Equilibrium contact angles and film thicknesses in the apolar and polar systems: role of intermolecular interactions in coexistence of drops with thin films

Abstract

A stable coexistence (or the lack of it) of a macroscopic drop with ita thin films on a substrate depends on the apolar (Lifshitz-van der Waals) and polar interactions near the contact zone. The augmented Young-Laplace (AYL) equation of capillarity incorporating the apolar and apolar interactions gives conditions for this coexistence, and the resulting equilibrium contact angles. The macroscopic (observed) contact angle is shown to depend only on the free energy of coexisting flat thin film, notwithstanding a significant curvature in the contact zone. The free energy of the film and, consequently, equilibrium contact angle are then shown to be related to the macroscopic parameters of wetting by a modified Young-Dupre equation with film pressure. For systems that are completely apolar, completely polar, or when components of the spreading coefficient, S, are of the same sign, the film pressure vanishes for S<0, and a spread thin film results for S>0. In the later case, the film thickness and ita edge morphology are determined from the AYL equation and conservation of initial volume. Conditions engendering the film pressure and ita magnitude are identified when the apolar and polar spreading coefficient are of opposite signs. An interesting possibility is that macrodrops of finite angles can coexist with thin films even when the total spreading coefficient is positive.