Reprinted from I&EC FUNDAMENTALS, Vol. 5, Page 379, August 1966 Copyright 1966 by the American Chemical Society and reprinted by permission of the copyright owner GRAVITATIONAL THINNING OF FILMS Effect of Surface Viscosity and Surface Elasticity THE role of surface active agents in stabilizing foams and emulsions is known to be complex (2, 6, 11, 15). Surface viscosity, surface elasticity, electrical forces, evaporation, and marginal regeneration are all governed by the adsorption of surface active agents at the gas-liquid interface, and each is capable of playing an influential role in stabilizing a foam. In this paper the effect of surface viscosity and surface elasticity on the drainage rate of a single free film is analyzed in the absence of electrical repulsive forces between the two surfaces, marginal regeneration, and evaporation. The calculated drainage rates cannot be valid for the latter STEPHEN WHITAKER Department of Chemical Engineering, University of California, Davis, Cali stages of film thinning where the electrical properties of the surface provide the major stabilizing force (6), and the neglect of marginal regeneration [which enhances the film thinning process by "pumping" fluid to the plateau border (75) 1 and evaporation requires that the calculated results be considered as a lower bound on real film-thinning rates. Discussion The analysis of the drainage of a single free film such as that indicated in Figure 1 requires application of: the laws of mechanics for both the bulk and surface fluids, conservation of VOL. 5 N O. 3 AUG U S T 1 9 6 6 379