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We study the local flow properties of various materials in a vane-in-cupgeometry. We use magnetic resonance imaging techniques to measure velocitiesand particle concentrations in flowing Newtonian fluid, yield stress fluid, andin a concentrated suspension of noncolloidal particles in a yield stress fluid.In the Newtonian fluid, we observe that the $\theta$-averaged strain ratecomponent $d_{r,\theta}$ decreases as the inverse squared radius in the gap, inagreement with a Couette analogy. This allows direct comparison (withoutend-effect corrections) of the resistances to shear in vane and Couettegeometries. Here, the mean shear stress in the vane-in-cup geometry is slightlylower than in a Couette cell of same dimensions, and a little higher than whenthe vane is embedded in an infinite medium. We also observe that the flowenters deeply the region between the blades, leading to significant extensionalflow. In the yield stress fluid, in contrast with the usually accepted picturebased on simulation results from the literature, we find that the layer ofmaterial that is sheared near the blades at low velocity is not cylindrical.There is thus a significant extensional component of shear that should be takeninto account in the analysis. Finally and surprisingly, in the suspension, weobserve that a thin non-cylindrical slip layer made of the pure interstitialyield stress fluid appears quickly at the interface between the shearedmaterial and the material that moves as a rigid body between the blades. Thisfeature can be attributed to the non-symmetric trajectories of the noncolloidalparticles around the edges of the blades. This new important observation is insharp contradiction with the common belief that the vane tool prevents slippageand may preclude the use of the vane tool for studying the flows of pastymaterials with large particles.

Flows and heterogeneities with a vane tool: Magnetic resonance imaging measurements