Morphodynamics during air injection into a confined granular suspension

We study the injection of air into a granular-suspension in a confined geometry. The grains and the suspending fluid are density matched allowing to vary the grain fraction phi over the whole range accessible: from phi = 0 up to close to the maximum packing fraction phi similar to 60%. In this situation, a large variety of patterns is observed, that we link directly to the rheology of the suspension in the confined geometry. For low grain fractions phi < 50%, we show that taking into account an effective viscosity for the suspension flow, resulting from particle migration, allows to link quantitatively the observed structures to the classical Saffman-Taylor selection mechanism. For higher grain fractions phi > 55% the granular matrix is jammed and we deal with a weak porous medium. In this case we link the observed patterns to the reorganization Occurring in the porous medium as a consequence of the injection process. (C) 2008 Elsevier B.V. All rights reserved.