Kinematics of granular heaps

It is widely believed that granular heaps form and grow in size by the capture of grains flowing down the free surface into the static layer below. The lack of knowledge about motion beneath the surface stems from the difficulty of visualizing the flow by optical imaging due to the opacity of granular media. Here we present the results of an experimental investigation of the internal flow of conical granular heaps. We create a 'steady' flowing heap by continuously pouring grains from a narrow source onto a transparent disk of finite diameter. We provide direct evidence of plastic flow occurring deep within an apparently static conical heap, challenging the conventional belief that flow is confined solely to the thin surface layer. We show radial outward motion at the base of the heap, with the flow transitioning from slow, frictional flow in the interior to fast, collisional flow at the surface. Our findings provide new insights into the internal mechanics of granular heaps and call into question theories for the stress in static granular heaps.