Capillary-driven instability of immiscible fluid interfaces flowing in parallel in porous media

When immiscible wetting and nonwetting 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 evolves, which has a well-defined thickness. This zone moves at constant average speed toward the nonwetting fluid. A diffusive current of bubbles of nonwetting fluid into the wetting fluid is set up.