Measurement of growing dynamical length scale on approach to jamming in granular systems

The flow of granular materials is of widespread practical and fundamental interest. One challenge to understanding and controlling behavior is that the response is nonlinear, with a forcing threshold below which the medium is static. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems brought close to jamming, where intermediate-time motion is correlated in the form of intermitted string-like swirls. Here this will be illustrated with experiments on air-driven beads, where jamming is approached by lowering the effective temperature, as well as by experiments on rapid heap flow, where jamming is approached as a function of depth from the free surface. Use of novel statistical quantities and optical spectroscopies reveal a growing dynamical length scale on approach to jamming. Collaborators: Adam Abate, Hiroaki Katsuragi, Aaron Keys, Sharon Glotzer.