Force and flow transition in plowed granular media.

We use plate drag to study the response of granular media to localized forcing as a function of volume fraction ϕ. A bifurcation in the force and flow occurs at the onset of dilatancy ϕc. Below ϕc rapid fluctuations in the drag force F(D) are observed. Above ϕc fluctuations in F(D) are periodic and increase in magnitude with ϕ. Velocity field measurements indicate that the bifurcation in F(D) results from the formation of stable shear bands above ϕc which are created and destroyed periodically during drag. A friction-based wedge flow model captures the dynamics for ϕ>ϕc.