Equilibrium thickness of large liquid lenses spreading over another liquid surface.

Abstract

This article discusses the equilibrium states and more particularly the equilibrium thickness of large lenses of a liquid spread over the surface of a denser liquid. Both liquids are supposed to be nonvolatile and immiscible. Taking into account the effect of intermolecular forces in addition to the sign of the spreading parameters leads to four possible states. The three first are similar to the states of equilibrium of a liquid spread on a solid surface: total wetting where the floating liquid spreads until it reaches an equilibrium thickness on the order of the molecular size, partial wetting where the floating liquid forms a lens of macroscopic thickness in equilibrium with a "dry" bath, and pseudopartial wetting where the floating liquid spreads as a lens of macroscopic thickness in equilibrium with a thin film covering the bath. The last regime, called pseudototal wetting, consists of a macroscopic lens of the floating liquid covered with a thin film of the bath. These four regimes are described through a free-energy minimization, and their equilibrium thicknesses are predicted. A comparison of this model with experimental results available in the literature and dedicated experiments for the pseudototal wetting state are reported.