Victor Bashtovoi

Magnetorheology of magnetic holes compared to magnetic particles

We compare the rheological behavior in a shear flow of two types of suspension of magnetic particles in the presence of a magnetic field. The first suspension is made of silica particles in a ferrofluid and the second one is made of carbonyl iron particles in silicone oil. The permeability curves of these two suspensions have been measured for different volume fractions as a function of the magnetic field in order to characterize the magnetic interactions. We show for both cases the existence of two different yield stresses: one associated with the solid friction of the particles on the plates of the rheometer and the second one with the rupture of the aggregates. This second yield stress presents a maximum with the volume fraction for the suspension of magnetic holes but grows faster than linearly with the volume fraction for the suspension of carbonyl iron. These features are explained by theoretical models based, respectively, on the deformation of aggregates of macroscopic size and on ruptures between...

Flow of magnetorheological fluid through porous media

Flow of a magneto-rheological (MR) fluid through different types of porous media (bundle of cylinders, packed beds of magnetic and non-magnetic spheres and cylinders) is considered, both theoretically and experimentally. The theory is based on averaging the magnetic and rheological properties of MR fluid in tortuous channels making different angles between local field and local velocity. A comparison of the pressure drop through porous beds and spiral channels is analyzed and practical recommendations are developed. It is shown that the mean yield stress of Bingham MR fluid (as well as the pressure drop, ΔP) depends on the mutual orientation of the external magnetic field and the main axis of the flow. This theory is tested against our experimental results and is shown to well predict the pressure drop obtained in different porous media.

Capillary ascension of magnetic fluids

Abstract The equilibrium of a magnetic fluid column inside a cylindrical capillary is investigated in the presence of the external uniform magnetic field. It is found that due to the fluid meniscus deformation, the surface pressure drop in the fluid decreases in the fields longitudinal and transverse to the capillary axis.

Boiling heat transfer in magnetic fluids

Abstract The results of an experimental study of heat transfer in magnetic fluids under magnetic fields normal and tangential to the cylindrical steel specimen axis are discussed. It is shown that the orientation of the magnetic field has a strong effect on the integral characteristics of boiling heat transfer under different boiling regimes.

Effect of the orientation of the magnetic field on the flow of magnetorheological fluid. II. Cylindrical channel

We consider the flow of a magnetorheological (MR) fluid in a cylindrical channel in the presence of a magnetic field inclined relatively to the channel axis (Oz). The stress tensor in the MR fluid is derived by assuming that chain structures are located in the planes parallel to the velocity and the magnetic field (plane Oxz). We show that the velocity field has two orthogonal axes of symmetry, Ox and Oy, in the plane perpendicular to Oz and that the plug zone can be approximated by an ellipsoid of major axis Ox. The plug zone is not defined in the usual way since inside the plug each line perpendicular to the Oy axis is moving at a different velocity. The experiments in a cylindrical capillary at high Mason numbers are compared for different angles between the flow and the field to theoretical predictions. We still recover Bingham behavior and all the pressure versus flow rate curves obtained at different field angles gather on a single straight line when represented against an average normalized yield s...

Effect of the orientation of the magnetic field on the flow of a magnetorheological fluid. I. Plane channel

Shear flow and Poiseuille flow of magnetorheological fluids in a plane channel under an inclined magnetic field are studied. The proposed theoretical model is based on stability analysis of chain-like clusters of dipolar magnetic particles. Hydrodynamic and magnetic torque acting on aggregates balances each other such that some misalignment between orientations of chains and the field takes place. Two magnetic field directions symmetric relative to the velocity gradient influence the aggregate behavior in different ways. In one case, the aggregates tend to turn along the flow whereas in the other case, they tend to turn transverse to the flow. The predicted peak value of the yield stress arises for θ0=−π/4 (where θ0 is the clockwise angle between the velocity gradient and the direction of the field) and is 1.8 times the value for θ0=0. If we consider Poiseuille flow between two parallel plates, the velocity gradient, being positive on one side and negative on the other, the stress is no longer symmetric r...

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.

Modelling of magnetic fluid support

One kind of elastic magnetic fluid support representing the magnetic fluid drop with permanent magnet inside is investigated experimentally and numerically. The dependencies between the magneto static force in support and the geometrical parameters and properties of the magnet and the magnetic fluid are established.

Dynamics of deformation of magnetic fluid flat drops in a homogeneous longitudinal magnetic field

Abstract The present work deals with the results of studying of the dynamics of shape of magnetic fluid drops and bubbles in magnetic fluid compressed between horizontal flat surfaces when a uniform magnetic field is oriented along these surfaces and can be turned on instantaneously.

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...