DYNAMIC FILM AND INTERFACIAL-TENSIONS IN EMULSION AND FOAM SYSTEMS

In concentrated fluid dispersions the liquid films are under dynamic c onditions during film rupture or drainage. Aqueous foam films stabiliz ed with sodium decylsulfonate and aqueous emulsion films stabilized wi th the nonionic Brij 58 surfactant were formed at the tip of a capilla ry and the film tension was measured under static and dynamic conditio ns. In the stress relaxation experiments the response of the film tens ion to a sudden film area expansion was studied. These experiments als o allowed the direct measurement of the Gibbs film elasticity. In the dynamic film tension experiments, the film area was continuously incre ased by a constant rate and the dynamic film tension was monitored. Th e measured film tensions were compared with the interfacial tensions o f the respective single air/water and oil/water interfaces, which were measured using the same radius of curvature, relative expansion, and expansion rate as in the film studies. It was found that under dynamic conditions the film tension is higher than twice the single interfaci al tension (IFT) and a mechanism was suggested to explain the differen ce. When the film, initially at equilibrium, is expanded and the inter facial area increases, a substantial surfactant depletion occurs insid e the film. As a result, the surfactant can be supplied only from the adjoining meniscus (Plateau border) by surface diffusion, and the film tension is controlled by the diffusion and adsorption of surfactant i n the meniscus. The results have important implications for the stabil ity and rheology of foams and emulsions with high dispersed phase rati os (polyhedral structure). (C) 1997 Academic Press.