VISCOUS AND ELASTIC FINGERING INSTABILITIES IN FOAM

We investigate pattern formation in the immiscible displacement of foa m from a confining geometry. The dominant length scales with the drivi ng pressure gradient as for a Newtonian fluid, but depends surprisingl y on the ratio of plate separation to bubble diameter. The pattern mor phology exhibits a transition from a jagged, presumably elastic, chara cter to a smooth, viscous character as the shear strain rate rises far above the natural internal relaxation time scale of the foam. These f eatures may result from the two-dimensional nature of the flows and th e unusual rheology of foams.