Wetting on Heterogeneous Surfaces

Depending on the liquid-surface affinity, a drop can either spread into a film (complete wetting), or form a spherical cap (partial wetting) on the surface. This chapter deals with the basics of wettability on heterogeneous surfaces. It first introduces the Young relationship describing the equilibrium contact angle of a droplet on an ideal, smooth surface. The chapter then describes how surface chemical and physical heterogeneities have an impact on the wetting properties. It focuses on the notion of contact angle hysteresis and the classic models from Wenzel, Cassie and Baxter. The latter describe how roughness can be used to amplify the wetting properties of a material, leading ultimately to a so-called fakir regime which has been the cornerstone in the development of super non-wetting surfaces.