Force Networks Examined with Photoelastic Imaging

How fast granular materials flow is related to how elastic energy in the materials is stored and released. Researchers at Duke University in Durham, N.C., have used a polariscope, a light source and a video camera to examine the movement of simulated grain -- made of a photoelastic material -- between two Perspex sheets. The purpose of the study was to test whether the stresses within densely packed granular materials depend on the rate of shearing.

The results, which were published in the Feb. 27 issue of Nature, may revise a long-standing tenet of granular materials. The study also increased the understanding of the extent to which disorder is associated with elastic energy. Further investigations will examine what happens as the stiffness of the particles is changed and why the stress network decays after it has been prepared in a pre-sheared state.