Probing non-locality in gravity-driven granular flows

Gravity-driven granular flows appear widely in nature, with avalanches, landslides and debris flows, changing the world around us and endangering lives. They are also crucial in countless important processes across many different types of industry, including pharmaceuticals, agriculture and construction. Despite their ubiquity, there are aspects of these flows that remain poorly understood. Specifically, we are interested in validating the concept of 'granular fluidity' that has been utilised in the development of theoretical non-local constitutive equations. Extensive experimental validation is currently lacking for these models, especially in determining the physical suitability of fluidity definitions. Using innovative experimental techniques that allow us to visualise the forces within a granular flow, we probe the origins of granular fluidity on a microscopic scale. Importantly, we show that boundary conditions have a significant impact on the flow and the suitability of the granular fluidity model used.