Energy Partitioning in Granular Flow Depends on Mineralogy via Nanoscale Plastic Work

Granular materials are central to a wide range of geologic concerns, but until now there has been no experimental or theoretical exploration of the relative influence of material characteristics typical of geological flows on overall rheology. Shearing samples from 50-300 rad/s under constant pressure 3.5 kPa, we measure dilation and fluctuation energy and establish what combination of parameters best predicts flow behavior. In fast granular flows, dilation results from fluctuation energy, but their precise relationship depends on dissipative processes. The best predictor of dissipation is characteristic length of plastic displacement, delta. Flows that have greater plastic deformation within grains (higher delta) dilate more for a given increase in fluctuation energy. This counterintuitive result likely stems from the reduced efficiency of energy transfer to more distant parts of the shear flow. The fact that mineralogy's effect on high-velocity granular flows is captured by the material's propensity to absorb energy through plastic damage or release energy through fracture is useful for understanding energy partitioning in granular flows and predicting the shear behavior of a wide range of geological materials. Plain Language Summary Granular materials like sands and powders can behave like a solid, a liquid, or a gas depending on how fast they are moving. For example, a sandy beach will support a person's weight like any other solid, but if someone starts to jog on the beach, the sand weakens and their feet sink slightly with each step. Flowing sands and powders form the basis of some of nature's most dangerous processes, such as landslides and earthquakes, but because of the complicated relationship between flow speed and granular strength, it can be difficult to predict the behavior of such events. We performed experiments in a lab to imitate very fast granular flows and tested how the mineralogy or type of sand changed the strength of such flows. We found that a material's tendency to incur very small-scale damage on individual grains has a direct effect on the larger-scale strength of the sand. Minerals that are more susceptible to damage expand more at high velocities and are therefore more gas-like rather than liquid-like. This general result is useful for geologists who want to compare different naturally occurring granular flows or investigate the energy balances that govern such events.