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Abstract

Water droplets may remain in a liquid metastable state when cooled to temperatures below 0 °C. Organic materials, such as proteins, non-proteinaceous biopolymers, or plastics, can initiate the freezing of supercooled droplets at various temperatures, ranging from –2 °C to –25 °C. Hence, these organic materials can impact the phase state of atmospheric clouds, the formation of ice crystals in biological organisms, and the ice formation in the aviation and cryo-preservation industry. The mechanisms of ice nucleation induced by organic materials remain poorly described, particularly regarding the specific sites where nucleation originates and the role of hydrophobic interfaces, such as the air-water interface or the plastic-water interface. These uncertainties limit our ability to predict the ice nucleation temperatures of organic matter in aerosol-cloud interactions, biological organisms, and technological applications. This thesis addresses this knowledge gap by (i) applying high-speed cryo-microscopic imaging to identify the onset locations of freezing in aqueous droplets, (ii) characterizing the size of ice nucleating nanoparticles with a microscopic scattering technique, and (iii) analyzing surface properties of materials (topography and hydrophobicity) with atomic force microscopy and contact angle measurements. The results showed that bacterial proteins triggered freezing at hydrophobic interfaces, such as the air-water interface of the droplet or the bacterial membrane. In contrast, non-proteinaceous biopolymers formed nanoparticles, which triggered freezing immersed in aqueous droplets. The dominant ice nucleation onset location for plastic materials was at the plastic-water-air contact line. Based on the mechanistic understanding, surface modification techniques were applied to influence the surface hydrophobicity and, therefore, the freezing temperatures of plastic interfaces used for ice nucleation experiments. In addition, ice nucleating materials were found to be an innovative tool in the field of point-of-care diagnostics to detect biomarkers using the freezing of water as an easily detectable signal. Overall, this thesis adds fundamental knowledge of the role of hydrophobic interfaces, such as the air-water interface of droplets, the nanoparticle-water interface, or the plastic-water interface to our understanding of heterogeneous ice nucleation induced by organic materials and extends the list of potential applications of ice nucleating particles with point-of-care diagnostics.