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Ferrofluid

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Ferrofluid on glass, with a magnet underneath.

A ferrofluid (from the Latin ferrum, meaning iron) is a liquid which becomes strongly polarised in the presence of a magnetic field. It is a colloidal mixture comprising extremely small magnetic particles suspended in a synthetic oil. The particles are coated with a a soap or detergent to prevent them from clumping together.

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 may suggest 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. True ferromagnetic fluids are difficult to create at present.

Description

A ferrofluid, influenced by a magnet underneath.

Ferrofluids comprise microscopic ferromagnetic nano-particles, usually magnetite, hematite or some other compound containing iron. The nano-particles are typically on the order of 10 nm. 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 sulfate 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 fluid's 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.

Normal-field instability

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 is known as the normal-field instability. 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.

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 in the absence of real-world friction. 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 fluid's magnetic properties (specifically, the fluid's 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.

Applications

Ferrofluid under the influence of a strong vertical magnetic field.

Electronic devices

Ferrofluids are used to form liquid seals (ferrofluidic seals) around the spinning drive shafts in hard disks. The rotating shaft is surrounded by magnets. A small amount of ferrofluid, placed in the gap between the magnet and the shaft, will be held in place by its attraction to the magnet. The fluid of magnetic particles forms a barrier which prevents debris from entering the interior of the hard drive. However, the ferrofluid is still similar enough in properties to a true liquid that it will not interfere with the spinning of the shaft.

Another common use of ferrofluids is as a liquid coolant. One commercial application for this usage is in megaphones and loudspeakers. Ferrofluid is put in the space (magnetic gap) between the permanent magnet and the voice coil of a speaker. Just as in the hard drive, the permanent magnet will hold the ferrofluid in place, keeping it in contact with the voice coil. Heat flows from the high current voice coil and into the ferrofluid. When the ferrofluid is heated above its Curie temperature it is no longer attracted to the magnet and is pushed out of the magnetic gap by cooler fluid nearby. When the hot ferrofluid cools below its critical temperature it resumes its paramagnetic behavior and will rush back into the magnetic gap. This forms an active, liquid, heat pump to prevent damage to the speaker.

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 cause the magnet to glide across smooth surfaces with minimal resistance.

Magnetorheological dampers of various applications have been and continue to be 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 resonance 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.

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.

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.

Optics

Magnetorheological Finishing, a magnetorheological fluid-based optical polishing method, has proven to be highly precise. It was used in the construction of the Hubble Space Telescope's corrective lens.

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.

Medicine

In medicine, a compatible ferrofluid can be used for cancer detection. There is also much experimentation with the use of ferrofluids to remove tumors. The ferrofluid would be forced into the tumor and then subjected to a quickly varying magnetic field. This would create friction, yielding heat, due to the movement of the ferrofluid inside the tumor which could destroy the tumor.

Additionally heavy metals used in MRI could be enclosed in carbon "cages" to protect the body from these possibly harmful metals.

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.

Automotive

If the shock absorbers of a vehicle's 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. As of 2007, BMW manufactures cars using their own proprietary version of this device, while GM (the first auto manufacturer to do so), Audi, and Ferrari offer the MagneRide on various models.

General Motors and other automotive companies are seeking to develop a magnetorheological fluid based clutch system for push-button four wheel drive systems. This clutch system would use electromagnets to solidify the fluid which would lock the driveshaft into the drive train.

See also

References

  1. T. Albrecht, C. Bührer; et al. (1997). "First observation of ferromagnetism and ferromagnetic domains in a liquid metal (abstract)". Applied Physics A: Materials Science & Processing. {{cite web}}: Explicit use of et al. in: |author= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. Elmars Rlums (1995). "New Applications of Heat and Mass Transfer Processes in Temperature Sensitive Magnetic Fluids". Brazilian Journal of Physics. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

Sources

  • Ferrohydrodynamics (1985), Ronald. E. Rosensweig. The usual starting reference for learning the details of ferrofluids.

External links

Preparation instructions

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