Dynamique et instabilités des interfaces grains/fluide dans les suspensions non-Browniennes

摘要

Since the first works of Einstein in 1906, many studies have focussed on the behaviour of granular suspensions, in particular on their rheology. However, the dynamical properties of interfaces or free-surfaces within or bounding such suspensions are quite unknown and are the subject of the present work. We propose to investigate the properties of such interfaces by injecting a pure fluid into an isodense granular suspension, whose grain fraction is systematically varied. The suspension is confined in a Hele-Shaw cell and either air or the pure suspending fluid can be injected. For grain fractions below 50%, air injection yields patterns that are visually identical to Saffman-Taylor viscous fingering. However, the width selection and the stability of the fingers are modified and we demonstrate that these modifications do not follow from consideringthe suspension as a homogeneous fluid with renormalized viscosity. For higher grain fractions, the media becomes partially jammed due to contacts between the grains and we obtain a weakly consolidated porous media. There are then two mechanisms of flow of the pure fluids: through the fixed granular structure or by mobilising this structure. For air injection, localized rearrangements lead to narrow air fingers whose width remains larger than the pore size. For the injection of suspending fluid, a more global mobilisation is observed leading to homogenous decompaction of the media and larger fingering structures.