Fig. 1.

Schematic of the experimental setup. a Side view of the setup. b Top view of the setup, which includes a square frame filled with ∼1300 bidisperse photo-elastic disks with a number ratio of 1:1 and a size ratio of 1.4:1. The four thick blue arrows represent the isotropic compression applied by simultaneously moving the four walls inwards. The small red square included in the stress image of a highly jammed packing contains a small portion amplified in c. c Top: image taken without a polarizer to record the packing’s configuration. Middle: corresponding stress image. Bottom: reconstructed stress image taken from the measured contact forces