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Ferrofluids — suspensions of magnetic particles in a fluid — were developed in the 1960s by NASA scientists who were seeking ways to pump liquid fuels around spacecraft safely, in zero-gravity and without any moving parts. The plan was to drive the ferrofluids with external magnetic fields, to displace and propel other non-magnetic fluids. Unfortunately, to do so required large permanent magnets that were incompatible with the weight constraints of space travel, and to prevent mixing only millimetre-sized fluid channels could be used, limiting flow rates to microlitres per minute.
Using decidedly un-space-age components — including a home-stereo amplifier, PVC piping bought from a local hardware store, and a commercially available ferrofluid of magnetite particles in mineral oil — Leidong Mao and colleagues demonstrate a solution to this problem. The setup consists of a series of solenoid coils wrapped around two sides of a closed loop of pipe, which is filled with ferrofluid. Rather than driving translational motion of a ferrofluid directly with a d.c. field, they apply a phased sequence of oscillating fields that sets its particles rotating. The resulting travelling wave drives the fluid around the loop at rates much faster than had been achieved previously.
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Gerstner, E. In the loop. Nature Phys 7, 743 (2011). https://doi.org/10.1038/nphys2121
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DOI: https://doi.org/10.1038/nphys2121