Ferrofluids are colloidal suspensions of microscopic permanent magnets. A striking combination of fundamental but tractable scientific problems, in addition to a substantial and growing list of applications, make ferrofluids attractive materials for study. They were first developed, and applied comercially, about 25 years ago. Their primary applications are as seals for rotating shafts and coatings for loudspeaker coils. Future applications could include high speed printer ink, and magneto-rheological fluids for automobile clutches and shock absorbers.
Ferrofluids are conceptually simple materials - they can be modeled nicely as a dilute gas of dipolar hard spheres. Despite their simple description, understanding their thermodynamic and hydrodynamic properties remains at the forefront of current research. The long spatial range of the dipole interaction, and its easily frustrated angular dependence, prevent accurate description of the thermodynamic equation of state. For strongly interacting ferrofluids the individual magnetic particles tend to align, head-to-tail, into chains, loops and branched networks as shown in the figure at the top of this page. When this occurs, weak-coupling, low-density virial expansions are entirely inadequate for deriving the magnetic susceptibility of the fluid. Instead, theories based on "living polymers" appear more successful.
Magnetic depolarization forces couple with bulk shape leading to a set of beautiful hydrodynamic instabilities. For example, confine a ferrofluid droplet between two glass slides and apply a perpendicular magnetic field. The fluid polarizes causing it to repel from itself. Surface tension maintains the topology of a single disk-like droplet, but the perimeter deforms into a "labyrinth" like the one shown below (taken from the T7 web page showing a simulated chemical reaction pattern).
Here are postscript copies of some recent papers by Widom and coworkers relating to ferrofluids:
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