By pouring equal balls into a container one obtains disordered packings with fascinating properties which might
shed light on several elusive properties of complex materials such as amorphous metals or colloids. In any real
experiment with equal-sized spheres one cannot reach packing fractions (fraction of volume occupied by the
spheres respect to the total volume, ρ) below the Random Loose Packing limit (RLP, ρ ~ 0.555) or above the
Random Close Packing limit (RCP, ρ ~ 0.645) unless order is externally induced. What is happening at these
two limits is an open unanswered question. In this paper we address this question by combining statistical
geometry and statistical mechanics methods. Evidences of phase transitions occurring at the RLP and RCP
limits are reported.
We investigate complex materials by performing "Virtual Experiments" starting from three-dimensional images of grain packs obtained by X-ray CT imaging . We apply this technique to granular materials by reconstructing a numerical samples of ideal spherical beads with desired (and tunable) properties. The resulting "virtual packing" has a structure that is almost identical to the experimental one. However,
from such a digital duplicate we can calculate several static and dynamical properties (e.g. the force network, avalanche precursors, stress paths, stability, fragility, etc.) which are otherwise not directly accessible from experiments. Our simulation code handles three-dimensional spherical grains and it takes into account repulsive elastic normal forces, frictional tangential forces, viscous damping and gravity. The system can be both simulated within a vessel or with periodic boundary conditions.