Image of ferrofluids is from Prof. Markus Zahn group at MIT.
Ferrofluids (sometimes referred as magnetic liquids) are colloidal suspensions of magnetic nanoparticles. They were originally invented independently around the same time in the early 1960s at NASA Lewis Laboratories, and aslo by Dr. R. E. Rosensweig et al. at AVCO Space Systems. Typically, particles within such colloidal suspension are about 10 nanometers (nm) in diameter and suspended in either water or oil.
There are a variety of ways in which nanometer-sized magnetic particles (for example, iron oxide) can be synthesized. One such straightforward reaction is the coprecipitation of magnetite with ammonia out of iron solutions:
2FeCl3 + FeCl2 + 8NH3+ 4H2O → Fe3O4 + 8NH4Cl
Under normal circumstances, a suspension of magnetic nanoparticles is not necessarily stable on its own. Magnetic attraction between the particles, combined with Van der Waal’s forces, will result in quick agglomeration and settling of the magnetic phase. In order to prevent the particles from coming too close to each other, various surfactants have been used for different liquid carriers. Once particles are prevented from sticking to each other, the colloidal suspension may be stabilized, provided that external forces, such as gravity, do not settle them down. Fortunately, at an average diameter of 10 nm, the magnetic nanoparticles have enough kinetic energy at room temperature to overcome gravity and can be uniformly distributed within their liquid medium.
Why do we care about ferrofluids?
If there is interest to a particular topic in scientific and engineering research, it means that there are plenty of potential industrial or healthcare applications that give the research effort a commercial significance. This is also the case for research in ferrofluids.
Below are several examples of ferrofluids applications.
(1) Liquid seals and bearings
Ferrofluids can be held in place simply via permanent magnets. This, in turn, enables their use as low-friction liquid seals against pressure differences (in turbo-pumps, for instance). They also act as highly efficient liquid bearings. (Ferrotec)
Ferrofluids are also utilized in loudspeakers to enable enhanced thermal contact (for cooling) and better damping for the voice coils. Over 50 million loudspeakers sold in the US each year use ferrofluids in this context. (Ferrotec)
(3) Cellular imaging
Functionalized magnetic nanoparticles can also be used as contrast agents in magnetic resonance imaging (MRI). Seen here as bright spots are the major lymph nodes in mice — such visualization could help in earlier and accurate diagnosis of cancer. Source: Kobayashi, et. al., Cancer Research (63), p. 271-276, 2003.
More information on ferrofluids –
Research groups working on ferrofluids: