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As the electrically-charged ionic tails of surfactants used to attach to the magnetic nanoparticles require the particle to be magnetized to stick to them; if a ferrofluid is passed has a solenoid DC current field, which causes demagnetization, will this cause the surfactants to leave the particles and in effect, make the particles insoluble again?

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  • $\begingroup$ I struggle to understand what you are asking here. Surfactants are not attracted to magnetic particles by magnetic effects. Static magnetic fields don't cause demagnetisation. And simple observation will demonstrate that ferrofluids don't fall apart under static magnetic fields (the spectacular 3D patterns the fluids produce in strong fields would not be possible if the solutions fell apart). $\endgroup$
    – matt_black
    Dec 4 '21 at 10:00
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Ferrofluids are not true solutions, but are stable colloidal suspensions. Brownian motion is sufficient to keep the tiny particles, on the order of 10 nm, from settling out. Other examples of colloids are milk (fatty globules in liquid) and colloidal gold in water.

Though an alternating magnetic field in an AC-powered solenoid would only stir up the particles and keep them suspended, a strong-enough steady magnetic field would separate the ferrofluid particles from the liquid, as would a centrifuge, just as whole milk is separated into cream and skimmed milk.

It is also possible to disrupt colloids by changing pH+. For example, cheeses are made by adding citric, acetic or lactic acids (or letting friendly microorganisms do that for you).

Bon appétit, but don't eat the ferrofluid.

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  • $\begingroup$ Ferrofluids would be pretty useless if static fields caused the fluids to break down and separate. None of their practical applications would even be possible if this were true. $\endgroup$
    – matt_black
    Dec 4 '21 at 10:03
  • $\begingroup$ @matt_black, read: a strong-enough field. Given sufficient strength, the iron particles adhere strongly enough to squeeze out the suspending fluid. It might take something from the Francis Bitter Magnet Lab (q.v., web.mit.edu/fbml) or a magnetar with field 10e11 T, but at some point the ferrofluid would precipitate. $\endgroup$ Dec 5 '21 at 1:20
  • $\begingroup$ I'm happy to clarify my claim to "under static magnetic field strengths routinely achievable on earth" ferrofluids won't separate. The record is currently ~50T which is 10 times the typical big fields in NMR or MRI machines. I don't think we know much about what happens to any material when fields go much above 10T. $\endgroup$
    – matt_black
    Dec 5 '21 at 14:34

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