Jamming in granular materials

Granular materials are one of a class of materials which undergo a transition from mechanically unstable to mechanically stable states as key system parameters change. Pioneering work by Liu and Nagel and O'Hern et al. focused on models consisting of frictionless grains. In this case, density, commonly expressed in terms of the packing fraction, phi, is of particular importance. For instance. O'Hern et al. found that there is a minimum phi = phi(J), such that below this value there are no jammed states, and that above this value, all stress-isotropic states are jammed. Recently, simulations and experiments have explored the case of grains with friction. This case is more subtle, and phi does not play such a simple role. Recently, several experiments have shown that there exists a range of relatively low phi's such that at the same phi it is possible to have jammed, unjammed, and fragile states in the sense of Cates et al. This review discusses some of this recent work, and contrasts the cases of jamming for frictionless and frictional granular systems. (C) 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.