Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the f irst normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. A pplication of the latter to the normal stresses requires that the droplet d iameter be known. At high shear rates the droplet diameter changes nearly i nversely proportional to the shear rate. This results in a first normal str ess difference proportional to shear rate and hence a 'normal viscosity' ca n be defined. This is used to compare the data with the available theoretic al predictions. At low shear rates deviations from a constant normal viscos ity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoela stic components have been used as well to investigate whether this would re sult in deviations from the behaviour observed for mixtures of Newtonian fl uids. (C) 1999 Elsevier Science B.V. All rights reserved.