The phenomenon of interfacial motion between two immiscible viscous fluids in the narrow gap between two parallel plates (Hele-Shaw cell) is considered. This flow is currently of interest because of its relation to pattern selection mechanisms and the formation of fractal structures in a number of physical applications. Attention is concentrated on the fingers that result from the instability when a less-viscous fluid drives a more-viscous one. The status of the problem is reviewed and progress with the thirty-year-old problem of explaining the shape and stability of the fingers is described. The paradoxes and controversies are both mathematical and physical. Theoretical results on the structure and stability of steady shapes are presented for a particular formulation of the boundary conditions at the interface and compared with the experimental phenomenon. Alternative boundary conditions and future approaches are discussed.
History and statusFlow in a porous medium is a challenging scientific problem of great technological importance. Even though it usually concerns viscous fluids under circumstances where the nonlinear convective terms in the equations of motion are negligible, the mathematical difficulties caused by the randomness and chaotic structure of the medium make fundamental theory as difficult as for turbulence. Moreover, experiments are hard as it is not easy to see into a porous medium or know exactly the position of a measuring probe. The difficulties are compounded when the fluid is not homogeneous. For the case of two immiscible fluids, the added complication of moving microscopic and macroscopic interfaces makes the problem even more intractable.In about 1956, Sir Geoffrey Taylor paid a visit to the Humble Oil Company and became interested in problems of two-phase flow in porous media. He worked out the macroscopic instability which can arise when a less-viscous fluid drives a more-viscous one and which is a t least partly responsible for the coneing in processes of secondary recovery in oil fields. He also realized that two-dimensional flow in a porous medium is modelled by flow in a Hele-Shaw (1898) apparatus consisting of two flat parallel plates separated by a small gap b. Then the average two-dimensional velocity u of a viscous fluid in the space between the plates is related to the pressure p by the formula grad p , div u = 0 ( 1 . 1 ) b2where p is the viscosity of the fluid. This is identical with Darcy's law for motion in a porous medium of permeability b2/12. But it is, of course, an approximation valid when the gap or transverse dimension b is small compared with variations of scale a , say, in the lateral dimension parallel to the plates; see also Lamb (1932, $330).