Rheology of Emulsions

Johann Zank and John White

Emulsions can be considered as complex fluids consisting of two immiscible phases, of which one is dispersed in the form of droplets in the other. Normally emulsions contain also a third component, an emulsifying agent. The two functions of the emulsifier are the decrease of the interfacial tension between the two phases, facilitating emulsion formation, and the stabilization of the dispersed phase towards coalescence. We are interested in the characterisation of high internal phase water-in-oil emulsions stabilized by novel polymeric surfactants. From the rheological behaviour of emulsions we can obtain information on their microstructure and stability. Flow properties of emulsions are determined by the continuous phase rheology and the droplet nature. The droplet nature is determined by size distribution, deformability, internal viscosity, concentration, and droplet-droplet interactions. These factors are significantly influenced by the surfactant used in the emulsion preparation process. The observed flow characteristics offer the possibility to rate the surfactants used in terms of their performance and can have a significant impact on the development of new surfactants.

For the temperature controlled rheology measurements a Brookfield RVDV-II+ cone/plate viscometer is used (see picture). A great advantage of this instrument is the fact that a very small sample volume (0.5 ml) is required for the rheological evaluations. Various cone spindles, defining the accessible viscosity range, are available. A CPE-52 cone spindle is used for standard measurements in wide viscosity ranges (e.g. maximum viscosity range: 49.2-983,000 mPa s at 0.1 rpm).