Capillary-Driven Instability of Immiscible Fluid Interfaces Flowing in Parallel in Porous Media

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.