Microemulsion rheology: Newtonian and non-Newtonian regimes.

Viscosity measurements were reported for microemulsions formed with pure and commercial surfactants. Approximate viscosities were measured with the multi-phase rolling ball viscometer. Viscosities as functions of shear rate were measured on a cone-and-plate rheometer with an environmental chamber designed to control both temperature and evaporation. At a fixed shear rate or shear stress, microemulsion viscosity often goes through two maximums as the salinity of the mixing brine increases and the microemulsion passes from a water- continuous to an oil-continuous phase. Microemulsions near these maximums were found in the present study to exhibit non-Newtonian behaviour: under shear their viscosity decreases by as much as a factor of six. Away from the maximum s, microemulsion appeared to be substantially Newtonian. Shear history effects were observed for several microemulsions even though temperature, environment and experimental procedure were carefully controlled. (A)