Studies of granular compaction

The work reported here is an overview of several studies of the compaction of granular material that we have conducted in our laboratories. We show that external vibrations lead to a slow, essentially logarithmic, approach of the packing density to a final steady-state value. Depending on the initial conditions and the magnitude of the vibration acceleration, the system can either reversibly move between steady-state densities or can become irreversibly trapped into metastable states. We find that the highest packing densities are obtained along the reversible branch. Finally, measurements of the spectrum of density fluctuations around the steady-state values provide a probe of the internal relaxation dynamics of the system and a link to recent thermodynamic theories for the settling of granular material.