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]
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]
https://en.wikipedia.org/wiki/Ferrofluid
A magnetorheological fluid (MR fluid, or MRF) 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 viscoelasticsolid. 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.[1]
MR fluid is different from a ferrofluid which has smaller particles. MR fluid particles are primarily on the micrometre-scale and are too dense for Brownian motion to keep them suspended (in the lower density carrier fluid). Ferrofluid particles are primarily nanoparticles that are suspended by Brownian motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications.
https://en.wikipedia.org/wiki/Magnetorheological_fluid
https://en.wikipedia.org/wiki/Thermodynamics
https://en.wikipedia.org/wiki/Chemical_thermodynamics
https://en.wikipedia.org/wiki/Vacuum_permeability
https://en.wikipedia.org/wiki/Covariant_formulation_of_classical_electromagnetism
https://en.wikipedia.org/wiki/Fine-structure_constant
https://en.wikipedia.org/wiki/Electromagnetic_spectrum
https://en.wikipedia.org/wiki/Helium
https://en.wikipedia.org/wiki/Spectral_line
https://en.wikipedia.org/wiki/Thermodynamic_cycle
https://en.wikipedia.org/wiki/Radiation
https://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure
https://en.wikipedia.org/wiki/Thermodynamic_equilibrium#Local_and_global_equilibrium
https://en.wikipedia.org/wiki/Transport_phenomena
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