Magnetic filtration of phase separating ferrofluids: from basic concepts to microfluidic device
Résumé
In this work, we briefly review magnetic separation of ferrofluids composed of large
magnetic particles (60 nm of the average size) possessing an induced dipole moment. Such
ferrofluids exhibit field-induced phase separation at relatively low particle concentrations
(∼0.8%vol.) and magnetic fields (∼10 kA/m). Particle aggregates appearing during the phase
separation are extracted from the suspending fluid by magnetic field gradients much easier
than individual nanoparticles in the absence of phase separation. Nanoparticle capture by a
single magnetized microbead and by multi-collector systems (packed bed of spheres and
micro-pillar array) has been studied both experimentally and theoretically. Under flow and
magnetic fields, the particle capture efficiency Λ decreases with an increasing Mason number
for all considered geometries. This decrease may become stronger for aggregated magnetic
particles (Λ ∝ Ma^−1.7 ) than for individual ones (Λ ∝ Ma^−1 ) if the shear fields are strong
enough to provoke aggregate rupture. These results can be useful for development of new
magneto-microfluidic immunoassays based on magnetic nanoparticles offering a much better
sensitivity as compared to presently used magnetic microbeads.