Liquid magnets, also known as ferrofluids, are colloidal suspensions of nanometer-scale magnetic materials in a liquid medium. A typical ferrofluid might comprise about 5% by volume of iron oxide particles roughly 10 nanometers in size, about 85% of a carrier fluid, such as kerosene, and 10% of a surfactant, perhaps oleic acid. A surfactant is typically a linear molecule which on one end has a chemical group that will bind to the iron oxide, and on the other end a group that is soluble in the carrier fluid. Soap is a surfactant for oil on one end and for water on the other, thereby allowing oily dirt to be placed into solution with the washing water. The surfactant not only makes formation of a colloidal suspension possible, but also prevents the magnetic particles from coalescing into larger particles which would change the behavior of the ferrofluid.
A ferrofluid exhibits very large magnetic susceptibility in the presence of a magnetic field. That is, in the presence of an external magnetic field, the ferrofluid takes on an induced magnetization. When the external field is removed, the magnetization of the ferrofluid returns to a very low value. Making a permanently magnet out of a ferrofluid is very difficult, but has rarely been accomplished by techniques beyond our present scope. However, a magnetized ferrofluid can be frozen to lock in the magnetization. Of course, one now has a solid magnet, but this is a fun thing to do.
Ferrofluids have many practical applications, based on the attraction of a ferrofluid to a magnet without risking formation of permanent agglomerations of the magnetic particles. A very common application is to form seals around the spinning drive shafts in computer hard disks. Such seals keep debris out of the extraordinarily delicate interior of the hard disk, and can maintain this seal against a pressure differential of 3-4 psi, thereby allowing the hard disk to survive at high altitude.
A new cancer treatment called magnetic hyperthermia is based on the physics of ferrofluids. When placed in an alternating magnetic field, the constant reorientation of the magnetic particles requires work, which heats the ferrofluid. Given that the ferrofluid can be beneficially placed within a tumor, it allows physicians to delicately control application of heat to the tumor. A constant magnetic field can also cause the ferrofluid to remain in the desired region.
A ferrofluid placed at a sliding contact between a magnet and a nonmagnetic surface will dramatically reduce the sliding friction of the contact, leading to more efficient mechanical devices. Ferrofluids are also used in radar absorbent materials, in spacecraft attitude control systems, and around loudspeaker sound coils to remove heat and to damp the movement of the cone, thereby avoid distortion caused by cone overtravel.
Having seen that ferrofluids, or liquid magnets, have fascinating and useful properties, let’s consider how to make such a beast. Although the details will not be listed for reasons of liability, the essential steps are:
1. Prepare a water-based suspension of nanoscale magnetite particles;
2. Coat the magnetite particles with a surfactant – let’s add oleic acid to the magnetite suspension and heat and stir, perhaps for an hour;
3. Add kerosene to the magnetite suspension, and stir to transfer the surfactant-coated magnetite particles from the water-based suspension into the kerosene.
The kerosene will eventually float on top of the water, and can be removed. This kerosene-based fluid is now a ferrofluid.
The most difficult step is to prepare nanoscale magnetite particles. Unfortunately, one cannot simply grind up magnetite – the resulting particles are far too large for a ferrofluid. Instead, extremely small clusters of magnetite are chemically synthesized, perhaps by reducing ferric chloride to ferrous chloride, then add twice as much ferric chloride to this solution, add ammonia, and heat to drive a chemical reaction which synthesizes magnetite, Fe3O4.
Ferrofluids are simple materials which exhibit a wide range of fascinating magnetic properties. The relative ease of preparing a ferrofluid makes this a reasonable project for a home chemical laboratory or a high school science project.