What is Ferrofluid? A Guide to the Theory, Properties and Applications of Magnetic Fluid

Ferrofluid: An overview
Theory behind Ferrofluid
Useful Properties and Applications of Ferrofluid
Sources and Further Reading

Ferrofluid: An overview

Ferrofluid sounds like a concept straight from a bad science-fiction film – a black, shape-shifting metallic liquid, that moves and forms spikes using magnetic fields. But ferrofluid is not the product of studio trickery; it is real and one of the most exciting materials to emerge in modern times.

NASA first developed ferrofluids in the 1960’s whilst researching methods of using liquids in space, but in the 21st century ferrofluid has found new levels of fame. In recent years, ferrofluid has also become somewhat of a YouTube sensation, due to the fantastical shapes that it can create simply using a magnetic field. A great example of this is shown below. However, ferrofluid is not just an internet curio – its inherent pliability opens up a whole range of applications which are discussed in more detail later.

Theory behind Ferrofluid

The term ‘ferrofluid’ is a portmanteau of ferromagnetic and fluid and is used to describe a fluid that is strongly magnetized by a magnetic field. This occurs because the fluid is composed of tiny magnetic particles, up to 100 times smaller than the wavelength of visible light. The most common minerals used in making these magnetic particles are iron oxides such as magnetite (Fe3O4) and hematite (Fe2O3), though other ferromagnetic or ferrimagnetic substances can be used. The particles are usually less than 10nm across.

These tiny particles are suspended in a liquid carrier fluid, which can be water or an organic solvent. Thus, ferrofluids can be termed colloidal liquids, as they contain evenly dispersed microscopic particles in another substance.

Once a magnetic field is applied to a ferrofluid, the nanoparticles are attracted and pull the entire liquid towards the magnetic field. However, if exposed to a strong magnetic force, some of the nanoparticles can be ripped out from the carrier fluid, forming an incredibly fine dust.

To stop the clumping of the nanoparticles via van der Waals forces, a surfactant (usually a hydrocarbon) coating is applied to the surface of each of the metallic particles, which overcomes the weak inter-particle attraction.

The particles suspended in a ferrofluid conform to Brownian motion, which means particle movement is generally random and the liquid will not settle under standard conditions.

Useful Properties and Applications of Ferrofluid

Aside from being used to create stunning sculptures, ferrofluid also has exciting real world applications. A major benefit of ferrofluid is that the liquid can be forced to flow via the positioning and strength of the magnetic field and so the ferrofluid can be positioned very exactly. Ferrofluids also have the capability of reducing friction, making them useful in a variety of electronic and transportation applications.

For example, ferrofluids can be used in hydraulic suspension pistons, with the strength of the magnetic field allowing the suspension to be hard or soft depending on what is necessary.

It can also be used as a liquid seal in many electronic devices. For example, in computer hard-drives ferrofluid can be used to form a seal around the rotating shaft. Furthermore, it can be used in loudspeakers to improve performance.

Ferrofluids could be used to keep us safe too: new body armour is being developed by MIT which utilises ferrofluid in hollow fibres. This body armour could act as a artificial splint in the heat of battle.

Ferrofluids also have medical applications and it is hoped that these will increase in the future. Two examples of on-going research related to ferrofluids are:

  • Carrying medications to exact locations within the body
  • Use as a contrasting agent for MRI scans

Currently, research on using ferrofluids to create an artificial heart with no mechanical parts is being undertaken by Suprock Technologies. By surrounding the heart with magnets, the ferrofluid fixed to frame of the heart will expand and contract when needed, imitating the pumping of the real thing. If developed correctly this system may be a better option than current heart assist devices because they do not have moving parts, meaning there will be less stress on the heart and they will also be cheaper.

Further disparate fields in which ferrofluids can be used are:

  • Heat transfer
  • Analytical instrumentation
  • Art
  • Aerospace

We are only just discovering the full potential of ferrofluids and we are now opening up a world of opportunity that will hopefully continue to grow rapidly in the next decade.

Sources and Further Reading

US Army seeks nanotech suits, New Scientist, 14/03/02

Institute of Making, University College London

Morphing mirror could clear the skies for astronomers, New Scientist, 07/11/08

Artificial heart uses ferrofluid to pump blood, New Scientist, 13/06/12

Madison Materials Research Science and Engineering Center, University of Wisconsin

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