Gardner-like crossover from variable to persistent force contacts in granular crystals

We report experimental evidence of a Gardner-like crossover from variable to persistent force contacts in a two-dimensional, bidisperse granular crystal by analyzing the variability of both particle positions and force networks formed under uniaxial compression. Starting from densities just above the freezing transition, and for variable amounts of additional compression, we compare configurations to both their own initial state, and to an ensemble of equivalent, reinitialized states. This protocol shows that force contacts are largely undetermined when the density is below a Gardner-like crossover, after which they gradually transition to being persistent, being fully so only above the jamming point. We associate the disorder that underlies this effect to the size of the microscopic asperities of the photoelastic disks used, by analogy to other mechanisms that have been previously predicted theoretically.

Methods
This dataset originated as digital images taken using both a polariscope (for photoelasticity) and unpolarized light. The data was processed using custom Matlab scripts to identify all particle locations in each image and to measure the similarity in photoelastic images.