In situ ice growth kinetics on water-repellent coatings under atmospheric icing conditions

Superhydrophobic surfaces represent a promising avenue to mitigate icing on aircraft from micrometric Supercooled Water Droplets (SWD). In the present work, we developed a novel laser-based in situ ice growth monitoring system to investigate, under realistic icing conditions, the role of wettability on the ice growth kinetics and the icephobic performance of water-repellent coatings during SWD ice accretion. This approach records the laser beam displacement and spreading, allowing one to measure ice thickness as well as assess ice microstructural changes in real time during the ice growth. Three surfaces were studied: hydrophilic Ti-6Al-4V alloy, hydrophobic spray coating, and superhydrophobic spray coating. The ice grows instantaneously on the hydrophilic and hydrophobic surfaces as pinned droplets freeze, and ice quickly covers the surface with a continuous layer. On the superhydrophobic sample, droplets bounce before freezing delaying ice nucleation by 26 +/- 5 s. In this case, the island-mode growth is driven by hydrophilic droplet-pinning defects in the coating. When ice has formed a continuous layer the growth rate is linear and it increases with increasing hydrophobicity, 28% faster on the superhydrophobic surface compared to the hydrophilic substrate. This new approach highlights the role of wettability during ice accretion and will guide the development of new icephobic surfaces for aeronautics.