Ferrofluid in a Magnetic Field

Wallpaper Description:

A superparamagnetic fluid, otherwise known as a ferrofluid, in a dish over a neodymium magnet.

A ferrofluid is a liquid which becomes strongly polarised in the presence of a magnetic field. It is a colloidal mixture comprising extremely small magnetic particles suspended in a liquid. The particles are coated with a surface active agent (surfactant) to prevent them from clumping together.

Ferrofluids are composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetization in the absence of an externally applied field. In fact, ferrofluids display (bulk-scale) paramagnetism, and are often referred as being "superparamagnetic" due to their large magnetic susceptibility. Permanently magnetized fluids are difficult to create at present.

More: http://en.wikipedia.org/wiki/Ferrofluid