Ferrofluid

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Ferrofluid on glass, with a magnet underneath.
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Ferrofluid on glass, with a magnet underneath.

A ferrofluid is a liquid that 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. Truly ferromagnetic fluids are difficult to create at present, requiring high temperatures and electromagnetic levitation.[1]

Contents

[edit] Description

A ferrofluid, influenced by a magnet underneath.
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A ferrofluid, influenced by a magnet underneath.

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 to contribute to the overall magnetic response of the fluid. This is analogous to the way 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.

[edit] Normal-field instability

A ferrofluid in a magnetic field showing normal-field instability caused by a neodymium magnet beneath the dish
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A ferrofluid in a magnetic field showing normal-field instability caused by a neodymium magnet beneath the dish

When a paramagnetic fluid 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. Ferrofluids have an exceptionally high magnetic susceptibility and the critical magnetic field for the onset of the corrugations can be realised by a small bar magnet ( see figure ).

[edit] Common ferrofluid surfactants

Ferrofluids often contain surfactants including, but not limited to:

These surfactants serve to decrease the rate of ferroparticle settling, of which a high rate is an unfavorable characteristic of ferrofluids. The ideal ferrofluid would never settle, but developing this ideal fluid is as highly improbable as developing a perpetual motion machine according to our current understanding of the laws of physics. Surfactant-aided prolonged settling is typically achieved in one of two ways. In the case of the addition of soy lecithin, the surfactant particles are nanospheres and prolong the onset of settling via Brownian motion. In the case of oleic acid and other micelle surfactants, the effective diameter of each ferroparticle is increased by the attachment of micelle molecules to each ferroparticle, thereby increasing particle diameter and making fluid remixing (particle redispersion) occur far faster and with less effort.

While surfactants are useful in prolonging the settling rate in ferrofluids, they also prove detrimental to the fluids magnetic properties (specifically, the fluids magnetic saturation), which is commonly a parameter which users wish to maximize (this is typically more of a concern when dealing with magnetorheological fluids). Whether or not the surfactant is nanosphere-based or micelle-based, the addition of surfactants (or any other foreign particles) decreases the packing density of the ferroparticles while in its activated state, thus decreasing the fluids on-state viscosity, resulting in a "softer" activated fluid. While the on-state viscosity (the "hardness" of the activated fluid) is less of a concern for some ferrofluid applications, it is a primary fluid property for the majority of their commercial and industrial applications and therefore a compromise must be met when considering on-state viscosity vs. the settling rate of a ferrofluid.

[edit] Applications

Ferrofluid under the influence of a strong vertical magnetic field.
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Ferrofluid under the influence of a strong vertical magnetic field.

[edit] Information Technologies

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

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

[edit] Mechanical Engineering

Ferrofluids have friction-reducing capabilities. 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.

Magnetorheological fluid-based dampers of various applications are being, and have been, developed. These dampers are mainly used in heavy industry with applications such as heavy motor dampening, operator seat/cab dampening in construction vehicles, and more.

As of 2006, materials scientists and mechanical engineers are collaborating to develop stand-alone seismic dampers which, when positioned anywhere within a building, will operate within the building's resonant frequency, absorbing detrimental shock waves and oscillations within the structure, giving these dampers the ability to make any building earthquake-proof, or at least earthquake-resistant.

[edit] Defense

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.

[edit] Aerospace

NASA has experimented using ferrofluids in a closed loop as the basis for a spacecraft's attitude control system. A magnetic field is applied to a loop of ferrofluid to change the angular momentum and influence the rotation of the spacecraft.

[edit] Optics

QED Technologies has developed a magnetorheological fluid-based optical polishing method which has proven to be highly precise. QED's polishing method was used in the construction of the Hubble Space Telescope's corrective lens.

[edit] Measurement

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.

[edit] Medicine

In medicine, a compatible ferrofluid can be used for cancer detection.

[edit] Heat transfer

An external magnetic field imposed on a ferrofluid with varying susceptibility, e.g., due to a temperature gradient, results in a nonuniform magnetic body force, which leads to a form of heat transfer called thermomagnetic convection. This form of heat transfer can be useful when conventional convection heat transfer is inadequate, e.g., in miniature microscale devices or under reduced gravity conditions.

Ferrofluids are commonly 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. Since ferrofluids are paramagnetic, they obey Curie's law, thus become less magnetic at higher temperatures. A strong magnet placed near the voice coil (which produces heat) will always attract colder ferrofluid towards it more than warmer ferrofluid thus forcing the heated ferrofluid away, towards the heat sink. This is an efficient cooling method which requires no additional energy input.

[edit] Automotive

If the shock absorbers of a vehicle' suspension are filled with ferrofluid instead of plain oil, and the whole device surrounded with an electromagnet, the viscosity of the fluid (and hence the amount of damping provided by the shock absorber) can be varied depending on driver preference or the weight being carried by the vehicle - or it may be dynamically varied in order to provide stability control. The MagneRide magnetic ride control or active suspension is one such system which permits the damping factor to be adjusted once every millisecond in response to conditions. BMW has implemented this in several of their performance coupes, and some American car makers hope to soon follow suit.

General Motors and other companies are seeking to develop a magnetorheological fluid based clutch system for push-button four wheel drive systems.

[edit] Home-made fluid

A simple ferrofluid can be home-made out of small magnetic particles mixed with mineral, vegetable or motor oil (sae10 or other lightweight). Iron filings do not work well; they are too big. Good sources for small magnetic particles are:

  • magnetic laser printer toner
  • magnetic inspection powder from welding shops
  • particles from burned steel wool (after mortar and pestle)
  • particles scraped from the surface of video tapes
  • particles "mined" from sand with a plastic bag and a magnet (see external links)
  • magnetic ink used to print checks

A 1:1 ratio between the oil and magnetic powder seems to work best

These fluids are not very stable, however. The particles will tend to clump and the fluid properties will be lost quickly. Fluids created for professional purposes use emulsifiers to suspend very small oily (octane or kerosene) magnetic particles in water. The particles are very fine; less than a micrometre in diameter.

Ferrofluid stains are practically impossible to clean, so caution should be used when mixing or using.

[edit] See also

[edit] References

  1. ^ First observation of ferromagnetism and ferromagnetic domains in a liquid metal
  • Ferrohydrodynamics (1985), Ronald. E. Rosensweig. The usual starting reference for learning the details of ferrofluids.

[edit] External links

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