From frictional to viscous behavior: three-dimensional imaging and rheology of gravitational suspensions.

We probe the three-dimensional flow structure and rheology of gravitational (nondensity matched) suspensions for a range of driving rates in a split-bottom geometry. We establish that for sufficiently slow flows, the suspension flows as if it were a dry granular medium, and confirm recent theoretical modeling on the rheology of split-bottom flows. For faster driving, the flow behavior is shown to be consistent with the rheological behavior predicted by the recently developed "inertial number" approaches for suspension flows.