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Complex fluids under shear rotation
When thinking about non-Newtonian fluids, phenomena that come to mind
are dependence of the viscosity on shear rate (shear thinning or
thickening) or effects related to anisotropic normal stresses, like the
Weissenberg (or "rod climbing") effect. Thanks to a two-dimensional
parallel plate rheometer where relative plate motion is possible in any
direction parallel to the gap, we show that under varying flow
directions (that we dub shear rotations), complex fluids routinely
exhibit shear forces misaligned with the flow direction. We measure
these forces on a few complex fluids : a simple non-Brownian suspension
of hard spheres, which otherwise has a viscosity which is independent of
the flow rate (a quasi-Newtonian fluid), a microgel suspension
(Carbopol), which is a simple yield stress fluid, and a mayonnaise. The
physical origin of forces orthogonal to the flow direction depends on
the system. For the hard sphere suspension, an interplay between contact
and hydrodynamic forces is at work, whereas for yield stress fluids, the
effects comes from the disorder in the local yield stress at the
microscopic scale.