Endo, H., Kimura, S., Hasegawa, M., Tanaka, T. et al., "Analysis of Minute Water Droplet's Freezing Process on Coated Surface," SAE Int. J. Aerosp. 6(2):465-474, 2013, doi:10.4271/2013-01-2177.
Unlike the conventional bleed-air method, using electro-thermal anti-/de-icing methods to completely evaporate all of the supercooled water droplets that collide with the leading edge wing surface of aircraft flying in a freezing environment is not easy in terms of technical feasibility and energy efficiency. If the leading edge is warm enough to stay free from frozen water droplets, the water moves backward while still maintaining the liquid phase. The droplets may freeze somewhere on an unheated surface after being halted for some reason and stick on the surface. Ice gradually accumulates as this process is repeated. Therefore, liquid water must be removed from the surface as soon as possible if the electrothermal method is employed for icing prevention. One answer to this problem is coating the surface with a superhydrophobic paint. Since the contact angle of a minute water droplet on a superhydrophobically coated surface becomes higher than 150° according to the definition, the water droplet becomes spherical. If the surface is inclined or the droplet receives an external force, the droplet moves along the surface with a rotational motion. When the electrothermal method is employed to prevent aircraft icing, the surface of the body of interest should be superhydrophobic in order to quickly remove liquid water. This type of icing prevention is called the electrothermal-coating method. To mechanize this concept into a device, correctly determining droplet behavior is important. In the present study, the freezing process of a minute water droplet moving on a hydrophobically coated surface in a sub-zero environment was analyzed experimentally. A simplified two-dimensional numerical droplet model was developed to calculate the temperature change inside the droplet.