WONDERMAGNET.COM - NdFeB Magnets, Magnet Wire, Books, Weird Science, Needful Things
A ferrofluid is a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier....basically it's a magnetic liquid. When it's exposed to a magnetic field the very small particles within it take the shape of the magnetic field. Accordingly, the fluid becomes more dense as the flux increases and vice versa.
Also used as a dampener, often in loudspeakers and on CD / DVD laser heads.
They also have it for purchase.