Ergodic nonergodic transition in tapped granular systems; The role of persistent contacts

Abstract

Static granular packs have been studied in the last three decades in the frame of a modi ed equilibrium statistical mechanics that assumes ergodicity as a basic postulate. The canonical example on which this framework is tested consists in the series of static con gurations visited by a granular column subjected to taps. By analyzing the response of a realistic model of grains, we demonstrate that volume and stress variables visit di erent regions of the phase space at low tap intensities in di erent realizations of the experiment. We show that the tap intensity beyond which sampling by tapping becomes ergodic coincides with the forcing necessary to break all particle{particle contacts during each tap. These results imply that the well-known \reversible" branch of tapped granular columns is only valid at relatively high tap intensities.