Visualizing Gardner-like effects in the transition from transient to persistent contacts in experiments on granular crystals.*

Recent numerical studies of granular materials suggest that the persistence of inter-particle contacts during cyclic loading depends on the lowest packing fraction (φ) to which they are decompressed. Contacts are transient at low φ, and increasingly conserved beyond a threshold φG. We present an experimental study of the conservation of particle contacts as a function of φ, for a nearly perfect bi-disperse granular crystal of photo-elastic discs. Particles were levitated by an air-cushion to limit basal friction, and agitated from compressed air streams to simulate thermal noise. Using auto-correlations in the force chain network (formed after compression beyond jamming) as a measure of particle contact conservation, we found marked transition from transient to conserved contacts. Moreover, we showed that this transition coincides with the onset of Gardner-like physics in amorphous solids.