(242d) Effect of Particle Roughness on Shear-Induced Diffusion

The shear-induced diffusivity of non-Brownian and neutrally buoyant spheres suspended in a viscous, sheared fluid was investigated using experiments and simulations. For experiments, monodisperse spheres were roughened using a mechanical process and used in a custom-built shear cell to measure the diffusivities. Two regimes of dependence of the diffusion coefficient on roughness were identified. Rougher particles were found to have a larger diffusivity at low volume fractions, but a lower diffusivity at high volume fractions; the cross-over occurred at a volume fraction of approximately 0.25. Simulations that incorporate a frictionless contact force confirmed this transition. At low volume fractions, the displacement induced by each collision is larger for higher values of roughness, and hence the diffusivity increases with particle roughness. At high volume fractions, the increased roughness promotes organization of the concentrated particles into layered structures aligned in the flow direction. This organization results in fewer particle collisions and a corresponding decrease in the diffusivity. Simulations with and without bounding walls predicted similar trends for the diffusion coefficient as a function of volume fraction, which revealed the layering of particles at high areal fractions and roughness are not due to the presence of the walls.