Density relaxation in a vibrated granular material

We report measurements of the density of a vibrated granular material as a function of time. The material studied consists of monodisperse spherical glass particles confined to a long, thin cylindrical tube. Vibrations cause the pile to evolve from a low density initial configuration toward a steady state with a final density that depends on the intensity of the vibrations. We find a complex time evolution that is incompatible with a single exponential relaxation. There appears to be a characteristic value of the acceleration that separates two regimes of packing behavior. The results are compared to current theories of compaction.