The chaotic dynamics of jamming

Granular materials are collections of discrete, macroscopic particles characterized by relatively straightforward interactions. Despite their apparent simplicity, these systems exhibit a number of intriguing phenomena, including the jamming transition, in which a disordered collection of grains becomes rigid when its density exceeds a critical value(1,2). Many aspects of this transition have been explored, but an explanation of the underlying dynamical mechanisms for the transition remains elusive. Here, applying nonlinear dynamical techniques(3-5) to simulated two-dimensional Couette shear cells(6-8), we reveal the mechanisms of jamming and find that they conflict with the prevailing picture of growing cooperative regions. In addition, at the density corresponding to random close packing(9,10), we find a dynamical transition from chaotic to non-chaotic states accompanied by diverging dynamical length- and timescales. Furthermore, we find that the dominant cooperative dynamical modes are strongly correlated with particle rearrangements and become increasingly unstable before stress jumps, providing a way to predict the times and locations of these striking stress-release events in our simulations.