PATTERN FORMATION IN NONCOHESIVE AND COHESIVE GRANULAR MEDIA

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 He le 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.