Numerical simulations of air-driven granular separation

Recently it has been shown that binary mixtures of equal-sized fine granular materials exhibit spontaneous separation under vertical vibration in the presence of air [Science 295, 1877 (2002)]. Here we describe a model of this behavior based on soft-sphere molecular dynamics coupled to the motion of the surrounding air. It exhibits many of the features observed experimentally including almost complete separation of the components into well defined regions with extremely sharp boundaries. The basic separation mechanism is robust and insensitive to many of the model parameters. Our results show that the forced flow of air through the bed, induced by vibration of the container, is responsible for this form of separation.