Effect of Surface Structure on the Sustainability of an Air Layer on Superhydrophobic Coatings in a Water−Ethanol Mixture

Using a laser beam and its reflection, we evaluated surface air layers on two superhydrophobic coatings with different roughnesses (Ra = 1900 and 74 nm) in a water-ethanol mixture. The reflected laser intensity decreased by increasing the ethanol concentration for the superhydrophobic coating with large roughness because of increased scattering by the surface structure of the solid. However, it was almost constant and slightly increased for the coating with small roughness, probably because the liquid-air interface becomes flatter as the liquid intrudes into the surface structure. Results demonstrated that the difference in surface structure provides different sustainability of the surface air layer.