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 γ=0 to γ_{max}, and then backwards to γ=0. During various cycle phases, fragile and jammed states form. Net intruder motion δ occurs during fragile periods generated by shear reversals. δ per cycle, e.g., the quasistatic rate c, is constant, linearly dependent on γ_{max} and f. It vanishes as c∝(ϕ_{c}-ϕ)^{a}, with a≃3 and ϕ_{c}≃ϕ_{J}, reflecting the stiffening of granular systems under shear [J. Ren, J. A. Dijksman, and R. P. Behringer, Phys. Rev. Lett. 110, 018302 (2013)]PRLTAO0031-900710.1103/PhysRevLett.110.018302 as ϕ→ϕ_{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.