Ferrofluid

A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field. Ferrofluids are colloidal mixtures composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nanoparticles are coated with a surfactant to prevent their agglomeration (due to van der Waals forces and magnetic forces). Ferrofluids usually do not retain magnetization in the absence of an externally applied field and thus are often classified as "superparamagnets" rather than ferromagnets. The particles in a ferrofluid primarily consist of nanoparticles which are suspended by Brownian motion and generally will not settle under normal conditions. MR fluid particles primarily consist of micrometre-scale particles which are too heavy for Brownian motion to keep them suspended, and thus will settle over time due to the inherent density difference between the particle and its carrier fluid. These two fluids have very different applications as a result. 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 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. In many applications, ferrofluid is an active component that contributes towards the enhanced performance of the device. These devices are either mechanical (e.g., seals, bearings and dampers) or electromechanical (e.g., loudspeakers, stepper motors and sensors) in nature. In other cases, ferrofluid is employed simply as a material for nondestructive testing of other components such as magnetic tapes, stainless steels and turbine blades. When correctly applied, Ferrofluid can produce dramatic improvements in a products' performance; or achieve a level of performance unattainable by any other technology or product.

Further information

1. T. Albrecht, C. Bührer et al. (1997), "First observation of ferromagnetism and ferromagnetic domains in a liquid metal (abstract)", Applied Physics A Materials Science & Processing (Applied Physics A: Materials Science & Processing). — Т. 65: 215.
2. Elmars Blums. New Applications of Heat and Mass Transfer Processes in Temperature Sensitive Magnetic Fluids. Brazilian Journal of Physics (1995).
3. Ватутин Э.И., Чевычелов С.Ю., Родионов А.А., Игнатенко Н.М. Некоторые результаты моделирования процесса генерации упругих волн переменным магнитным полем в магнитоупорядоченных композитах. Сборник научных трудов "Сварка и родственные технологии в машиностроении и электронике" (2002).
4. Статья Ferrofluid из Wikipedia, свободной энциклопедии. Доступно под лицензией Creative Commons Attribution-Share Alike.
5. Статья Ферромагнитная жидкость из Wikipedia, свободной энциклопедии. Доступно под лицензией Creative Commons Attribution-Share Alike.

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