Fluctuating Ferrofluids

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Ferrofluids–liquids seeded with magnetically sensitive ferrous nanoparticles–demonstrate some beautiful and bizarre behaviors when exposed to magnetic fields. This video shows the reaction of a pool of ferrofluid to the magnetic field generated by an alternating current through a simple wire coil. At 1 Hz, the fluid response is not unlike the normal-field instability–the characteristic spikes–the fluid develops when exposed to a permanent magnet. But because field is fluctuating, the spikes pop out and fade again. At 10 Hz, the behavior gets even more interesting. As the frequency of the magnetic field’s oscillation increases, the time the fluid has to respond to changes in the magnetic field decreases. Eventually, one can imagine a point where the magnetic field oscillates faster than the molecules in the fluid can rearrange themselves to respond. It’s unclear if such a mismatch in timescales is the cause of the increasing violence of the ferrofluid’s response in the later clips or whether this results from an unmentioned change to the current through the coil. For something even wilder, check out Nick’s video of the ferrofluid’s response to music. (Video credit: N. Moore)

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