Wetting

Written by Grant England AP225, Fall 2011

Introduction

Wetting refers to how well or poorly a liquid contacts a surface. Usually the term applies to water, where if a surface is hydrophobic it will not wet well while if it is hydrophilic it will wet well. The relative hydrophobicity or hydrophilicity of a substrate can be determined by measuring the contact angle of water with the surface. A liquid wets a surface better if it has a low contact angle with that surface. In general, if the contact angle is lower than 90 degrees, the liquid is considered to be wetting for that surface; while, if the contact angle is greater than 90 degrees, the liquid is non-wetting for the surface.

Diagram showing states for a liquid on a surface with (A) little wetting, (B) moderate wetting, and (C) high wetting. (Taken fromWikipedia Article )

Cassie and Wenzel States

Surfaces can be made to be superhydrophobic or superhydrophilic by modification to have high aspect-ratio structures (micro-posts) on their surfaces, and chemical modification of these surfaces with hydrophobic or hydrophilic functional groups. Depending on the energetics of the surface, a drop of liquid on such a surface can be in either of two states--sitting on top of the micro-posts or sitting with the micro-posts embedded within it. The former is the Cassie state and the latter is the Wenzel state.

Ways to Change Wetting

Surfactants can change the wetting properties of a liquid, since they change the energetics of the surface of the liquid. Also, changing the properties of the surface by chemical functionalization or modification of the surface (etching or other methods of attaining high aspect-ratio structures on the surface which increase the microscopic surface area) can change a liquid-solid interaction from wetting to non-wetting or vice versa.