Seeing Inside Sand: Visualizing Force Chains with Photoelastic Disks

As their name suggests, so-called “granular materials” are made up of “grains” -- small (but macroscopic) pieces of sand, glass beads, coffee grounds, or almost any other solid you can think of. Granular materials can flow like a liquid (like sand in an hourglass), resist deformation like a solid (like the sand under your feet at the beach), or quickly transition between these states (like pebbles in a rockslide).