Negative magnetophoresis of submicron species in magnetic nanofluids

Abstract

In this work we studied the focusing and trapping of submicron, nonmagnetic species immersed in a magnetic nanofluid under applied magnetic fields. Focusing was achieved using two pairs of permanent magnets, which forced submicron fluorescently-tagged polystyrene beads to focus in the region between the two magnet pairs. Size-based trapping was achieved using a microchip that produced spatially increasing magnetic field gradients that trapped flowing polystyrene beads at different locations, depending on their relative sizes. In the focusing experiments, a mixture of magnetic nanoparticles and nonmagnetic, fluorescently tagged latex beads (435 nm and 910 nm in diameter) were loaded into a capillary tube and placed in-between the magnet pairs. The concentration profiles of the latex beads were measured using fluorescence imaging and simulated results were obtained using continuum modeling. Good quantitative agreement was found between experiments and theory for both latex-bead sizes at various experimental conditions. Size-based trapping of latex beads was accomplished by balancing drag and magnetic buoyancy forces in such a way that smaller and larger nonmagnetic species were trapped at different locations. A microfabricated device with two external magnets was used to generate the trapping forces, and a syringe pump was used to flow the mixture of magnetic fluid and nonmagnetic particles through the device. Size-based trapping was achieved for a feed mixture of 435 nm and 865 nm latex beads, as measured using fluorescence imaging. Semi-quantitative agreement was found between experiments and Brownian-dynamics simulations. Our work shows that negative magnetophoresis in magnetic nanofluids can be used to size-selectively trap and focus submicron, nonmagnetic species.