# physics4me

physicsgg

## … makes liquid climb like spaghetti winding up a fork

The Weissenberg Effect happens when a fluid behaves in a very counter-intuitive way. Instead of being driven away from a spinning object, it starts climbing. Take a look at something you may have seen at home—but you’ve probably never noticed.

If you’ve cooked at all, you know what happens when you put a spinning electric mixer in a batch of dough. The dough get driven out to the sides of the bowl. But you’ll probably also see a little bit of liquid clinging to the central rod of the beater—maybe even forming a little bump there.

This is because what you’re stirring is probably not a strongly viscoelastic fluid. Viscoelastic fluids have the property of viscosity—they will resist forces applied to change their current shape—and elasticity—they will attempt to regain a shape after they’ve been stretched out of it. When these properties are present, and a spinning mixer is placed into the fluid this happens:

It’s not always that dramatic. Sometimes the fluid only climbs slightly, gathering into a top-heavy bump around the base of the rod. If the fluid were not viscous or elastic, the part of the liquid being driven in a circle could just slip by the rest of the liquid. Instead, as the liquid resists being deformed, a component of force is created perpendicular to the force in the rod. Instead of moving back and forth, some of the liquid moves up, and since it’s elastic, it takes the rest of the liquid with it.

Nearly all liquids have some degree of viscosity or elasticity, which is why, when you’re stirring dough, a little will cling, but most will get thrown off. Only strongly viscoelastic fluids will be impressive when stirred by a rod. There is, however, a twist. Physicists have discovered the Rodless Weissenberg Effect. Stir a viscoelastic fluid without a rod, and you’ll still get a slight climb around the central vortex created by the stirrer.