Substrate effects from force chain dynamics in dense granular flows

Granular materials are composed of solid, discrete particles and exhibit mechanical properties that range from fluid to solid behavior. Some of the complexity exhibited by granular systems arises due to the long-range order that develops due to particle-particle contact. Inter-particle forces in granular materials often form a distributive network of filamentary force-accommodating chains (i.e., force chains), such that a fraction of the total number of particles accommodates the majority of the forces in the system. The force chain network inherent to a system composed of granular materials controls the macroscopic behavior of the granular material. Force transmission by these filamentary chains is focused (or localized) to the grain scale at boundaries such as the granular flow substrate. This investigation addresses the effects of force localization on the substrate by dynamic force chain processes and the implications for bed entrainment in dense, unconfined, two-dimensional, gravity-driven granular flows. Our experimental system employs photoelastic techniques and provides an avenue for quantitative force analysis via image processing and provides a data set that can be used validate discrete element modeling approaches. We show that force chains cause extreme bed-force localization in dynamic granular systems, and that these localized forces can propagate extensively into the substrate, even ahead of the flow front.