PHYSICA Jamming and Stress Propagation in Particulate

Abst rac t We present simple models of particulate materials whose mechanical integrity arises from a jamming process. We argue that such media are generically "fragile", that is, they are unable to support certain types of incremental loading without plastic rearrangement. In such models, fragility is naturally linked to the marginal stability of force chain networks (granular skeletons) within the material. Fragile matter exhibits novel mechanical responses that may be relevant o both jammed colloids and cohesionless a semblies of poured, rigid grains. 1 In t roduct ion In this paper, we consider the relation between jamming (a kinetic process) and the laws of static mechanical equilibrium of particulate media. We describe first a simple model of jamming in a colloid, sheared between parallel plates. We assume that force chains (linear arrays of rigid particles in contact) develop along the major stress compression axis and span the sample. The resulting jammed state can support a shear stress indefinitely, but does this by virtue of a self-organized internal structure (the force chain array) which has developed in direct response to the applied load itself. If a different load is now applie