The behaviour of water-immersed glass-bronze particulate systems under vertical vibration

We report a detailed investigation of the behaviour of water-immersed binary mixtures of glass and bronze spheres under sinusoidal vertical vibration. Excellent separation with a bronze layer above a glass layer is observed for a range of particle sizes and size ratios, with very sharp boundaries between the bronze and glass regions. Convection cells exist within each separated region but convection does not cause global mixing. Separation is maintained in the presence of convection due to gaps which appear between the bronze and glass beds for part of the vibration cycle. We vary the size of the spheres and the size ratio, investigating the separation behaviour as a function of frequency and amplitude of vibration. We also discuss a number of meta-stable granular configurations. The Faraday tilting observed at low frequencies enhances the speed of separation. For very fine particles, the amplitude of vibration necessary for separation becomes very large and the time scale for separation becomes very long. For large particle diameters, the fluid damping of the particle motion weakens and separation may fail through global convective mixing.