Ferrofluid

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Ferrofluid on glass, with a magnet underneath Ferrofluid Magnet under glass edit.jpg
Ferrofluid on glass, with a magnet underneath
Steve Papell invented ferrofluid for NASA in 1963 Steve Papell NASA ferrofluid developer in 1963.JPG
Steve Papell invented ferrofluid for NASA in 1963

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. [1]

Contents

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. [2] 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. [3] [4] [5] [6] 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. [7]

Description

R. E. Rosensweig with ferrofluid in his lab (1965) R. E. Rosensweig with ferrofluid in his lab (1965).jpg
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. [8]

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. [9] 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 the magnet which is underneath the white dish. Ferrofluid poles.jpg
Ferrofluid is the oily substance collecting at the poles of the magnet which is underneath the white dish.

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. [10]

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. [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 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. [12]

Ferrofluid simulations for different parameters of surface tension and magnetic field strengths Ferrofluid simulations for different parameters of surface tension and magnetic field strengths.png
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. Ferrofluid close.jpg
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 Ferrofluid in magnetic field.jpg
A ferrofluid in a magnetic field showing normal-field instability caused by a neodymium magnet beneath the dish

Applications

Actual

Electronic devices

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;[ citation needed ] 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.

Ferrofluids can also be used in semi-active dampers in mechanical and aerospace applications. While passive dampers are generally bulkier and designed for a particular vibration source in mind, active dampers consume more power. Ferrofluid based dampers solve both of these issues and are becoming popular in the helicopter community, which has to deal with large inertial and aerodynamic vibrations.

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. [13]

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 pulling 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. [14]

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 Ferrotec and publishing a paper in 1980. [15] In concert sound, Showco began using ferrofluid in 1979 for cooling woofers. [16] 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. [17]

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. [18] 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.

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. [19] [20] Several agents were introduced and then withdrawn from the market, including Feridex I.V. (also known as Endorem and ferumoxides), discontinued in 2008; [21] resovist (also known as Cliavist), 2001 to 2009; [22] Sinerem (also known as Combidex), withdrawn in 2007; [23] Lumirem (also known as Gastromark), 1996 [24] to 2012; [25] [26] Clariscan (also known as PEG-fero, Feruglose, and NC100150), development of which was discontinued due to safety concerns. [27]

Future

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. [28]

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. [29]

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. [30]

It has also been proposed for targeted magnetic hyperthermia to convert electromagnetic energy into heat. [31]

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. [19]

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). [32]

Optics

Research is under way to create an adaptive optics shape-shifting magnetic mirror from ferrofluid for Earth-based astronomical telescopes. [33]

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. [34]

Energy harvesting

Ferrofluids enable an interesting opportunity to harvest 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 simple 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. [35] 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. [35]

See also

Related Research Articles

Superparamagnetism

Superparamagnetism is a form of magnetism which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Néel relaxation time. In the absence of an external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Néel relaxation time, their magnetization appears to be in average zero; they are said to be in the superparamagnetic state. In this state, an external magnetic field is able to magnetize the nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is much larger than that of paramagnets.

Magnetohydrodynamics Study of the magnetic properties of electrically conducting fluids

Magnetohydrodynamics is the study of the magnetic properties and behaviour of electrically conducting fluids. Examples of such magneto­fluids include plasmas, liquid metals, salt water, and electrolytes. The word "magneto­hydro­dynamics" is derived from magneto- meaning magnetic field, hydro- meaning water, and dynamics meaning movement. The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in Physics in 1970.

Magnet Material or object that produces a magnetic field

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets.

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism. The word remanence is from remanent + -ence, meaning "that which remains".

Coercivity

Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC.

Magnetorheological fluid

A magnetorheological fluid is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications. Extensive discussions of the physics and applications of MR fluids can be found in a recent book.

Halbach array

A Halbach array is a special arrangement of permanent magnets that augments the magnetic field on one side of the array while cancelling the field to near zero on the other side. This is achieved by having a spatially rotating pattern of magnetisation.

Smart fluid

A smart fluid is a fluid whose properties can be changed by applying an electric field or a magnetic field.

Electrorheological fluid

Electrorheological (ER) fluids are suspensions of extremely fine non-conducting but electrically active particles in an electrically insulating fluid. The apparent viscosity of these fluids changes reversibly by an order of up to 100,000 in response to an electric field. For example, a typical ER fluid can go from the consistency of a liquid to that of a gel, and back, with response times on the order of milliseconds. The effect is sometimes called the Winslow effect after its discoverer, the American inventor Willis Winslow, who obtained a US patent on the effect in 1947 and wrote an article published in 1949.

MagneRide is an automotive adaptive suspension with magnetorheological damper system developed by the Delphi Automotive corporation, during a period when the company was a subsidiary of General Motors (GM), that uses magnetically controlled dampers, or shock absorbers, for a highly adaptive ride. As opposed to traditional suspension systems, MagneRide has no mechanical valves or even small moving parts that can wear out. This system consists of four monotube dampers, one on each corner of the vehicle, a sensor set, and an ECU to maintain the system.

Ferrofluids can be used to transfer heat, since heat and mass transport in such magnetic fluids can be controlled using an external magnetic field.

A magnetorheological damper or magnetorheological shock absorber is a damper filled with magnetorheological fluid, which is controlled by a magnetic field, usually using an electromagnet. This allows the damping characteristics of the shock absorber to be continuously controlled by varying the power of the electromagnet. Fluid viscosity increases within the damper as electromagnet intensity increases. This type of shock absorber has several applications, most notably in semi-active vehicle suspensions which may adapt to road conditions, as they are monitored through sensors in the vehicle, and in prosthetic limbs.

Magnetic nanoparticles are a class of nanoparticle that can be manipulated using magnetic fields. Such particles commonly consist of two components, a magnetic material, often iron, nickel and cobalt, and a chemical component that has functionality. While nanoparticles are smaller than 1 micrometer in diameter, the larger microbeads are 0.5–500 micrometer in diameter. Magnetic nanoparticle clusters that are composed of a number of individual magnetic nanoparticles are known as magnetic nanobeads with a diameter of 50–200 nanometers. Magnetic nanoparticle clusters are a basis for their further magnetic assembly into magnetic nanochains. The magnetic nanoparticles have been the focus of much research recently because they possess attractive properties which could see potential use in catalysis including nanomaterial-based catalysts, biomedicine and tissue specific targeting, magnetically tunable colloidal photonic crystals, microfluidics, magnetic resonance imaging, magnetic particle imaging, data storage, environmental remediation, nanofluids, optical filters, defect sensor, magnetic cooling and cation sensors.

Flux pumping is a method for magnetising superconductors to fields in excess of 15 teslas. The method can be applied to any type II superconductor and exploits a fundamental property of superconductors, namely their ability to support and maintain currents on the length scale of the superconductor. Conventional magnetic materials are magnetised on a molecular scale which means that superconductors can maintain a flux density orders of magnitude bigger than conventional materials. Flux pumping is especially significant when one bears in mind that all other methods of magnetising superconductors require application of a magnetic flux density at least as high as the final required field. This is not true of flux pumping.

Iron oxide nanoparticles are iron oxide particles with diameters between about 1 and 100 nanometers. The two main forms are magnetite (Fe3O4) and its oxidized form maghemite (γ-Fe2O3). They have attracted extensive interest due to their superparamagnetic properties and their potential applications in many fields (although Co and Ni are also highly magnetic materials, they are toxic and easily oxidized).

Physics of magnetic resonance imaging

The physics of magnetic resonance imaging (MRI) concerns fundamental physical considerations of MRI techniques and technological aspects of MRI devices. MRI is a medical imaging technique mostly used in radiology and nuclear medicine in order to investigate the anatomy and physiology of the body, and to detect pathologies including tumors, inflammation, neurological conditions such as stroke, disorders of muscles and joints, and abnormalities in the heart and blood vessels among others. Contrast agents may be injected intravenously or into a joint to enhance the image and facilitate diagnosis. Unlike CT and X-ray, MRI uses no ionizing radiation and is, therefore, a safe procedure suitable for diagnosis in children and repeated runs. Patients with specific non-ferromagnetic metal implants, cochlear implants, and cardiac pacemakers nowadays may also have an MRI in spite of effects of the strong magnetic fields. This does not apply on older devices, details for medical professionals are provided by the device's manufacturer.

A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil.

Ferrofluid mirror

A ferrofluid mirror is a type of deformable mirror with a reflective liquid surface, commonly used in adaptive optics. It is made of ferrofluid and magnetic iron particles in ethylene glycol, the basis of automotive antifreeze. The ferrofluid mirror changes shape instantly when a magnetic field is applied. As the ferromagnetic particles align with the magnetic field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Since any shapes can be produced by changing the magnetic field geometries, wavefront control and correction can be achieved.

Magnetic field-assisted finishing, sometimes called magnetic abrasive finishing, is a surface finishing technique in which a magnetic field is used to force abrasive particles against the target surface. As such, finishing of conventionally inaccessible surfaces is possible. Magnetic field-assisted finishing (MAF) processes have been developed for a wide variety of applications including the manufacturing of medical components, fluid systems, optics, dies and molds, electronic components, microelectromechanical systems, and mechanical components.

Superparamagnetic relaxometry (SPMR) is a technology combining the use of sensitive magnetic sensors and the superparamagnetic properties of magnetite nanoparticles (NP). For NP of a sufficiently small size, on the order of tens of nanometers (nm), the NP exhibit paramagnetic properties, i.e., they have little or no magnetic moment. When they are exposed to a small external magnetic field, on the order of a few millitesla (mT), the NP align with that field and exhibit ferromagnetic properties with large magnetic moments. Following removal of the magnetizing field, the NP slowly become thermalized, decaying with a distinct time constant from the ferromagnetic state back to the paramagnetic state. This time constant depends strongly upon the NP diameter and whether they are unbound or bound to an external surface such as a cell. Measurement of this decaying magnetic field is typically done by superconducting quantum interference detectors (SQUIDs). The magnitude of the field during the decay process determines the magnetic moment of the NPs in the source. A spatial contour map of the field distribution determines the location of the source in three dimensions as well as the magnetic moment.

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