Rheology of emulsions — a review

The flow properties of an emulsion are obviously among some of its more important physical attributes in either technical or aesthetic terms. Hence the ability to measure, adjust and, if possible, predict such properties is very important. The linear viscoelastic properties are first discussed, i.e. frequency-dependent storage and loss moduli and the time-dependent creep modulus. Then the non-linear, steady state properties such as shear viscosity and normal stress differences are covered, the latter being measurable with care, but because they are so small, it is concluded that for all practical purposes normal stresses — and thus overt elastic effects like die swell and the Weissenberg effect — are irrelevant for most emulsions. The basic rheology-determining parameters of an emulsion are considered next: continuous phase rheology and the nature of the drops (size distribution, deformability, internal viscosity, concentration and nature of particle—particle interaction). These are best discussed as they change the parameters of the so-called Krieger—Dougherty microstructure/ viscosity equation. In qualitative terms the rheology of emulsions is well understood, i.e. the effects brought about by various formulation and processing variables that result in a change of dispersed phase volume, size distribution and particle—particle interaction as well as particle deformability. What we do not have, however, is a very good quantitative understanding — at least compared with dispersions of solid particles — because of the lack of good model systems ranging from monodispersed to known and controlled degrees of polydispersity, together with an independent change in particle deformability. Until this happens, emulsion rheology will continue to be the study of multifunctional effects with best guesses at effects found. For instance, we shall not be able easily to distinguish between the effects of particle deformation and particle-size-distribution width.