Ferrofluid

What’s That? (64)
Originally uploaded by jurvetson.

As the photographer explains:

More specifically, it's a superparamagnetic fluid, otherwise known as a Ferrofluid, in a dish over a neodymium magnet.

The key to a funky fluid like this [:] 10 nanometer magnetic particles in suspension. At that critical particle size, the particles’ magnetic fields balance with the surface tension of the liquid. So they don’t clump together, even in strong magnetic fields (in this case a N40 magnet).

Ferrofluids are used as heat sinks in loudspeakers and liquid seals around the fast-spinning drive shaft in a hard disk.