Manipulating particles in Stokes flow: Interaction with Interfaces and Symmetry Breaking

Density-matched, force-free particles in Stokes flow are typically assumed to passively follow streamlines. When encountering an obstacle or boundary, steric requirements forbidding penetration enforce deviations from the initial streamline, leading to well-defined changes in normal and tangential velocity components. If the particle’s approach to and departure from the interface occurs in a symmetric fashion, however, no net displacement will be observed. However, if the geometry of the boundary or the geometry of the flow field break this symmetry, net displacement, and thus meaningful manipulation of particle transport, is possible. We investigate the occurrence and magnitude of such a permanent displacement both in prototypical vortical Stokes flow fields and transport flows. Of particular interest are scenarios where the particle is driven towards the interface so that the fluid layer between particle and boundary becomes extremely thin, so that in a practical application adhesion to the interface results. Comparing such effects of particle displacement at zero Reynolds number to those exploiting finite particle inertia may lead to new protocols of deterministic displacement or particle capture in filters.