THERMODYNAMICS OF CONTACT-ANGLE PHENOMENA IN THE PRESENCE OF A THIN LIQUID-FILM

The effects of a thin liquid film on contact angles are studied using a simplified thermodynamic model. (in this model, the small transition zone between the liquid-vapour interface and the flat thin liquid film is neglected). A set of mechanical equilibrium conditions have been derived for contact angle systems with a flat thin liquid film. The equilibrium condition at the three-phase intersection explicitly predicts the effects of the film tension, the disjoining pressure and the film thickness, on contact angles. The number of degrees of freedom for a two-component solid-liquid-vapour surface system with a flat thin liquid film is shown to be three, implying the existence of an equation-of-state-type relationship among the solid-liquid interfacial tension, gamma-sl, liquid surface tension, gamma-l-nu, the disjoining pressure, PI, and the film tension, gamma-f. An approximate, explicit form of such an equation of state has been derived. The combination of this equation of state with the equilibrium condition of the the three-phase intersection can be used to estimate the film tension, gamma-f, and the solid-liquid interfacial tension, gamma-sl, from the measured data for the vapour pressure, P-nu, the film thickness, h, the curvature of the liquid-vapour meniscus, J, the liquid surface tension, gamma-l-nu, and the contact angle, theta. The effect of the thin film on the drop-size dependence of contact angles is also investigated and found to be negligible.