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A rotating magnetic field is running through a fluid that is reactive to magnetic fields that probably also has isopropyl alcohol or some other fluid in the space the ferrofluid does not occupy. The field is 3D, but the fluid is kept flat to show a slice of that field and the surface tension of the fluids, and the reactions that causes.[it is called] a labyrinth instability. Its not just a magnetic field cross section, but a lot of reactions.