Freezing delay of water droplets on metallic hydrophobic surfaces in a cold environment

In the subfreezing environment, unwanted ice formation and accretion on industrial metallic structures contribute to significant operational risks and failure. A (super)hydrophobic metallic surface can delay the water droplet freezing process on these surfaces. However, the heat transfer between the water droplet and the (super) hydrophobic surface during the freezing process is not fully understood. Most previous experimental studies were limited to droplet impact and freezing on cold surfaces at room temperatures. In this study, the anti-icing performance of stainless-steel-based hydrophobic and superhydrophobic surfaces was evaluated experimentally. The freezing delay of water droplets on the metal surfaces is measured in a cold room below the freezing point of water. Several fabrication methods, including sandblasting (S), Zinc electrodeposition (ED), stearic acid coating (SA), and their combinations, are used to achieve different water wettabilities on the metal surfaces. The results demonstrate that the superhydrophobic surfaces can significantly delay water droplet freezing, with up to a 57.47 +/- 5.22 s total freezing delay time. We also found that the freezing delay time increases when the static contact angle theta(s) of water on the sample surfaces increases. Furthermore, droplet impact from a higher distance on the same target surface leads to faster freezing. The heat transfer analysis demonstrates that a poorer wetting condition (e.g., on the superhydrophobic surfaces) contributes to a smaller final contact area A(c) of a water droplet on the surface. The average freezing rate per unit mass is approximately proportional to the final contact area. This explains the longer freezing delay on the superhydrophobic surfaces.