Abstract
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 that emulsion stability is controlled primarily by the concentration of particles adsorbed at the oil-water interface. At sufficiently high concentrations of particles, colloid-laden oil-water 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 colloid-laden oil-water interfaces. Measurements of the interfacial dilational properties of such colloid-laden oil-water interfaces are also presented. The results clearly show that oil-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.
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