HYDRODYNAMIC THEORY FOR SPONTANEOUSLY GROWING DIMPLE IN EMULSION FILMS WITH SURFACTANT MASS-TRANSFER

In this paper we propose a theory for the recently discovered phenomen on of spontaneous cycling dimpling. The latter is responsible for the high stability of non-equilibrium aqueous emulsion films when surfacta nt is transferred across the interfaces toward the surrounding oil pha ses. Our treatment is based on the lubrication approximation for the t hin film hydrodynamics, with account for the surfactant Awes due to co nvection and diffusion. A fourth-order differential equation is derive d for the time evolution of the surface shape. This equation is solved numerically, and the results are compared with experimental profiles (determined by means of interferometry). Very good agreement is observ ed. It is proved that the surface viscosity and the surface diffusion are insignificant for the dimple growth. Quite essential is the Marang oni effect: the exhaustion of surfactant from the region around the fi lm center gives rise to gradients of the interfacial tension which set the interface into motion, and consequently, liquid is dragged into t he film, feeding the dimple. (C) 1997 Academic Press.