A liquid-metal blob reacting to a simulated animated magnetic field — spikes rise, fall, and travel across the surface. Drag to orbit, scroll to zoom.
Ferrofluid is a colloidal suspension of nanoscale ferromagnetic particles (typically magnetite, Fe3O4) coated with a surfactant and dispersed in a carrier fluid such as oil or water, which keeps the particles from clumping via Brownian motion. When an external magnetic field grows strong enough, the fluid's surface becomes unstable: magnetic forces trying to concentrate field lines through peaks overcome surface tension and gravity, which normally want to keep the surface flat. Past this critical threshold the surface spontaneously buckles into a regular array of spikes — the Rosensweig, or normal-field, instability — with spike spacing and sharpness set by the balance between magnetic pressure and surface tension. The spikes continuously rise, split, and fall as the field pattern shifts, since the fluid remains liquid and simply redistributes itself along field lines. Beyond its striking appearance, ferrofluid is used in loudspeaker voice-coil cooling and damping, rotary shaft seals, and precision damping/sensing devices.