Editing Ferrofluid (section)

==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"/>