Fundamental principles of emulsion rheology and their applications

This review starts with an introduction summarizing the factors that affect the flow characteristics (rheology) of emulsions. These factors are the volume fraction of the disperse phase, the viscosity of the disperse droplets, the droplet size distribution, the viscosity and chemical composition (pH, electrolyte concentration, etc.) of the medium, the interfacial rheology of the emulsifier film and the concentration and nature of the emulsifier. A section on interfacial rheology describes the basic equations and the methods that may be applied to the measurement of interfacial elasticity and viscosity. This is followed by a discussion on the correlation of interfacial rheology with emulsion stability. Examples are given showing the correlation between interfacial viscosity and elasticity with emulsion coalescence. The second part of the review deals with the bulk rheology of emulsions. It starts with a section on the viscosity-volume fraction relationship, which clearly showed that emulsions stabilized with surfactant films behave like hard sphere dispersions provided the adsorbed layer thickness is much smaller than the droplet radius. The viscoelastic properties of concentrated sterically stabilized oil-in-water (o/w) and water-in-oil (w/o) emulsions are then described. These emulsions show a transition from predominantly viscous to predominantly elastic response as the frequency of oscillation exceeds a critical value. This allows one to obtain the relaxation time of the emulsion system, which was shown to increase with increase in the volume fraction φ. Plots of the complex modulus (G*), the storage modulus (G′) and the loss modulus (G″) showed a transition from predominantly viscous to predominantly elastic response at a critical φ value. This reflected the steric interaction which increased with increase in φ. When the surface-to-surface separation between the droplets became smaller than twice the adsorbed layer thickness, the steric repulsion became strong, increasing in magnitude with increase in φ. The last part of the review describes the viscoelastic properties of weakly flocculated emulsions. These were obtained by addition of a "free" (non-adsorbing) polymer to a sterically stabilized emulsion (depletion flocculation). Again the viscoelastic behaviour reflected the weak attraction between the droplets in the emulsion. Above a critical volume fraction of free polymer (which decreased with increase of the molecular weight of the polymer) there was a rapid increase in the storage modulus and this determined the onset of depletion flocculation. The results obtained were similar to those obtained with suspensions, thus showing that viscoelastic measurements can be applied to investigate the interaction between emulsion droplets in the same manner as for suspensions.