Cécilia Magnet

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

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, the size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in detail. In experiments, a 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 50 μm) 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.

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

In this work, we consider magnetic separation of iron oxide nanoparticles when a nanoparticle suspension (diluted ferrofluid) passes through a closed-loop filter composed of a packed bed of micro-beads magnetized by an externally applied magnetic field. We show that the capture of nanoparticles of a size as small as 60 nm is easily achieved at low-to-moderate magnetic fields (15 kA/m) thanks to relatively strong magnetic interactions between them. The key parameter governing the capture process is the Mason number-the ratio of hydrodynamic-to-magnetic forces exerted to nanoparticles. The filter efficiency, Λ, defined through the ratio of the inlet-to-outlet concentration shows a power-law dependency on Mason number, 0.83 Ma − Λ ∝ , in the range 2 4 10 10 Ma < <. The proposed theoretical model allows a correct prediction of the Mason number dependency of the filter efficiency. The obtained results could be of potential interest for water purification systems based on chemical adsorption of micro-pollutants on magnetic nanoparticles, followed by magnetic separation of the nanoparticles.

Rotational diffusion may govern the rheology of magnetic suspensions

This paper is focused on the theoretical modeling of the rheological properties of the magnetic suspensions in shear flows under an external magnetic field aligned with the streamlines. The conventional theory postulates that the field-induced aggregates of magnetic particles are highly anisotropic and aligned with the flow direction. Therefore, no substantial variation in suspension viscosity would be expected in the presence of field. However, experiments reveal a strong Bingham rheological behavior of the suspensions with a dynamic yield stress of the same order of magnitude that the one measured in the magnetic fields perpendicular to the flow. We explain the high level of shear stress, generated in longitudinal magnetic fields, by stochastic rotary oscillations of the aggregates caused by many-body magnetic interactions with neighboring aggregates. The interaggregate interactions are accounted for by an effective rotational diffusion process with a diffusion constant proportional to the mean square i...

Magnetorheological effect in the magnetic field oriented along the vorticity

In this work, we have studied the magnetorheological (MR) fluid rheology in the magnetic field parallel to the fluid vorticity. Experimentally, the MR fluid flow was realized in the Couette coaxial cylinder geometry with the magnetic field parallel to the symmetry axis. The rheological measurements were compared to those obtained in the cone-plate geometry with the magnetic field perpendicular to the lower rheometer plate. Experiments revealed a quasi-Bingham behavior in both geometries with the stress level being just a few dozens of percent smaller in the Couette cylindrical geometry at the same internal magnetic field. The unexpectedly high MR response in the magnetic field parallel to the fluid vorticity is explained by stochastic fluctuations of positions and orientations of the particle aggregates. These fluctuations are induced by magnetic interactions between them. Once misaligned from the vorticity direction, the aggregates generate a high stress independent of the shear rate, and thus assimilated to the suspension apparent (dynamic) yield stress. Quantitatively, the fluctuations of the aggregate orientation are modeled as a rotary diffusion process with a diffusion constant proportional to the mean square interaction torque. The model gives a satisfactory agreement with the experimental field dependency of the apparent yield stress and confirms the nearly quadratic concentration dependency Sigma_Y proportional to Phi^2.2, revealed in experiments. The practical interest of this study lies in the development of MR smart devices with the magnetic field non-perpendicular to the channel walls.

CHAPTER 1:Importance of Interparticle Friction and Rotational Diffusion to Explain Recent Experimental Results in the Rheology of Magnetic Suspensions

We present a review of recent experimental and theoretical results on the magnetorheology of fiber suspensions in magnetic fields perpendicular to the shear as well as of suspensions of spherical magnetic particles in longitudinal magnetic fields. Both these problems reveal essentially similar physics. Upon magnetic field application, both spherical particles and fibers form strongly elongated aggregates exhibiting a similar behavior in shear flows. The differences lie in a stronger magnetic permeability of the aggregates of fibers and a presumably stronger solid friction between fibers. This leads to a few times enhancement of the yield stress and shear moduli of magnetic fiber suspensions as compared to suspensions of spherical particles. A number of theoretical models have been proposed to predict viscoelastic properties of magnetic fiber suspensions, employing either friction or permeability enhancement scenarios. Applied to appropriate ranges of Mason numbers, these theories agree with experiments at least semi-quantitatively. Concerning the flows of magnetorheological (MR) suspensions in longitudinal fields, one observes an unexpectedly strong MR effect in this geometry: the suspension yield stress appears to be of the same order of magnitude that the one in the perpendicular field. Such a “longitudinal” MR effect has been explained by many-body magnetic interactions between aggregates, which induce misalignments of particle aggregates from the streamlines and result in stochastic oscillations of their orientation. Both experiments and theory suggest a strong concentration dependence of the yield stress, σY∞Φ3, in longitudinal fields.