Rheology of non-Brownian suspensions: a rough contact story

Abstract

This paper presents a compilation of experimental and numerical results that both show that particles in a suspension under shear flow come into contact through surface roughness. Accounting for those frictional contacts captures well the measured viscosity values as well as the shear-thinning observed in many concentrated non-Brownian suspensions. More specifically, the precise analysis of the asymmetric microstructure of the suspension clearly shows the role played by surface asperities in the interparticle contact. Numerical simulation results are then recalled and show the major impact of solid friction between particles on the viscosity. It is then pointed out that shear reversal experiments are a good way to assess this impact and to deduce useful information on the frictional properties of contacts. Finally, when considering together contacts via asperities and the close link between friction and rheology, a natural explanation for the shear-thinning observed in most suspensions can be proposed. According to it, as the shear stress increases, the friction coefficient decreases which leads to a viscosity decrease. This scenario first implemented in numerical simulations is verified experimentally through coupled AFM and rheometry measurements.

The asymmetry of the shear-induced microstructure of non-Brownian and non-colloidal suspensions reflects the presence of solid contacts between particles, enabled by the surface asperities of the particles. The peculiar characteristics of these asperity contacts open the way to explain the shear-thinning observed for many suspensions by a decrease in the friction coefficient with increasing shear stress.