  When immiscible wetting and non-wetting fluids move in parallel in a porous
medium, an instability may occur at sufficiently high capillary numbers so that
interfaces between the fluids initially held in place by the porous medium are
mobilized. A boundary zone containing bubbles of both fluids evolve which has a
well defined thickness. This zone moves at constant average speed towards the
non-wetting fluid. A diffusive current of bubbles of non-wetting fluid into the
wetting fluid is set up.
