Capturing stress in motion: Novel photoelastic techniques for high-inertia granular flows

Rapid granular flows occur in a huge variety of natural and industrial processes, from snow avalanches to pharmaceutics. Despite their importance, our ability to model these flows is limited by shortcomings in theory, and inherent challenges in high-speed experimentation. Specifically, extracting quantitative stress data from dynamic flows is often impossible. To overcome this, we use advanced techniques to detect forces between particles in high-inertia experiments. In this work, we present a novel framework to harness photoelastic effects of accelerating particles, unlocking photoelasticity as a tool for collisional flows. We introduce unique experiments to subject particles to impulsive forces and record their optical response with remarkable temporal resolution. These experiments validate our innovative theory and illustrate challenges in high-speed photoelasticity, where viscoelasticity can lead to surprising effects. We also highlight results from large-scale avalanches performed in our lab. Here, we employ our advanced methods to give unprecedented insight into the spatial and temporal evolution of stresses within gravity-driven flows. The high-resolution data lets us determine the suitability of granular rheologies, including those that introduce descriptions of non-local effects. Our findings shed light on the micro-scale physics underpinning these models and indicate ways we can improve our ability to predict the dynamics of granular flows in nature and industry.