Rheology of Non-Dilute Emulsions: A Comprehensive Review

Non-dilute emulsions are emulsions where the concentration of the droplets is high enough for the neighbouring droplets to interact with each other hydrodynamically but is still smaller than the packed bed concentration where the droplets are packed and deformed against each other. Thus, they cover a broad range of droplet concentrations. Many emulsions encountered in industrial applications fall under this category. Non-dilute emulsions exhibit rich rheological behaviour, from a simple Newtonian fluid to a highly non-Newtonian fluid, reflecting shear-thinning, shear-thickening, yield stress, viscoelasticity, etc. In this article, the rheology of non-dilute emulsions is reviewed comprehensively. Emulsions of hard-sphere-type droplets and deformable droplets, with and without surfactants, are covered. The mathematical models describing the rheological behaviour of non-dilute emulsions are discussed. The influences of electric charge and interfacial rheology on the rheological behaviour of emulsions are covered in detail. The flocculation of droplets caused by different mechanisms, such as depletion and bridging induced by additives, and their effect on emulsion rheology are investigated thoroughly. Finally, the dynamic rheology of non-dilute emulsions is discussed, covering both pure oil–water interfaces and additive-laden interfaces. The mathematical models describing the dynamic rheological behaviour of non-dilute emulsions are described. Based on the existing theoretical and empirical models, it is possible to a priori predict the rheology of non-dilute emulsions. However, serious gaps in the existing knowledge on non-dilute emulsion rheology remain. This review identifies the gaps in existing knowledge and points out future directions in research related to non-dilute emulsion rheology.