Jamming and stress propagation in particulate matter

We present simple models of particulate materials whose mechanical integrit y 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 natural ly linked to the marginal stability of force chain networks (granular skele tons) within the material. Fragile matter exhibits novel mechanical respons es that may be relevant to both jammed colloids and cohesionless assemblies of poured, rigid grains.