Fluidization, segregation and stress propagation in granular materials

Results from recent experimental investigations into the fluidization, size-segregation and stress-transmission behaviour of model granular materials are described. Digital high-speed photography and particle-tracking software are used to study the vibro-fluidization of a two-dimensional powder. The dependence of granular temperature on vibration frequency, amplitude and number of grains is compared with simulations and a simple analytical model. Particle tracking at low base accelerations has also shown that a single large intruder in a two-dimensional bed of monodisperse particles moves upwards at the same speed as the smaller grains over a wide range of accelerations, suggesting that convection is the key to size segregation under these conditions. Force profiles have been measured under conical sandpiles by using a simple elasto-optical technique, which provides a spatial resolution comparable to the grain diameter. The results confirm the existence of counter-intuitive pressure dips at the centre of the pile. Simple two-dimensional models for force transmission in granular materials are reviewed; these are extended to three dimensions and to lattices with perturbations induced by deformation of the grains or by polydispersity.