Elisabeth Lemaire - InPhyNi, UCA- CNRS - Keywords: Suspension Rheology, Particle interaction, Contact forces

Elisabeth Lemaire - InPhyNi, UCA- CNRS - Keywords: Suspension Rheology, Particle interaction, Contact forces

Contribution title: Concentrated suspension dynamivs: a contact story

Suspensions of rigid particles in low Reynolds number flows are ubiquitous in industry (food transport, cosmetic products, civil engineering, etc.) or natural flows (such as mud or lava flows) to mention but a few. Until recently, it was assumed that the rheology of non-Brownian (athermal) suspensions was dominated by hydrodynamic interactions between particles. This idea was supported by the fact that lubrication forces which diverge as particle separation tends to zero, prevent solid contacts between particles. But, this should exclusively apply to perfectly smooth particles although particles used in industrial formulations (paints, cosmetics, food, etc.) or in fundamental rheological studies generally exhibit roughnesses greater than one thousandth of their diameter. By studying shear-induced microstructure in suspensions, we have experimentally shown that suspended particles come into direct contact by means of the asperities located at their surface. A major concern was then to quantify the influence of solid contacts between particles on the rheological properties of non-Brownian suspensions. In this scope, we have developed a computational code that takes into account both hydrodynamic (long range and short range) interactions and frictional contact between particles. For the first time, we have shown numerically the key role of solid friction in suspension rheology demonstrating that the introduction of solid friction enables to recover the experimental values of the viscosity of concentrated suspensions whereas the older simulations which do not take into account the friction greatly underestimate the viscosity (by a factor of 5 for a 45 vol% concentrated suspension). Contact forces and friction between particles provide insights into many phenomena which cannot be explained if only hydrodynamic forces are involved, i.e. irreversibility of particle trajectories in oscillating shear flows, shear-induced particle migration or discontinuous shear-thickening.
We will first show how direct solid contacts between particles can be evidenced by looking at the shear-induced microstructure in suspensions. Then, we will discrete numerical simulation that illustrate the influence of friction on rheology and dynamics of athermal suspensions.