Editing Ferrofluid (section)

==Description==
[[File:R. E. Rosensweig with ferrofluid in his lab (1965).jpg|thumb|R. E. Rosensweig with ferrofluid in his lab (1965)]]
Ferrofluids are composed of very small nanoscale particles  (diameter usually 10 nanometers or less) of [[magnetite]], [[hematite]] or some other compound containing [[iron]], and a liquid  (usually [[oil]]). 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 similar 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. The composition of a typical ferrofluid is about 5% magnetic solids, 10% [[surfactant]] and 85% carrier, by volume.<ref>{{Cite news|url=http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm|title=How to Make Liquid Magnets|work=ThoughtCo|access-date=2018-07-09|last=Helmenstine|first=Anne Marie|archive-date=2007-02-03|archive-url=https://web.archive.org/web/20070203194303/http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm|url-status=dead}}</ref>

Particles in ferrofluids are dispersed in a liquid, often using a [[surfactant]], and thus ferrofluids are [[Colloid|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.<ref>{{Cite web|url=http://education.jlab.org/beamsactivity/6thgrade/vocabulary/index.html|title=Vocabulary List|website=education.jlab.org|language=en-us|access-date=2018-07-09}}</ref> This ability to change phases with the application of a magnetic field allows them to be used as [[seal (mechanical)|seals]], [[lubricant]]s, 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]].

===Normal-field instability===
[[File:Ferrofluid poles.jpg|thumb|right|Ferrofluid is the oily substance collecting at the poles of a magnet which is underneath the brown oil.]]

When a paramagnetic fluid is subjected to a strong vertical [[magnetic field]], the surface forms a regular pattern of peaks and valleys. This effect is known as the ''Rosensweig'' or ''normal-field instability''. The instability is driven by the magnetic field; it can be explained by considering which shape of the fluid minimizes the total energy of the system.{{sfn|Andelman|Rosensweig|2009|pp= 20–21}}

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.{{sfn|Andelman|Rosensweig|2009|pp= 21, 23; Fig. 11}}

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 [[specific surface energy|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.{{sfn|Andelman|Rosensweig|2009|pp= 21}}
[[File:Ferrofluid_simulations_for_different_parameters_of_surface_tension_and_magnetic_field_strengths.png|thumb|Ferrofluid simulations for different parameters of surface tension and magnetic field strengths]]

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.

[[File:Ferrofluid close.jpg|thumb|[[Macrophotograph]] of ferrofluid influenced by a magnet.]]

===Common ferrofluid surfactants===
The soapy [[surfactant]]s used to coat the nanoparticles include, but are not limited to:

* [[oleic acid]]
* [[tetramethylammonium hydroxide]]
* [[citric acid]]
* [[soy lecithin]]

These [[surfactant]]s 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 [[chemical polarity|polar]] head and non-polar tail (or vice versa), one of which [[adsorption|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. 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.

[[File:Ferrofluid in magnetic field.jpg|right|thumb|A ferrofluid in a [[magnetic field]] showing normal-field instability caused by a [[neodymium magnet]] beneath the dish]]