  It is shown that low Reynolds number fluid flows can cause suspended
particles to respond as though they were in an equilibrium system with an
effective potential. This general result follows naturally from the fact that
different methods of moving particles in viscous fluids give rise to very
different long-range flows. Two examples are discussed: electrophoretic
`levitation' of a heavy charged sphere, for which a hydrodynamic
`pseudo-potential' can be written in closed form, and quasi-two dimensional
crystals of like-charged colloidal spheres which form near charged walls, whose
apparent attraction arises not from a force but from persistent fluid flows.
