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 FD are observed. Above φc fluctuations in FD are periodic and increase in magnitude with φ. Velocity field measurements indicate that the bifurcation in FD 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.
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Gravish et al. (2010) studied this question.
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