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

• Received 25 May 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.128301