  We study the local flow properties of various materials in a vane-in-cup
geometry. We use magnetic resonance imaging techniques to measure velocities
and particle concentrations in flowing Newtonian fluid, yield stress fluid, and
in a concentrated suspension of noncolloidal particles in a yield stress fluid.
In the Newtonian fluid, we observe that the $\theta$-averaged strain rate
component $d_{r,\theta}$ decreases as the inverse squared radius in the gap, in
agreement with a Couette analogy. This allows direct comparison (without
end-effect corrections) of the resistances to shear in vane and Couette
geometries. Here, the mean shear stress in the vane-in-cup geometry is slightly
lower than in a Couette cell of same dimensions, and a little higher than when
the vane is embedded in an infinite medium. We also observe that the flow
enters deeply the region between the blades, leading to significant extensional
flow. In the yield stress fluid, in contrast with the usually accepted picture
based on simulation results from the literature, we find that the layer of
material that is sheared near the blades at low velocity is not cylindrical.
There is thus a significant extensional component of shear that should be taken
into account in the analysis. Finally and surprisingly, in the suspension, we
observe that a thin non-cylindrical slip layer made of the pure interstitial
yield stress fluid appears quickly at the interface between the sheared
material and the material that moves as a rigid body between the blades. This
feature can be attributed to the non-symmetric trajectories of the noncolloidal
particles around the edges of the blades. This new important observation is in
sharp contradiction with the common belief that the vane tool prevents slippage
and may preclude the use of the vane tool for studying the flows of pasty
materials with large particles.
