Size segregation and convection

– The size segregation of granular materials in a vibrating container is investigated using Molecular Dynamics. We find that the rising of larger particles is accompanied by the existence of convection cells even in the case of the lowest possible frequencies. The convection can, however, also be triggered by the larger particle itself. The possibility of rising through this mechanism strongly depends on the depth of the larger particle. One of the most puzzling phenomena encountered in granular matter is size segregation: When a mixture of grains of the same material (equal density) but different size is shaken in a container the larger particles rise to the top. This effect has been extensively studied experimentally [1, 2, 3] and has much importance in numerous industrial processes[4]. Recently this so called " Brazil nut effect " has attracted much interest among physicists[5]. Size segregation inevitably seems to contradict equilibrium statistical mechanics since the density of the overall packing increases with polydispersivity and so gravity makes situations with larger particles on the bottom energetically more favourable. Rosato et al.[6] proposed a Monte Carlo algorithm and put forward a kinetic argument to explain segregation using the fact that smaller particles are more mobile. In the same year Haff and Werner[7] did Molecular Dynamics simulations of rather small systems and claimed that segregation was essentially a consequence of solid friction and the rotation of the particles. Jullien et al.[8] used a piling technique which is non–stochastic as compared to the one of ref. 6 and found a critical ratio R for the radia of spherical particles below which no segregation occurs. Based on these ideas Duran et al.[9] formulated a geometrical theory for segregation in which the small particles glide down along the surface of the larger particle, the critical ratio R of radia being between continuous and discontinuous gliding. They also claimed experimental evidence for two types of dynamics and visualized the discontinuous ascent of the larger particle through stroboscopic photos. Jullien et al.[10] reproduced the discontinuous dynamics by including horizontal random fluctuations into their model. Parallel to these local theories there has been the " convection connection " : It is known Typeset using EURO-T E X