When does spinning a magnet in a jar of water produce a thin tube of air that reaches all the way down to the bottom of the jar?

Problem in full

This is problem 10 of 17 for IYPT 2027

“Place a small rod-shaped magnet on the bottom of a vessel filled with liquid and set it into rotation using a magnetic stirrer. Above a certain rotation speed, a thin air channel forms, extending from the liquid surface down to the rotating magnet itself. Investigate and explain this phenomenon.”

Specialist equipment

• A magnetic stirrer as used in school science labs. These consist of a consisting of a rod shaped magnet that goes in a container of liquid, and an electronic baseplate that the container sits on. It is possible to make your own – but that isn’t the point of the experiment.
• A clear sided jar so you can see the vortex form.

Safety first!

There are no specific safety concerns with this experiment, however the type and state of the liquid used may require special care. Consider this in your safety plan.