Granular flow of 3D mixtures of soft and hard spheres

The discharge of granular mixtures composed of hard frictional beads and soft low-friction beads was investigated in a cylindrical silo in experiments and numerical simulations. In the two limits, we find a fill height dependent flow rate for 100% low friction soft grains and a height independent flow rate for 100% hard frictional grains. When mixing the two types of grains, the transition between the two limiting cases occurs rather abruptly. Namely, adding only 20% of hard frictional grains to a sample of low friction soft grains changes the dependence of the flow rate on the discharged mass significantly, i.e. causes the slope of the curve to decrease by 50-70%. Our numerical simulations reveal that the main factor leading to the strong change in the flow rate behavior at low hard grain concentration is the high sensitivity of the stress conditions in the orifice region to the mixture composition. Since frictional dissipation can be an important factor influencing the flow rate, we also analyze the frictional properties of our samples in two additional experiments: (i) quasistatic shear tests in a split-bottom shear cell and (ii) drag force measurements on an object moved in the mixture. The mixtures show increasing dissipation as a function of increasing hard grain concentration in both of these measurements, but the increase is rather modest in the low concentration range, thus it does not explain the abrupt change in the silo discharge rate.