Packing of Compressible Granular Materials

3D computer simulations and experiments are employed to study random packings of compressible spherical grains under external confining stress. In the rigid ball limit, we find a continuous transition in which the stress vanishes as $(\phi -{\phi }_c{)}^{\beta }$, where $\phi$ is the (solid phase) volume density. The value of ${\phi }_c$ depends on whether the grains interact via only normal forces (giving rise to random close packings) or by a combination of normal and friction generated transverse forces (producing random loose packings). In both cases, near the transition, the system's response is controlled by localized force chains.