# Ergodic-nonergodic transition in tapped granular systems: The role of persistent contacts

## TL;DR

This paper investigates the transition between ergodic and non-ergodic behavior in tapped granular systems, showing that ergodicity depends on contact-breaking and tap intensity, challenging traditional statistical mechanics assumptions.

## Contribution

It demonstrates that ergodicity in tapped granular systems is linked to contact-breaking, revealing a transition point dependent on tap intensity and challenging existing equilibrium models.

## Key findings

- Ergodicity occurs only above a certain tap intensity.
- Breaking particle contacts is essential for ergodic sampling.
- Reversible behavior is limited to high tap intensities.

## Abstract

Static granular packs have been studied in the last three decades in the frame of a modified 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 configurations 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 different regions of the phase space at low tap intensities in different 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.