Motion of water-immersed superparamagnetic microparticles on top of a topographically flat thin film substrate imprinted with engineered magnetic stripe domains of gradually increasing/decreasing stripe width and alternating head-to-head/tail-to-tail magnetization configuration

The controlled transport and fractionation of magnetic microparticles according to their structural and magnetic properties is of great importance for the realization of miniaturized Lab-on-a-chip devices, applicable, e.g., for medical point-of-care diagnostics. We designed a transport and pre-sorting system for superparamagnetic beads based on an engineered magnetic stray field landscape emerging from a magnetic stripe domain pattern inside an exchange-biased thin film system. The magnetic stripe domains were imprinted into the system via ion bombardment induced magnetic patterning. The pattern consists of stripe domains with gradually increasing and decreasing stripe width, periodically repeated across the whole substrate. Directed motion of superparamagnetic beads above the substrate inside a quiescent liquid (water) is induced by applying weak external magnetic field pulses, superposing the static magnetic stray field landscape. The beads are forced to perform a jumping motion with increasing/decreasing jump length owing to the arrangement of the underlying domain pattern. Our data shows a simultaneous fractionation of all particles into mobile and immobile units according to their magnetophoretic mobilities. The motion velocity of the particles is thereby a function of jump length (stripe domain width) and the magnitude of the external field pulses. Superparamagnetic beads of significantly different sizes can be spatially separated using a short sequence of constant field pulses.

Videos of the particle motion were recorded using high-speed cameras attached to an optical microscope. The water-dispersed particles were placed on top of the magnetically patterned substrate inside a microfluidic chamber and motion was initiated by applying periodically repeated magnetic field pulses (pulse lengths in the ms-range; pulse magnitudes in the mT-range) using orthogonally arranged Helmholtz coils. The videos were analyzed qualitatively and quantitatively (particle velocity) using a custom Python code.