The Jamming point street-lamp in the world of granular media

TL;DR

This paper investigates the relevance of the Jamming point, well-studied in thermal soft spheres, to real granular media by analyzing contact network dynamics in horizontally shaken grains near the transition.

Contribution

It provides experimental evidence that granular media exhibit behaviors near the Jamming transition similar to thermal soft spheres, extending the applicability of the J-point concept.

Findings

Contact network heterogeneities peak at a specific packing fraction

Two cross-over points converge at the Jamming point J in the zero excitation limit

Granular dynamics can be mapped onto thermal soft sphere behaviors

Abstract

The Jamming of soft spheres at zero temperature, the J-point, has been extensively studied both numerically and theoretically and can now be considered as a safe location in the space of models, where a street lamp has been lit up. However, a recent work by Ikeda et al, 2013 reveals that, in the Temperature/Packing fraction parameter space, experiments on colloids are actually rather far away from the scaling regime illuminated by this lamp. Is it that the J-point has little to say about real system? What about granular media? Such a-thermal, frictional, systems are a-priori even further away from the idealized case of thermal soft spheres. In the past ten years, we have systematically investigated horizontally shaken grains in the vicinity of the Jamming transition. We discuss the above issue in the light of very recent experimental results. First, we demonstrate that the contact network exhibits a remarkable dynamics, with strong heterogeneities, which are maximum at a packing fraction phi star, distinct and smaller than the packing fraction phi dagger, where the average number of contact per particle starts to increase. The two cross-overs converge at point J in the zero mechanical excitation limit. Second, a careful analysis of the dynamics on time scales ranging from a minute fraction of the vibration cycle to several thousands of cycles allows us to map the behaviors of this shaken granular system onto those observed for thermal soft spheres and demonstrate that some light of the J-point street-lamp indeed reaches the granular universe.