Editing Ferrofluid (section)

===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]]