Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields

Abstract

When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in details. In experiments, dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50µm) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces -the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter α ≥2), the Brownian motion seems not to affect the cloud behavior.

Two-stage kinetics of field-induced aggregation of medium-sized magnetic nanoparticles

“…Qualitatively, the aggregate width progressively decreases when moving from the aggregate center to the tips, as in the case of ellipsoidal shape supposed in the model. Conical spikes have not been detected on aggregate tips, as opposed to the experiments of Promislow and Gast 30 on drop-like aggregates of magnetorheological fluids and to our previous experiments on phase condensation around a magnetized micro-bead 55,57 . In fact, the characteristic period…”

“…The technique described in Sec. III does not allow determination with confidence of the second coexistence curve ''( ) H  magnetically driven phase transition, as inferred from our previous study 57 . In our case, this curve was fitted by the following empirical formula, valid in the concentration range .…”

“…So, these particles are usually porous nanoclusters composed of smaller 10-nm superparamagnetic nanoparticles [52][53][54][55][56] . Similarly to small nanoparticles, medium-sized colloids exhibit condensation phase transitions 54,55,57 which enhance drastically their magnetic separation on magnetized collectors [57][58][59][60][61] . Kinetics of phase separation of medium-sized nanoparticles has been studied only scarcely.…”

Closed-loop magnetic separation of nanoparticles on a packed bed of spheres

“…4 show that the filtration process accelerates as the Mason number Ma / u=H 2 0 decreases, in qualitative accordance with the theory [Eq. (6)]. As the filtration speed decreases, the nanoclusters have enough time to enter into contact with the collectors and be captured.…”

“…Upon an application of an external magnetic field, the nanoparticles reversibly gather into drop-like aggregates, which are extracted from the suspending liquid much easily than individual nanoparticles. 5 In our previous works, 5,6 we have studied in details the capture of iron oxide nanoclusters by a single collector-an iron or nickel micro-bead magnetized by an external applied magnetic field. The nanoclusters were permanent quasi-spherical agglomerates of individual nanoparticles stabilized in water by an oleic acid double layer.…”