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Poperechny I.S.,RAS Institute of Continuous Media Mechanics | Raikher Y.L.,RAS Institute of Continuous Media Mechanics | Stepanov V.I.,RAS Institute of Continuous Media Mechanics
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

On the basis of Brown's kinetic equation a consistent study of the regimes of cyclic magnetization reversal of single-domain particles with a uniaxial anisotropy is performed. The applied field is harmonic and linearly polarized, its amplitude equals the maximal coercive force of a Stoner-Wohlfarth particle. The dynamic magnetic hysteresis loops are obtained for a particle (oriented particle ensemble), whose easy-magnetization axis is tilted to the field direction under an arbitrary angle. It is shown that the Stoner-Wohlfarth regime (often termed as quasistatic) is able to describe the behavior of a nanoparticle only in a quite limited material and external parameter range. The developed approach has at least two major merits: it enables one to consider dynamic magnetic hysteresis in the temperature-frequency domains inaccessible with the aid of approximate methods, and provides a tool to test the accuracy of the latter. © 2010 The American Physical Society.

Shklyaev S.,RAS Institute of Continuous Media Mechanics
EPL | Year: 2015

Self-propulsion of a Janus droplet in a solution of surfactant, which reacts on a half of a drop surface, is studied theoretically. The droplet acts as a catalytic motor creating a concentration gradient, which generates its surface-tension-driven motion; the self-propulsion speed is rather high, and more. This catalytic motor has several advantages over other micromotors: simple manufacturing, easily attained neutral buoyancy. In contrast to a single-fluid droplet, which demonstrates a self-propulsion as a result of symmetry breaking instability, for the Janus one no stability threshold exists; hence, the droplet radius can be scaled down to micrometers. © CopyrightEPLA, 2015.

Ivanov A.S.,RAS Institute of Continuous Media Mechanics | Pshenichnikov A.F.,RAS Institute of Continuous Media Mechanics
Physics of Fluids | Year: 2014

The paper reports a new phenomenon-vortex flows in isothermal magnetic fluids in the vicinity of the localized source of magnetic field (magnetized iron sphere) induced by the drift of drop-like aggregates. Although the observed magnetic precipitation of drop-like aggregates resembles an ordinary rainfall in the Earth atmosphere, its origin and nature are quite different. In magnetic fluids this "rain" is induced by the non-uniform magnetic field and occurs at the scale of 1 mm, not at the scale of several kilometers as in the Earth atmosphere. The reason of this phenomenon is that the applied magnetic field initiates phase transition of "gas-liquid" type which is accompanied by formation of condensed phase represented by drop-like aggregates with the characteristic dimension of about tens of micrometers elongated along the field lines. Inhomogeneous spatial distribution of drop-like aggregates leads to deviation of the ponderomotive force, which is responsible for the formation of vortex flows in the fluid. The "rain" is the primary reason for the vortex flows and it lasts until all magnetic particles capable of condensing into drop-like aggregates precipitate at the surface of the condensation core (iron sphere). Thus, vortex flows induced by drop-like aggregate magnetophoresis represent one variant of "gas-liquid" phase transition. Hydrodynamic flows intensify mass transfer in vicinity of magnetic condensation core and considerably speed it up. © 2014 AIP Publishing LLC.

Stolbov O.V.,RAS Institute of Continuous Media Mechanics | Raikher Y.L.,RAS Institute of Continuous Media Mechanics | Balasoiu M.,Horia Hulubei National Institute of Physics and Nuclear Engineering
Soft Matter | Year: 2011

The origin of the so called magnetostriction effect, i.e., the deformation of a soft magnetic elastomer (SME) in response to a uniform magnetic field is discussed. We note that up to now there exists a number of cases, where the theory is unable to explain the facts well established for real SMEs. In our view, the essential issue, formerly not adequately accounted for, is the type of structural short-range order in the magnetic microparticle assembly. Two-dimensional model problems are formulated and solved, which demonstrate the validity of the hypothesis. © 2011 The Royal Society of Chemistry.

Morozov I.A.,RAS Institute of Continuous Media Mechanics
KGK Kautschuk Gummi Kunststoffe | Year: 2011

A new method for quantitative analysis of the structure of filled rubbers by atomic force microscopy (AFM) has been developed. A distinctive feature of this approach is the splitting of the detected surface microrelief, at a first stage, into separate structures, designated as clusters. The height level, where the splitting is done, is controlled by the total number of clusters per 1 μm2 (this value must be maximum), their compactness (its minimum value is attributed to branched structures) and their volume fraction (which is approximately equal to the given filler volume fraction). Further stages of investigations include a statistical characterization of the structure and distribution of the clusters. Furthermore, calculations of the Morishita index (a measure of cluster distribution), area, size, perimeter, degree of orientation and fractal characteristics of the examined structures are presented. The validity of the method has been verified by studying EPDM-rubbers filled with N330 carbon black, Silica Aerosil 200 and Aerosil R974 of filler fractions 20 and 40 phr, and by investigating a stretched natural rubber samples filled with N339 carbon black (50 phr).

Lebedev A.V.,RAS Institute of Continuous Media Mechanics
Colloid Journal | Year: 2010

It is established that the working temperature range of a magnetic fluid (MF) stabilized with a mixture of oleic and linoleic acids is widened to -100°C and, when the linoleic acid mass fraction in the mixed surfactant is 10%, the fluid becomes unpolymerizable. The stability of the MF with respect to a coagulant (isopropanol) is studied. The dependence of the magnetic susceptibility of the stable fraction of the fluid on the volume fraction of the alcohol almost coincides with the data obtained for an MF stabilized with oleic acid. The MF is separated into coarse and finely dispersed fractions. The temperature dependence of the magnetic susceptibility is investigated for the coarse fraction at low temperatures. The magnetic susceptibility is shown to drastically decrease below -80°C because of the mechanical blocking of particles. © 2010 Pleiades Publishing, Ltd.

Lebedev A.V.,RAS Institute of Continuous Media Mechanics
Colloid Journal | Year: 2014

The temperature dependence of the saturation magnetization of a magnetite-based magnetic fluid has been directly measured with a vibrating-coil magnetometer equipped with a superconducting solenoid. The magnetization varies in accordance with the 1 - αT 2 law. Coefficient α = 1.4 × 10-6 is almost twice as high as that of monolithic magnetite. The results of measuring the susceptibility of magnetic fluids stabilized with oleic and linoleic acids have been analyzed using novel corrections to the temperature dependence of particle magnetization. The susceptibility of ultimately concentrated samples is in good agreement with the Ivanov-Huke-Lücke and Morozov theories. The susceptibility of samples with a medium concentration is adequately described by the Ivanov theory alone. The susceptibility of low-concentrated samples increases to the level predicted by the Morozov theory in the case of particle aggregation. The widening of the particle size distribution leads to a reduction in the level of the interparticle interactions. © 2014 Pleiades Publishing, Ltd.

Morozov I.A.,RAS Institute of Continuous Media Mechanics
Polymer Composites | Year: 2013

Segmentation algorithms have been developed to study the surface of filled rubbers using atomic force microscopy (AFM) images. The analysis reveals two types of segments in the material: micropellets (large, dense inclusions of a insufficiently ground filler) and aggregates (small primary filler inclusions). The proposed approach allows us to find the minimum size of a representative area, starting from the filler distribution can be treated as homogeneous (mesoscale). Secondary structures (clusters) created by aggregates are identified and their fractal parameters are calculated. The validity of the approach for the analysis of rubbers filled with carbon black is demonstrated. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers Copyright © 2013 Society of Plastics Engineers.

Raikher Y.L.,Ural Federal University | Stepanov V.I.,RAS Institute of Continuous Media Mechanics
Journal of Magnetism and Magnetic Materials | Year: 2014

A uniaxially anisotropic superparamagnetic particle suspended in a viscous fluid and subjected to an ac field is considered. Consistently taking into account both internal (Néel) and external (Brownian) magnetic relaxations, a simple expression for the dynamic susceptibility is obtained. This result, with regard to the ac field energy absorption, is compared to the common heuristic approach. This is done for a model polydisperse colloid containing maghemite nanoparticles, which are assumed to posses either bulk or surface magnetic anisotropy. It is shown that viscous losses caused by the particle motion in a fluid matrix make important contribution to the full magnetic response of a ferrocolloid and, thus, its ability to absorb the ac field energy. The obtained exact expression, which takes in both dissipation mechanisms, paves the way to correct optimization of the nanoparticle-mediated heating effect. © 2014 Elsevier B.V.

Pshenichnikov A.F.,RAS Institute of Continuous Media Mechanics
Journal of Magnetism and Magnetic Materials | Year: 2012

A new algorithm for calculating magnetic fields in a concentrated magnetic fluid with inhomogeneous density is proposed. Inhomogeneity of the fluid is caused by magnetophoresis. In this case, the diffusion and magnetostatic parts of the problem are tightly linked together and are solved jointly. The dynamic diffusion equation is solved by the finite volume method and, to calculate the magnetic field inside the fluid, an iterative process is performed in parallel. The solution to the problem is sought in Cartesian coordinates, and the computational domain is decomposed into rectangular elements. This technique eliminates the need to solve the related boundary-value problem for magnetic fields, accelerates computations and eliminates the error caused by the finite sizes of the outer region. Formulas describing the contribution of the rectangular element to the field intensity in the case of a plane problem are given. Magnetic and concentration fields inside the magnetic fluid filling a rectangular cavity generated under the action of the uniform external filed are calculated. © 2011 Elsevier B.V. All rights reserved.

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