Bouncing behavior of a water droplet on a super-hydrophobic surface near freezing temperatures

Super-hydrophobic surfaces (SHSs) have demonstrated outstanding capability to mitigate the problem of ice nucleation in various engineering applications. Hence, experiments were conducted to understand the bouncing behavior of a water droplet on SHS under the condition of large temperature difference between the SHS and the impinging water droplet. The SHS was fabricated by spray-coating of a suspension of acetylene black and trimethylsiloxysilicate on a copper substrate. The ice-phobic nature of SHS was evaluated for a wide range of SHS temperature maintained around the freezing point of water (283 K–248 K). In addition, the effects of increasing the droplet temperature and droplet size on the droplet rebounding behavior were also analyzed. To evaluate the dynamics of water droplet during impact, the contact time and rebounding height were measured and employed as performance parameters. The results revealed that either by lowering the SHS temperature (from 283 K to 248 K) or by increasing the droplet temperature (from 278 K to 293 K) or droplet diameter (from D_S = 2.07 mm to D_L = 2.99 mm), an increase in the droplet-SHS contact time or a decrease in rebounding height were observed. Above findings were elucidated by the fact that when an impinging water droplet of relatively higher temperature came into contact with an SHS maintained at much lower temperature, evaporation took place from the droplet and at low temperature of SHS, the empty space inside the micro/nano surface textures of SHS was filled due to the condensation of evaporated water vapor. Thus, water bridges were formed within the micro/nano surface cavities and the wetting transition from the Cassie-Baxter state to the Wenzel state took place and the super-hydrophobicity of the surface could no longer be preserved. To overcome the adhesion between the impinging droplet and water bridges within micro/nano surface cavities, a larger amount of kinetic energy was spent during the droplet rebound. Consequently, for an impinging droplet, a noticeable decrease in the residual energy was observed.