ABSTRACT
Emulsions and foams are complex fluid dispersions generally formed and stabi lized by adsorbing surfactants, polymers, proteins, solid particles, or their mix tures onto fluid-fluid interfaces. These fluid dispersions containing small droplets or bubbles possess large specific surfaces. Surfactants stabilize such systems largely by imparting an intrinsic rheological behavior to the fluid interfaces, with respect to both its viscous and elastic natures. At surfactant concentrations below or around the critical micelle concentration (cmc), the adsorption of surfaceactive molecules on the film surfaces and the properties of the adsorbed layers control the drainage and stability of the film separating the droplets or bubbles. The lifetime (stability) of foam or emulsion film is determined primarily by its rate of thinning. The thinning rate and stability of the thin lamella are governed by the hydrodynamic and thermodynamic interactions between the two film sur faces. The first stage of film thinning, at a thickness greater than 100 nm, is determined by hydrodynamic interactions, which are greatly influenced by the deformation and mobility of the surfaces. Consequently, the interfacial and film rheological properties play a significant role. When the film has thinned to less than 100 nm, thermodynamic interactions begin to dominate.
