An Imaging Reflectometry Study of the Effect of Electrolyte on the Drainage and Profile of an Aqueous Film between an Oil Droplet and a Hydrophilic Silica Surface

Abstract

Imaging reflectometry was used to study the effect of electrolyte concentration on the profile and drainage of an aqueous film confined by an oil droplet and a hydrophilic silica surface. Droplets were observed to adopt a dimpled profile at the film's center on approach to the silica surface due to hydrodynamic effects. The intervening film eventually evolves to an equilibrium film of uniform thickness stabilized by the double layer repulsion between the drop and the silica surface, which also determines the film thickness at equilibrium. The range over which double layer interactions are detectable is much larger than measurable by mechanical force measuring techniques such as SFA and AFM, indicating the high sensitivity of this system to the detection of surface interactions. In effect, the deformable oil/water interface acts as an infinitely weak spring. The drainage rate at the film's periphery (barrier ring) is controlled initially by hydrodynamic effects. However, as the film thins and double layer overlap occurs, the drainage rate becomes hindered, since double layer repulsion acts to stabilize the draining film, thus opposing drainage. Drainage at the film's center is driven by the Laplace pressure across the oil/water interface while spreading of the film at the barrier ring leads to a large pressure drop across the film from its center across the barrier ring and hence hindered fluid flow.