Abrupt contraction flow of magnetorheological fluids

International audienceContraction and expansion flows of magnetorheological fluids occur in a variety of smart devices. It is important therefore to learn how these flows can be controlled by means of applied magnetic fields. This paper presents a first investigation into the axisymmetric flow of a magnetorheological fluid through an orifice so-called abrupt contraction flow. The effect of an external magnetic field, longitudinal or transverse to the flow, is examined. In experiments, the pressure-flow rate curves were measured, and the excess pressure drop associated with entrance and exit losses was derived from experimental data through the Bagley correction procedure. The effect of the longitudinal magnetic field is manifested through a significant increase in the slope of the pressure-flow rate curves, while no discernible yield stress occurs. This behavior, observed at shear Mason numbers 10Mnshear100, is interpreted in terms of an enhanced extensional response of magnetorheological fluids accompanied by shrinkage of the entrance flow into a conical funnel. At the same range of Mason numbers, the transverse magnetic field appears not to influence the pressure drop. This can be explained by a total destruction of magnetic particle aggregates by large hydrodynamic forces acting on them when they are perpendicular to the flow. To support these findings, we have developed a theoretical model connecting the microstructure of the magnetorheological fluid to its extensional rheological properties and predicting the pressure-flow rate relations through the solution of the flow equations. In the case of the longitudinal magnetic field, our model describes the experimental results reasonably well

Effect of drop-like aggregates on the viscous stress in magnetic suspensions

We present results of theoretical and experimental study of effect of dense drop-like aggregates on the magnetoviscous effects in suspensions of non-Brownian magnetizable particles. Unlike the previous works on this subject, we do not restrict ourselves by the limiting case of highly elongated drops. This allows us to reproduce the experimental rheological curve in wide region of the shear rate of the suspension flow.This work has been supported by the Russian Fund of Fundamental Investigations, Grants 12-01-00132, 13-02-91052, 13-01-96047 and 14-08-00283; by the Act 211 Government of the Russian Federation No. 02.A03.21.0006; by the Junta de Andalucía (Spain), Project P09-FQM-4787; and by the University of Granada (Acción Integrada con Russia; Plan Propio 2011); and CNRS PICS No. 6102 is also acknowledged

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, 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 50$\mu$m) 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 $\alpha \geq 2$), the Brownian motion seems not to affect the cloud behavior

How nonmagnetic particles intensify rotational diffusion in magnetorheological fluids

In this work we propose a mechanism to explain the enhancement of the magnetic field-induced yield stress when non-magnetic particles are added to magnetic particulate suspensions –i.e., bi-component suspensions. Our main hypothesis is that the non-magnetic particles collide with the field-induced magnetic aggregates under shear flow. Consequently, supplementary fluctuations of the orientations of the magnetic aggregates occur, resulting in an effective rotary diffusion process, which increases the dynamic yield stress of the suspension. Furthermore, the collision rate and the rotary diffusivity of the aggregates should increase with the concentration of non-magnetic particles. Rheological measurements in plate-plate and cylindrical Couette geometries confirm the increase of the yield stress with the volume fraction of non-magnetic particles. In addition, such an effect appears to be more important in Couette geometry, for which orientation fluctuations of the magnetic aggregates play a more significant role. Finally, a theoretical model based on this rotary diffusion mechanism is developed, providing with a quantitative explanation to the experimentally-observed trends.This work was supported by Project No. FIS2013-41821-R (Ministerio de Economía y Competitividad, Spain) and the project “Factories of the Future” (Grant No. 260073, DynExpert FP7). In addition, L.R.-A. acknowledges financial support from Secretaría de Estado de Educacion, Formación´ Profesional y Universidades (MECD, Spain) through its FPU and Estancias Breves programs

Normal stresses in a shear flow of magnetorheological suspensions: viscoelastic versus Maxwell stresses

International audienceThis work reports an experimental and theoretical study on the normal force developed by suspensions of magnetic microparticles subjected to magnetic fields. Experimental values of the normal force were obtained using a rotational rheometer, for a broad range of particle concentration in the suspensions. Applied magnetic fields up to 343 kA/m were generated in the plate-plate measuring geometry. It was found that the normal force exhibited a high-value plateau at low shear, followed by a decrease as the suspensions started to flow and a final low-value plateau at high shear. These three regions in the normal force vs. shear rate curve were well correlated with the microscopic regimes in the suspensions: field-aligned structures filling the gap; inclined structures still filling the gap; and structures non-filling the gap. The theoretical model developed is based on the equilibrium between hydrodynamic and magnetostatic torques and forces in a field-induced aggregate of particles subjected to shear. The stress tensor was obtained and the normal force calculated as the integral of the stress over the total surface of the rotational plate. A good correspondence among theoretical and experimental values was obtained

On the theory of magnetoviscous effect in magnetorheological suspensions

Copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.The following article appeared in Zubarev, A.; et al. On the theory of magnetoviscous effect in magnetorheological suspensions. Journal of Rheology, 58: 1673 (2014) and may be found at http://dx.doi.org/10.1122/1.4889902A theoretical model of magnetoviscous effect in a suspension of nonBrownian linearly magnetizable particles is suggested. A simple shear flow in the presence of an external magnetic field aligned with the velocity gradient is considered. Under the action of the applied field, the particles are supposed to form dense highly elongated droplike aggregates. Two different scenarios of the aggregates’ destruction under shearing forces are considered, namely, a “bulk” destruction of aggregates into pieces and an “erosive” destruction connected to the rupture of individual particles from the aggregate surface. Both models are based on a balance of forces acting either on the whole aggregate or on individual particles. The two approaches lead to qualitatively different Mason number (Ma) behaviors of the magnetic suspensions: The suspension viscosity scales as either Ma^-2/3 for the bulk destruction of aggregates or Ma^-4/5 for the erosive destruction. In any case, we do not recover Bingham behavior (Ma^-1) often predicted by chain models of the magneto- or electrorheology. Our theoretical results are discussed in view of comparison with existing theories and experimental results in the wide range of Mason numbers.This work has been done under support of Russian Fund of Fundamental Investigations, Grant Nos. 12-01-00132, 13-02-91052, 13-01-96047, and 14-08-00283; by the Act 211 Government of the Russian Federation No. 02.A03.21.0006. The University of Granada (Acción Integrada con Rusia; Plan Propio 2011), as well as project CNRS PICS No. 6102 are also acknowledged for their financial support

Normal stress differences in Non-Brownian fibre suspensions

In this paper, we present an experimental study of the normal stress differences that arise in non-Brownian rigid fibre suspensions subject to a shear flow. The first and the second normal stress differences are measured using a single experiment which consists of determining the radial profile of the second normal stress, along the velocity gradient direction, Σ 22,, in a torsional flow between two parallel discs. Suspensions are made of monodispersed fibres immersed a neutrally buoyant Newtonian fluid. Two lengths and three aspect ratios of the fibers and a wide range of concentrations have been tested. N1 is found to be positive while N2 is negative and the magnitude of both normal stress differences increases when nL2d increases (n is the number of fibres per unite volume, L, the fibre length and d, their diameter). The magnitude of N2 is found to smaller than N1, but not negligible, which is in contradiction with the theoretical prediction of [Shaqfeh and Fredrickson, 1990] in which they predicted that N2=-1/7 N1 but consistent with the recent numerical work of [Snook et al., 2014]

Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

International audienceThis paper is focused on the rheology of magnetic fiber suspensions in the presence of a magnetic field applied perpendicular to the flow. At low Mason numbers, Mn<0.1, the experimental flow curves show a steep initial section corresponding to the inclination and stretching of the gap-spanning aggregates formed upon magnetic field application. At higher Mason numbers, aggregates no longer stick to the walls and the flow curves reach a Bingham regime, with the dynamic yield stress growing with the magnetic field intensity. This yield stress appears to be about three times higher for the fiber suspensions than for the suspensions of spherical particles. Such difference, measured at relatively low magnetic field intensities, H0<30 kA/m, is explained in terms of the enhanced magnetic susceptibility of the aggregates composed of fibers compared to the aggregates composed of spherical particles. For weak magnetic fields, the forces of solid friction between fibers are expected to play a minor role on the stress level of the suspension. In order to confirm these findings, we propose a new theoretical model, taking into account hydrodynamic interactions. The flow curve and the yield stress predictions are in a good agreement with the experimental results for semi-diluted suspensions

Magnetic and magnetorheological properties of nanofiber suspensions

International audienceIn this work the preparation and characterization of magnetorheological (MR) fluids constituted by CoNi nanofibers (56 nm length, 6.6 nm width) are reported. The properties of these new fluids were characterized by usual techniques (including magnetometry and magnetorheology). The results were compared with those obtained for conventional suspensions constituted by CoNi nanosperes

Yield stress in magnetorheological suspensions near the limit of maximum-packing fraction

International audienceThis work deals with the magnetic field-induced static yield stress of magnetorheological (MR) suspensions with concentration near the limit of maximum-packing fraction. With this aim, homogeneous suspensions of iron microparticles with 50 vol.% concentration were prepared, and their yield stress measured as a function of the applied magnetic field. In view of the failure of existing models to predict, on the basis of realistic hypotheses, the values of the yield stress of highly concentrated MR suspensions, we developed a new model. Our model considers that field application induces body-centered tetragonal (BCT) structures. Upon shearing, these structures deform in such a way that interparticle gaps appear between neighboring particles of the same chain, whereas the approach of particles of parallel chains ensures the mechanical stability of the whole multi-chain structure. Based on this hypothesis, and using finite element method simulations of interparticle magnetic interactions, our model is able to quantitatively predict the yield stress of highly concentrated MR suspensions. Furthermore, estimations show that the main contribution to the field-dependent part of the yield stress comes from the change in the permeability of the structures as interparticle gaps are enlarged by the shear