Coating has been recently considered as having good potential for use in preventing in-cloud icing on the leading edge of the lifting surfaces of an aircraft in cold climates. In terms of wettability, a coat may exhibit hydrophobicity or hydrophilicity depending on its specific properties. The same applies to the ice adhesion strength, which may be either high or low. It is thus necessary to determine which type of anti-icing or de-icing coat would be appropriate for a particular application in order to fully utilize its specific properties. Notwithstanding, a coat is incapable of preventing ice accretion by itself, and a perfect icephobic coat is yet to be developed. Coating is also sometimes applied to the surfaces of electrical heaters and load-applying machines to enable them to function more effectively and use less energy. The coating used for an electric heater, for instance, should be hydrophobic because of the need for rapid removal of molten water from the surface. The nanostructured surface of a coating of this type produces a lotus-like effect.
During in-cloud icing on the surfaces of the wings of an aircraft, minute supercooled airborne water droplets collide with the leading-edges of the wings at almost the same speed as that of the aircraft. If the wing surface is hydrophobically coated and heated by an installed heating system, the droplets would remain in the liquid state on the leading edge immediately after impingement, and then begin to move toward the trailing edge. It has been observed from icing wind tunnel tests conducted on scale airfoil models with hydrophobically coated surfaces that the molten water droplets detach themselves from the surface after attaining a particular size. In contrast, if the surface is hydrophilic, the water would move backward in rivulets along the surface. In the present study, we conducted wind tunnel tests at room temperature using circular cylindrical models to determine the precise behavior of water droplets on the model surfaces, which were variously modified for superhydrophobicity and hydrophilicity. The models were designed to ooze water from their windward surfaces and were set inside the closed test section of the wind tunnel. A high-speed camera was used to monitor how the water droplets behaved on the surfaces of the cylinders for different air speeds. The results provide some fundamental insights into the behavior of water on a coated cylindrical surface in a flow field, and would be useful for the design of coating-type aircraft ice protection systems.