Relation between ordering and shear thinning in colloidal suspensions

Abstract

Colloidal suspensions exhibit shear thinning and shear thickening. The most common interpretation of these phenomena identifies layering of the fluid perpendicular to the shear gradient as the driver for the observed behavior. However, studies of the particle configurations associated with shear thinning and thickening cast doubt on that conclusion and leave unsettled whether these nonequilibrium phenomena are caused primarily by correlated particle motions or by changes in particle packing structure. We report the results of Stokesian dynamics simulations of suspensions of hard spheres that illuminate the relation among the suspension viscosity, shear rate, and particle configuration. Using a recently introduced sampling technique for nonequilibrium systems, we show that shear thinning can be decoupled from layering, thereby eliminating layering as the driver for shear thinning. In contrast, we find that there is a strong correlation between shear thinning and a two-particle measure of the shear stress. Our results are consistent with a recent experimental study.colloids | rare-event sampling | Nonequilibrium Umbrella Sampling | rheology F lowing colloidal suspensions exhibit many nonlinear response phenomena, prominent among which are shear thinning and thickening, i.e., the decrease and increase, respectively, of the suspension viscosity with increasing shear strength (1-5). In the last four decades, many experimental studies and computer simulations have attempted to identify changes in particle configurations responsible for these phenomena (6-16), but no conclusive signature has yet been identified. In early experimental studies of dense colloidal suspensions of hard spheres (8, 17) (solid or glass at equilibrium) and of dilute charge-stabilized suspensions that crystallize at equilibrium (18, 19), sliding layers were observed when the systems were subjected to shear flow. This layering structure, characterized by a nonuniform density distribution along the shear gradient direction, was independently predicted to occur by nonequilibrium molecular dynamics (NEMD) simulations without hydrodynamic interactions (20,21). The organization of the suspension into layers was thought to explain the shear-thinning behavior observed, but later experimental studies of less dense systems (with liquid-like order at equilibrium) using light scattering (22, 23) or small-angle neutron scattering (24-27) yielded mixed signals relevant to the existence of this layering structure in the shear-thinning regime, thereby implying that layering might not be necessary for shear thinning. This implication is supported by Stokesian dynamics simulations (28, 29), which differ from earlier NEMD simulations by incorporating hydrodynamic interactions between the particles. The Stokesian dynamics simulations find no shear-induced layering of hard spheres in the shear-thinning regime in bulk systems that are liquid-like at equilibrium (9, 30).It is certainly the case that diffraction from an assembly of particles is sensitive to ordering an...

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