Imagine you put a lot of magnetic particles inside an oily substance, so that they get “stuck”, but they can flow. Then you got a ferrofluid. These fluids have very strange behaviour. Look at the almost alive forms in these videos…
Why do they behave like that? The magnetic particles get attracted by the magnetic field, and they “drag” the oil. Of course, a special kind of mixing is necessary here, but we won’t go into that. But, why the spikes? Because of the surface tension. As in any liquid, ferrofluids want to minimise their free-surface area, so the optimum shape is this array of conical shapes.
They do have practical applications. For example, in your hard drive, they are a good lubricant to the magnetic shafts. They are normally used as seals, since they will attach to any magnetized surface very tightly.
But, despite their name, ferrofluids are not ferromagnetic! The magnetic particles inside are not interacting, so it does not acquire a net magnetization in absence of external fields. I am developing myself some theory regarding possible “truly” ferromagnetic fluids, if they might exist… you know how theorists work, we have the idea, and then we pray the experimentalists to find it out…
With thanks to Rodolfo
Physics Napkins 