DENSITY-FLUCTUATIONS IN VIBRATED GRANULAR-MATERIALS

We report systematic measurements of the density of a vibrated granula r material as a function of time. Monodisperse spherical beads were co nfined to a cylindrical container and shaken vertically. Under vibrati ons, the density of the pile slowly reaches a final steady-state value about which the density fluctuates. We have investigated the frequenc y dependence and amplitude of these fluctuations as a function of vibr ation intensity Gamma. The spectrum of density fluctuations around the steady state value provides a probe of the internal relaxation dynami cs of the system and a link to recent thermodynamic theories for the s ettling of granular material. In particular, we propose a method to ev aluate the compactivity of a powder, first put forth by Edwards and co -workers, that is the analog to temperature for a quasistatic powder. We also propose a stochastic model based on free volume considerations that captures the essential mechanism underlying the slow relaxation. We compare our experimental results with simulations of a one-dimensi onal model for random adsorption and desorption. [S1063-651X(98)07602- 8].