Friday, May 27, 2016

The "Weissenberg Effect" or Rod Climbing Experiment

You may have noticed when baking that fluids don’t always behave as expected when you agitate them. If you put a spinning rod into a fluid, we’d expect the rod to fling fluid away, creating a little vortex that stirs everything around. And for a typical Newtonian fluid, this is what we see. The fluid’s viscosity tries to resist deforming the fluid, but the momentum imparted by the rod wins out.

With a viscoelastic fluid, on the other hand, the story is much different. As before, the spinning of the rod deforms the fluid. But the viscoelastic fluid contains long chains of polymers. As those polymers get stretched by the deformation, they generate their own forces, including forces parallel to the rod. Instead of being flung outward, the viscoelastic fluid starts climbing up the rod, with the stretchy elasticity of the polymers helping pull more fluid up and up.

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