What is a ferrofluid?

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A ferrofluid is a liquid which becomes highly magnetized in the presence of a magnetic field. The distinctive ‘spikey’ shape of a magnetized ferrofluid is caused by the need to find the most stable shape in order to minimize the total energy of the system, an effect known as the normal-field instability. The fluid is more easily magnetized than the surrounding air, so is drawn out along the magnetic field lines, resulting in the formation of peaks and troughs. However, the extension of the ferrofluid is resisted by gravity and surface tension. The formation of the corrugations lowers the magnetic energy of the system but raises the gravitational energy and surface free energy. When these forces are balanced, the minimum energy configuration is achieved. Because ferrofluids are very easily magnetized (they have an incredibly high magnetic susceptibility), the peaks can be produced using a small bar magnet.

Ferrofluids are known as colloidal fluids and are composed of nanoscale ferromagnetic particles suspended in a carrier fluid, usually water or an organic solvent like kerosene, and coated with a surfactant to stop them clumping together in the liquid. A typical composition would be 5% magnetic particles, 10% surfactant and 85% carrier fluid.

The particles in a ferrofluid have a diameter of 10 nanometers or less and are composed of a ferromagnetic, highly magnetically susceptible compound such as magnetite (Fe 3O4) or hematite (Fe 2O3). The particle size has to be small enough to allow them to be evenly dispersed through the liquid by Brownian motion (the random motion of particles in a liquid due to collisions which other molecules) but large enough for them to each make a significant contribution to the magnetic response of the fluid. Upon application of an external magnetic field, the nanoparticles align with the field. However, once the external field is turned off, the particles return to a random alignment. For this reason, ferrofluids are classed as superparamagnets rather than ferromagnets.

The surfactant's van der Waals forces stop the magnetic nanoparticles aggregating in the solution. Different surfactants work in different ways but the general principle is that the surfactant creates a layer around the particle which will repel other coated nanoparticles. The diagram below illustrates the principles of an ionic surfactant – the surfactant ions form a layer of charge around the nanoparticle, repelling other charged, surfactant coated particles. Whilst the addition of a surfactant is crucial, it has the negative effect of decreasing the viscosity of the fluid in the magnetized state and making it ‘softer’. As most applications require a ‘hard’ fluid in the magnetized form, this is an important factor to consider when choosing the ferrofluid composition.

In 1963, Steve Papell of NASA created ferrofluid for use as rocket fuel. His team of NASA scientists were investigating methods of directing fluids in space and realized that magnetic fluids could be completely controlled by the application and variation of a magnetic field. The ferrofluid was mixed with liquid fuel and drawn towards the ignition system with an external magnetic field. Ferrofluids have now found use in many applications from small electronic devices to space crafts to cancer treatments to art. In fact, ferrofluids are found in many common household devices, including hard drives where they are used to seal the interior of the device. When magnetized they form a barrier to dust and dirt which could damage the delicate plates.

Ferrofluids can have very high thermal conductivities and their heat transfer properties are exploited in devices such as loud speakers where they are used to cool the voice coil. In a loudspeaker, sound is produced when the voice coil vibrates but this also generates unwanted heat. Ferrofluids lose their magnetism as they are heated, fully losing their magnetic properties when heated to a high enough temperature, known as the Curie temperature. If ferrofluid is placed around the voice coil, a magnet placed near the coil will attract more cold ferrofluid than hot ferrofluid because the colder ferrofluid will be more strongly magnetized. This cold ferrofluid will absorb heat around the voice coil and then be moved towards a heat sink as it is replaced by cooler ferrofluid.

Ferrofluids are also the focus of current scientific research and have the potential to be used in many medical applications. In magnetic drug targeting for example, where drugs could be enclosed by ferrofluid and, once injected into the specific body area requiring treatment, a magnetic field could be applied to keep the drugs in this target area. The localization would limit exposure to the rest of the body and enable the dosage level to be decreased, reducing the adverse side effects experienced by the patient.

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