Avalanches in fine, cohesive powders

We have investigated the onset of avalanches in fine, cohesive granular mat erials. In our experiments sheer stress is generated by tilting an initiali zed bed of powder and increasing the angle of tilt until the powder avalanc hes. We find that the angle alpha of the avalanche decreases with increasin g bed width. The avalanche depth increases with the bed width and, in all c ases, is of the order of several millimeters, which is much greater than th e particle size. We carry out a macroscopic analysis of the avalanche proce ss based on Coulomb's method of wedges. This analysis shows the fundamental role played by powder cohesion and boundary conditions on avalanches in fi ne cohesive powders. This behavior contrasts with the behavior of noncohesi ve grains, such as dry sand, where avalanches consist of superficial layers of about ten grains. The reason behind this is that fur our experimental p owders (particle diameter similar to 10 mum) the van der Waals interparticl e adhesive force exceeds several orders of magnitude particle weight. Adhes ive forces oppose gravity, and as a result fine cohesive powders settle in very open structures as compared to noncohesive granular materials. Because of the dominance of adhesive forces over particle weight, our materials be have more like wet sand.