Temporal force fluctuations measured by tracking individual particles in granular materials under shear

The network of forces between contacts in a granular material determines the response of the material to an external perturbation. As the shear stress is increased, a static, rigid material can start to yield and flow as it becomes unjammed. The time- and ensemble-averaged distribution of forces shows a marked change in character at this jamming-unjamming transition. Here, we investigate the temporal fluctuations of the forces between individual particles and the external boundary in a dense, slowly sheared system. We find that the fluctuations occur over a very broad range of time scales, resulting in power spectra characterized by a 1/f-type shape and a rollover to 1/f(2) behavior above a turnover frequency f(*). At very low frequencies additional enhancements over the 1/f contributions are observed in regions of significant in-plane shear. Moreover, the magnitude of the force fluctuation spectra produced by one particle is different from those produced by its neighbors. Such heterogeneities persist over periods of time exceeding our measurement interval (60 h). In this respect they are reminiscent of the dynamic heterogeneities that occur in liquids supercooled toward the glass transition temperature.