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 softwar e are used to study the vibro-fluidization of a two-dimensional powder. The dependence of granular temperature on vibration frequency, amplitude and n umber of grains is compared with simulations and a simple analytical model. Particle tracking at low base accelerations has also shown that a single l arge intruder in a two-dimensional bed of monodisperse particles moves upwa rds at the same speed as the smaller grains over a wide range of accelerati ons, suggesting that convection is the key to size segregation under these conditions. Force profiles have been measured under conical sandpiles by us ing a simple elasto-optical technique, which provides a spatial resolution comparable to the grain diameter. The results confirm the existence of coun ter-intuitive pressure dips at the centre of the pile. Simple two-dimension al 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.