Editing Ferrofluid (section)

{{Short description|Liquid that is attracted by poles of a magnet}}
[[File:Ferrofluid Magnet under glass edit.jpg|thumb|upright=1.3|Ferrofluid on glass, with a [[neodymium magnet]] underneath]]
[[File:Steve Papell NASA ferrofluid developer in 1963.JPG|thumb|Steve Papell invented ferrofluid for NASA in 1963]]
{{Electromagnetism}}
{{Continuum mechanics|rheology}}
'''Ferrofluid''' is a dark liquid that is attracted to the poles of a [[magnet]]. It is a [[colloidal]] liquid made of [[Nanoscopic scale|nanoscale]] [[Ferromagnetism|ferromagnetic]] or [[Ferrimagnetic interaction|ferrimagnetic]] particles suspended inside a
[[Wiktionary:carrier|carrier]] [[fluid]] (usually an [[organic solvent]] or water).<ref>{{Cite web |title=Ferrofluid Product |url=https://ferrofluid.com/index.php/en/ |access-date=2023-10-29 |website=Ferrofluid.com |language=en-gb}}</ref> Each magnetic particle is thoroughly coated with a [[surfactant]] to inhibit clumping.  Large ferromagnetic particles can be ripped out of the homogeneous colloidal mixture, forming a separate clump of magnetic dust when exposed to strong magnetic fields. The magnetic attraction of tiny [[nanoparticle]]s is weak enough that the surfactant's [[Van der Waals force]] is sufficient to prevent magnetic clumping or [[Flocculation|agglomeration]].  Ferrofluids usually do not retain [[magnetization]] in the absence of an externally applied field and thus are often classified as "[[Superparamagnetism|superparamagnets]]" rather than ferromagnets.<ref>{{cite journal|last1=Voit|first1=W.|last2=Kim|first2=D. K.|last3=Zapka|first3=W.|last4=Muhammed|first4=M.|last5=Rao|first5=K. V.|title=Magnetic behavior of coated superparamagnetic iron oxide nanoparticles in ferrofluids|journal=MRS Proceedings|date=21 March 2011|volume=676|doi=10.1557/PROC-676-Y7.8}}</ref>

In contrast to ferrofluids, [[magnetorheological fluid]]s (MR fluids) are magnetic fluids with larger particles. That is, a ferrofluid contains primarily nanoparticles, while an MR fluid contains primarily micrometre-scale particles.  The particles in a ferrofluid are [[suspension (chemistry)|suspended]] by [[Brownian motion]] and generally will not settle under normal conditions, while particles in an MR fluid are too heavy to be suspended by Brownian motion.  Particles in an MR fluid will therefore settle over time because of the inherent density difference between the particles and their carrier fluid. As a result, ferrofluids and MR fluids have very different applications.

A process for making a ferrofluid was invented in 1963 by NASA's [[Steve Papell]] to create liquid [[rocket fuel]] that could be drawn toward a fuel pump in a weightless environment by applying a magnetic field.<ref>{{cite patent |country=US |status=Patent |number=3215572 |fdate=Oct 9, 1963 }}</ref> The name ferrofluid was introduced, the process improved, more highly magnetic liquids synthesized, additional carrier liquids discovered, and the physical chemistry elucidated by R.{{nbsp}}E. Rosensweig and colleagues. In addition Rosensweig evolved a new branch of fluid mechanics termed ferrohydrodynamics which sparked further theoretical research on intriguing physical phenomena in ferrofluids.<ref>{{citation|title=Ferrohydrodynamics|author=Rosensweig, R.E.|publisher=Dover Books on Physics, Courier Corporation|year=1997|isbn=9780486678344|url=https://books.google.com/books?id=uSa5nJGXYicC}}</ref><ref>{{citation|title=Ferrohydrodynamics: Testing a third magnetization equation|journal=Physical Review|volume=64|year=2001|author=Shliomis, Mark I.|issue=6|page=060501|doi=10.1103/PhysRevE.64.060501|pmid=11736163|arxiv=cond-mat/0106415|bibcode=2001PhRvE..64f0501S|s2cid=37161240}}</ref><ref>{{citation|title=Surface instabilities and magnetic soft matter|journal=Soft Matter|issue=10|year=2009|author=Gollwitzer, Christian|author2=Krekhova, Marina|author3=Lattermann, Günter
|author4=Rehberg, Ingo|author5=Richter, Reinhard|volume=5|page=2093|doi=10.1039/b820090d|arxiv=0811.1526|bibcode=2009SMat....5.2093G|s2cid=17537054|url=https://pubs.rsc.org/en/content/articlelanding/2009/SM/b820090d
}}</ref><ref>{{citation|title=Flow restrictive and shear reducing effect of magnetization relaxation in ferrofluid cavity flow|journal=Physics of Fluids|volume=28|issue=8|author=Singh, Chamkor|author2=Das, Arup K.|author3=Das, Prasanta K.|
year=2016|page=087103|doi=10.1063/1.4960085|bibcode=2016PhFl...28h7103S}}</ref> In 2019, researchers at the [[University of Massachusetts]] and Beijing University of Chemical Technology succeeded in creating a permanently magnetic ferrofluid which retains its magnetism when the external magnetic field is removed. The researchers also found that the droplet's magnetic properties were preserved even if the shape was physically changed or it was divided.<ref>{{Cite web|url=https://phys.org/news/2019-07-laws-scientists-magnetic-liquid-droplets.html|title=New laws of attraction: Scientists print magnetic liquid droplets|last=Lawrence Berkeley National Laboratory|date=July 18, 2019|website=phys.org|language=en-us|access-date=2019-07-19}}</ref>