Flowing granular materials arise everywhere around us, in industry from pharmaceutical processes to bulk good transport lines, and in nature from snow avalanches to captivating dune fields.

In landslides, we have an interesting interplay between microscale (grain-grain contacts) and macroscale processes (continuum behavior). In order to understand critical macroscale processes such as stability of a slope, creep and failure, we need to be able to visualize and characterize the microscale interactions.

In this talk, I will introduce a laboratory technique called photoelasticity to visualize grain-grain contacts in time and space. Collisions between grains create a fascinating network of so-called force chains, which are responsible for the inhomogeneous distribution of stresses in a granular medium. We discover stress distributions in 2D granular avalanches, visualized with bespoke, superior-quality birefringent photoelastic particles. This technique gives us for the first time access to the full velocity, density and stress fields inside of a dynamic avalanche, and allows us to experimentally validate granular rheological models.