Granular segregation in a tilted-rotating drum

In this manuscript we present experimental results for the rotation of a bi-disperse mixture of granular material (glass beads) in a titled cylindrical drum. The system parameters are the tilt angle (35° ≤ α ≤ 65°) measured from the horizontal where α = 0 is a vertically oriented right circular cylindrical drum, concentration of smaller beads based on volume (ϕ = 0.30, 0.40 or 0.50), and the constant angular rotation rate of a 66 mm diameter drum (0.25 Hz ≤ Ω ≤ 1.1 Hz) resulting in rotational Froude numbers in the range 0.0002 < Fr < 0.004. The rotation rate range is chosen so that the mixture undergoes a continuous avalanche process resulting in a cylindrical wedge shaped granular domain. A horizontally oriented rotating drum typically yields segregation of the smaller beads into multiple bands that can merge and split, but in a tilted system it is observed that the smaller particles form a single-segregated band at a fixed axial location. It is unclear though if this segregation is mostly axial or radial and which parameters influence the axial location of the band which is observed to form at both extremum depending on the tilt angle and Froude number. In order to estimate the segregation quality a semi-analytical solution for the steady shape of the flowing granular media based on assumptions made for the total volume of beads, and subsequently the apparent volume of smaller diameter beads based on the segregated band width, is proposed. The segregated band location appears to be driven by both the rotation rate and tilt angle in the range of parameter used in this study. Stronger data trends appear when the tilt angle is used as the discriminating parameter.