Shear-induced diffusion of smooth and roughened particles*

The shear-induced diffusivity of non-Brownian and neutrally buoyant spheres suspended in a viscous fluid were measured using a custom-built shear cell. Measurements were performed on monodisperse and relatively smooth spheres, then the measurements were repeated after the spheres were roughened using a mechanical process. Unexpectedly, the roughened spheres were more diffusive than the original, smooth spheres for volume fractions in excess of 0.25. Simulations that balance viscous drag and contact forces on each particle predict two regimes of dependence of the diffusion coefficient on roughness: below a volume fraction of 0.2, rougher particles were found to have a larger diffusivity, but a lower diffusivity at volume fractions higher than 0.25. 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 for rougher particles.