A Brief Consideration of Hydrophilicity and Hydrophobicity: A Simplified Derivation of Young, Wenzel, and Cassie–Baxter Equations

Abstract

Superhydrophobic surfaces show high water repellency, as they are not wetted by water (in contrast, superhydrophilic surfaces are wetted completely). The most important requirements for preparing a superhydrophobic surface are the combination of the material's surface properties and its topography, i.e., surface roughness. In this communication, we present a simplified derivation of fundamental equations that govern hydrophobicity/hydrophilicity and explain when and why roughness increases the hydrophobicity or the hydrophilicity of surfaces. Instead of free-energy arguments usually used in such derivations, we rely on forces and work to make the derivations more accessible to high-school students.

Keywords: (super) hydrophobicity, (super) hydrophilicity, Young’s equation, Wenzel model, Cassie–Baxter mode