"Super-diffusive and long-ranged dynamical heterogeneities: Criticality at the Jamming transition"

Jamming occurs when the volume fraction of hard grains is increased beyond a certain point, above which the system is able to support mechanical stresses. It has been argued that this rigidity transition is akin to phase transitions in thermal systems, and that the jamming density is a genuine critical point possibly displaying similarities with the glass transition.

In this work, a dense amorphous monolayer of hard disks is horizontally driven by a glass plate oscillating underneath while confined in a fixed rectangular cell. As the packing fraction is decreased, the system exhibits a transition between a totally jammed state in which the pressure is driven by the contact network and a "supercooled" regime in which the kinetic contribution becomes dominant.

We characterize the dynamics of individual grains, which become super-diffusive at the jamming transition, signaling long-ranged temporal correlations. Correspondingly, fourpoint dynamical correlations become spatially extended at the critical density and obey critical scaling.
Correlation time and length soar on both sides of the transition, as the volume fraction varies over a remarkably tiny range. Altogether, our experimental results provide strong evidence that the appearance of macroscopic rigidity is indeed a critical phenomenon, with increasingly collective and heterogeneous dynamics.