Comprehensive Rheology of Dilute Emulsions

The rheology of dilute emulsions is reviewed comprehensively. The fundamental equations governing the flow fields inside and outside the droplets are discussed, along with the boundary conditions. The rheological constitutive law for dilute emulsions with pure interfaces characterized by interfacial tension is developed using the flow field external to the droplets. Both zero-order and first-order deformations of droplets are considered. Dilute emulsions exhibit non-Newtonian behavior. The influences of surface charge and surfactants on the emulsion rheology are covered in detail. The rheology of emulsions of double droplets and droplets covered with elastic membranes is covered as well. Finally, a significant section of the review is focused on the dynamic rheology of dilute emulsions. Emulsions with pure interfaces and additive-laden interfaces are considered. The theories developed for the dynamic rheology of emulsions consisting of different types of interfaces are reviewed, including purely viscous interfaces, purely elastic interfaces, viscoelastic interfaces, and interfaces possessing bending rigidity. In general, the theory for dilute emulsion rheology is well developed. Our current understanding of dilute emulsion rheology is good from a theoretical point of view. A priori predictions of dilute emulsion rheology are possible using the existing theories. However, serious gaps in the existing knowledge on dilute emulsion rheology remain. This review identifies the gaps in existing knowledge and points out future directions in research related to dilute emulsion rheology.