A special photoactive material is cut into small disks and viewed between crossed polarizer filters. When loaded, the stress in the material causes the disks to light up. The setup creates a simplified 2D granular pile and allows us to visualize they way in which forces are transmitted through a granular medium. The surprising thing is that forces are carried by branching chains where some elements are under a lot of stress while others are under little or no stress whatsoever.
I made this small prototype in 2003 and am currently working on a much larger version supported by the University of Chicago, Materials Research Science and Engineering Center.
Original inspiration for this and much help provided by Bob Berhinger, Duke University, whose research group uses this technique to look at many aspects of granular behavior.