Rheology of Emulsions - The Relationship to Structure and Stability

Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK 

Even simply for the purpose of classifying rheological behaviour, it is convenient to distinguish between flocculated and non-flocculated systems. This is because, firstly, the theoretical position is much more well developed as a function of oil volume fraction for dispersed (non-flocculated) systems than for flocculated ones, an4 secondly, the experimental behaviour of many emulsion systems can be interpreted most effectively at the mechanistic level from a detailed consideration of the type and extent of flocculation. Much of the experimental work published recently on emulsion rheology has been concerned with the role of water-soluble polymers in controlling the structure and stability of flocculated systems. Of particular importance in such systems is the viscoelasticity of the polymer-containing aqueous continuous phase and the nature of the interaction between polymer and emulsion droplets. 

This chapter first reviews general theoretical concepts and then proceeds to discuss relevant experimental results. It is assumed that readers are already reasonably familiar with basic rheological principles and techniques as set out in various texts. Any review of this sort, of course, has some inevitable bias in coverage. In this instance it is towards the rheology of milk protein-stabilized emulsions—a topic of particular interest to the author. 

Theoretical treatments of emulsion rheology are less well developed than those devised for particulate dispersions. It therefore seems appropriate to give a brief survey of classical work on particulate dispersion rheology as a prelude to discussing the rheology of dispersions of droplets. A further justification for this approach is the observation that, in practice, many a fine emulsion system can be reliably treated in rheological terms as if it were a dispersion of solid spherical particles. 

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There is no shortage of published experimental data on emulsion rheology. In terms of understanding the relationship of rheological data to structure and stability, however, some studies are more informative than others. The examples selected for consideration below are taken from the recent literature with the specific purpose of illustrating some of the key theoretical issues mentioned above. 

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