Dynamical effects in the segregation of granular mixtures in quasi 2D piles

• Dynamical mixing of glass beads with larger ones or coriander grains is compared.
• Layering is only observed for the mixtures of glass beads of different diameters.
• Complete segregation only occurs for coriander grains and smaller glass beads.
• Avalanches play a key part in layering and only occur for mixtures of glass beads.
• Spatiotemporal diagrams analyze local displacements at the surface of the piles.

The dynamics of segregation, mixing and layering is studied during the build-up of quasi-2D piles of two different species of grains of different diameters (i.e. 1 mm glass beads and either 3 mm glass beads or 3.1 mm coriander seeds). The ranges of existence of the different flow regimes are studied as a function of the relative volume flow rates of the two species and of the drop height; the variation of the mean slope of the pile with time is shown to depend on the flow regime. Complete segregation occurs when the larger grains are coriander seeds while there is always some mixing for glass beads, although the ratio between the sizes of the particles is the same in both cases. A layered regime is observed for glass beads of same geometry (i.e. spheres) and density but of two different diameters: it is accompanied by variations of large amplitude of the slope of the pile. No layering occurs when the large particles are coriander seeds instead of glass beads. The local processes underlying these regimes are analyzed quantitatively from spatiotemporal diagrams of the profile of the free surface and the characteristic velocities of the different types of displacements of the grains are determined. Avalanches are only observed for glass beads and in the layered regime and play an important part in the development of the layers.

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