Ferrofluid

Ferrofluid is a liquid that becomes magnetized in the presence of a magnetic field. It consists of nanoscale ferromagnetic or ferrimagnetic particles, typically magnetite (Fe3O4) or maghemite (γ-Fe2O3), dispersed in a carrier liquid with a surfactant coating to prevent agglomeration. Particle diameters are usually 6 to 20 nanometers, small enough for Brownian motion to counteract sedimentation and keep the suspension stable without fields. In zero field the fluid behaves as a normal liquid; when a magnetic field is applied, the particles align with the field and can form chain-like structures along the field lines.

Under magnetic fields, ferrofluids exhibit surface deformations and spike patterns along field lines, a phenomenon known as the normal-field instability. The magnetic forces interact with surface tension and viscous forces to create spikes and arrays. The particles are typically superparamagnetic at these sizes, so the fluid does not retain magnetization after the field is removed. Applied fields can also increase the fluid’s apparent viscosity, a magnetoviscous effect that slows flow.

Applications include seals, dampers, and lubricants in rotating machinery, where a ferrofluid can form a magnetic seal or provide vibration damping. Ferrofluid is used in loudspeakers to damp movement and improve cooling, and in laboratory demonstrations of magnetically controlled fluids. Research explores ferrofluids for targeted drug delivery, magnetic hyperthermia, and microfluidic systems, with biocompatible coatings under development for medical use.

Ferrofluids are produced by dispersing nanoscale magnetic particles in carrier liquids with surface coatings. Oil-based ferrofluids are common in industry; water-based types exist but are less common. The concept originated in the 1960s at NASA for spacecraft sealing and guidance, and has since found applications in science and engineering.