Experiments on Granular Fluctuation and Dissipation

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NON-TECHNICAL ABSTRACT: The goal of this project at the University of Pennsylvania is to provide fundamental guidance for how granular materials flow and deform in response to applied forces. The experimental approach will leverage modern optical technologies developed in-house. Modern society relies on transporting, storing, mixing, crushing, and processing such diverse granular materials as foods, seeds, minerals, ores, building materials, and pharmaceutical pills & powders. However, actual applications are inefficient and subject to such catastrophic failures as clogging, mispacking, and demixing. This project will lay the basis for improved engineering practices by establishing the fundamental link between microscopic grain-scale motion and the macroscopic flows that result from applied forces. Besides applications, this topic is of basic interest both because granular materials are nonlinear far-from equilibrium systems at the boundary of the world of known physics, and because of their rich connections to geophysical problems such as earthquakes, mudslides, erosion, and desertification. The project will bring modern physics and measurement techniques into the educational experience of graduate and undergraduate students by involving them directly in cutting-edge scientific research.TECHNICAL ABSTRACT: The goal of this project at the University of Pennsylvania is to measure grain-scale dynamics and to connect with the unusual macroscopic mechanical properties of granular media. For strong forcing, both the microscopic dynamics and the macroscopic flows are homogeneous, and are reasonably well described by hydrodynamic approaches. However, for low forcing, the response can be intermittent in time and localized in space. Such heterogeneities are exacerbated on approach to jamming, and create havoc both for modeling as well as for applications. This project will elucidate grain-scale behavior using high-speed video microscopy, as well as a multispeckle dynamic light scattering method developed in-house and known as Speckle-Visibility Spectroscopy. These probes will be applied to heap and hopper flows, to flows involving impact cratering, and to gas-fluidized grains and rods. The results will be correlated with bulk flow behavior and to rheological properties. Altogether this will establish the fundamental microscopic fluctuation mechanisms responsible for the dissipation of energy injected at the macroscopic scale, and their consequences for deformation and flow. The project will bring modern physics and measurement techniques into the educational experience of graduate and undergraduate students by involving them directly in cutting-edge scientific research.