Can you predictably manipulate a magnetic gas?

Submitted by hufsa7 t3_zgm8q4 in askscience

Does there exist a gas that changes in response to magnetic fields in its surrounding?

Could one theoretically manipulate such a gas in a meaningful way with magnets? I.e. if gas was placed in a hermetically sealed cylinder with some spinning magnets at the top and bottom, or on the sides or whatever, could you create a predictable vortex / any predictable fluid flow within the tube? Assuming you find some smart person to piece out all the complicated math of spinning magnetic fields and fluids etc.

What's the limiting technology for this?

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jonzornow t1_izhvc7r wrote

Surprisingly, magnetism has actually been observed in gasses but only at extremely low temperatures. The magnetic gas can be manipulated, in the sense that they can make the atoms repel each other differently, but probably not controlled in the way you're hoping.

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dukesdj t1_izifbak wrote

You have two things to consider here which is materials science and magnetohydrodynamics (MHD). I am not a materials scientist so can not comment on the possibility of an inherently magnetic gas. However, I am an active researcher of MHD so can comment on that aspect of the problem.

> Could one theoretically manipulate such a gas in a meaningful way with magnets? I.e. if gas was placed in a hermetically sealed cylinder with some spinning magnets at the top and bottom, or on the sides or whatever, could you create a predictable vortex / any predictable fluid flow within the tube? Assuming you find some smart person to piece out all the complicated math of spinning magnetic fields and fluids etc.

Yes if such an electrically conducting gas exists (all gasses are electrically conducting actually but only weakly so. They can be ionized to be more so but then they are plasmas). The flow of any electrically conducting fluid can change in response to a magnetic field. This is a major aspect of astrophysical fluid dynamics. We can consider two regimes which are categorized by the magnetic Reynolds number (Rm). This non-dimensional number can be thought of as a measure of electrical conductivity. In the simple case with low Rm then the field strongly influences the flow but the flow does not strongly influence the field. The high Rm case is more difficult as the field and flow are strongly coupled and Alfvens frozen flux theorem is applicable (which essentially says that the fluid flow is frozen to the magnetic field and vice versa). So both cases result in the ability for the field to manipulate the fluid flow. The latter being significantly more challenging.

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forte2718 t1_izjbahw wrote

> ... We can consider two regimes which are categorized by the magnetic Reynolds number (Rm). ... In the simple case with low Rm then the field strongly influences the flow but the flow does not strongly influence the field. The high Rm case is more difficult as the field and flow are strongly coupled and Alfvens frozen flux theorem is applicable (which essentially says that the fluid flow is frozen to the magnetic field and vice versa).

Huh ... I'm curious, is there any intermediate regime, however small or poorly-understood it might be, where there is some substantial back-reaction of the flow onto the field but not enough to freeze the fluid flow to the magnetic field ... or is there essentially a hard phase transition between the two behaviors?

Cheers!

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dukesdj t1_izjhh15 wrote

It varies continuously. We typically define these regimes in the asymptotic limit of infinite or infinitely small. The frozen flux theorem is strictly applicable for the case of infinite conductivity (infinite Rm) which is non-physical but a very good approximation for most stellar applications.

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Hapankaali t1_izivoas wrote

Yes, you can manipulate gases using magnetic fields with magneto-optical traps. Among other applications, this is essential for creating Bose-Einstein condensates, and for certain quantum computing and quantum simulation architectures. One neat thing they have done with such systems is to create vortex lattices, see e.g. Phys. Rev. Lett. 83, 2498 (1999).

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