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{{Short description|Liquid that is attracted by poles of a magnet}}
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] underneath]]
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{{Electromagnetism}}
{{Continuum mechanics|rheology}}
'''Ferrofluid''' is a liquid that is attracted to the poles of a ]. It is a ] liquid made of ] ] or ] particles suspended in a ] ] (usually an ] 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 ] 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 ]s is weak enough that the surfactant's ] is sufficient to prevent magnetic clumping or ]. Ferrofluids usually do not retain ] in the absence of an externally applied field and thus are often classified as "]" 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, ]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 ] by ] 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 '''ferrofluid''' (from the Latin '']'', meaning iron) is a liquid which becomes strongly polarised in the presence of a ]. It is a ] mixture comprising extremely small magnetic particles suspended in a synthetic oil. The particles are coated with a soap or detergent to prevent them from clumping together.


A process for making a ferrofluid was invented in 1963 by NASA's ] to create liquid ] 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
Ferrofluids are composed of ] ] particles suspended in a ] ], usually an organic solvent or water. The ferromagnetic nano-particles are coated with a ] to prevent their agglomeration (due to ] and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ], since they do not retain magnetisation in the absence of an externally applied field. In fact, ferrofluids display ], and are often referred as being "superparamagnetic" due to their large ]. True ferromagnetic fluids are difficult to create at present.<ref name="two">{{Cite web|url=http://dx.doi.org/10.1007/s003390050569|title=First observation of ferromagnetism and ferromagnetic domains in a liquid metal (abstract)|accessyear=2007|accessmonthday=August 31|publisher=Applied Physics A: Materials Science & Processing|year=1997|author=T. Albrecht, C. Bührer et al.|language=English}}</ref>
|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 ] 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== ==Description==
]
]
Ferrofluids comprise microscopic ferromagnetic nano-particles, usually ], ] or some other compound containing iron. The nano-particles are typically on the order of 10 nm. 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 analogous to the way that the ions in an aqueous ] salt solution (such as an aqueous solution of copper sulfate or manganese chloride) make the solution paramagnetic. Ferrofluids are composed of very small nanoscale particles (diameter usually 10 nanometers or less) of ], ] or some other compound containing ], and a liquid (usually ]). 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 ] salt solution (such as an aqueous solution of ] or ]) make the solution paramagnetic. The composition of a typical ferrofluid is about 5% magnetic solids, 10% ] 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 ], and thus ferrofluids are ] – 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 ], ]s, and may open up further applications in future ].
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 ] (MRF) refers to liquids similar to ferrofluids (FF) that solidify in the presence of a magnetic field. Magnetorheological fluids have ] scale magnetic particles that are 1–3 orders of magnitude larger than those of ferrofluids.
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 ] (MRF) refers to liquids similar to ferrofluids (FF) that solidify in the presence of a magnetic field. Magnetorheological fluids have ] 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 ].


===Normal-field instability=== ===Normal-field instability===
] showing normal-field instability caused by a ] beneath the dish]] ]

When a paramagnetic fluid is subjected to a strong vertical ], 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 ], 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 ] and ]. 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 ] and the ] of the liquid, but reduces the magnetic energy. The corrugations will only form above a critical magnetic ], when the reduction in magnetic energy outweighs the increase in surface and gravitation energy terms.{{sfn|Andelman|Rosensweig|2009|pp= 21}}
]

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


] of ferrofluid influenced by a magnet.]]
When a paramagnetic fluid is subjected to a sufficiently strong vertical magnetic field, the surface spontaneously forms a regular pattern of corrugations; this effect is known as the normal-field instability. The formation of the corrugations increases the 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, when the reduction in magnetic energy outweighs the increase in surface and gravitation energy terms. 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.


===Common ferrofluid surfactants=== ===Common ferrofluid surfactants===
Ferrofluids often contain surfactants including, but not limited to: The soapy ]s used to coat the nanoparticles include, but are not limited to:


* ] * ]
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* ] * ]


These ]s prevent the nanoparticles from clumping together, so the particles can not fall out of suspension nor clump into a pile of magnetic dust on 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 ] head and non-polar tail (or vice versa), one of which ] to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular ], respectively, around the particle. Electrostatic repulsion then prevents agglomeration of the particles.
These ]s serve to decrease the rate of ferroparticle settling, of which a high rate is an unfavorable characteristic of ferrofluids. The ideal ferrofluid would never settle in the absence of real-world friction. Surfactant-aided prolonged settling is typically achieved in one of two ways. In the case of the addition of soy lecithin, the surfactant particles are nanospheres and prolong the onset of settling via ]. In the case of oleic acid and other ] surfactants, the effective diameter of each ferroparticle is increased by the attachment of micelle molecules to each ferroparticle, thereby increasing particle diameter and making fluid remixing (particle redispersion) occur far faster and with less effort.


While surfactants are useful in prolonging the settling rate in ferrofluids, they also prove detrimental to the fluid's magnetic properties (specifically, the fluid's magnetic saturation), which is commonly a parameter which users wish to maximize (this is typically more of a concern when dealing with ]). Whether or not the surfactant is nanosphere-based or micelle-based, the addition of surfactants (or any other foreign particles) decreases the packing density of the ferroparticles while in its activated state, thus decreasing the fluids 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 vs. the settling rate of a ferrofluid. While surfactants are useful in prolonging the settling rate in ferrofluids, they also hinder the fluid's magnetic properties (specifically, the fluid's ]). The addition of surfactants (or any other foreign particles) decreases the ] of the ferroparticles while in its activated state, thus decreasing the fluid's on-state ], 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.

] showing normal-field instability caused by a ] beneath the dish]]


==Applications== ==Applications==
===Current===
]
====Electronic devices====
{{main|Ferrofluidic seal}}
Ferrofluids are used to form liquid ] around the spinning drive shafts in ]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.


===Electronic devices=== ====Mechanical engineering====
Ferrofluids have ]-reducing capabilities. If applied to the surface of a strong enough magnet, such as one made of ], it can cause the magnet to glide across smooth surfaces with minimal resistance.
Ferrofluids are used to form liquid ] (]) around the spinning drive shafts in ]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. However, the ferrofluid is still similar enough in
properties to a true liquid that it will not interfere with the spinning of the shaft.


==== Materials science research ====
Another common use of ferrofluids is as a liquid coolant. One commercial
Ferrofluids can be used to image magnetic domain structures on the surface of ferromagnetic materials using a technique developed by ].<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}}</ref>
application for this usage is in ]s and ]. Ferrofluid is put in the
space (magnetic gap) between the permanent magnet and the ] of a ]. Just as in the hard drive, the permanent magnet will hold the ferrofluid in place, keeping it in contact with
the voice coil. Heat flows from the high current voice coil and into the ferrofluid. When the ferrofluid is heated above its ] it is no longer attracted to the magnet and is pushed out of the magnetic gap by cooler fluid nearby. When the hot ferrofluid cools below its critical temperature it resumes its paramagnetic behavior and will rush back into the magnetic gap. This forms an active, liquid, heat pump to prevent damage to the speaker.<ref name="one">{{Cite web|url=http://64.233.167.104/search?q=cache:suVXfrtIuZkJ:www.sbfisica.org.br/bjp/download/v25/v25a10.pdf+ferrofluid+curie+heat+pump&hl=en&ct=clnk&cd=5&gl=us&lr=lang_en&client=firefox-a|title=New Applications of Heat and Mass Transfer Processes in Temperature Sensitive Magnetic Fluids|accessyear=2007|accessmonthday=August 31|publisher=Brazilian Journal of Physics|year=1995|author=Elmars Rlums|language=English}}</ref>


===Mechanical engineering=== ====Loudspeakers====
Starting in 1973, ferrofluids have been used in ]s to remove heat from the ], and to passively ] 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 ] 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 ]. 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>
Ferrofluids have friction-reducing capabilities. If applied to the surface of a strong enough magnet, such as one made of ], it can cause the magnet to glide across smooth surfaces with minimal resistance.


Bob Berkowitz of ] 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, ] 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=] }}</ref> ] 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>
]s of various applications have been and continue to be developed. These dampers are mainly used in heavy industry with applications such as heavy motor dampening, operator seat/cab dampening in construction vehicles, and more.


====Cell separations====
As of 2006, materials scientists and mechanical engineers are collaborating to develop stand-alone ] dampers which, when positioned anywhere within a building, will operate within the building's ], absorbing detrimental ] and ] within the structure, giving these dampers the ability to make any building earthquake-proof, or at least earthquake-resistant.
Ferrofluids conjugated with antibodies or common capture agents such as ] (SA) or rat anti-mouse Ig (RAM) are used in ], a subset of ].<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 ], ], ], among others.


===Defense=== ====Audio-visualization====
On the aesthetic side, ferrofluids can be displayed to ]. 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=] | date=8 April 2021 }}</ref>
The ] introduced a ] (RAM) '''paint''' made from both ferrofluidic and non-magnetic substances. By reducing the ] of ], this material helps to reduce the ] of ].


===Aerospace=== ====Ferrolens====
A magneto-optic device and magnetic-field flux viewer dynamic lens can be created by using a ] thin-film encapsulated and sealed between two optic flat glasses. The ] is made of a heavily diluted, almost transparent ferrofluid that is several microns thick. The ferrolens has an ] 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 ]. 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>
NASA has experimented using ferrofluids in a closed loop as the basis for a spacecraft's ] system. A ] is applied to a loop of ferrofluid to change the ] and influence the rotation of the spacecraft.


===Optics=== ===Former===
====Medical applications====
], a ]-based optical polishing method, has proven to be highly precise. It was used in the construction of the ]'s corrective lens.
Several ferrofluids were marketed for use as ] in ], 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|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>
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===Measurement=== ===Future===
====Spacecraft propulsion====
Ferrofluids have numerous ] applications due to their ] properties; that is, each grain, a ], reflects light. These applications include measuring ] of a liquid placed between a ] and an ], illuminated by a ]-] ].
{{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 ]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====
===Medicine===
Ferrofluids have numerous ] applications because of their ] properties; that is, each grain, a ], reflects light. These applications include measuring ] of a liquid placed between a ] and an ], illuminated by a ].<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>
In ], a ] ferrofluid can be used for ] detection. There is also much experimentation with the use of ferrofluids to remove tumors. The ferrofluid would be forced into the tumor and then subjected to a quickly varying magnetic field. This would create friction, yielding heat, due to the movement of the ferrofluid inside the tumor which could destroy the tumor.


====Medical applications====
Additionally heavy metals used in MRI could be enclosed in carbon "cages" to protect the body from these possibly harmful metals.
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 ] 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>
===Heat transfer===
An external magnetic field imposed on a ferrofluid with varying susceptibility, e.g., due to a temperature gradient, results in a nonuniform magnetic body force, which leads to a form of ] called ]. This form of heat transfer can be useful when conventional convection heat transfer is inadequate, e.g., in miniature microscale devices or under reduced gravity conditions.


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>
Ferrofluids are commonly used in ]s to sink heat between the voice coil and the magnet assembly, and to passively 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 ], thus become less magnetic at higher temperatures. A strong magnet placed near the voice coil (which produces heat) will always attract colder ferrofluid towards it more than warmer ferrofluid thus forcing the heated ferrofluid away, towards the heat sink. This is an efficient cooling method which requires no additional energy input.


===Automotive=== ====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 ] called ]. This form of heat transfer can be useful when conventional convection heat transfer is inadequate; e.g., in miniature microscale devices or under ] conditions.
If the ]s of a vehicle's ] are filled with ferrofluid instead of plain oil, and the whole device surrounded with an ], the viscosity of the fluid (and hence the amount of ] provided by the shock absorber) can be varied depending on driver preference or the weight being carried by the vehicle - or it may be dynamically varied in order to provide stability control. The ] ''magnetic ride control'' or ''active suspension'' is one such system which permits the damping factor to be adjusted once every millisecond in response to conditions. As of 2007, BMW manufactures cars using their own proprietary version of this device, while GM (the first auto manufacturer to do so), Audi, and Ferrari offer the MagneRide on various models.


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 ]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 (]).<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>
] and other automotive companies are seeking to develop a ] based clutch system for push-button four wheel drive systems. This clutch system would use ] to solidify the fluid which would lock the ] into the ].


==See also== ====Optics====
Research is under way to create an ] from ferrofluid for Earth-based astronomical ]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>
{{Commons|Ferrofluids|Ferrofluids}}
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==References==
{{reflist|2}}


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>
==Sources==
* Ferrohydrodynamics (1985), Ronald. E. Rosensweig. The usual starting reference for learning the details of ferrofluids.


====Energy harvesting====
==External links==
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 ] 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 ], voltage is induced in the coil of wire due to change in magnetic flux.<ref name="Bibo2012"/>
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== See also ==
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{{portal|Physics}}
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* {{annotated link|Continuum mechanics}}
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* {{annotated link|Electrorheological fluid}}
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* {{annotated link|Fluid mechanics}}
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* {{annotated link|Magnetic field viewing film}}
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* {{annotated link|Magnetic ionic liquid}}
* {{annotated link|Magnetohydrodynamics}}
* {{annotated link|Magnetorheological fluid}}
* {{annotated link|Plasma physics}}
* {{annotated link|Smart fluid}}
* ]

{{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|Ferrofluids}}
* {{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}}
* {{youTube|PvtUt02zVAs|title= How ferrofluid works video}}
*
* (subscription required)
* Sachiko Kodama art projects: {{Webarchive|url=https://web.archive.org/web/20060805165506/http://video.google.com/videoplay?docid=7932498063864415301 |date=2006-08-05 }},
* *
* Marketing material at INVENTUS Engineering GmbH website:
*
* {{youTube|id=21WzdjqAG0s |title=Liquid seal for Stirling piston (video)}}
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* {{Webarchive|url=https://web.archive.org/web/20070203194303/http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm |date=2007-02-03 }}
* Teaching materials: (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}}
*


{{Authority control}}
===Preparation instructions===
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* {{cite journal
| last = Berger
| first = Patricia
| coauthors = Nicholas B. Adelman, Katie J. Beckman, Dean J. Campbell, et al
| year = 1999
| month = July
| title = Preparation and properties of an aqueous ferrofluid
| journal = Journal of Chemical Education
| volume = 76
| issue = 7
| pages = pp. 943-948
| id = {{ISSN|00219584}}
| url = http://proquest.umi.com/pqdweb?did=42639843&Fmt=4&clientId=11263&RQT=309&VName=PQD
| accessdate = 2007-01-02
}}
* (contains videos and a lab for synthesis of ferrofluid)
* — instructions in ] and ] format


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Latest revision as of 18:50, 20 December 2024

Liquid that is attracted by poles of a magnet
Ferrofluid on glass, with a neodymium magnet underneath
Steve Papell invented ferrofluid for NASA in 1963
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Ferrofluid is a liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid (usually an organic solvent or water). 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 nanoparticles is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping or agglomeration. Ferrofluids usually do not retain magnetization in the absence of an externally applied field and thus are often classified as "superparamagnets" rather than ferromagnets.

In contrast to ferrofluids, magnetorheological fluids (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 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. The name ferrofluid was introduced, the process improved, more highly magnetic liquids synthesized, additional carrier liquids discovered, and the physical chemistry elucidated by R. 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. 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.

Description

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.

Particles in ferrofluids are dispersed in a liquid, often using a surfactant, and thus ferrofluids are 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. This ability to change phases with the application of a magnetic field allows them to be used as seals, lubricants, 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

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.

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.

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 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.

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.

Macrophotograph of ferrofluid influenced by a magnet.

Common ferrofluid surfactants

The soapy surfactants used to coat the nanoparticles include, but are not limited to:

These surfactants prevent the nanoparticles from clumping together, so the particles can not fall out of suspension nor clump into a pile of magnetic dust on 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 polar head and non-polar tail (or vice versa), one of which 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.

A ferrofluid in a magnetic field showing normal-field instability caused by a neodymium magnet beneath the dish

Applications

Current

Electronic devices

Main article: Ferrofluidic seal

Ferrofluids are used to form liquid seals around the spinning drive shafts in hard disks. 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; 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.

Loudspeakers

Starting in 1973, ferrofluids have been used in loudspeakers to remove heat from the voice coil, and to passively 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.

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. In concert sound, Showco began using ferrofluid in 1979 for cooling woofers. 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 million sound-generating transducers per year are produced with ferrofluid inside, including speakers installed in laptops, cell phones, headphones and earbuds.

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. 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 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.

Ferrolens

A magneto-optic device and magnetic-field flux viewer dynamic lens can be created by using a 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 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 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).

Former

Medical applications

Several ferrofluids were marketed for use as contrast agents in magnetic resonance imaging, which depend on the difference in magnetic relaxation times of different tissues to provide contrast. Several agents were introduced and then withdrawn from the market, including Feridex I.V. (also known as Endorem and ferumoxides), discontinued in 2008; resovist (also known as Cliavist), 2001 to 2009; Sinerem (also known as Combidex), withdrawn in 2007; Lumirem (also known as Gastromark), 1996 to 2012; Clariscan (also known as PEG-fero, Feruglose, and NC100150), development of which was discontinued due to safety concerns.

Future

Spacecraft propulsion

Further information: 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 CubeSats.

Analytical instrumentation

Ferrofluids have numerous optical applications because of their refractive properties; that is, each grain, a 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.

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.

It has also been proposed for targeted magnetic hyperthermia to convert electromagnetic energy into heat.

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.

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 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 nanofluids 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).

Optics

Research is under way to create an adaptive optics shape-shifting magnetic mirror from ferrofluid for Earth-based astronomical telescopes.

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.

Energy harvesting

Ferrofluids enable the harvesting of vibration energy from the environment. Existing methods of harvesting low frequency (<100 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. 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 electromagnetic induction, voltage is induced in the coil of wire due to change in magnetic flux.

See also

References

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  2. Voit, W.; Kim, D. K.; Zapka, W.; Muhammed, M.; Rao, K. V. (21 March 2011). "Magnetic behavior of coated superparamagnetic iron oxide nanoparticles in ferrofluids". MRS Proceedings. 676. doi:10.1557/PROC-676-Y7.8.
  3. US Patent 3215572 
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  6. Gollwitzer, Christian; Krekhova, Marina; Lattermann, Günter; Rehberg, Ingo; Richter, Reinhard (2009), "Surface instabilities and magnetic soft matter", Soft Matter, 5 (10): 2093, arXiv:0811.1526, Bibcode:2009SMat....5.2093G, doi:10.1039/b820090d, S2CID 17537054
  7. Singh, Chamkor; Das, Arup K.; Das, Prasanta K. (2016), "Flow restrictive and shear reducing effect of magnetization relaxation in ferrofluid cavity flow", Physics of Fluids, 28 (8): 087103, Bibcode:2016PhFl...28h7103S, doi:10.1063/1.4960085
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  11. Andelman & Rosensweig 2009, pp. 20–21.
  12. Andelman & Rosensweig 2009, pp. 21, 23, Fig. 11.
  13. Andelman & Rosensweig 2009, pp. 21.
  14. US 4478424A, issued 1984-01-27 
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  17. Melillo, Louis; Raj, K. (1981-03-01). "Ferrofluids as a Means of Controlling Woofer Design Parameters". Journal of the Audio Engineering Society. 29 (3). Audio Engineering Society: 132–139.
  18. Free, John (June 1979). "Magnetic Fluids". Popular Science. p. 61.
  19. "Brief History of Ferrofluid". Ferrofluid Displays, Art, and Sculptures | Concept Zero.
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  22. "Ferrofluid display cell bluetooth speaker". YouTube. 8 April 2021.
  23. Markoulakis, Emmanouil; Vanderelli, Timm; Frantzeskakis, Lambros (2022). "Real time display with the ferrolens of homogeneous magnetic fields". Journal of Magnetism and Magnetic Materials. 541: 168576. arXiv:2109.12044. Bibcode:2022JMMM..54168576M. doi:10.1016/j.jmmm.2021.168576. ISSN 0304-8853.
  24. ^ Scherer, C.; Figueiredo Neto, A. M. (2005). "Ferrofluids: Properties and Applications" (PDF). Brazilian Journal of Physics. 35 (3A): 718–727. Bibcode:2005BrJPh..35..718S. doi:10.1590/S0103-97332005000400018.
  25. Wang, YX (December 2011). "Superparamagnetic iron oxide based MRI contrast agents: Current status of clinical application". Quantitative Imaging in Medicine and Surgery. 1 (1): 35–40. doi:10.3978/j.issn.2223-4292.2011.08.03. PMC 3496483. PMID 23256052.
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