[Kevin Sevcik]

The technical term is actually a magnetorheological fluid (MRF). Ferrofluids just attracted to magnetic fields, they don't change viscosity or anything. I'm unsure about using it as a brake, however. While it solidifies under a magnetic field, that's only while the pressure applied to it doesn't break the chains of nanoparticles that are solidifying it. Think of it as really quick setting jello as opposed to ice. So as a brake, it'd be somewhat less that useful. Viscous braking isn't typically useful anyways, as viscous resistance is dependent on velocity, so you wouldn't have much luck sitting in place not moving.