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 as similar sharp transition with a small increase in disorder. A measurable change in $S$, but not $\varphi$ and $Z$, was detected under tapping. We find disorder-induced metastability does not show configurational relaxation, but can be detected via force-chain reorientations.

• Received 20 November 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.128001