Liquid surfaces: theory of surface tension

Abstract

The theory of equilibrium liquid surfaces is reviewed from the macroscopic (thermodynamic) and microscopic (statistical) points of view. Although emphasis is placed on the surface tension, other surface thermodynamic quantities are treated, especially the contact angle and work of adhesion. The Gibbs scheme is used to describe the surface thermodynamics. The equivalent hydrostatic approach, the curvature dependence of the surface tension and its relation to other surface quantities are presented. The effect of solid surface topology on the contact angle is discussed, together with the controversial Young's equation. Good's and Fowkes' theories are described and criticised from the statistical point of view. The surface tension theories for simple liquids (Kirkwood and Buff), simple metals (Evans) and solid-fluid interphases (Navascues and Berry) are deduced from a single statistical mechanical formalism. The van der Waals theory of the liquid vapour interphase is presented. Thermodynamic perturbation theory for the interphase (Toxvaerd, Abraham) and the analysis of the free energy density as a functional of the interphase density (Yang, Flemming and Gibbs) are discussed.