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. (Image credit: Ewoldt Research Group, source)
Tag: Weissenberg effect
Stirring Up
When a viscoelastic non-Newtonian fluid is stirred, it climbs up the stirring rod. This behavior is known as the Weissenberg effect and results from the polymers in the fluid getting tangled and bunched due to the stirring. You may have noticed this effect in the kitchen when beating egg whites. In this video, researchers explore the effect using rodless stirring. The first example in the video shows a viscous Newtonian fluid being stirred. The stirring action creates a concave shape in the glycerin-air interface, and dye injection shows a toroidal vortex formed over the stirrer. Fluid near the center of the vortex is pulled downward and circulates out to the sides. In contrast, the viscoelastic fluid bulges outward when stirred. Dye visualization reveals fluid being pulled up the center into the bulge. It then travels outward, forming a mushroom-cap-like shape before sinking down the outside. This is also a toroidal vortex, but it rotates opposite the direction of the Newtonian one. Exactly how the polymers create this change in flow behavior is a matter of active research. (Video credit: E. Soto et al.)
The Weissenberg Effect
Non-Newtonian fluids exhibit all kinds of odd behaviors, even climbing up a spinning rod! This is known as the Weissenberg effect and is associated with polymers in the fluid.
