Compaction of anisotropic granular materials: experiments and simulations.

We present both experimental and numerical investigations of compaction in granular materials composed of rods. As a function of the particles size and with respect to the container diameter, we have observed large variations of the asymptotic packing volume fraction. The relevant parameter is the ratio between the rod length l and the tube diameter D . Even the compaction dynamics remains unchanged for various particle lengths, and a transition between 3d and 2d ordering for grain orientations is observed for l/D=1 . A toy model for the compaction of needles on a lattice is also proposed. This toy model gives a complementary view of our experimental results and leads to behaviors similar to experimental ones.