Sensitivity of Granular Force Chain Orientation to Disorder-induced Metastable Relaxation

A two-dimensional system of photoelastic disks subject to vertical tapping against gravity was experimentally monitored from ordered to disordered configurations by varying bidispersity. The packing fraction $\varphi$, coordination number $Z$, and an appropriately defined force chain orientational order parameter $S$, all exhibit similar sharp transition with small increase in disorder. A measurable change in $S$, but not $\varphi \mathrm{\&}Z$, was detected under tapping. We find disorder-induced metastability does not show configurational relaxation, but can be detected via force chain reorientations.