Roughness and topology of ultra-hydrophobic surfaces

Ultra-hydrophobic surfaces have considerable technological potential for various applications due to their extreme water repellent properties. When water contacts such surfaces, contact angles approaching 180° may be obtained. These surfaces owe their unique wetting properties to a subtle interplay between surface chemistry and mesoscopic topological structure. In this paper, we discuss the requirements regarding surface roughness for ultra-hydrophobicity. We demonstrate by numerical calculations of contact angles on idealized surfaces that the topological nature of the surface roughness has a major influence on the water repellency of hydrophobic surfaces. In order to validate this theory, we present an electrochemical method to experimentally determine important model parameters, characterizing the solid/liquid interface. Furthermore, we substantiate our theoretical findings with experimental data on contact angles on hydrophobic surfaces with well-characterized surface roughness and topology.