(326d) Experiments and Numerical Simulations of the Shear Rheology of Particulate Suspensions in Shear-Thinning Elastic Fluids

The rheological properties of viscoelastic suspensions are important to many engineering applications, such as oil recovery, additive manufacturing, and pharmaceutical formulations. One particular system of interest is the injection of drug-free micro-scale plates into arthritic joints. The synovial fluid of the joint is a highly shear-thinning viscoelastic fluid that is less viscous upon the onset of arthritis. The addition of inert microparticles to the synovial fluid of joints has been shown to alleviate arthritic symptoms. Therefore, it is important to understand the underlying mechanism of how the particles affect the bulk stress of the synovial fluid. To this end, we perform rheological shear measurement experiments and direct numerical simulations of particles in a shear thinning fluid at low Reynolds numbers. Our experiments are completed in suspensions of spherical particles at concentrations <10% in a model polyacrylamide fluid system to mimic the rheology of arthritic synovial fluid. We find that the per-particle shear viscosity increases with particle volume fraction. Our numerical studies use a fully parallelized 3D finite volume solver to simulate shear flow past a single sphere, with the Giesekus constitutive model used to describe the polymeric fluid rheology. We show good quantitative agreement between experiments and simulations. We find that relative per particle shear viscosity is non-monotonic with respect to Wi and is driven by the polymer-induced fluid stress. Note that we contrast these results to previous studies where the polymer concentration in the fluid was relatively low and thus the suspending fluid was a “Boger fluid”. We demonstrate that the distribution of this polymer-induced fluid stress is less localized at lower values of the concentration parameter β, which is the fractional solvent contribution to fluid viscosity. Thus, the particle-induced stress field surrounding a given particle decays algebraically slowly and is significant even many radii from the included sphere. This would imply that suspensions of particles in highly shear thinning, low β polymeric fluids demonstrate inter-particle interactions at lower volume fractions and we discuss the consequences of these findings for suspension rheology.