Binary Collisions: Suspension Ordering Instead of Hydrodynamic Diffusion

Binary collisions between rough (or repulsive) particles in dilute suspension flows are assumed to lead to particle diffusion that relaxes suspension non-uniformities. Here we show that binary collisions in wall-bounded shear flow may result in an opposite effect, i.e., development of layered particle ordering. We demonstrate that, depending on particle roughness (or repulsion range), the ordering may either extend through the whole channel width or start from the walls and decay exponentially towards the channel center. The key factor in formation of the layered behavior is existence of the swapping (reversing) binary trajectories [J. Fluid Mech. 592].