Mechanics of Granular Materials: Rigidity, Nonlocality, and Activated Failure

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Non-technical AbstractGranular materials are integral to many parts of our daily lives, from the coffee beans that fuel our mornings, to the pharmaceutical pills that heal us, to the coal that fuels our power plants. Yet, this class of materials remains very difficult to successfully handle in industrial contexts, to provide hazard estimation on our landslide-prone hillsides, or to ensure stability of civil infrastructure such as embankments or levees. Through this project, the PI will perform controlled laboratory experiments aimed at determining the processes by which a stable granular material begins to flow. Large datasets measuring the details of loading and slipping for individual particles will allow us to test the efficacy of, and draw connections between, a variety of approaches drawn from both the engineering and physics communities. Together with the NC State Science House, they will organize an annual workshop called LEAP! (Launch your Excellent Adventures with Physics) to both provide high school girls with hands-on exposure to cutting-edge physics, and train physics researchers in the translation of their research to publicly-accessible formats, and also to our lab’s collaborators working on industrial and geophysical applications. Finally, they will continue work with undergraduate researchers, including a particular emphasis on recruiting and supporting students from under-represented and historically marginalized groups. These activities will equip their trainees to find jobs in education, industry, or governments as their interests align, with key expertise in both experimental techniques and data science. Technical AbstractWhen a granular material loses rigidity, it collectively rearranges its particles and internal forces, subject to known constraints. The material does not transition to a new configuration spontaneously, but must cross some threshold, frictional or energetic. Through this project, the PI will perform experiments on slow flows in order to determine the mechanical process by which a granular material begins to deform/flow from one valid state to another, and how this series of states is selected. For a given configuration of particles, the internal forces are not simply determined by the external load: many degenerate solutions are possible. This indicates that approaches drawn from statistical physics will be necessary to capture granular phenomena. The group aims to test multiple frameworks and seek connections between engineering approaches to the problem (nonlocal rheology, Mohr-Coulomb failure) and those of statistical physics (jamming, rigidity percolation, athermal statistical ensembles). They will drive the material to lose rigidity both by mechanical forcing from a boundary, as well as by internal forcing from particle-scale activity, in order to make these connections. Experiments will provide fundamental knowledge key to a variety of applications: geotechnical engineering and hazard estimation; bulk materials handling in the pharmaceutical, agricultural, and mining industries; and solar system exploration missions. Together with the NC State Science House, the PI will organize an annual workshop called LEAP! (Launch your Excellent Adventures with Physics) to both provide high school girls with hands-on exposure to cutting-edge physics (including granular materials), and train physics researchers in the translation of their research to publicly accessible formats. Finally, they will continue work with undergraduate researchers, including a particular emphasis on recruiting and supporting students from under-represented and historically marginalized groups.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.