Reversible Aggregation and Nonlinear Rheology of Fine Suspensions

Abstract

Abstract In this paper, we develop a tentative rheological model for finely dispersed systems whose particles are capable of forming stable aggregates under shear flow conditions. The main achievement of this paper consists in putting forward a physical model for an aggregative structural transition in a flowing dispersion. When shear rate reaches a certain threshold value, rapid aggregation occurs, the aggregates originate as a result of reversible coagulation of particles in a secondary potential minimum characteristic of the molecular interparticle interaction. The aggregated system consists of an assemblage of aggregates that contain many particles and that are suspended in an ambient media that represents a suspension of single individual particles. Aggregation terminates and aggregates disappear when shear rate exceeds another critical value. The aggregated system effective viscosity is determined by the volume concentration of the aggregates and by the volume concentration of single particles in the ambient suspension within the interstices. Since these concentrations, as well as the aggregate size, depend on the shear rate, the effective viscosity is a nonlinear function of shear rate. Our model offers an opportunity to describe both shear-thickening and shear-thinning of aggregating dispersions within the frames of a unified approach.