A ferrofluid is a liquid which becomes strongly polarised in the presence of a magnetic field. Ferrofluids are composed of nanoscale ferromagnetic particles suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals and magnetic forces). Although the name suggests otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetisation in the absence of an externally-applied field. In fact, ferrofluids display paramagnetism, and are often referred as being "superparamagnetic" due to their large magnetic susceptibility. Although current theory does not preclude the possibility of a truly ferromagnetic fluid, no such fluid has yet been observed.

Ferrofluids are comprised of microscopic ferromagnetic nano-particles, usually magnetite, hematite or some other compound containing Fe2+ or Fe3+. The nano-particles are typically of order 10nm. This is small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them contribute to the overall magnetic response of the fluid. This is analogous to the way the that the ions in an aqueous paramagnetic salt solution (such as an aqueous solution of copper sulphate or manganese chloride) make the solution paramagnetic.

True ferrofluids are stable. This means that the solid particles do not agglomerate or phase separate even in extremely strong magnetic fields. However, the surfactant tends to break down over time (a few years), and eventually the nano-particles will agglomerate, and they will separate out and no longer contribute to the fluids magnetic response. The term magnetorheological fluid (MRF) refers to liquids similar to ferrofluids (FF) that solidify in the presence of a magnetic field. Magnetorheological fluids have micrometre scale magnetic particles that are 1–3 orders of magnitude larger than those of ferrofluids.

When a ferrofluid is subjected to a sufficiently strong vertical magnetic field, the surface spontaneously forms a regular pattern of corrugations. This effect, known as the normal-field instability, is truly remarkable. The formation of the corrugations increases the surface free energy and the gravitational energy of the liquid, but reduces the magnetic energy. The corrugations will only form above a critical magnetic field, when the reduction in magnetic energy outweighs the increase in surface and gravitation energy terms.

A FF on the contrary does not form chains. The random movement of the particles is larger than the force pulling them together. Their viscosity doesn't change, but they like to stay in high magnetic fields. The magnetorheological effect starts above a particle size of 10 nanometers.

Ferrofluids are superparamagnetic and have very low hysteresis.

The particles are usually iron, magnetite, or cobalt, and are smaller than a magnetic domain, typically 10 nanometers in diameter. The surrounding liquid is typically oil or water (or possibly wax). Surfactants are used to make the suspension more stable, so that the micelle-trapped particles repel each other due to steric hindrance effects.

Ferrofluids form intriguing three-dimensional shapes in the presence of magnetic fields, and patterns of stripes when confined to a thin sheet (as between two plates of glass) due to the individual particles' magnetic fields aligning and repelling each other and the opposing surface tension forces of the liquid holding them together.

They are used in loudspeakers to sink heat between the voice coil and the magnet assembly, and to passively damp the movement of the cone. They reside in what would normally be the air gap around the voice coil, held in place by the speaker's magnet.

Ferrofluids are similarly used to form liquid seals around the spinning drive shafts in hard disks.

Using electromagnets and sensors, the viscosity of magnetorheological fluids can be controlled dynamically, allowing for active damping (in car shock absorbers like Delphi Corporation's MagneRide, for instance). This allows hundreds of watts of mechanical power to be controlled with a few watts of electrical power, which is much more efficient than other methods of vibration control, such as piezoelectric crystals.

They also have friction-reducing capabilities as well. If applied to the surface of a strong enough magnet, such as one made of neodymium, it can glide across smooth surfaces with minimal resistance.

Matsushita Electric Industry produced a printer capable of printing 5 pages per minute using a ferrofluid ink.

The United States Air Force introduced a Radar Absorbent Material (RAM) paint made from both ferrofluidic and non-magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce the RADAR cross-section of aircraft.

Ferrofluids have numerous optical applications due to their refractive properties; that is, each grain, a micromagnet, reflects light. These applications include measuring specific viscosity of a liquid placed between a polarizer and an analyzer, illuminated by a helium-neon laser.Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.
Virtual Magic is a human knowledge database blog. Text Based On Information From Wikipedia, Under The GNU Free Documentation License. Copyright (c) 2007 Virtual Magic. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

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