spiking of ferrofluids

spiking of ferrofluids

Ferrofluids Spiking – It is a version of the Rayleigh-Taylor instability. This is an instability of an interface between two fluids of different densities which occurs when the lighter fluid is pushing the heavier fluid.

Ferrofluids Spiking Redditor Specktagon has a wonderful analogy.

This happens because maple syrup layer is quasi-statically unstable even if there’s a tiny bit of extra maple syrup in one place, gravity pulls harder on that part of the layer, and the slightly-thicker place stretches down to become a stalactite and then a drip.

The ferrofluids behave in similar fashion. All the cool ferrofluid stuff happens because it is energetically favorable for the magnetic field to pass through ferromagnetic particles, rather than empty space.

So if you have a uniform surface of ferrofluid, with a field passing through it, the field “likes” to stay in the ferrofluid. But you can get more field lines spending more distance inside the ferrofluid, by bunching up field lines and stuffing them in spikes.

So what happens is, even if a small bit of ferrofluid is thicker than the other parts of the fluid, then field lines bunch up there. That draws more ferrofluid in and builds up a spike. The instability, at least in its early growth stages, is almost exactly the same as the R-T instability acting on maple syrup.

The Ferrofluids Spiking have limited length because pulling fluid upwards out of the mass costs some energy, and eventually that energy cost outpaces the energy benefit of keeping the field lines in the ferrofluid.

The spikes have limited lateral extent because of the energy cost of bunching field lines together as they repel one another. So when the field gets too concentrated a particular spike can’t grow any more and other nearby spikes grow instead. Those two effects “shut down” the instability, which is why you don’t get infinitely thin spikes that get infinitely long.

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