Surfactant wetting layer driven instability in a Hele–Shaw cell

Usually, an air–liquid interface moving in a Hele–Shaw cell will become unstable only when the less viscous air is displacing the more viscous liquid. However, if the liquid under consideration is an aqueous surfactant solution and the walls of the cell are coated with a wetting layer of the liquid, an instability in the motion of the interface can be produced even when the liquid is displacing air. Experimental results of this unusual behavior are briefly reviewed. A model based on the diffusion of the surfactants in the wetting layer on the wall is developed to explain the observed phenomena.