Particle size-segregation in granular ows driven by a conveyor belt.

Particle motion within the leading edge of bidisperse gravity-driven granular flows is affected by particle size segregation. Shear-induced segregation causes coarse particles to move towards the upper layers. Then, these are advected down to the tip region, in which they are overrun and dragged back by the flow, while being stuck on the bottom. This phenomenon, called breaking size-segregation wave has been studied from a theoretical perspective, but there is a lack of experimental evidence1. Large particle displacement along the bottom signicantly a affects the flow composition, generating higher bed friction. At the laboratory scale, visualizing what happens within the leading edge made of bidisperse particles is a signicant challenge, especially when the objective is to observe breaking size-segregation waves. In this talk we address a particular point: how the rheological behavior depends on local concentrations? We present experiments performed using a conveyor belt (with a corrugated surface) and a bidisperse mixture of spherical grains. By employing the refractive index match technique we were able to measure the local dynamics of a self-sustained granular avalanche front. Experiments involving beads of 6 and 14 mm in diameter were carried out under different slope angles, with the mean large-particle concentration kept at 50%. We found that uniform flow conditions were achieved on average, but locally within the leading edge, the flow depth varied as a function of the coarse-particle concentration. Two main regimes were observed. In the first regime, the leading edge was composed of coarse particles, resembling the boulder-rich front described by theory. The second regime occurred when the coarsest particles were dragged back by the belt and replaced with fine particles. The change in the leading-edge composition led to instabilities driven by size segregation, which fostered the transition to the first regime. Local changes in the bulk composition evidently led to a varying slope of the bulk free surface, which reected the rheological changes (bed friction, and probably internal energy dissipation).