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Ferrofluid

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 liquid. The particles are coated with a surface active agent (surfactant) to prevent them from clumping together.

Ferrofluids are composed of nanoscale ferromagnetic, or ferrimagnetic, 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 magnetization in the absence of an externally applied field. In fact, ferrofluids display (bulk-scale) paramagnetism, and are often referred as being "superparamagnetic" due to their large magnetic susceptibility. Permanently magnetized fluids are difficult to create at present.

It is important to note the difference between ferrofluids and magnetorheological fluids (MR fluids). The particles in a ferrofluid primarily consist of nanoparticles which are suspended by Brownian motion and generally will not settle under normal conditions. MR fluid particles primarily consist of micrometre-scale particles which are too heavy for Brownian motion to keep them suspended, and thus will settle over time due to the inherent density difference between the particle and its carrier fluid. These two fluids have very different applications as a result.

Description

Ferrofluids are composed of nanoscale particles (diameter usually 10 nanometers or less) of magnetite, hematite or some other compound containing iron. 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(II) sulfate or manganese(II) chloride) make the solution paramagnetic.

Ferrofluids are tiny iron particles covered with a liquid coating, also surfactant that are then added to water or oil, which gives them their liquid properties. Ferrofluids are colloidal suspensions -- materials with properties of more than one state of matter. In this case, the two states of matter are the solid metal and liquid it is in. This ability to change phases with the application of a magnetic field allows them to be used as seals, lubricants, and may open up further applications in future nanoelectromechanical systems.

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 one to three orders of magnitude larger than those of ferrofluids.

However, ferrofluids lose their magnetic properties at sufficiently high temperatures, known as the curie temperature. The specific temperature required varies depending on the specific compounds used for the nano-particles, surfactant, and carrier fluid.

Normal-field instability

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 strength, 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

The surfactants used to coat the nanoparticles include, but are not limited to:

These surfactants prevent the nanoparticles from clumping together, ensuring that the particles do not form aggregates that become too heavy to be held in suspension by Brownian motion. The magnetic particles in an ideal ferrofluid do not settle out, even when exposed to a strong magnetic, or gravitational field. A surfactant has a polar head and non-polar tail (or vice versa), one of which adsorbs to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular micelle,respectively, around the particle. Steric repulsion then prevents agglomeration of the particles.

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). 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

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. According to engineers at Ferrotec, ferrofluid seals on rotating shafts typically withstand 3 to 4 psi, but these seals don't work well in translational seals for pistons because the fluid gets mechanically dragged out of the magnetic gap.

Ferrofluids are also used in many high-frequency speaker drivers (tweeters) where they provide heat conduction from the voice coil to the surrounding assembly as well as mechanical damping to reduce undesired resonances. The ferrofluid is kept in place in the magnetic gap due to the strong magnetic field and is in contact with both the magnetic surfaces as well as the coil.

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.

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.

Analytical Instrumentation

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.

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 remove heat from the voice coil, 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 attract cold ferrofluid more than hot ferrofluid thus forcing the heated ferrofluid away from the electric voice coil and toward a heat sink. This is an efficient cooling method which requires no additional energy input.

References

Sources

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

External links

Optical and magnetic properties

Dynamic Etalon utilizing ferrofluid- image gallery, references, published papers

Preparation instructions

FerroFluid Synthesis
Berger, Patricia; Nicholas B. Adelman, Katie J. Beckman, Dean J. Campbell, et al (1999). "Preparation and properties of an aqueous ferrofluid". Journal of Chemical Education 76 (7): pp. 943–948.
- Interdisciplinary education group: Ferrofluids (contains videos and a lab for synthesis of ferrofluid)
- Synthesis of an Aqueous Ferrofluid — instructions in PDF and DOC format