A ferrofluid (from the Latin ferrum, meaning iron) is a liquid which becomes strongly polarised in the presence of a magnetic field. It is a colloidal mixture comprising extremely small magnetic particles suspended in a liquid. The particles are coated with a surface active agent (surfactant) to prevent them from clumping together.
Ferrofluids are composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetization in the absence of an externally applied field. In fact, ferrofluids display (bulk-scale) paramagnetism, and are often referred as being "superparamagnetic" due to their large magnetic susceptibility. Permanently magnetized fluids are difficult to create at present.
It is important to note the difference between ferrofluids and magnetorheological fluids (MR fluids). 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.
Ferrofluids are composed of nanoscale particles (diameter usually 10 nanometers or less) of magnetite, hematite or some other compound containing iron. 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 analogous 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.
Ferrofluids are tiny iron particles covered with a liquid coating, also surfactant that are then added to water or oil, which gives them their liquid properties. 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. 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. The specific temperature required varies depending on the specific compounds used for the nano-particles, surfactant, and carrier fluid.
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.
These surfactants prevent the nanoparticles from clumping together, ensuring that the particles do not form aggregates that become too heavy to be held in suspension by Brownian motion. The magnetic particles in an ideal ferrofluid do not settle out, even when exposed to a strong magnetic, or gravitational 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. Steric repulsion then prevents agglomeration of the particles.
While surfactants are useful in prolonging the settling rate in ferrofluids, they also prove detrimental to 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 fluids 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 vs. the settling rate of a ferrofluid.
Ferrofluids are also used in many high-frequency speaker drivers (tweeters) where they provide heat conduction from the voice coil to the surrounding assembly as well as mechanical damping to reduce undesired resonances. The ferrofluid is kept in place in the magnetic gap due to the strong magnetic field and is in contact with both the magnetic surfaces as well as the coil.
Ferrofluids are commonly 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, 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 forcing the heated ferrofluid away from the electric voice coil and toward a heat sink. This is an efficient cooling method which requires no additional energy input.