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

Shear elasticity of isotropic magnetic gels

The paper deals with a theoretical study of the effective shear modulus of a magnetic gel, consisting of magnetizable particles randomly and isotropically distributed in an elastic matrix. The effect of an external magnetic field on the composite modulus is the focus of our consideration. We take into account that magnetic interaction between the particles can induce their spatial rearrangement and lead to internal anisotropy of the system. Our results show that, if this magnetically induced anisotropy is insignificant, the applied field reduces the total shear modulus of the composite. Strong anisotropy can qualitatively change the magnetomechanic effect and induce an increase of this modulus with the field.A.Y.Z. is grateful for financial support from the Russian Fund for Basic Research, through Grant No. 16-58-12003, the Program of Russian Federation Ministry of Science and Education, Project No. 3.1438.2017/4.6. M.T.L.-L. was supported by Project No. FIS2013-41821-R of Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica, Ministerio de Economía y Competitividad, Spain, co-funded by ERDF, European Union. D.Y.B. would like to acknowledge support of from Deutsche Forschungsgemeinschaft under Grant No. Bo 3343/1-1

Rheology of magnetic alginate hydrogels

Según Sherpa/Romeo el periodo de embargo es de 12 mesesMagnetic hydrogels are becoming increasingly in demand for technical and biomedical applications, especially for tissue engineering purposes. Among them, alginate-based magnetic hydrogels emerge as one of the preferred formulations, due to the abundance, low cost, and biocompatibility of alginate polymers. However, their relatively slow gelation kinetics provokes strong particle settling, resulting in nonhomogeneous magnetic hydrogels. Here, we study magnetic hydrogels prepared by a novel two-step protocol that allows obtaining macroscopically homogeneous systems, consisting of magnetic microparticles embedded within the alginate network. We describe a comprehensive characterization (morphology, microstructure, and mechanical properties under shear stresses) of the resulting magnetic hydrogels. We pay special attention to the effects of particle volume fraction (up to 0.33) and strength of the magnetic field on the viscoelastic properties of the magnetic hydrogels. Our results indicate that magnetic hydrogels are strongly strengthened against shear stresses as magnetic particle concentration and applied field intensity increase. Finally, we report an adaptation of the two-step protocol for the injection of the magnetic hydrogels that might be adequate for implementation in vivo. Interestingly, injected magnetic hydrogels present similar morphology and mechanical properties to noninjected hydrogels. To conclude, we report magnetic alginate hydrogels with adequate homogeneity and injectability character. These characteristics, together with the broad range of their mechanical properties, make them perfect candidates for cutting-edge technology.FIS2013-41821-R (Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica, MINECO, Spain, cofunded by ERDF, European Union) and FIS2017-85954-R (Ministerio de Economía, Industria y competitividad, MINECO, and Agencia Estatal de Investigación, AEI, Spain, cofunded by Fondo Europeo de Desarrollo Regional, FEDER, European Union). Ministry of Education and Science of the Russian Federation, projects 02.A03.21.0006, 3.1438.2017/4.6, and 3.5214.2017/6.7, as well as to the Russian Fund of Basic Researches, project 18-08-00178. French government, piloted by the National Research Agency (ANR) in the framework of the project Future Investments UCA JEDI, Ref. No. ANR-15-IDEX-01 (RheoGels).

To the theory of mechano-magnetic effects in ferrogels

The paper deals with theoretical study of effect of ferrogels uniaxial elongation on magnetic susceptibilities of these composite materials. We have considered the systems with magnetically soft ellipsoidal and spherical particles. The results show that elongation of the composites with the ellipsoidal particles enhances the susceptibility in the direction of the elongation, whereas the deformation of ferrogels with the spherical particles decreases the susceptibility when the particles concentration is small enough and increases it when the concentration exceeds some threshold magnitude.This work has been supported by Russian Foundation for Basic Research (projects 18-08-00178, 19-52-12028); by the Program of the Ministry of Education and Science of the Russian Federation, projects 02.A03.21.0006; 3.1438.2017/4.6; 3.5214.2017/6.7. MTLL is grateful to proyect FIS2017-85954-R (Agencia Estatal de Investigación, AEI, Spain, co-funded by Fondo Europeo de Desarrollo Regional, ERDF, European Union)

Magnetorheological behavior of magnetite covered clay particles in aqueous suspensions

Montmorillonite clay particles coated with magnetite nanoparticles suspended in aqueous media behave as magnetorheological fluids with enhanced stability as compared to conventional ones. In this work, the study of the magnetorheological behavior of these suspensions of magnetite-clay composite particles has been carried out. For this purpose, both steady and dynamic rheological measurements were carried out in the absence and in the presence of external magnetic fields. In the first kind of experiments, the rheograms of the suspensions (shear stress versus shear rate plot) are analyzed as a function of the strength of the magnetic field applied. In the second one, oscillatory stresses are applied to the system, and the storage modulus is studied as a function of the external magnetic field. In the absence of magnetic field, the suspensions develop a weak yield stress due to the aggregation of the magnetite covered clay particles. In the presence of magnetic field, the yield stress is strongly dependent on the magnetic field strength inside the samples, demonstrating that the suspensions experience a magnetorheological effect, moderate when the magnetic field strength is weak and stronger for values of magnetic field higher than 150–200 kA/m. Actually, the most intriguing result is the change of the trend in the dependence of the yield stress with the field. This dependence is approximately linear with the field for strength values smaller than 150–200 kA/m. On the other hand, for higher values, the yield stress increases with magnetic field following a power law with exponent 4.5.The results are interpreted by means of a model that relates the structure of the particles in the suspensions to the magnetic field applied and using the interaction energy between particles calculated by the extended DLVO theory to include magnetic interaction.Financial support by Ministerio de Ciencia e Innovación (Spain) under project No FIS2009-07321, and Junta de Andalucía (Spain) under Project Nos. P08-FQM-3993 and P09-FQM-4787 is gratefully acknowledge

Mechanics of Magnetopolymer Composites: A Review

Magnetopolymer materials (ferrogels, magnetic hydrogels, magnetic elastomers) are new kind of smart materials whose physical and mechanical properties can be controlled by an external magnetic field. We present an overview of experimental, theoretical and computer studies of magentomechanical properties and behavior of these systems.A. Yu. Zubarev and L. Yu. Iskakova thank for support the Russian Scientific Foundation, project 14-19-00989. Financial support by project FIS2013-41821-R, MINECO, Spain, co-funded by ERDF, European Union, is also acknowledged

Shear modulus of isotropic ferrogels

We present results of theoretical study of magnetorheological effect in ferrogels with magnetizable spherical particles chaotically distributed in a current gel. To avoid intuitive constructions with uncontrolled accuracy and adequacy, the analysis is done in the frames of the mathematically regular pair approximation. Our results demonstrate non monotonic increase of the composite shear modulus with the applied magnetic field. This effect is stronger for the systems with the soft gel, than for the relatively rigid ones.A.Z, L.I. and A.M are grateful to the program of the Ministry of Education and Science of the Russian Federation, projects 02.A03.21.0006; 3.1438.2017/4.6; 3.5214.2017/6.7 as well as to the Russian Fund of Basic Researches, projects 18-08-00178 and 19-52- 12028. M.T.L-L acknowledges financial support by project FIS2017- 85954-R (Ministerio de Economía, Industria y Competitividad, MINECO, and Agencia Estatal de Investigación, AEI, Spain, cofunded by Fondo Europeo de Desarrollo Regional, FEDER, European Union)

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

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

Role of particle clusters on the rheology of magneto-polymer fluids and gels

Even in absence of cross-linking, at large enough concentration, long polymer strands have a strong influence on the rheology of aqueous systems. In this work, we show that solutions of medium molecular weight (120,000 – 190,000 g/mol) alginate polymer retained a liquid-like behaviour even for concentrations as large as 20 % w/v. On the contrary, solutions of alginate polymer of larger (and also polydisperse) molecular weight (up to 600,000 g/mol) presented a gel-like behaviour already at concentrations of 7 % w/v. We dispersed micron-sized iron particles at a concentration of 5 % v/v in these solutions, which resulted either in stable magnetic fluids or gels, depending on the type of alginate polymer employed (medium or large molecular weight, respectively). These magneto-polymer composites presented a shear-thinning behaviour that allowed injection through a syringe and recovery of the original properties afterwards. More interestingly, application of a magnetic field resulted in the formation of particle clusters elongated along the field direction. The presence of these clusters intensely affected the rheology of the systems, allowing a reversible control of their stiffness. We finally developed theoretical modelling for the prediction of the magnetic-sensitive rheological properties of these magneto-polymer colloids.Ministerio de Economía, Industria y Competitividad, MINECO, and Agencia Estatal de Investigación, AEI, Spain, cofounded by Fondo Europeo de Desarrollo Regional, FEDER, European Union, project FIS2017-85954-R. Ministry of Education and Science of the Russian Federation, projects 02.A03.21.0006; 3.1438.2017/4.6; 3.5214.2017/6.7 as well as to the Russian Fund of Basic Researches, project 19-52-1202