Magnetoviscosity

Magnetoviscosity is the dependence of a magnetizable fluid’s viscosity on an applied magnetic field. It is most commonly studied in ferrofluids, which are colloidal suspensions of nanoscale magnetic particles stabilized by surfactants. When a magnetic field is applied, the particles acquire dipole moments that interact and tend to align along the field, forming transient chain- and column-like structures. This field-induced structuring increases the fluid’s resistance to flow, leading to a higher effective viscosity and, in some cases, a yield-stress-like response in concentrated suspensions. The effect is generally anisotropic, often producing greater resistance to shear parallel to the field than perpendicular to it.

Mechanisms and factors contributing to magnetoviscosity include the strength of the magnetic field, particle concentration and size, surface coatings, and the balance between magnetic interactions and thermal motion. In dilute ferrofluids, Brownian motion competes with dipolar interactions, so viscosity changes are moderate at low fields but become more pronounced as the field grows. In oscillatory or time-dependent shear, the dynamic viscosity also depends on the field frequency and relaxation dynamics of the magnetization.

Theoretical descriptions of magnetoviscosity are encompassed by ferrohydrodynamics, with models such as Shliomis and related approaches that couple magnetization relaxation to fluid flow and predict field-dependent viscosity and anisotropy. Magnetoviscosity is related to, but distinct from, magnetorheological effects: MR fluids typically exhibit large, field-induced yield stresses due to solid-like chain structures in concentrated suspensions, while ferrofluids show more modest, but still important, viscosity changes.

Applications include vibration damping, sealing, and controllable fluidic systems where a magnetic field tunes rheological properties. Measurements are usually performed with rheometers under applied magnetic fields to characterize viscosity versus field strength and orientation.