Slow relaxation and compaction dynamics of granular piles

We present the results of an experimental study of compaction dynamics at three different scales. The density of the packing is measured during compaction. Extremely slow dynamics towards an asymptotic density has been evidenced. The motion of every grain in a pile is also tracked. The mobility of the grains is found to vanish when the packing approaches its asymptotic density. The mobility of the grains is related to the increasing density of the packing. The domains of grains ideally ordered, i.e. hexagonal structures, are tracked. The growth of these domains obeys a crystalization kinetics, described by the Avrami theory. This allows us to propose a new law for the compaction dynamics. The application of those results to 3d sphere packings, as well as anisotropic grains, is also discussed.