Nonlocal granular rheology: Role of pressure and anisotropy

We probe the secondary rheology of granular media, by imposing a main flow and immersing a vane-shaped probe into the slowly flowing granulate. The secondary rheology is then the relation between the exerted torque T and rotation rate w of our probe. In the absence of any main flow, the probe experiences a clear yield-stress, whereas for any finite flow rate, the yield stress disappears and the secondary rheology takes on the form of a double-exponential relation between w and T. This secondary rheology does not only depend on the magnitude of T, but it is also anisotropic -which we show by varying the relative orientation of the probe and main flow. By studying the depth dependence of the three characteristic torques that characterize the secondary rheology, we show that for counterflow, the dominant contribution is frictional-like -i.e., T and pressure are proportional for given w- whereas for coflow, the situation is more complex. Our experiments thus reveal the crucial role of anisotropy for the rheology of granular media. Copyright (c) EPLA, 2014