This experimental research is directed at the response of noncohesive granular materials to applied forces. Strong forcing can create smooth hydrodynamic-like flow, but with microscopic fluctuations and inelastic grain-grain collisions that limit the speed of flow. Weak forcing can lead to intermittent avalanche-like flow or no flow at all. Similar behavior occurs in other materials, like foams or dense colloids or supercooled liquids; the detailed phenomenology and extent of analogy are at the forefront of condensed matter physics, under the name of jamming. Here the grain-scale fluctuations will be measured by high-speed video and by a new quasielastic laser light scattering method we call "speckle-visibility spectroscopy". One thust is to explore the extent to which the microscopic dynamics are quantitatively analogous to fluctuations in a thermal equilibrium system. Another thrust is to relate the fluctuations to dissipation, particularly in the context of impact cratering. The graduate and undergraduate students involved in this research will be trained in cutting-edge experimental technology; this will prepare them for a range of careers in academia, industry or government. %%% This research is aimed at fundamental puzzles in how granular systems, like sand, respond to applied forces. Modern society relies on handling all sorts of granular materials, ranging from foods to building supplies to pharmaceuticals and more. Yet actual equipment is surprisingly prone to sudden failure, like clogging or jamming or demixing. Ultimately, this is because we still don't know how applied forces on large systems cause relative motion between neighboring grains, which is how energy is dissipated. Here the proposed work will break new ground by combining high-speed video with a newly invented laser light scattering method. This will allow study of grain motion and energy dissipation at short enough scales to capture the crucial physics, which was not possible with prior means. Students in this program receive rigorous training in experimental methods involving modern electronic and optical physics, and can pursue careers in academic or industrial science or engineering. ***
