Three-dimensional close-to-substrate trajectories of magnetic microparticles in dynamically changing magnetic field landscapes

TL;DR

This study develops a method to accurately track three-dimensional trajectories of magnetic microparticles near substrates in dynamic magnetic fields, revealing hopping motions and enabling improved optical detection in lab-on-a-chip systems.

Contribution

It introduces a label-free optical technique to measure vertical particle positions in dynamic magnetic fields, advancing understanding of particle transport mechanisms.

Findings

Observed maximum vertical jumps of several micrometers.

Confirmed theoretical predictions of particle-substrate distances.

Demonstrated potential for optical detection of particle-substrate interactions.

Abstract

The transport of magnetic nano- or microparticles in microfluidic devices using artificially designed magnetic field landscapes (MFL) is promising for the implementation of key functionalities in Lab-on-a-chip (LOC) systems. A close-to-substrate transport is hereby instrumental to use changing particle-substrate interactions upon analyte binding for analytics and diagnostics. Here, we present an essential prerequisite for such an application, namely the label-free quantitative experimental determination of the three-dimensional trajectories of superparamagnetic particles (SPP) transported by a dynamically changing MFL above a topographically flat substrate. The evaluation of the SPP sharpness within defocused video-recorded images, acquired by an optical bright-field microscope, was employed to obtain a vertical z-coordinate. This method applied to a prototypical transport scheme, using the static MFL of parallel-stripe domains superposed by a particular magnetic field pulse sequence, revealed a hopping-like motion of the magnetic particles, previously predicted by theory. Maximum vertical particle jumps of several micrometers have been observed experimentally, corroborating theoretical estimates for the particle-substrate distance. As our findings pave the way towards precise quantification of particle-substrate separations in the discussed transport system, they bear deep implications for future LOC detection schemes using only optical microscopy.