We use plate drag to study the response of granular media to localized forcing as a function of volume fraction, $\phi$. A bifurcation in the force and flow occurs at the onset of dilatancy, $\phi_c.$ Below $\phi_c$ rapid fluctuations in the drag force, $F_D,$ are observed. Above $\phi_c$ fluctuations in $F_D$ are periodic and increase with $\phi$. Velocity field measurements indicate that the bifurcation in $F_D$ results from the formation of stable shear bands above $\phi_c$ which are created and destroyed periodically during drag. A friction-based model captures the dynamics for $\phi>\phi_c$.
Categories: cond-mat.soft physics.flu-dyn physics.geo-ph