The interpretation of forces involving deformable interfaces

Quantitative understanding of the forces acting at liquid-liquid interfaces and how they are influenced by various additives is important in many chemical engineering applications. For example, the control of emulsion stability is a major concern in many commercial products and industrial processes. The measurement of interfacial forces between two surfaces as a function of separation provides direct information on the relationship between effects of specific additives and system stability. The interaction forces between a silica colloidal particle and an immobilized hydrocarbon droplet in an aqueous medium have been measured using the Atomic Force Microscope. Force-distance relationships for an anionic surfactant have been measured over a range of concentrations. A theoretical model that calculates the force between a rigid spherical probe particle (attached to an AFM cantilever) and a liquid interface has been examined. A key parameter in this model, the electrostatic surface potential, was inferred from measurement of electrophoretic mobilities of dispersed oil droplets. The interpretation of an oil droplet's surface potential from its mobility in an electric filed is the same as for a hard particle, except that the effect of interfacial momentum transfer on the droplet drag coefficient must be considered. Although the theoretical solution for a fluid droplet's drag coefficient is well known, its experimental measurement is difficult and its interpretation is complicated due to adsorbed surfactant and the possibility of droplet deformation. Interpretation of experimental electrophoretic mobilities requires independent measurements of particle size, shape, and amount of adsorbed surfactant