(271i) Mechanism of Shear-Induced Diffusion in Suspensions of Non-Brownian Particles

Mechanisms of shear-induced diffusion of non-Brownian neutrally buoyant spheres suspended in a viscous fluid are investigated. Our recent experiments and simulations [1] revealed a surprising effect of particle roughness on their diffusivity in dense suspensions under shear. It was observed that increasing particle roughness leads to a decrease in their diffusivity, which contradicts expectations based upon two-particle interactions. The simulations reveal that increased roughness promotes organization of the concentrated particles into layered structures aligned in the flow direction at high volume fractions. This organization results in fewer particle collisions and a corresponding decrease in the diffusivity.

Despite qualitative agreement between these simulations and experiments, there is a substantial quantitative discrepancy due to a minimalist model that neglected all hydrodynamic interactions except normal lubrication and assumed a simple approximation for the particle contact forces. In this contribution, we report results of simulations with a more detailed model that includes additional components of lubrication, as well as a more accurate model for particle contacts that includes friction. We perform a systematic study of effect of different types of interactions (normal vs. tangential component of lubrication, normal contact force, friction force) on diffusivity and pattern formation. This is accomplished by measuring contributions of individual forces to the random force acting on the particles, as well as performing simulations in which some of the interactions are disabled. Diffusivity and suspension microstructure obtained from the simulations are compared with the experimental data.

[1] H. Zhang, P. Pham, B. Metzger, D. I. Kopelevich, J. E. Butler, Effect of particle roughness on shear-induced diffusion, submitted to Phys. Rev. Fluids.