Wetting: Gibbs’ superficial tension revisited

According to Gibbs the surface of discontinuity between a solid and a fluid phase could be split into two parts: one of them belongs to the solid and the other belongs to the fluid. Correspondingly, ω, the work of forming of the interface could be represented as a sum of ωs, the work of forming of the solid surface in vacuum and ρ, the work of forming of the fluid part of the interface. The quantityρ, called by Gibbs superficial tension of the fluid in contact with solid can be treated either as energy or as force like the surface tension of liquids.We have applied this Gibbs’ approach to the case of thin liquid film formed on plane solid surface. Thus, a new quantity in the wetting thermodynamics: superficial tension of the film in contact with solid ρf is introduced. The conditions of mechanical equilibrium in the case of partial wetting are examined. Two contacts angles, θ and θf, are distinguished. The contact angle θ is determined by the well-known Young equation, obtained according to Gibbs by energy minimization approach. The contact angle θf manifests the action of the long-range interaction forces in the thin wetting film.