Dynamical behaviour of fine granular glass/bronze mixtures under vertical vibration

It has recently been reported that mixtures of fine equal-sized bronze and glass spheres separate into glass-rich and bronze-rich regions under sinusoidal vertical vibration [Burtally et al., Science 295, 1877 (2002)]. Here we report a study of these separation effects, varying both the bronze/glass size ratio and the composition of the mixture. For a wide range of mixtures we observe separation into bronze-rich and almost pure glass regions separated by extremely sharp boundaries. Either a bronze-rich region forms above a glass layer or a bronze-rich layer is sandwiched between two glass layers. Convection exists within each layer but does not act to cause mixing of the two regions. The separation into two regions is maintained even in the presence of a variety of spatial oscillations. The variations of behaviour with differing glass/bronze size ratios support the proposal of a differential air-damping separation mechanism, while experiments using a porous bottomed box indicate that separation requires air to be driven through the granular bed by the vibration.