequentially F10 ; set feed rate to 10 units per second G1 X30 G1 Y22 G1 X-30 G1 Y-22 G1 Z $ZDROP ENDREPEAT BEAMOFF G1 Z -($ZDROP*$REPS) G1 X45 ; 45mm linear move between squares at a speed of 10 unit per second ENDREPEAT ; end of command to be repeated 120 M2 ; end of program G code #2 Milling circular area DVAR $RINTERVAL $REPS $RADIUS1 DVAR $PASS_COUNT G91 ; Set to incremental mode G71 ; Set to metric mode $RADIUS1 = 2 $RINTERVAL = 0.5 $REPS= ($RADIUS1 / $RINTERVAL)+1 $PASS_COUNT = 1 REPEAT $REPS G1 Y - ($PASS_COUNT-1)*$RINTERVAL $RADIUS1 = $RADIUS1-($PASS_COUNT-1)*$RINTERVAL $PASS_COUNT = $PASS_COUNT + 1 F10 BEAMON G2 X $RADIUS1 Y 0 I 0 J -$RADIUS1 G2 X 0 Y -$RADIUS1 I -$RADIUS1 J 0 121 G2 X -$RADIUS1 Y 0 I 0 J $RADIUS1 G2 X 0 Y $RADIUS1 I $RADIUS1 J 0 BEAMOFF ENDREPEAT G1 Y +($RINTERVAL*($REPS-1)) M2 ; end of program G code #3 Milling wedge-shaped geometry Down left DVAR $WIDTH $XINTERVAL $REPS DVAR $PASS_COUNT G91 ; Set to incremental mode G71 ; Set to metric mode $WIDTH = 0.625 $XINTERVAL = 0.005 $REPS= $WIDTH / $XINTERVAL $PASS_COUNT = 1 122 ; Turn the laser on REPEAT $REPS ; Start of command block that is to be processed ten times sequentially F5 ; set feed rate to 1 units per second G1 X -$XINTERVAL BEAMON G1 Y (0.625-$PASS_COUNT*$XINTERVAL)*8 BEAMOFF G1 Y-(0.625-$PASS_COUNT*$XINTERVAL)*8 $PASS_COUNT = $PASS_COUNT + 1 ENDREPEAT BEAMOFF G1 X +($XINTERVAL*$REPS) M2 ; end of program Down right DVAR $WIDTH $XINTERVAL $REPS DVAR $PASS_COUNT 123 G91 ; Set to incremental mode G71 ; Set to metric mode $WIDTH = 0.625 $XINTERVAL = 0.005 $REPS= $WIDTH / $XINTERVAL $PASS_COUNT = 1 ; Turn the laser on REPEAT $REPS ; Start of command block that is to be processed ten times sequentially F5 ; set feed rate to 1 units per second G1 X $XINTERVAL BEAMON G1 Y (0.625-$PASS_COUNT*$XINTERVAL)*8 BEAMOFF G1 Y-(0.625-$PASS_COUNT*$XINTERVAL)*8 $PASS_COUNT = $PASS_COUNT + 1 ENDREPEAT BEAMOFF G1 X -($XINTERVAL*$REPS) M2 ; end of program 124 Appendix C Surface wettability modification by silanization grafting. (a) HO-PDMS surface Schematic of the setup for chemical reaction (b) HO-PFOS surface Schematic of the setup for chemical reaction 125 126 Appendix D Temperature PID control code (DS18B20Arduino) /********************************************************************/ // First we include the libraries #include <OneWire.h> #include <DallasTemperature.h> /********************************************************************/ // Data wire is plugged into pin 2 on the Arduino #define ONE_WIRE_BUS 2 /********************************************************************/ // Setup a oneWire instance to communicate with any OneWire devices // (not just Maxim/Dallas temperature ICs) OneWire oneWire(ONE_WIRE_BUS); /********************************************************************/ // Pass our oneWire reference to Dallas Temperature. DallasTemperature sensors(&oneWire); /********************************************************************/ float temp0 = 0; //Floating point value to hold temperature from probe 0. float temp1 = 0; //Floating point value to hold temperature from probe 1. float setTemp = 90.0; //This is the desired temperature of the heaters. int contact0 = 3; //Digital pin assignment of contactor 0. int contact1 = 4; //Digital pin assignment of contactor 1. 127 void setup(void) { // start serial port Serial.begin(9600); Serial.println("Dallas Temperature IC Control Library Demo"); // Start up the library sensors.begin(); pinMode(contact0, OUTPUT); //Assigns pin denoted by contact0 as an output pin. pinMode(contact1, OUTPUT); //Assigns pin denoted by contact1 as an output pin. } void loop(void) { // call sensors.requestTemperatures() to issue a global temperature // request to all devices on the bus /********************************************************************/ Serial.print(" Requesting temperatures..."); sensors.requestTemperatures(); // Send the command to get temperature readings Serial.println("DONE"); /********************************************************************/ temp0 = sensors.getTempCByIndex(0); //Aquires temperature of probe 0. temp1 = sensors.getTempCByIndex(1); //Aquires temperature of probe 1, 128 Serial.print("Temperature is: "); Serial.print(temp0); //Prints temperature 0. Serial.print('\t'); Serial.print(temp1); //Prints temperature 1. // Why "byIndex"? // You can have more than one DS18B20 on the same bus. // 0 refers to the first IC on the wire delay(1000); if(temp0 < setTemp){ digitalWrite(contact0, LOW); //Sets contact0 to high, turning on the heater. }else{ digitalWrite(contact0, HIGH); //Sets contact0 to low, turning off the heater. } if(temp1 < setTemp){ digitalWrite(contact1, LOW); //Sets contact1 to high, turning on the heater. }else{ digitalWrite(contact1, HIGH); //Sets contact1 to low, turning off the heater. } }
Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Li, Ri
Influence of surface characteristics on pool boiling heat transfer
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