Beautiful and useful

This beautiful thing has practical applications. Here are some examples:

Electronic devices
Ferrofluids are used to form liquid seals around the spinning drive shafts in hard disks. The rotating shaft is surrounded by magnets. A small amount of ferrofluid, placed in the gap between the magnet and the shaft, will be held in place by its attraction to the magnet. The fluid of magnetic particles forms a barrier which prevents debris from entering the interior of the hard drive.

Aerospace
NASA has experimented using ferrofluids in a closed loop as the basis for a spacecraft's attitude control system. A magnetic field is applied to a loop of ferrofluid to change the angular momentum and influence the rotation of the spacecraft.

Medicine
In medicine, ferrofluids are used as contrast agents for magnetic resonance imaging and can be used for cancer detection. The ferrofluids are in this case composed of iron oxide nanoparticles and called SPION, for "Superparamagnetic Iron Oxide Nanoparticles."

There is also much experimentation with the use of ferrofluids in an experimental cancer treatment called magnetic hyperthermia. It is based on the fact that a ferrofluid placed in an alternating magnetic field releases heat.

(Music: Gluck's Dance of the Spirits, from Orfeo et Euridice)