Imaging Ellipsometry/Reflectometry for Profiling the Shape of a Deformable Droplet as It Approaches an Interface

Abstract

The application of imaging ellipsometry/reflectometry to study the system of an oil droplet approaching a silica surface in a continuous aqueous medium is described for the first time. Imaging ellipsometry/reflectometry is shown to be capable of accurately determining changes in the profile of a droplet as film drainage occurs between a droplet and an interface. The technique has the advantages that it is sensitive to the measurement of both thick and thin films and yields a film profile in real time without the need to scan across the surface. It is shown that imaging ellipsometry/reflectometry provides a valuable alternative to interferometric and photometric techniques, which have previously been employed to study a system of this type. The approach of a squalene droplet to a silica surface in a continuous aqueous phase was investigated. The droplet was observed to dimple on approach to the silica plate. The shape of the droplet evolved from a dimpled to flattened profile. The equilibrium film thicknesses and film drainage data obtained were observed to be dependent upon electrolyte concentration because of double-layer interactions between the droplet and the silica surface.