A new cross - streamline particle migration mechanism in a dilute suspension of spheres in shear flow confined by planar walls

The effects of confinement on the dynamics of binary encounters between spherical particles in shear flow are studied for a system bounded by a single planar wall or two parallel planar walls under creeping flow conditions. We show that wall proximity gives rise to a new class of binary trajectories resulting in cross-streamline migration of the particles. In contrast, in unbounded space spherical particles on open trajectories return to their original streamlines after a binary encounter is completed (when there are no non-hydrodynamic forces). Thus, the new type of trajectories constitutes the dominant cross-streamline migration mechanism in dilute wall-bounded suspensions. The physical origin of the new trajectories is explained in terms of counter-rotation of particle pairs that is driven by the dynamic pressure distribution. The effect of the new migration mechanism in dilute suspensions is illustrated using a Boltzmann–Monte Carlo simulation technique. Some other implications of our findings are indicated.