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Ferrofluid is a dark liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of Nanoscopic scale Ferromagnetism or ferrimagnetic particles suspended inside a 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 is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping or Flocculation. 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.
Particles in ferrofluids are dispersed in a liquid, often using a surfactant, and thus ferrofluids are Colloid – 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, , 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.
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.
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.
These prevent the nanoparticles from clumping together, so the particles neither fall out of suspension nor clump into a pile of magnetic dust near the magnet. The magnetic particles in an ideal ferrofluid never settle out, even when exposed to a strong magnetic field. A surfactant has a polar head and non-polar tail (or vice versa), one of which adsorption 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.
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.
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.
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 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).
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.
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