1. Looking Inside a Granular Material R.P. Behringer, PI, Duke University DMR-0137119 In Nature, 435, 1079 (2005) and Powders and Grains, 65 (2005) we present the first experimental results for internal granular contact forces. These experimental images show the characteristic force chains of stressed granular matter. Our experiments exploit photoelasticity to obtain contact forces in 2D systems that have either been compressed (above left) or sheared (above right and far right). We obtain P(f) and show that it depends on the stress state. Likewise, spatial correlations appear to be long-range along the strong force direction of sheared systems and short-range otherwise.

2. Looking Inside a Granular Material R.P. Behringer, PI, Duke University DMR-0137119 Photoelasticity is based on the idea that stresses within a material affect the way light propagates depending on the polarization of the light and the forces. By placing a stressed photoelastic object between crossed polarizers, it is possible to deduce the forces that the object feels. These studies are particularly novel in their use of photoelasticity to extract the forces that particles exert on each other for large enough collections of particles that one can obtain important statistical information. This information, such as the number of contact forces have value f, called P(f), forms the basis of new models for granular materials.

3. Looking Inside a Granular Material R.P. Behringer, PI, Duke University DMR-0137119 Education: Three under- graduates, and a high school student have been involved in related aspects of this work. These include undergrad Joyce Coppock, who will write a senior thesis based on her work, Annie Thebprasith, a visitor from Mount Holyoke College, and David Marks, now a physics graduate student at Stanford. High school Andy Lane is now a Biomedical Engineering student at UNC Chapel Hill. Summer work has involved first year graduate students Katie Mahoney and Jun Uehara. Jun will continue her Ph.D. work in this lab. Trushant Majmudar will complete His Ph.D. thesis based on these studies. Societal Impact: The handling of granular materials is of utmost importance to many commercial processes from the handling of ores and food grains to making pills. The current predictive capability for designing these processes is woefully inadequate, and leads to substantial losses. For instance, it has been estimated that up to $1Trillion a year is spent handling granular materials, and typical handling facilities operate around 2/3 of the design capability. Our work addresses the crucial need to develop a fundamental understanding of granular physics to rectify this deficiency.