Q: What is ferrofluid and how does it work?

The word “ferrofluid” is a blend of the words “ferromagnetic” and “fluid.”  The “ferromagnetic” part refers to nanoscale particles that contain iron compounds, such as magnetite or hematite.  These particles are tiny- on the order of one billionth of a meter in diameter.  The iron nanoparticles are suspended in a liquid (the “fluid” in “ferrofluid”), which could be water or an organic solvent.  The nanoparticles are coated with a surfactant, such as oleic acid or tetramethylammonium hydroxide, which lowers the surface tension between the nanoparticles and the liquid.  This prevents the nanoparticles from clumping together, allowing the ferrofluid to flow and maintain its liquid-like qualities.

When ferrofluid is exposed to a magnet, the nanoparticles are magnetized, and the fluid arranges into a peaks and valleys pattern that minimizes the energy of the system.  Basically, the magnetic field would prefer to travel through the ferrofluid as much as possible, as it’s easier to magnetize the ferrofluid than it is to magnetize the air.  By stretching the ferrofluid out into peaks, magnetic field lines can travel farther through the fluid.  Slightly thicker parts of the ferrofluid attract more ferrofluid, bunching the magnetic field lines together and building up into spikes.  Gravity and surface tension keeps the fluid together and limits how high the peaks can form.

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When the magnetic field is taken away, the ferrofluid loses its magnetic properties and reverts back to a shapeless blob.

Like many other technologies, we can thank NASA for ferrofluid.  In the 1960s, NASA scientist Steve Papell was working on a way to control liquid rocket fuel in outer space, where the lack of gravity made managing the fuel difficult.  Papell thought that fuel with ferrofluid magnetic properties could be controlled in gravity-free space by using magnets and magnetic fields.  While the idea was novel, NASA decided to go in another direction involving solid rocket fuel.

Even though the NASA plan never worked out, there are other practical applications for ferrofluid.  In loudspeakers, voice coils are sometimes surrounded by ferrofluid, which helps to conduct heat away and improve efficiency.  Medical researchers are exploring using the magnetic ferrofluid as a contrast agent for magnetic resonance imaging (MRI) to improve visibility in the images.  Ferrofluid is also used as a liquid seal to provide low friction in moving parts, such as in turbo pumps.

Plus, you know, ferrofluid makes for a really cool physics demo.

Keep calm and science on.

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