Breaking size segregation waves in experimental bidisperse granular flows.

In the leading edge of a bidisperse gravity-driven granular flow, two opposing mechanisms coexist: size-segregation and mixing. As a result of shear-induced particle segregation, large particles are displaced to upper layers of the flow where they move faster towards the tip. Rapidly, they are dragged back, sticked to the bottom where they mix with small particles and are driven again to the upper layers, hence repeating the process. This recirculation phenomenon called breaking size-segregation wave is well defined theoretically but with no clear experimental evidence. Species local concentration and size ratio greatly affect particle size segregation. The present experiments addresses the following question: how does local concentration of particles affect the development and characteristics of breaking size segregation waves? We carried out experiments in a conveyor belt facility, consisting of a closed inclined channel with movable rough bed. The granular material is a mixture of 6 and 14 mm glass beads with the large particle concentration varying from 0 to 1 relative to the bulk concentration. Using the refractive index match technique and particle tracking velocimetry, we are able to visualise and measure internal movement and dynamics of the bulk. Experimental results confirm the existence of breaking size segregation waves. Local particle concentration induces different flow heights and positions for the mixing region, therefore determining two clear flow regimes and the transition between them. Besides the flow is partially uniform and unstable, waves are characterized in terms of the species’ bulk concentration.