Granular materials like sand and beads can shift and flow in fluid-like ways, but they’re much harder to predict. Part of this is due to the way friction between individual grains transmits force through the network. Here, we see photoelastic beads responding to the intrusion of a narrow rod. The lightning-like flashes show how stress is traveling between neighboring grains. Notice how the lower grains are essentially frozen into a state of high stress, but the movable upper grains shift and readjust themselves to try and relieve stress.
This experiment took place under lunar gravitational conditions. Lower gravity means that it takes a larger pile of grains on top to create a given stress. But it also means it’s easier for those movable top grains to shift or even get thrown up by a hastily applied force. The purpose of experiments like this is to better understand how rovers and probes should dig in low-gravity environments without kicking up a cloud of regolith and dust. (Image credit: K. Daniels et al., source)