PATTERN FORMATION IN NONCOHESIVE AND COHESIVE GRANULAR MEDIA

Abstract

We studied pattern formation in granular media in two distinct states: in the dry and non-cohesive (powder) state, on the one hand, and in the wet and cohesive (paste) state, on the other. In the first case, we have shown that gas injection in a thin layer of powder within a Hele Shaw cell leads to fractal patterns remarkably similar to viscous fingering patterns obtained with complex fluids. In the second case, we have shown that the tensile cohesive viscoplastic fracture of a layer of paste leads to self-affine rough surfaces with a Hurst exponent close to 0.88, very close to the value obtained for fragile fracture by other authors.18 Our observations reinforce the universality of two fractal growth processes and add a new element to the ambivalent nature of the granular state of matter.