Sinking in a bed of grains activated by shearing

We show how a weak force f enables intruder motion through dense granular materials subject to external mechanical excitations, in the present case, stepwise shearing. A force acts on a Teflon disk in a two-dimensional system of photoelastic disks. This force is much smaller than the smallest force needed to move the disk without any external excitation. In a cycle, the material plus intruder are sheared quasistatically from gamma = 0 to gamma(max), and then backwards to gamma = 0. During various cycle phases, fragile and jammed states form. Net intruder motion delta occurs during fragile periods generated by shear reversals. delta per cycle, e.g., the quasistatic rate c, is constant, linearly dependent on gamma(max) and f. It vanishes as c alpha (phi(c) - phi)(a), with a similar or equal to 3 and phi(c) similar or equal to phi(J), reflecting the stiffening of granular systems under shear [J. Ren, J. A. Dijksman, and R. P. Behringer, Phys. Rev. Lett. 110, 018302 (2013)] as phi -> phi(J). The intruder motion induces large-scale grain circulation. In the intruder frame, this motion is a granular analog to fluid flow past a cylinder, where f is the drag force exerted by the flow.