  The concept of effective depletion forces between two fixed big colloidal
particles in a bath of small particles is generalized to a non-equilibrium
situation where the bath of small Brownian particles is flowing around the big
particles with a prescribed velocity. In striking contrast to the equilibrium
case, the non-equilibrium forces violate Newton's third law, are
non-conservative and strongly anisotropic, featuring both strong attractive and
repulsive domains.
