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Ferrofluid is a dark coloured liquid that is attracted to the poles of a magnet.

It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended inside 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. 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 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.

The magnetic particles in an ideal ferrofluid never settle out, even when exposed to a strong magnetic field. 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 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.

In this process the drugs would be attached to or enclosed within a ferrofluid and could be targeted and selectively released using magnetic fields.

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.

Optical filters tunable for different wavelengths by varying the magnetic field can be built using ferrofluid emulsion.

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.

Quick Facts

• That is, a ferrofluid contains primarily nanoparticles, while an MR fluid contains primarily micrometre-scale particles.
• In contrast to ferrofluids, magnetorheological fluids (MR fluids) are magnetic fluids with larger particles.
• 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.
• 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.
• 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.

Source material: Wikipedia - "Ferrofluid". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.