Students prepare a ferrofluid – a liquid that contains small particles, approximately 10 nanometers in diameter, which spontaneously magnetizes in the presence of a magnetic field – through solution chemistry materials.

A ferrofluid (magnetic fluid) is a collection of superparamagnetic nanoparticles that are suspended in a liquid.  These nanoparticles are approximately 10 nm in diameter.  The majority of nanoparticles, like the ones you make in this lab, are iron-based, such as magnetite (Fe3O4). If as-synthesized nanoparticles are put into solution, they aggregate and form clusters due to van der Waal interactions. These clusters are too large to be kept in suspension by Brownian motion and settle to the bottom of the container. The use of a magnet can accelerate the settling of these clusters allowing for easy washing of the particles. 

Nanoparticles will remain suspended in a solution as long as they do not aggregate.  A technique to prevent aggregation is to ‘stabilize’ the particles by encapsulating them with an outer shell.  The general method to achieve this is to use a surfactant.  Surfactants are molecules with two contrasting properties.  They can be a linear molecule with a hydrophilic region and a hydrophobic region, or a cation anion pair.  The former works by having the hydrophilic end of the molecules attach to the magnetite nanoparticles positioning the hydrophobic end to form a ‘greasy’ layer around the particle.  This ‘greasy’ layer prevents nanoparticles from getting close enough to each other to aggregate.  The later is the method used in this experiment.  The ion pair keeps particles separated through Coulombic repulsion by encapsulating the particles with a cationic outter shell.  The anions adhere to the surface of the magnetite nanoparticles, and they attract its counter cations to form the positively charged outer shell.  Since like charges repel, the positively charged outer shell prevent magnetite nanoparticles from aggregating.

Applications of Ferrofluids

There are many applications of ferrofluids.  Most applications are based on these properties:
1) The ferrorfluid will go to where the strongest magnetic field is and stay there (this is called localizability).
2) Ferrofluids absorb electromagnetic energy at convenient frequencies and heat up.
3) The physical properties of ferrofluids change when a magnetic field is applied.

Mechanical Applications

Ferrofluids are used as seals in gas lasers, motors, and blowers. In these applications the ferrofluid is held in place by a strong magnet and separate two different pressured chambers. They have also been used to as a media for vibrational dampaning in electronic applications, such as hard drives, graphic plotters, and instrument gauges.  Ferrofluids are also found in almost all high power loudspeakers.  The ferrofluid is kept in place by the magnet on the horn of the loudspeaker, and serves to seal the speaker chamber, cool the speaker, and dampen unwanted overtones.¹

Medical Applications

There is also a substantial interest in using ferrofluids for medical applications.  For example, if a drug used to treat cancer was injected into a person with the ferrofluid, a magnetic field could be used to focus the drug onto the tumor.  Or, a ferrofluid, once injected, could be easily heated locally to burn a tumor.  These methods have already been successful at destroying cancers in rats.

Other Application

Perhaps one of the most interesting applications is in ink; ferrofluid based ink is used by the US government to print the one dollar bill.²  This helps the government to identify counterfeit bills.