Cereals in grain silos, coal in freight cars, and powders in processing vats are all examples of granular materials that show similar flow properties despite the differences in size and shape of the particles. During flow, loosely packed granular materials are similar to fluids in that the particles are not closely connected but nonetheless interact with each other through periodic collisions. Above a critical packing fraction, the number of contacts between neighboring particles increases and creates mechanical stability leading to a jamming transition. In a set of elegant experiments, Majmudar et al. have tracked the jamming transition in two dimensions for a bidisperse mixture of disk-shaped particles, with a size ratio and composition designed to guarantee a disordered system. The particles were made from a birefringent polymer so that contacts between particles and their stress fields could be measured with polarized light, while a second image taken without polarizers tracked the particle centers. The authors observed critical values at the jamming transition consistent with recent simulations, although the sharpness of the transition was diminished because of residual stress effects from the walls of the container. -- MSL
Phys. Rev. Lett. 98, 058001 (2007).
