10.9 Ferrofluids
Ferrofluid is a unique material that exhibits both liquid and magnetic properties. It operates through a combination of magnetic and fluid dynamics principles.
Essentially, the ferrofluid is a colloidal suspension of magnetic particles in a carrier fluid (such as oil or water). Typically, the magnetic particles are iron oxide, ground to the nano-scale and approximately nanometers in size. These particles are coated with a surfactant, a substance that reduces surface tension. This coating prevents the particles from clumping together, ensuring they remain evenly dispersed in the fluid. The viscosity of the fluid, the nanometre size of the particles, and their constant movement prevent the particles from settling down.
When there is no magnet around, a ferrofluid acts like a liquid, but when there is a magnet nearby, the particles are temporarily magnetized and the fluid becomes a magnet. They form structures within the fluid causing the ferrofluid to act more like a solid. When the magnet is removed, the particles are demagnetized and the ferrofluid acts like a liquid again.
The first ferrofluid developed by NASA in 1960, was ground from natural magnetite. Ferrofluid was invented to move liquids through space.
This phenomenon is due to the competition between magnetic forces, surface tension and gravity. In the presence of strong magnetic field, the formation of chain-like structures is a result of magnetic forces pulling the fluid upwards while gravity and surface tension work to pull it back down. These chains align along the magnetic field lines and increase the viscosity, making it behave like a solid bulging in certain directions. These are commonly known as spikes. However, the spikes are formed where the magnetic forces overcome the other forces (Fig. 10.17).