Wall attraction and repulsion of hydrodynamically interacting particles

We investigate hydrodynamic interaction effects between colloidal particles in the vicinity of a wall in the low Reynolds-number limit. Hydrodynamically interacting pairs of beads being dragged by a force parallel to a wall, as for instance during sedimentation, are repelled by the boundary. If a pair of beads is trapped by harmonic potentials parallel to the external flow and at the same distance to a wall, then the particle upstream is repelled from the boundary while its neighbor downstream is attracted. The free end of a semiflexible bead-spring polymer-model, which is fixed at one end in a flow near a wall, is bent towards the wall by the same reason. The results obtained for point-like particles are exemplarily confirmed by fluid particle dynamics simulations for beads of finite radii, where the shear induced particle rotations either weaken or enhance the the effects obtained for point-like particles.