Experiments on force fluctuations and the jamming transition
Using photoelastic experimental techniques, we probe the nature of jamming in dense 2D granular systems. This approach allows a complete characterization of the system, including contact forces, and particle motion. Key findings for systems of disks include: force distributions that are sensitive to the stress state of the system (e.g. isotropic or not), The distributions have a roughly exponential tail following pure shear, but transition to a more gaussian-like distribution for isotropic compression. Near jamming, we find that the contact number per particle, Z, rises sharply as a function of packing fraction, f, at jamming, and then continues to rise above critical with an exponent that is consistent with 1/2. The pressure also rises as a function of f above jamming with an exponent slightly larger than 1.0, which is consistent with predictions. We also find that under cyclic shear, a dense granular material undergoes a different type of jamming transition, and we present initial results showing that P is a function of Z above this transition. © 2009 American Institute of Physics.
Zhang, J; Behringer, RP; Majmudar, TS; Sperl, M
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