Editing Ferrofluid (section)

===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>