A ferrofluid is a stable colloidal suspension of ferromagnetic particles in a liquid carrier.  Ferrofluids contain tiny particles (~10 nm diameter) of a magnetic solid suspended in a liquid medium.

Ferrofluids were discovered at the National Aeronautics and Space Administration (NASA) Research Center in the 1960s.  NASA scientists were investigating different possible methods of controlling liquids in space.  NASA scientists discovered that they could precisely control the location of a ferrofluid through the application of a magnetic field.  Additionally, by varying the strength of the field, the fluids could be made to flow.