  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.
