Film shapes for deformable drops at liquid-liquid interfaces. II. The mechanisms of film drainage

Abstract

The light interference pattern produced by illuminating the thin liquid film of water trapped between a rising oil drop and the bulk oil/water interface with white light was observed for various sizes of oil drops for four oil/water systems. Various concentrations of a surfactant, sodium lauryl sulfate, were used in the water phase. The interference patterns are presented and are broken into mechanisms to explain the different patterns observed. These mechanisms are based on analyses of the shear stress at the bulk (surfactant-bearing) interface and of the interfacial concentration gradient of the surfactant and consequent interfacial tension gradient that this produces in the interface. The mechanisms are rapid approach of the drop to the interface, dimple formation, even drainage, uneven drainage, and film rupture. Uneven drainage is discussed in detail, with particular reference to toluene and anisole drops at the bulk interface. Measurements gave 300–500 Å as the film thickness at rupture.