Ferrofluids

Ferrofluids are strange materials that are part liquid, part magnet, and which solidifies when exposed to magnetic fields. They consist of nanometer sized magnetic particles dispersed in a nonmagnetic fluid such as water or kerosene and form a magnetic liquid.

The magnetic particles consist of several thousand magnetic atoms of iron (Fe) or cobalt (Co) all aligned in the same direction and make up a tiny magnet. Since the particles are so small, they are continously hit by the liquid molecules and are thrown around in a random thermal motion (Brownian motion). This vigorous motion prevents the nanoparticles from aggregating into larger units. In some cases the attractive forces may be so strong that it is necessary to coat the particles with a protective soap layer (a surfactant) in order to keep them separated.

The ferrofluid is strongly attracted to magnetic fields and will move to the position where the magnetic field is strongest. Such magnetic fluids may therefore be moved or trapped by magnetic fields. They have important applications in bearings between moving parts and in sealings. Ordinary lubricants will soon be moved away by the moving part, but ferrofluid lubricants can be kept in position by magnetic fields. They also allow an air pressure difference across the bearing, thus acting as a sealing. Other types of ferrofluid commercial applications include vibration shock absorbers and resonance absorbers in hi-fi loadspeakers.

Much of their usefulness is related to the effect that the liquid stiffens when it is exposed to a magnetic field. In technical terms, the viscosity of the fluid increases with the magnetic field strength. In strong magnetic fields the ferrofluid may look like gelly or rubber and thus be used to reduce vibrations.

Microscopic particles, like colloidal microspheres or silicate nanoplates, are acted on by magnetic forces when they are dispersed in a ferrofluid. By using an external magnetic field, we may move these suspended nonmagnetic microparticles, or by a proper combination of several magnetic fields, we may position the microparticles in ordered patterns. Such ordered microparticle patterns can be easily observed using an optical microscope and may be useful in laser-optical applications.