Factors Controlling the Stability of Colloid-Stabilized Emulsions III. Measurements of the Rheological Properties of Colloid-Laden Interfaces

Finely divided insoluble solid particles constitute an important class of emulsifying agents. Colloidal particles that are partially wetted by both the aqueous and the oleic phases are capable of effectively stabilizing emulsions. We have shown in the past taht emulsion stability is controlled primarily by the concentration of particles adsorbed  at the oil-water interface. At sufficiently high concentrations of particles, colloid-laden oi0water interfaces tend to exhibit non-Newtonian behavior. In Part II of this series, we presented a model to show that colloid-laden oil-water interfaces behave viscoelastically.  Here we present an experimental setup that we have designed and built to measure the dilational rheological properties of surfactant and/or coloid-laden oil0water interfaces.  Measurements of the interfacial dilational properties of such colloid-laden oil-water interfaces are also presented. The results clearly show that oi-water interfaces containing adsorbed surfactants and/or colloidal particles exhibit viscoelastic behavior. Such viscoelastic interfaces enhance emulsion stability by increasing the magnitude of steric hindrance (i.e., the energy required to displace particles away from the drop-drop contact region) and by decreasing the rate of film thinning between coalescing emulsion droplets.