INTERFACIAL VISCOELASTICITY IN EMULSIONS AND FOAMS

Both the generation and the stability of emulsions and foams depend on interfacial properties induced by adsorbed surfactants. The essential function of surfactants during emulsification and foaming is not that they reduce the equilibrium interfacial tension, but they impart specific dynamic properties to the interface. A surfactant-covered interface behaves as a two-dimensional body with its own elasticity and viscosity, which are related to the non-equilibrium values of the interfacial tension. Such viscoelasticity is a significant factor in any liquid flow near the interface, and in the stability of the thin films separating emulsion drops or foam bubbles. Numerically, values of the viscoelasticity vary over a wide range, depending on surfactant parameters, on rate of surface deformation and on any relaxation processes driving the interfacial tension to its equilibrium. A quantitative framework is available for the evaluation of these rheological parameters, and of the dynamic surface tension of surfactant solutions, if the relaxation mechanism is diffusional interchange between surface and solution. We review both theoretical and experimental information on the dynamic surface behaviour of surfactants, and indicate possible routes to a quantitative model for their functionality in food emulsions and foams.