Sol-gel derived icephobic coatings for large outdoor structures
Date of Issue2016-02-10
School of Materials Science and Engineering
Ice accretion possesses serious challenges for maintaining the operation and performance of large outdoor structures in cold climate. Icephobic materials have attracted much attention recently because they have demonstrated good promise in mitigating the problems. Superhydrophobic materials provide a viable option for the development of icephobic materials, as they have several advantages including slower heat transfer and lower sliding angle, which could potentially be exploited for decreasing the icing temperature, increasing the icing lag time, and reducing the ice adhesion strength, etc. However, based on the literatures, not all superhydrophobic materials show icephobic properties. The effect of surface energy, roughness, morphology and condensation should be carefully considered. In this report, investigation of ice nucleation behaviour and ice adhesion has been presented. The commonly adopted anti-icing tests and materials system for anti-icing applications were summarized based on existing literatures. In order to investigate the effect of surface properties on icephobicity, several series of samples were prepared by mechanical grinding and spraying sol-gel coating. The surface energy, roughness, wettability at different temperatures, ice adhesion and icing temperatures were measured. It was found out that the intrinsic surface energy and surface morphology of the coatings play important roles together in terms of anti-icing performance. Not all superhydrophobic surfaces, as indicated by the water contact angle at room temperatures, could remain water repellent with reduced ice adhesion at low temperatures. The anti-icing performance are closely correlated with the surface properties at lower temperatures instead of that at room temperature. The two aspects of the anti-icing performance, i.e., ice adhesion and icing temperature, are not necessarily correlated on rough surfaces because of the different mechanism for them. It depends on the size and surface energy of the added particles in the coatings. An optimized icephobic coating was achieved by a fluorinated sample F-16 which can maintain superhydrophobic at sub-zero temperatures. The effect of different test environments on the icing behaviour of droplets on different samples was also investigated. It was found out that low humidity would lead to higher contact angle especially on hydrophobic or superhydrophobic surfaces that has a hydrophilic component. This would also lead to a decrease of icing temperature on these surfaces because of restrained condensation at the adjacent of the three-phase contact line. For those samples which are hydrophilic or intrinsically hydrophobic, the influence of humidity is not so much when cooling from the bottom of the water droplets. However, all the icing temperatures are lowered to a great extent by isothermal cooling without airflow in the environmental chamber because of humidity was lowered even further. When the droplets are exposed to shear airflow, the icing temperatures would be greatly increased because of the turbulence it causes. The trend of icing temperatures with airflow is that the higher the contact angle, the higher the icing temperature. When it comes to icing lag time, the repeatability and reliability of the measurement is strongly dependent on individual samples and the icing temperatures, and thus render it unsuitable for measurement in current study. The icing temperatures of 10 L droplets on selected series of sol-get coatings which cover a wide range of roughness, apparent surface energy, and different wetting modes at low temperatures were statistically investigated. The results are in good agreement with the wetting behaviour of the samples at low temperatures. Through analysis based on classical nucleation theory, the geometric factors were found out to be closely related to the surface property and wetting mode. The pre-factors for the nucleation on the samples was found to be closely related to the geometric factors. For most samples, the lower the geometric factor, the lower the pre-factor. Our fitting results support the conclusion that the dominant heterogeneous nucleation sites in our study are not on the whole water-substrate interface, but mainly on the water-air-substrate three-phase contact line. Finally, based on the investigation and conclusion from the study, a future work plan was proposed including a more thorough investigation on the effect of test environments on anti-icing performance, a more direct observation of the contact line nucleation, a field test for the icephobic coating, and the investigation of materials with precisely controlled surface properties.