Magnetophoresis of Paramagnetic and Diamagnetic Particles in Suspensions under Nonuniform Magnetic Fields: Effects of Convection and Sedimentation

Magnetophoresis, or the magnetic field gradient driven movement of solutes or particles through a fluid medium, presents an efficient means of controlling the transport of suspended particles with various engineering, environmental, and medical applications. While magnetophoresis of ferromagnetic and superparamagnetic particles has been extensively studied, the movement of paramagnetic and diamagnetic particles under nonuniform magnetic fields remains poorly understood. In this work, we use a combination of experiments and numerical simulations to study the capture of paramagnetic manganese oxide and diamagnetic bismuth oxide particles in an aqueous suspension by nonuniform magnetic fields. We find that for magnetic Grashof numbers beyond the order of unity, the concentration gradients produced by magnetophoresis induce a bulk flow that accelerates the capture of paramagnetic particles. We also compare magnetophoresis with particle sedimentation and find that in cases where they act in opposite directions, the flow splits into two distinct regions: one where magnetophoresis drives particle capture, and one where particles settle due to gravity. The extent of this region can be predicted by comparing the magnetic Péclet number with the gravitational Péclet number.