Capture of magnetic nanoparticles does not depend on the compaction of microparticles in a chip*

Magnetic nanoparticles can be captured in a magnetized porous medium composed of ferromagnetic microparticles. But, how does the microparticles' packing state affect the nanoparticle capture? In this work, we present experiments of a magnetic trap in a microfluidic chip where we study the capture efficiency as a function of the compaction of the microparticles column. We also present a simple analytical model based on the competition of drag and magnetic forces. Our work can help optimize environmental and biomedical applications based on high-gradient magnetic nanoparticle separation.