Editing Ferrofluid (section)

===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.]]