Effect Of Surface Mobility On The Dynamic Behavior Of Thin Liquid Films

Abstract

The role of various factors such as bulk and surface diffusion of surfactant, surface viscosity and, in the case of emulsions, liquid flow in the drops, in determining the surface mobility and hence the velocity of thinning and the lifetime of liquid films are discussed. It is shown that: 1) With relatively low concentrations of surfactant soluble in the continuous phase surface diffusion plays a dominant role. 2) The velocity of thinning of a plane-parallel emulsion film depends strongly on the partition coefficient of the surfactant: when the surfactant is soluble in the drops rather than in the continuous phase it has no influence on the surface mobility and the velocity of film thinning is very high. 3) With surfactant free emulsions or with surfactant soluble in the drops the dissipation of energy in the film is negligible so that the velocity of thinning does not depend on film viscosity. 4) Surface mobility increases the amplitude of capillary waves in the films but does not affect the critical thickness of rupture when surface diffusion and surface viscosity can be neglected; the allowance for the latter two effects may help in explaining the experimentally observed dependence of the critical thickness of rupture upon surfactant concentration. © 1980, Walter de Gruyter. All rights reserved.