Editing Ferrofluid

{{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>

==Description==
[[File:R. E. Rosensweig with ferrofluid in his lab (1965).jpg|thumb|R. E. Rosensweig with ferrofluid in his lab (1965)]]
Ferrofluids are composed of very small nanoscale particles  (diameter usually 10 nanometers or less) of [[magnetite]], [[hematite]] or some other compound containing [[iron]], and a liquid  (usually [[oil]]). This is small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them to contribute to the overall magnetic response of the fluid. This is similar to the way that the ions in an aqueous [[paramagnetic]] salt solution (such as an aqueous solution of [[copper(II) sulfate]] or [[manganese(II) chloride]]) make the solution paramagnetic. The composition of a typical ferrofluid is about 5% magnetic solids, 10% [[surfactant]] and 85% carrier, by volume.<ref>{{Cite news|url=http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm|title=How to Make Liquid Magnets|work=ThoughtCo|access-date=2018-07-09|last=Helmenstine|first=Anne Marie|archive-date=2007-02-03|archive-url=https://web.archive.org/web/20070203194303/http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm|url-status=dead}}</ref>

Particles in ferrofluids are dispersed in a liquid, often using a [[surfactant]], and thus ferrofluids are [[Colloid|colloidal suspensions]] – materials with properties of more than one state of matter. In this case, the two states of matter are the solid metal and liquid it is in.<ref>{{Cite web|url=http://education.jlab.org/beamsactivity/6thgrade/vocabulary/index.html|title=Vocabulary List|website=education.jlab.org|language=en-us|access-date=2018-07-09}}</ref> This ability to change phases with the application of a magnetic field allows them to be used as [[seal (mechanical)|seals]], [[lubricant]]s, and may open up further applications in future [[nanoelectromechanical systems]].

True ferrofluids are stable. This means that the solid particles do not agglomerate or phase separate even in extremely strong magnetic fields. However, the surfactant tends to break down over time (a few years), and eventually the nano-particles will agglomerate, and they will separate out and no longer contribute to the fluid's magnetic response.

The term [[magnetorheological fluid]] (MRF) refers to liquids similar to ferrofluids (FF) that solidify in the presence of a magnetic field.  Magnetorheological fluids have [[micrometre]] scale magnetic particles that are one to three orders of magnitude larger than those of ferrofluids.

However, ferrofluids lose their magnetic properties at sufficiently high temperatures, known as the [[Curie temperature]].

===Normal-field instability===
[[File:Ferrofluid poles.jpg|thumb|right|Ferrofluid is the oily substance collecting at the poles of a magnet which is underneath the brown oil.]]

When a paramagnetic fluid is subjected to a strong vertical [[magnetic field]], the surface forms a regular pattern of peaks and valleys. This effect is known as the ''Rosensweig'' or ''normal-field instability''. The instability is driven by the magnetic field; it can be explained by considering which shape of the fluid minimizes the total energy of the system.{{sfn|Andelman|Rosensweig|2009|pp= 20–21}}

From the point of view of [[magnetic energy]], peaks and valleys are energetically favorable. In the corrugated configuration, the magnetic field is concentrated in the peaks; since the fluid is more easily magnetized than the air, this lowers the magnetic energy. In consequence the spikes of fluid ride the field lines out into space until there is a balance of the forces involved.{{sfn|Andelman|Rosensweig|2009|pp= 21, 23; Fig. 11}}

At the same time the formation of peaks and valleys is resisted by [[gravity]] and [[surface tension]]. It requires energy both to move fluid out of the valleys and up into the spikes, and to increase the surface area of the fluid. In summary, the formation of the corrugations increases the [[specific surface energy|surface free energy]] and the [[gravitational energy]] of the liquid, but reduces the magnetic energy. The corrugations will only form above a critical magnetic [[field strength]], when the reduction in magnetic energy outweighs the increase in surface and gravitation energy terms.{{sfn|Andelman|Rosensweig|2009|pp= 21}}
[[File:Ferrofluid_simulations_for_different_parameters_of_surface_tension_and_magnetic_field_strengths.png|thumb|Ferrofluid simulations for different parameters of surface tension and magnetic field strengths]]

Ferrofluids have an exceptionally high [[magnetic susceptibility]] and the critical magnetic field for the onset of the corrugations can be realised by a small bar magnet.

[[File:Ferrofluid close.jpg|thumb|[[Macrophotograph]] of ferrofluid influenced by a magnet.]]

===Common ferrofluid surfactants===
The soapy [[surfactant]]s used to coat the nanoparticles include, but are not limited to:

* [[oleic acid]]
* [[tetramethylammonium hydroxide]]
* [[citric acid]]
* [[soy lecithin]]

These [[surfactant]]s prevent the nanoparticles from clumping together, so the particles neither fall out of suspension nor clump into a pile of magnetic dust near the magnet. The magnetic particles in an ideal ferrofluid never settle out, even when exposed to a strong magnetic field. A surfactant has a [[chemical polarity|polar]] head and non-polar tail (or vice versa), one of which [[adsorption|adsorbs]] to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular [[micelle]], respectively, around the particle. Electrostatic repulsion then prevents agglomeration of the particles.

While surfactants are useful in prolonging the settling rate in ferrofluids, they also hinder the fluid's magnetic properties (specifically, the fluid's [[magnetic saturation]]). The addition of surfactants (or any other foreign particles) decreases the [[packing density]] of the ferroparticles while in its activated state, thus decreasing the fluid's on-state [[viscosity]], resulting in a "softer" activated fluid. While the on-state viscosity (the "hardness" of the activated fluid) is less of a concern for some ferrofluid applications, it is a primary fluid property for the majority of their commercial and industrial applications and therefore a compromise must be met when considering on-state viscosity versus the settling rate of a ferrofluid.

[[File:Ferrofluid in magnetic field.jpg|right|thumb|A ferrofluid in a [[magnetic field]] showing normal-field instability caused by a [[neodymium magnet]] beneath the dish]]

==Applications==
===Current===
====Electronic devices====
{{main|Ferrofluidic seal}}
Ferrofluids are used to form liquid [[Seal (mechanical)|seals]] around the spinning drive shafts in [[hard disk]]s.  The rotating shaft is surrounded by magnets. A small amount of ferrofluid, placed in the gap between the magnet and the shaft, will be held in place by its attraction
to the magnet. The fluid of magnetic particles forms a barrier which prevents debris from entering the interior of the hard drive. According to engineers at Ferrotec, ferrofluid seals on rotating shafts typically withstand 3 to 4 psi;<ref>{{cite patent|country=US|number=4478424A|gdate=1984-01-27}}</ref> additional seals can be stacked to form assemblies capable of withstanding higher pressures.

====Mechanical engineering====
Ferrofluids have [[friction]]-reducing capabilities. If applied to the surface of a strong enough magnet, such as one made of [[neodymium]], it can cause the magnet to glide across smooth surfaces with minimal resistance.

==== Materials science research ====
Ferrofluids can be used to image magnetic domain structures on the surface of ferromagnetic materials using a technique developed by [[Francis Bitter]].<ref>{{Cite journal|last=Mee|first=C D|date=1950-08-01|title=The Mechanism of Colloid Agglomeration in the Formation of Bitter Patterns|url=http://stacks.iop.org/0370-1298/63/i=8/a=122?key=crossref.d5dd5c87e293fe8b0c3b380fdec6d174|journal=Proceedings of the Physical Society, Section A|volume=63|issue=8|pages=922|doi=10.1088/0370-1298/63/8/122|issn=0370-1298|bibcode=1950PPSA...63..922M|url-access=subscription}}</ref>

====Loudspeakers====
Starting in 1973, ferrofluids have been used in [[loudspeaker]]s to remove heat from the [[voice coil]], and to passively [[Damping ratio|damp]] the movement of the cone. They reside in what would normally be the air gap around the voice coil, held in place by the speaker's magnet. Since ferrofluids are paramagnetic, they obey [[Curie's law]] and thus become less magnetic at higher temperatures. A strong magnet placed near the voice coil (which produces heat) will attract cold ferrofluid more than hot ferrofluid thus pushing the heated ferrofluid away from the electric voice coil and toward a [[heat sink]]. This is a relatively efficient cooling method which requires no additional energy input.<ref>{{cite journal|author=Rlums, Elmars|url=http://www.sbfisica.org.br/bjp/download/v25/v25a10.pdf|journal=Brazilian Journal of Physics|volume=25|issue=2|date=1995|title=New Applications of Heat and Mass Transfer Processes in Temperature Sensitive Magnetic Fluids}}</ref>

Bob Berkowitz of [[Acoustic Research]] began studying ferrofluid in 1972, using it to damp resonance of a tweeter. Dana Hathaway of Epicure in Massachusetts was using ferrofluid for tweeter damping in 1974, and he noticed the cooling mechanism. Fred Becker and Lou Melillo of Becker Electronics were also early adopters in 1976, with Melillo joining Ferrofluidics and publishing a paper in 1980.<ref>{{cite journal | last1=Melillo | first1=Louis | last2=Raj | first2=K. | title=Ferrofluids as a Means of Controlling Woofer Design Parameters | journal=Journal of the Audio Engineering Society | publisher=Audio Engineering Society | volume=29 | issue=3 | date=1981-03-01 | pages=132–139 }}</ref> In concert sound, [[Showco]] began using ferrofluid in 1979 for cooling woofers.<ref>{{cite magazine |url=https://books.google.com/books?id=fAEAAAAAMBAJ&pg=PA61 |page=61 |date=June 1979 |title=Magnetic Fluids |last=Free |first=John |magazine=[[Popular Science]] }}</ref> [[Panasonic]] was the first Asian manufacturer to put ferrofluid in commercial loudspeakers, in 1979. The field grew rapidly in the early 1980s. Today, some 300&nbsp;million sound-generating transducers per year are produced with ferrofluid inside, including speakers installed in laptops, cell phones, headphones and earbuds.<ref>{{Cite web|url=https://www.czferro.com/ferrofluid-history|title=Brief History of Ferrofluid|website=Ferrofluid Displays, Art, and Sculptures &#124; Concept Zero}}</ref>

====Cell separations====
Ferrofluids conjugated with antibodies or common capture agents such as [[Streptavidin]] (SA) or rat anti-mouse Ig (RAM) are used in [[immunomagnetic separation]], a subset of [[cell sorting]].<ref>{{cite web |url=https://biomagneticsolutions.com/pages/ferrofluid |title=Ferrofluid – BioMagnetic Solutions |website=biomagneticsolutions.com |url-status=dead |archive-url=https://web.archive.org/web/20200714163030/https://biomagneticsolutions.com/pages/ferrofluid |archive-date=2020-07-14}} </ref> These conjugated ferrofluids are used to bind to target cells, and then magnetically separate them from a cell mixture using a low-gradient magnetic separator. These ferrofluids have applications such as [[cell therapy]], [[gene therapy]], [[cellular manufacturing]], among others.

====Audio-visualization====
On the aesthetic side, ferrofluids can be displayed to [[Music visualization|visualize sound]]. For that purpose, the blob of ferrofluid is suspended in a clear liquid. An electromagnet acts on the shape of the ferrofluid in response to the volume or the audio frequency of the music, allowing it to selectively react to a song’s treble or bass.<ref>{{cite web | url=https://gizmodo.com/sound-reactive-bluetooth-speaker-uses-magnetic-ferroflu-1846729756 | title=Sound Reactive Bluetooth Speaker Uses Magnetic Ferrofluid to Become a Real-Life Winamp Visualizer|first1=Andrew|last1=Liszewski | date=21 April 2021|website=Gizmodo }}</ref><ref>{{cite web | url=https://www.youtube.com/watch?v=pgp2sp0EB7w | title=Ferrofluid display cell bluetooth speaker | website=[[YouTube]] | date=8 April 2021 }}</ref>

====Ferrolens====
A magneto-optic device and magnetic-field flux viewer dynamic lens can be created by using a [[Superparamagnetism|superparamagnetic]] thin-film encapsulated and sealed between two optic flat glasses. The [[thin film]] is made of a heavily diluted, almost transparent ferrofluid that is several microns thick. The ferrolens has an [[Light-emitting diode|LED]] ring array around its perimeter that illuminates it. When an external magnetic field is projected onto the surface of the thin film, it produces a 2D flux magnetic field imprint pattern, similar to the Faraday's classical [[:File:Magnet0873.png|iron filings experiment]]. This pattern includes depth of field information of the external field being displayed by the ferrolens device, despite the thin film having a finite thickness only of several microns (i.e. 10 to 20 μm).<ref>{{Cite journal |last1=Markoulakis |first1=Emmanouil |last2=Vanderelli |first2=Timm |last3=Frantzeskakis |first3=Lambros |date=2022 |title=Real time display with the ferrolens of homogeneous magnetic fields |url=https://doi.org/10.1016/j.jmmm.2021.168576 |journal=Journal of Magnetism and Magnetic Materials |volume=541 |pages=168576 |arxiv=2109.12044 |doi=10.1016/j.jmmm.2021.168576 |bibcode=2022JMMM..54168576M |issn=0304-8853}}</ref>

===Former===
====Medical applications====
Several ferrofluids were marketed for use as [[MRI contrast agent|contrast agents]] in [[magnetic resonance imaging]], which depend on the difference in magnetic relaxation times of different tissues to provide contrast.<ref name=app/><ref>{{cite journal|last1=Wang|first1=YX|title=Superparamagnetic iron oxide based MRI contrast agents: Current status of clinical application|journal=Quantitative Imaging in Medicine and Surgery|date=December 2011|volume=1|issue=1|pages=35–40|pmid=23256052|pmc=3496483|doi=10.3978/j.issn.2223-4292.2011.08.03}}</ref> Several agents were introduced and then withdrawn from the market, including Feridex I.V. (also known as Endorem and ferumoxides), discontinued in 2008;<ref>{{cite web |url=http://www.amagpharma.com/products/feridex_iv.php |title=Feridex - Products - AMAG Pharmaceuticals |publisher=Amagpharma.com |access-date=2012-06-20 |url-status=dead |archive-url=https://web.archive.org/web/20120615182847/http://www.amagpharma.com/products/feridex_iv.php |archive-date=2012-06-15 }}</ref> resovist (also known as Cliavist), 2001 to 2009;<ref>{{cite web|author=Softways |url=http://www.mr-tip.com/serv1.php?type=db1&dbs=Resovist |title=Magnetic Resonance TIP - MRI Database : Resovist |publisher=Mr-tip.com |access-date=2012-06-20}}</ref> Sinerem (also known as Combidex), withdrawn in 2007;<ref>{{cite web |url=http://www.thefreelibrary.com/AMAG+Pharmaceuticals,+Inc.+Announces+Update+on+Sinerem(TM)+in+Europe.-a0172378541 |title=AMAG Pharmaceuticals, Inc. Announces Update on Sinerem(TM) in Europe. - Free Online Library |publisher=Thefreelibrary.com |date=2007-12-13 |access-date=2012-06-20 |archive-date=2019-03-23 |archive-url=https://web.archive.org/web/20190323160818/https://www.thefreelibrary.com/AMAG+Pharmaceuticals%2c+Inc.+Announces+Update+on+Sinerem(TM)+in+Europe.-a0172378541 |url-status=dead }}</ref> Lumirem (also known as Gastromark), 1996<ref>{{cite web |url=http://www.centerwatch.com/drug-information/fda-approvals/drug-details.aspx?DrugID=105 |title=Newly Approved Drug Therapies (105) GastroMARK, Advanced Magnetics |publisher=CenterWatch |access-date=2012-06-20 |archive-date=2011-12-29 |archive-url=https://web.archive.org/web/20111229114636/http://centerwatch.com/drug-information/fda-approvals/drug-details.aspx?DrugID=105 |url-status=dead }}</ref> to 2012;<ref>{{cite web|title=AMAG Form 10-K For the Fiscal Year Ended December 31, 2013|url=https://www.sec.gov/Archives/edgar/data/792977/000104746914000718/a2218084z10-k.htm|publisher=SEC Edgar}}</ref><ref>{{cite web|title=NDA 020410 for GastroMark|url=https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=020410|archive-url=https://web.archive.org/web/20170212171148/https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=020410|url-status=dead|archive-date=February 12, 2017|publisher=FDA|access-date=12 February 2017}}</ref> Clariscan (also known as PEG-fero, Feruglose, and NC100150), development of which was discontinued due to safety concerns.<ref>{{cite journal |doi=10.3978/j.issn.2223-4292.2011.08.03 |first1=Yi-Xiang J. |last1=Wang |year=2011 |title=Superparamagnetic iron oxide based MRI contrast agents: Current status of clinical application |journal=Quantitative Imaging in Medicine and Surgery |pmid=23256052 |volume=1 |issue=1 |pages=35–40 |pmc=3496483 }}</ref>

===Future===
====Spacecraft propulsion====
{{further|Spacecraft propulsion}}
Ferrofluids can be made to self-assemble nanometer-scale needle-like sharp tips under the influence of a magnetic field. When they reach a critical thinness, the needles begin emitting jets that might be used in the future as a thruster mechanism to propel small satellites such as [[CubeSat]]s.<ref>{{Cite news|url=http://www.spacesafetymagazine.com/2013/10/17/novel-thrusters-developed-nanosats|title=Novel Thrusters Being Developed for Nanosats|date=2013-10-17|work=Space Safety Magazine|access-date=2018-07-09|language=en-US|last=Raval |first=Siddharth }}</ref>

====Analytical instrumentation====
Ferrofluids have numerous [[optical]] applications because of their [[refractive]] properties; that is, each grain, a [[magnet|micromagnet]], reflects light. These applications include measuring [[specific viscosity]] of a liquid placed between a [[polarizer]] and an [[analyzer]], illuminated by a [[helium–neon laser]].<ref>{{cite journal|doi=10.1016/j.proeng.2013.09.250|title=Transient Optical Phenomenon in Ferrofluids|journal=Procedia Engineering|volume=76|pages=74–79|year=2014|last1=Pai|first1=Chintamani|last2=Shalini|first2=M|last3=Radha|first3=S|doi-access=free}}</ref>

====Medical applications====
Ferrofluids have been proposed for magnetic drug targeting. In this process the drugs would be attached to or enclosed within a ferrofluid and could be targeted and selectively released using magnetic fields.<ref>{{cite journal|last1=Kumar|first1=CS|last2=Mohammad|first2=F|title=Magnetic nanomaterials for hyperthermia-based therapy and controlled drug delivery|journal=Advanced Drug Delivery Reviews|date=14 August 2011|volume=63|issue=9|pages=789–808|pmid=21447363|pmc=3138885|doi=10.1016/j.addr.2011.03.008}}</ref>

It has also been proposed for targeted [[magnetic hyperthermia]] to convert electromagnetic energy into heat.<ref>{{cite journal|last1=Kafrouni|first1=L|last2=Savadogo|first2=O|title=Recent progress on magnetic nanoparticles for magnetic hyperthermia|journal=Progress in Biomaterials|date=December 2016|volume=5|issue=3–4|pages=147–160|doi=10.1007/s40204-016-0054-6|pmid=27995583|pmc=5304434}}</ref>

It has also been proposed in a form of nanosurgery to separate one tissue from another—for example a tumor from the tissue in which it has grown.<ref name=app>{{cite journal|author1=Scherer, C.  |author2=Figueiredo Neto, A. M. |title=Ferrofluids: Properties and Applications|journal= Brazilian Journal of Physics|volume=35|issue=3A|pages=718–727|date=2005|url=http://www.sbfisica.org.br/bjp/files/v35_718.pdf|doi=10.1590/S0103-97332005000400018|bibcode = 2005BrJPh..35..718S |doi-access=free}}</ref>

====Heat transfer====
An external magnetic field imposed on a ferrofluid with varying susceptibility (e.g., because of a temperature gradient) results in a nonuniform magnetic body force, which leads to a form of [[heat transfer]] called [[thermomagnetic convection]].  This form of heat transfer can be useful when conventional convection heat transfer is inadequate; e.g., in miniature microscale devices or under [[microgravity|reduced gravity]] conditions.

Ferrofluids of suitable composition can exhibit extremely large enhancement in thermal conductivity (k; ~300% of the base fluid thermal conductivity). The large enhancement in k is due to the efficient transport of heat through percolating nanoparticle paths. Special magnetic [[nanofluid]]s with tunable thermal conductivity to viscosity ratio can be used as multifunctional ‘smart materials’ that can remove heat and also arrest vibrations (damper). Such fluids may find applications in microfluidic devices and microelectromechanical systems ([[MEMS]]).<ref>{{cite journal|doi=10.1021/jp204827q |title=Tuning of Thermal Conductivity and Rheology of Nanofluids Using an External Stimulus|date=2011|last1=Shima|first1=P. D.|last2=Philip|first2=John|journal=The Journal of Physical Chemistry C|volume=115|issue=41|page=20097}}</ref>

====Optics====
Research is under way to create an [[Ferrofluid mirror|adaptive optics shape-shifting magnetic mirror]] from ferrofluid for Earth-based astronomical [[Optical telescope|telescope]]s.<ref>{{cite news|publisher=New Scientist|title=Morphing mirror could clear the skies for astronomers|date=7 November 2008|author=Hecht, Jeff |url=https://www.newscientist.com/article/dn15154-morphing-mirror-could-clear-the-skies-for-astronomers.html}}</ref>

Optical filters are used to select different wavelengths of light. The replacement of filters is cumbersome, especially when the wavelength is changed continuously with tunable-type lasers. Optical filters tunable for different wavelengths by varying the magnetic field can be built using ferrofluid emulsion.<ref>{{cite journal|doi=10.1088/0957-0233/14/8/314|title=A tunable optical filter|date=2003|last1=Philip|first1=John|last2=Jaykumar|first2=T|last3=Kalyanasundaram|first3=P|last4=Raj|first4=Baldev|journal=Measurement Science and Technology|volume=14|issue=8|page=1289|bibcode = 2003MeScT..14.1289P |s2cid=250923543 }}</ref>

====Energy harvesting====
Ferrofluids enable the harvesting of vibration energy from the environment. Existing methods of harvesting low frequency (<100&nbsp;Hz) vibrations require the use of solid resonant structures. With ferrofluids, energy harvester designs no longer need solid structure. One example of ferrofluid based [[energy harvesting]] is to place the ferrofluid inside a container to use external mechanical vibrations to generate electricity inside a coil wrapped around the container surrounded by a permanent magnet.<ref name="Bibo2012">{{cite journal|last1=Bibo|first1=A.|last2=Masana|first2=R.|last3=King|first3=A.|last4=Li|first4=G.|last5=Daqaq|first5=M.F.|title=Electromagnetic ferrofluid-based energy harvester|journal=Physics Letters A|date=June 2012|volume=376|issue=32|pages=2163–2166|doi=10.1016/j.physleta.2012.05.033|bibcode=2012PhLA..376.2163B}}</ref> First a ferrofluid is placed inside a container that is wrapped with a coil of wire. The ferrofluid is then externally magnetized using a permanent magnet. When external vibrations cause the ferrofluid to slosh around in the container, there is a change in magnetic flux fields with respect to the coil of wire. Through [[Faraday's Law of Induction|Faraday's law of electromagnetic induction]], voltage is induced in the coil of wire due to change in magnetic flux.<ref name="Bibo2012"/>

== See also ==
{{portal|Physics}}
{{div col}}
* {{annotated link|Continuum mechanics}}
* {{annotated link|Electrorheological fluid}}
* {{annotated link|Fluid mechanics}}
* {{annotated link|Magnetic field viewing film}}
* {{annotated link|Magnetic ionic liquid}}
* {{annotated link|Magnetohydrodynamics}}
* {{annotated link|Magnetorheological fluid}}
* {{annotated link|Plasma physics}}
* {{annotated link|Smart fluid}}
* [[List of textbooks in electromagnetism]]

{{div col end}}

== References ==
{{Reflist|30em}}

==Bibliography==

* {{ cite book |first1=David |last1=Andelman |first2=Ronald E. |last2=Rosensweig |chapter=The Phenomenology of Modulated Phases: From Magnetic Solids and Fluids to Organic Films and Polymers |pages=1–56 |editor-first=Yoav |editor-last=Tsori |editor2-first=Ullrich |editor2-last=Steiner |date=2009 |title=Polymers, liquids and colloids in electric fields: interfacial instabilities, orientation and phase transitions |isbn=978-981-4271-68-4 |bibcode=2009plce.book.....T |doi=10.1142/7266 }}
* {{cite journal | last1=Berger | first1=Patricia | last2=Adelman | first2=Nicholas B. | last3=Beckman | first3=Katie J. | last4=Campbell | first4=Dean J. | last5=Ellis | first5=Arthur B. | last6=Lisensky | first6=George C. | title=Preparation and Properties of an Aqueous Ferrofluid | journal=Journal of Chemical Education | publisher=American Chemical Society (ACS) | volume=76 | issue=7 | year=1999 | issn=0021-9584 | doi=10.1021/ed076p943 | page=943| bibcode = 1999JChEd..76..943B}}

== External links ==
{{Commons}}
* {{Cite journal |last=Rosensweig |first=Ronald E. |date=1982 |title=Magnetic Fluids |url=https://www.jstor.org/stable/24966707 |journal=Scientific American |volume=247 |issue=4 |pages=136–145 |doi=10.1038/scientificamerican1082-136 |jstor=24966707 |bibcode=1982SciAm.247d.136R |issn=0036-8733|url-access=subscription }}
* {{youTube|PvtUt02zVAs|title= How ferrofluid works video}}
* [https://web.archive.org/web/20040505105319/http://www.ifs.tohoku.ac.jp/nishiyama-lab/Research.html A comparison of ferrofluid and MR fluid (at the bottom of the page)]
* [https://www.chemedx.org/search/result/ferrofluid Chemistry comes alive: Ferrofluid] (subscription required)
* Sachiko Kodama art projects: [https://web.archive.org/web/20140222164028/http://www.kodama.hc.uec.ac.jp/protrudeflow/index.html Ferrofluid Sculptures] [http://video.google.com/videoplay?docid=7932498063864415301 (Google Video)] {{Webarchive|url=https://web.archive.org/web/20060805165506/http://video.google.com/videoplay?docid=7932498063864415301 |date=2006-08-05 }}, [https://web.archive.org/web/20080827055531/http://www.flypmedia.com/issues/12/#15/1 Ferrofluid Sculptures] 
* [http://www.dansdata.com/magnets.htm#ff Daniel Rutter has some fun with Ferrofluid]
* Marketing material at INVENTUS Engineering GmbH website: [http://www.inventus.at/index.php?id=74 High pressure valve]
* {{youTube|id=21WzdjqAG0s |title=Liquid seal for Stirling piston (video)}}
* [http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm FerroFluid Synthesis] {{Webarchive|url=https://web.archive.org/web/20070203194303/http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm |date=2007-02-03 }}
* Teaching materials: [https://web.archive.org/web/20080303175816/http://mrsec.wisc.edu/Edetc/nanolab/ffexp/index.html Interdisciplinary education group: Ferrofluids] (contains videos and a lab for synthesis of ferrofluid)
* {{Cite web|url=http://voh.chem.ucla.edu/classes/Magnetic_fluids/|title=Synthesis of an Aqueous Ferrofluid|website=voh.chem.ucla.edu|access-date=2018-07-09}}
*  [https://www.dignitymemorial.com/obituaries/cleveland-heights-oh/solomon-papell-6510706 Solomon Papell Obituary - Cleveland Heights, OH]

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[[Category:Magnetism]]
[[Category:Fluid mechanics]]
[[Category:Nanomaterials]]
[[Category:Articles containing video clips]]