Footprints in Sand: The Response of a Granular Material to Local   Perturbations
  We experimentally determine ensemble-averaged responses of granular packings to point forces, and we compare these results to recent models for force propagation in a granular material. We used 2D granular arrays consisting of photoelastic particles: either disks or pentagons, thus spanning the range from ordered to disordered packings. A key finding is that spatial ordering of the particles is a key factor in the force response. Ordered packings have a propagative component that does not occur in disordered packings. 
Junfei Geng (1), D. Howell (1), E. Longhi (1), and R. P. Behringer   (1), G. Reydellet (2), L. Vanel (2) and E. Clement (2),...