magnetophoresis

Magnetophoresis is the motion of magnetic particles suspended in a fluid under the influence of a spatially varying magnetic field. The force on a particle arises from the contrast between the magnetic susceptibility of the particle and that of its surrounding medium, causing the particle to migrate toward regions of higher or lower magnetic field depending on whether it is more paramagnetic or more diamagnetic than the medium.

In a simple view, the magnetic force scales with the particle volume and the gradient of the magnetic field (often described as proportional to ∇(H^2) or ∇(B^2)). The effect is more pronounced for magnetically susceptible materials such as iron oxide nanoparticles or magnetic beads, which are commonly used in laboratory applications. Thermal motion and fluid shear compete with magnetic forces, so effective separation or manipulation is achieved by balancing field strength, gradient geometry, and flow conditions.

Applications include magnetic separation and purification of cells, proteins, or other labeled species; magnetic bead-based immunomagnetic sorting (MACS) in biomedical research; and the guided transport of magnetic nanoparticles in microfluidic devices. In these systems, external magnets or electromagnets create the field gradient that pulls target particles along a desired path, enabling rapid enrichment or isolation from complex mixtures.

Limitations include the need for sufficient magnetic susceptibility contrast, potential aggregation of beads, heating from strong fields, and reduced efficiency in very viscous media or at high flow rates. Magnetophoresis is a key tool in bioseparation and nanotechnology, often used in conjunction with chemical or biological recognition elements to achieve selectivity.