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.
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.
Our findings all come from experiments on the behavior of disordered far-from-equilibrium materials such as dense collections of grains, bubbles, and colloidal particles. This is a broad and forefront topic, because, while we fundamentally understand ordered and equilibrium materials, such seemingly mundane materials as "sand" and "foam" exhibit surprising and mysterious behavior that cannot be approached using traditional physics tools. Yet such media are all around us, in everyday life and in earth and environmental sciences, and are crucial for a vast number of industries. Funds were spent primarily on graduate student salaries and benefits. Besides the education of students in state of the art physics and experimental techniques, products include seventeen peer-reviewed journal articles, sixteen invited conference presentations, and fourteen seminars or colloquia; these numbers reflect the productivity and importance of the sponsored work. Further presentations were given as outreach to elementary school students and teachers. Some of our findings include new force laws for granular impact cratering and new laws for the nature of the motion of grains and particles in a variety of situations where they are close to jamming into a solid-like state. This new information ought to be useful for developing rules for processing / transporting / manipulating grains and powders, which currently is done with not nearly the same level of reliability and efficiency as for ordinary equilibrium fluids and solids.
