Break

Dispersions of paramagnetic nanoparticles can be manipulated with external magnetic fields and are employed in magnetorheological fluids, drug delivery vehicles, and separation processes. A key transport quantity for these applications is the magnetophoretic mobility, i.e. how fast nanoparticles move in response to a magnetic field gradient. Though magnetophoretic transport is well understood in the dilute limit, the mobility is challenging to predict in concentrated dispersions, the regime relevant in practice. Induced dipole interactions among nanoparticles result in self-assembly, and the resulting anisotropic microstructure drastically alters the hydrodynamic drag on aggregates and hydrodynamic interactions among particles. Here, we investigate the transport of paramagnetic nanoparticles using coarse-grained Brownian dynamics simulations in volume fraction and field strength ranges where these collective effects dominate. We find that magnetic-field-induced structure of field-aligned nanoparticle chains tends to enhance the magnetophoretic velocity compared to gravity-driven sedimentation of colloidal hard spheres, particularly in the direction parallel to the self-assembled chains. At sufficiently large magnetic field strengths, nanoparticle chains aggregate laterally and arrest in a percolated network. For a range of field strengths, the shear forces generated by magnetophoretic translation overpower the interparticle dipolar forces holding the network together, and the network ruptures, leading to a dramatic increase in the magnetophoretic mobility. This phenomenon is fully analogous to the rapid collapse of metastable colloidal gels after network rupture during gravity-driven sedimentation. Because the shear forces scale linearly with the field magnitude and the interparticle forces are proportional to the square of the field, cohesive forces dominate at large fields and the network resists rupture. With this fundamental understanding of magnetophoretic transport, we design schemes leveraging magnetic fields for separation and recovery of metal ions from electronic waste.