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. The spheres on these new trajectories do not pass each other (as
they would in free space) but instead they swap their cross-streamline
positions. To determine the significance of the wall-induced particle
migration, we have evaluated the hydrodynamic self-diffusion coefficient
associated with a sequence of uncorrelated particle displacements due to binary
particle encounters. The results of our calculations quantitatively agree with
the experimental value obtained by \cite{Zarraga-Leighton:2002} for the
self-diffusivity in a dilute suspension of spheres undergoing shear flow in a
Couette device. We thus show that the wall-induced cross-streamline particle
migration is the source of the anomalously large self-diffusivity revealed by
their experiments.Comment: submited to JF