  Magnetic nanoparticles (MNPs) have many applications which require MNPs to be
captured and immobilized for their manipulation and sensing. For example, MNP
sensors based on detecting changes to the ferromagnetic resonances of an
antidot nanostructure exhibit better performance when the nanoparticles are
captured within the antidot inclusions. This study investigates the influence
of microfluidics upon the capture of MNPs by four geometries of antidot array
nanostructures hollowed into 30 nm-thick Permalloy films. The nanostructures
were exposed to a dispersion of 130 nm MNP clusters which passed through PDMS
microfluidic channels with a 400 {\mu}m circular cross-section fabricated from
wire molds. With the microfluidic flow of MNPs, the capture efficiency - the
ratio between the number of nanoparticles captured inside of the antidot
inclusions to the number outside the inclusions - decreased for all four
geometries compared to previous results introducing the particles via droplets
on the film surface. This indicates that most MNPs were passing over the
nanostructures, since there were no significant magnetophoretic forces acting
upon the particles. However, when a static magnetic field is applied, the
magnetophoretic forces generated by the nanostructure are stronger and the
capture efficiencies are significantly higher than those obtained using
droplets. In particular, circular antidots demonstrated the highest capture
efficiency among the four geometries of almost 83.1% when the magnetic field is
parallel to the film plane. In a magnetic field perpendicular to the film, the
circle antidots again show the highest capture efficiency of about 77%. These
results suggest that the proportion of nanoparticles captured inside antidot
inclusions is highest under a parallel magnetic field. Clearly, the geometry of
the nanostructure has a strong influence on the capture of MNPs.
