| dc.description.abstract | Ice formation on surfaces imposes numerous challenges in the industry and everyday life, ranging from icy windshields to airplanes needing de-icing before take-off. Today, the main methods to mitigate these issues revolve around removing already formed surface ice, using heating, chemicals, mechanical removal or a combination of these. However, these methods are often costly, energy-consuming and hazardous. Thus, this has become a growing field of interest in science, with numerous studies on potential anti-icing surfaces and coatings. However, ice formation is a complex phenomenon still not fully understood and with no complete theory to predict ice formation on different surfaces. The most commonly used theory, the classical nucleation theory (CNT), fails to predict the ice nucleation in some cases, especially concerning textured surfaces. Hence, a modified nucleation theory has been proposed in recent studies, in which the existence of a quasi-liquid layer (QLL) in the ice embryo-surface interface might explain deviations from the CNT. The aim of this thesis is thus to investigate and increase our understanding of ice formation on sapphire surfaces with the basis of the newly proposed modified nucleation theory. Sapphire has been chosen for its excellent durability and usability in optical applications and lenses, making it a relevant material for the fluorinated graphene coating under development at UiB. Two different surface finishes are investigated and compared, micro-scaled rough and smooth epipolished and an additional smooth silica surface as a comparable reference. The investigations are performed macroscopically with contact angle and freezing onset measurements and microscopically with atomic force microscopy (AFM) and freezing out layer-by-layer nuclear magnetic resonance (NMR) spectroscopy, using the more rare flat coil probe head. The freezing onset measurements showed a promoting freezing effect for sessile droplets on rough sapphire (RMS 120-180nm) versus the smooth sapphire (RMS 0.13-4.3) and smooth silica (RMS 0.59-4.3), at a relative humidity of 20-60 % and with different washing procedures. The results show that roughness impacts the freezing temperatures while the surface wetting properties were more inconclusive. However, based on uncertainties in the curvature analysis, it cannot be concluded if this is in line with CNT. The flat coil 1H-NMR measurements performed on double-plated sapphire samples were inconclusive in detecting a QLL, due to the interference of background noise signals. This, together with freezing temperature differences, means that the thesis did not find results that substantiate the modified nucleation theory. However, the NMR experiments did show interesting results of gradual freezing of water near the freezing temperature for rough sapphire and smooth silica samples. | |
