ferrohydrodynamics

Ferrohydrodynamics is the branch of fluid dynamics that studies the motion of ferrofluids—stable colloidal suspensions of magnetic nanoparticles dispersed in a carrier liquid—under the influence of magnetic fields. The magnetic particles, typically magnetite or similar ferromagnetic materials, are coated with surfactants to prevent aggregation, giving the fluid unique magneto-hydrodynamic properties. The magnetic field induces forces and stresses that couple with viscous, inertial, and capillary effects, producing behaviors not seen in ordinary fluids.

The governing framework combines the Navier–Stokes equations with magnetostatics and a suitable constitutive relation for the fluid magnetization. The magnetic field exerts a body force on the ferrofluid tied to its magnetization and contributes a magnetic stress to the total stress balance. In many studies, field-dependent effects such as magnetoviscosity—changes in apparent viscosity with magnetic field strength—and modifications to interfacial tension are important for predicting flow patterns and stability.

A hallmark phenomenon is the Rosensweig instability, or normal-field instability, in which the surface of a ferrofluid under a perpendicular magnetic field destabilizes and forms a pattern of peaks. Other effects include field-assisted pumping, levitation of interfaces, and magnetically induced structuring within microchannels.

Applications of ferrohydrodynamics encompass damping and sealing devices, microfluidic actuators, and cooling systems where magnetic control of liquid motion is advantageous. The field underpins studies of interfacial instabilities and the interplay of magnetism with complex fluids. It has its roots in the work of Ronald Rosensweig and colleagues, and remains active in both theoretical and experimental research.