Size segregation fluxes in oscillating shear cells.

Gravity-driven granular free-surface flows (or granular avalanches) provide ideal conditions for particles to separate based on their size. This size segregation process is caused by shear-induced dilatation that allows the percolation of small particles under gravity (kinetic sieving), after which large particles are pushed upwards by surrounding small particles (squeeze expulsion). Consider able progress has been made in developing simple continuum models for the segregation process, however, the functional dependence of the segregation rates is still relatively poorly understood. The objective of the present work is to gain an understanding of the segregation rates, by using simple shear experiments. Simple shear cells have been widely used to study granular segregation and consist of two tilted parallel plates that rotate about pivot points to create an oscillatory shear within the bulk material. For this work, two different shear cells have been used: a 2D and a 3D cell. The 2D cell width ranges from 55 to 280 mm, in 15 mm steps. The cell is filled with Polyoxymethylene (POM) disks of different diameters but of the same thickness as the cell gap (∼ 5 mm). The 3D shear cell consists in two parallel plates that pivot in an upper point located at 80 mm from the movable bottom of the load cell. The separation between the plates is set to 45 mm while the plate thickness is 68 mm. For the 3D shear cell, we used the refractive index match (RIM) technique for visualizing the intruder. Poly methyl methacrylate (PMMA) spheres, transparent for the substrate and opaque for the intruders, are used in the experiments along with Triton X-100 to achieve the refractive index matched mixture. We ran experiments using single intruders immersed in a differently sized medium in both shear cells. Quantification of the intruders and bulk local velocities is provided. An estimation of the segregation rates as a function of the size ratio is also obtained. Size segregation in the 2D shear cell is only observed for large intruders because of the constraints imposed by the planar configuration. In contrast, in the 3D shear cell small particle percolation occurs and it may stop at middle positions in specific cases. Under certain conditions, large intruders didn’t segregate. These observations raised the question: what is initiating and restraining segregation? Within our experimental framework, we suggest the interstitial fluid and size ratio may prevent and restrain segregation.