Dispersion Of Magnetic Particles Research Articles

Third harmonic of the dynamic magnetic susceptibility of a concentrated ferrofluid: Numerical calculation and simple approximation formula.

Information about the nonlinear magnetic response of dispersions of magnetic particles is the basis for biomedical applications. In this paper, using analytical and numerical methods, the third harmonic of the dynamic susceptibility of an ensemble of moving magnetic particles in an ac magnetic field with an arbitrary amplitude is studied, taking into account interparticle interactions. A simple approximation formula is proposed to predict the third harmonic as a function of two parameters: the Langevin susceptibility χ_{L}, which is used to estimate the particle dipole-dipole interactions, and the Langevin parameter ξ, which represents the ratio of the energy of the magnetic moment interacting with the magnetic field to the thermal energy. The derived approximation formula corresponds with the known single-particle theories in the limit case of a small particle's concentration and is valid for concentrated dispersions of magnetic particles (with the Langevin susceptibility up to χ_{L}≤3) in high-amplitude ac fields (with the Langevin parameter up to ξ≤10).

Environmentally safe technology of waste processing of galvanic production

It is known that waste is difficult to purify using existing reagent methods to the standards of the MPC. In addition, the formed sediments contain poorly soluble hydroxy compounds of heavy metals and gypsum, which makes it impossible to dispose of iron- and copper-containing electroplating sludges. The purpose of the study was to study the possibility of ion exchange separate and simultaneous extraction of iron and copper ions from acidic solutions of pickling and copper plating in galvanic production to obtain ferrites. In the course of research, the highly acidic cation exchange resin Dowex HCR S/S in the H+ form was chosen as the cationite. The choice of a strongly acidic cationite is justified, based on the fact that cationites of this type are able to easily lose hydrogen ions due to their displacement from the cationite matrix by other cations of the solution. The sorption processes of iron (II) and copper (II) ions were carried out under dynamic conditions. In the course of experimental studies, it was shown that with an increase in the concentration of sulfuric acid, the full exchangeable dynamic capacity of the cationite significantly decreases both during the sorption of iron ions and during the sorption of copper ions. For iron-containing solutions, full dynamic exchange capacity decreases from 1.39 g-eq/dm3 in the absence of H2SO4 in the initial solution and decreases to 0.3-0.46 g-eq/dm3 at a concentration of sulfuric acid at the level of 1-3 g/dm3. An increase in the concentration of iron (II) ions leads to a significant increase in full dynamic exchange capacity, even with a concentration of sulfuric acid of 8 - 13 g/dm3, full dynamic exchange capacity was at the level of 1.35 g-eq/dm3, which corresponds to the level of sorption of diluted solutions even in the absence of acid. However, the total concentration shows that the efficiency of sorption of metals from acidic solutions remains quite high and increases with the increase of the initial total content of sorbed ions. The main indicator used when choosing an ion exchange method is the possibility of effective regeneration of the cation exchange material. When using 5% solution of sulfuric acid already at the specific consumption of the regeneration solution, it was possible to achieve a degree of regeneration at the level of 95-98%, and when using 10% solution, the degree of regeneration reached 100%. According to the developed technological scheme, regeneration solutions are processed by precipitation into magnetite in a ferritizer reactor. To remove iron ions from concentrated iron-containing regeneration solutions by the ferrite method in order to obtain ferrite particles (magnetite) with maximum magnetic properties, a mixture of Fe (II) and Fe (III) sulfates was used with their concentration ratio K = [Fe2+]/[Fe3+] = 0.5, as a stable relationship between magnetic properties and sediment volume was noted. At maximum magnetic properties, the sediment volume is minimal, and vice versa. Obtaining highly dispersed magnetic particles from a mixture of ferric and ferric sulfates by precipitating them with 10% NaOH solution during heating allows to significantly reduce the duration of the process, simplify it and increase the productivity of the equipment. To obtain magnetic particles of copper ferrites from regeneration solutions by precipitation with alkali at normal temperatures, the concentration ratio K1 = [Fe2+]/[Cu2+] = 3.15-7.76 should be maintained. With an increase in the concentration of Cu2+ ions, there is an increase in the period of crystallization of the obtained sediments and their acquisition of magnetic properties. The formed ferrite material can be used for environmental purposes as a valuable raw material for carbon monoxide neutralization catalysts, as part of alkaline cement for concrete products, or sent for safe disposal. The obtained results on the application of ion-exchange extraction of iron and copper ions from acidic solutions of galvanic production are expedient as the first stage of complex ecologically safe processing of galvanic wastes with the production of ferritic materials for industrial use.

A Brief Review of Surface Modification of Carbonyl Iron Powders (CIPs) for Magnetorheological Fluid Applications

Magnetorheological fluids (MRFs) is a smart fluid system that exhibits swift and reversible alterations in their rheological characteristics when exposed to an external magnetic field. MRFs are used for applications in various areas, including automotive systems, robotics, aerospace, and civil engineering. The performance of MRFs depends on the behavior of the dispersed magnetic particles, necessitating thoughtful consideration of particle traits to optimize fluid performance. Carbonyl Iron Powders (CIPs), high purity iron (>98%) reduced from penta carbonyl iron, are widely employed in MRFs due to their exceptional magnetic characteristics. Nevertheless, the innate surfaces of CIPs tend to conglomerate, leading to compromises in fluid stability and rheological performance. To overcome the challenges, an intensive research has been devoted to advancing surface modification techniques that augment the dispersion, stability, and overall functionality of MRFs based on CIPs. This review describes the comprehensive approach to surface modification of CIPs for highly stable MRFs. We discuss the various surface modification methodologies that have been explored to optimize the behavior of carbonyl iron-based MRFs. Coating techniques, surfactant functionalization, magnetic coatings, and emerging approaches such as nanocoatings and electrochemical modification are also summarized. Moreover, insights into potential applications and future prospects of these modified MRFs are provided.

The effect of incorporating magnetic fly ash in asphalt binders with respect to permanent deformation and fatigue

The search for efficient and environmentally viable materials to improve the quality and durability of asphalt binders used in roadways has been growing in recent years. This is especially true giving the increase in the volume and magnitude of traffic loads, as well as the changes in the earth's temperature that cause frequent pavement distresses, such as permanent deformation and fatigue. In this context, the objective of this work is to evaluate the potential of magnetic particles from fly ash, as a residual resource with high added value and low-cost raw material for modifying asphalt binders. Magnetic particles were incorporated into the asphalt binder at 2%, 4%, 6% and 8%, content by weight. The materials were characterized by chemical, empirical and rheological tests, specifically the Multiple Stress Creep and Recovery and the Linear Amplitude Sweep test. X-ray Fluorescence data revealed silicon, aluminum and iron as the main elements present in the fly ash, as well as the increase in iron concentration after magnetic separation in the magnetic particle sample. A homogeneous dispersion of magnetic particles in the asphalt binder observed by Scanning Electron Microscopy and shifts in infrared bands characteristic of iron (ca. 450 cm-1) and of asphalt binder (ca. 1456 cm-1) clearly indicated a significative interaction between the magnetic particles and the asphalt binder. The rheological results indicated that the incorporation of particles provide better performance to the binder in terms of both permanent deformation and fatigue. The modified binders were less susceptible to permanent deformation than the neat binder, and this is reflected in the increase in adequate traffic levels, moving from standard traffic to heavy traffic at a temperature of 64 °C. This economical, easy, and sustainable approach to using magnetic particles in hot asphalt binders can contribute to producing asphalts that are more efficient with changes in temperature and traffic.

3D Printed Magneto-Active Microfiber Scaffolds for Remote Stimulation and Guided Organization of 3D In Vitro Skeletal Muscle Models.

This work reports the rational design and fabrication of magneto-active microfiber meshes with controlled hexagonal microstructures via melt electrowriting (MEW) of a magnetized polycaprolactone-based composite. In situ iron oxide nanoparticle deposition on oxidized graphene yields homogeneously dispersed magnetic particles with sizes above 0.5µm and low aspect ratio, preventing cellular internalization and toxicity. With these fillers, homogeneous magnetic composites with high magnetic content (up to 20weight %) are obtained and processed in a solvent-free manner for the first time. MEW of magnetic composites enabled the creation of skeletal muscle-inspired design of hexagonal scaffolds with tunable fiber diameter, reconfigurable modularity, and zonal distribution of magneto-active and nonactive material, with elastic tensile deformability. External magnetic fields below 300mT are sufficient to trigger out-of-plane reversible deformation. In vitro culture of C2C12 myoblasts on three-dimensional (3D) Matrigel/collagen/MEW scaffolds showed that microfibers guided the formation of 3D myotube architectures, and the presence of magnetic particles does not significantly affect viability or differentiation rates after 8 days. Centimeter-sized skeletal muscle constructs allowed for reversible, continued, and dynamic magneto-mechanical stimulation. Overall, these innovative microfiber scaffolds provide magnetically deformable platforms suitable for dynamic culture of skeletal muscle, offering potential for in vitro disease modeling.

Gelation in Alginate-Based Magnetic Suspensions Favored by Poor Interaction among Sodium Alginate and Embedded Particles

Alginate gels are extensively tested in biomedical applications for tissue regeneration and engineering. In this regard, the modification of alginate gels and solutions with dispersed magnetic particles gives extra options to control the rheo-elastic properties both for the fluidic and gel forms of alginate. Rheological properties of magnetic suspensions based on Na-alginate water solution with embedded magnetic particles were studied with respect to the interfacial adhesion of alginate polymer to the surface of particles. Particles of magnetite (Fe3O4), metallic iron (Fe), metallic nickel (Ni), and metallic nickel with a deposited carbon layer (Ni@C) were taken into consideration. Storage modulus, loss modulus, and the shift angle between the stress and the strain were characterized by the dynamic mechanical analysis in the oscillatory mode. The intensity of molecular interactions between alginate and the surface of the particles was characterized by the enthalpy of adhesion which was determined from calorimetric measurements using a thermodynamic cycle. Strong interaction at the surface of the particles resulted in the dominance of the “fluidic” rheological properties: the prevalence of the loss modulus over the storage modulus and the high value of the shift angle. Meanwhile, poor interaction of alginate polymer with the surface of the embedded particles favored the “elastic” gel-like properties with the dominance of the storage modulus over the loss modulus and low values of the shift angle.

Synthesis and characterization of magnetic organic montmorillonite: efficient adsorption of hexavalent chromium.

This research compared two potential adsorbents for the efficient adsorption of toxic hexavalent chromium. The non-magnetic material STAC-Mt and the magnetic material FeSO4-STAC-Mt were synthesized by a simple impregnation method using montmorillonite (Mt), octadearyl dimethyl ammonium chloride (STAC) and ferrous sulfate as raw materials. The structural and morphological characteristics of both adsorbents were investigated by BET, XRD, FTIR, Zeta, VSM, TEM, SEM and XPS techniques. SEM and TEM results clearly revealed that FeSO4-STAC-Mt had a more loosely curled structure than STAC-Mt and the existence of well dispersed diamond-shaped magnetic particles. The saturation magnetization intensity of 17.949 emu/g obtained by VSM further confirmed the presence of magnetite particles in FeSO4-STAC-Mt. Due to the superparamagnetic properties of magnetite, the adsorption performance of FeSO4-STAC-Mt was better than STAC-Mt. FeSO4-STAC-Mt adsorbed up to 43.98 mg/g of Cr(VI), meanwhile it was easily separated from the reaction mixture by an external magnetic field. Intermittent adsorption studies at pH, adsorbent dosage and time revealed a rapid Cr(VI) adsorption process. In combination with response surface optimization analysis, a removal rate of 98.03% of Cr(VI) was obtained at pH 5-6. The adsorption process was properly described by the pseudo-second-order kinetic equation and the Langmuir equation, and the adsorption process was chemisorption and single molecular layer adsorption. In addition, the removal of Cr(VI) reached 72.68% after five cycles, demonstrating the good stability of the FeSO4-STAC-Mt.

Metal-organic frameworks derived carbon nanotube and carbonyl iron composite materials for broadband microwave absorbers with a wide filling range

• CI/CNT composite was obtained by epitaxial growth-pyrolysis scheme. • Embedding CNT between CI sheets, it exhibits great EMWA in a wide filling range. • By the meta-structure design, the composite achieved RL < −10 dB in 2–18 GHz. Magnetic carbon-based composites derived from metal organic frameworks (MOFs), with tunable morphology and highly dispersed magnetic particles, show great potential as electromagnetic wave absorbing (EMWA) materials. Here, a composite with carbon nanotube (CNT) embedded only between the gaps of carbonyl iron (CI) sheets were obtained, which was achieved by controlling the collapse state of the MOF skeleton during pyrolysis. The unique distribution can prevent the production of global conduction channels, resulting in the impedance matching between air and the composites in the broadened filling range. The composite thus can achieve a broadband absorption in the frequency range of 2–18 GHz by structure design. This work provides a new approach for EMWA materials to reduce the preparation difficulty of broadband absorbers.

Core/shell magnetite/copolymer composite nanoparticles enabling highly stable magnetorheological response

AbstractMagnetorheological fluids (MRFs) have been successfully used in a variety of smart control systems, but are still limited due to their relatively poor settling stability. Herein, a core/shell‐structured Fe3O4/copolymer composite nanoparticle is synthesized as a new candidate material for stimulus‐responsive MRFs to tackle the limitation of the long‐term dispersion stability. Aniline‐co‐diphenylamine copolymers (PANI‐co‐PDPA) are loaded onto the surface of Fe3O4 nanoparticles, providing a lighter density and sufficient active interface for the dispersion of magnetic particles in the carrier medium. The features of the Fe3O4/copolymer composite nanoparticles, including morphology, compositional, and crystalline properties, are characterized. An MRF is prepared by suspending Fe3O4/copolymer composite nanoparticles in a nonmagnetic medium oil, and its rheological properties are assessed using a controlled shear rate test and dynamic oscillation tests using a rotational rheometer. Rheological models including the Bingham model and the Herschel–Bulkley model are fitted to the flow curves of the MRF. The obtained Fe3O4/copolymer composite shows soft‐magnetic properties, as well as greater density adaptability and higher stability, compared to Fe3O4. Moreover, the sedimentation testing provides information about the dispersion stability characteristics of MRF and shows a good correlation with high‐stability magnetorheological (MR) response. The Fe3O4/copolymer‐based MRF with a tunable and instantaneous MR response is considered a promising material for smart control applications.

Particle dispersion and mixing characteristics on the surface of a closed microchamber with magnetic artificial cilia

Inspired by the versatility of cilia, magnetic artificial cilia (MAC) are used to mimic cilia on the surface of biological organs. The application of MAC is currently focused on micro mixing, micro pumps, surface self-cleaning, etc. In order to imitate and study molecular dispersion during biological detection, MAC are utilized in this study to realize enhanced mixing and dispersion of magnetic particles suspended in a customized microchamber and the mixing efficiency is measured experimentally. With permanent magnets rotating outside the microchamber, the top of MAC is actuated to move periodically in a scallop-shape path, which can introduce internal circulation flow and particle migration in the chamber. Mixing conducted with MAC rows and arrays in different spacing are evaluated and compared. Four kinds of micro particle are chosen to display the performance of particles under the interference of MAC. It is found that arrays MAC with larger density would render higher mixing efficiency and less mixing time due to the larger flow velocity near the MAC. The alternating magnetic field generated by dual magnets combined with MAC can make the magnetic particles overcome the agglomeration and enhance mixing. Microfluidic chip with MAC in microchamber is demonstrated to be convenient and efficient for repeating separation-mixing processes of micro particles and it can be a promising micro device for mixing progress in various biological and chemical applications in the future.

Electromagnetic attenuation distribution in a three-dimensional amorphous carbon matrix with highly dispersed Fe/Fe3C@graphite-C nanoparticles

The dielectric-magnetic synergistic effect can be achieved by dielectric relaxation and magnetic resonances in composites of carbon materials and magnetic particles. However, there is no direct evidence demonstrating the distribution of electromagnetic energy attenuation in the microstructure of composites. Herein, an amorphous carbon matrix with dispersed Fe/Fe3C@graphite-C was fabricated via a facile route. The distribution of the magnetic vector loss and electric vector loss in the hierarchical structures was revealed by numerical simulation. The content and dispersion of magnetic particles as well as the morphology of the amorphous carbon can be tuned. The 3D amorphous carbon matrix with highly dispersed Fe/Fe3C@graphite-C nanoparticles provided the strongest absorption of −69.0 dB and an effective response bandwidth (RL ≤ −10 dB) of 7.06 GHz. The numerical simulation revealed that the magnetic vector loss originated only from the core of Fe/Fe3C, and the electric vector loss was distributed throughout the structure, especially in the graphite-C shell, the interface between graphite-C and carbon nanoflakes, and the junction of different carbon nanoflakes. This work not only reveals the distribution of electromagnetic loss but also supports the theoretical analysis of the electromagnetic wave attenuation mechanism in composites, which provides direction for the future design of composites.

The role of thermal diffusion, particle clusters, hydrodynamic and magnetic forces on the flow behaviour of magneto-polymer composites.

In this paper, we study the shear-induced flow of magneto-polymer composites, consisting of dispersions of magnetic particles in solutions of polymers, as a competition between the colloidal forces amid particles and their bulk transport induced by the hydrodynamic forces. For this aim, we analyse the role of different experimental parameters. Firstly, by using only solutions of a well-known anionic polymer (sodium alginate), we provoke a moderate hindering of particle movement, but keeping the liquid-like state of the samples. On the contrary, a gel-like behaviour is conferred to the samples when a cationic polymer (chitosan) is additionally added, which further reduces the particle movement. We analyse the effect of an applied magnetic field, which is opposed to particle transport by hydrodynamic forces, by inducing magnetic attraction between the particles. We perform the analysis under both stationary and oscillatory shear. We show that by using dimensionless numbers the differences between samples and experimental conditions are emphasized. In all cases, as expected, the transport of particles driven by bulk hydrodynamic forces dominates at high values of the shear rate. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

Ultralight monolithic magnetite aerogel

We report the successful synthesis of magnetite aerogel, which has been a long-standing un-resolved challenge in magnetic materials and in aerogel materials. It is the first purely magnetic oxide-ceramic aerogel. So far, a leading approach in inducing magnetic properties in aerogels has been the dispersion of magnetic particles in various aerogel supports. This new aerogel has an ultralow density of 55 mg/mL, containing up to 98.7% air. The synthetic challenge required careful tailoring of the various steps, from the magnetite sol, through the gel, and up to the final aerogel. The rationale behind the various synthetic modifications is described in detail. We find that the ability to form monoliths is associated with dual porosity of the aerogel – interstitial microporosity of small magnetite aggregates that build the walls of the material, along with the wide macropores, typical of aerogels. Detailed mechanistic description is provided, along with intensive characterization of the material properties, the superparamagnetic properties, and more. Modifications of the magnetite are described, such as control of its density, mechanical reinforcement with glass fibers and inducing anisotropy by gelation under an external magnetic field. Magnetite aerogel was found to be very dark with extremely low reflectance in the IR range. Various applications of this ultra-light magnetic material are proposed, including lightweight magnetic actuator, support for separable catalysts and more.

Steady rheological properties of a magnetorheological fluid from mining waste

Magnetorheological fluids have a viscosity which can be controlled by the application of magnetic field. Usually they have dispersions of magnetic particles with size micrometric in a carrier liquid. After purification process, extracted material from mining waste allowed us to obtain the magnetite that was used as the magnetic component of a magnetorheological fluid. The carrier continuous medium for such fluid was 2.5 and 20 W oils. These oils are used for damping systems in high-performance motorcycles and are readily available. The magnetics particle’s size ranged from 2 to 100 µm. Magnetic fields from 50 to 1200 gauss, as well as structural, morphological, and rheological studies were performed to the analyzed fluids. We observed that the model that best fits the stress vs. shear rate curves was the Herschel–Bulkley model.

Selective Laser Sintering-Based 4D Printing of Magnetism-Responsive Grippers.

Components fabricated by four-dimensional (4D) printing hold the potential for applications in soft robotics because of their characteristics of responding to external stimuli. Grippers, being the common structures used in robotics, were fabricated by the selective laser sintering (SLS)-based 4D printing of magnetism-responsive materials and tested for remote-controllable deformation in an external magnetic field. A composite material consisting of magnetic Nd2Fe14B powder and thermoplastic polyurethane powder was selected as the raw material for the SLS; the magnetic particle acquired permanent magnetism by magnetization after the SLS process. Microscopic characterization showed the homogeneous dispersion of magnetic particles inside the polymer matrix. The magnetic induction intensity distribution was systematically investigated by both experiments and numerical simulations. The reliability of the numerical model proposed was justified by the excellent consistency between them. The deformation of the grippers could be regulated by tuning the magnetic particle content and the distance from the external magnet; the deformation mechanism is investigated numerically. The magnetic driving force and the corresponding horizontal displacement are calculated, thus having high accuracy compared with the existing research that obtained the deformation amount by only visual inspection. Mechanical properties of the SLS-fabricated magnetic polymer composite specimens were also studied because of their close relationship with the deformation behaviors. These findings provide guidance for future research on controllable deformation and driving force calculation for 4D printing.

Magnetorheological Elastomers: Fabrication, Characteristics, and Applications.

Magnetorheological (MR) elastomers become one of the most powerful smart and advanced materials that can be tuned reversibly, finely, and quickly in terms of their mechanical and viscoelastic properties by an input magnetic field. They are composite materials in which magnetizable particles are dispersed in solid base elastomers. Their distinctive behaviors are relying on the type and size of dispersed magnetic particles, the type of elastomer matrix, and the type of non-magnetic fillers such as plasticizer, carbon black, and crosslink agent. With these controllable characteristics, they can be applied to various applications such as vibration absorber, isolator, magnetoresistor, and electromagnetic wave absorption. This review provides a summary of the fabrication, properties, and applications of MR elastomers made of various elastomeric materials.

Influence of Additives on Thermal Properties and Morphological of Magnetorheological Elastomer

Magnetorheological elastomers (MREs) are categorized as part of the smart materials class whose rheological properties can be altered under the influence of a magnetic field. MREs are fabricated by embedding soft magnetic particles such as carbonyl iron particles (CIPs) in a rubber matrix such as silicone and natural rubber. In this project, epoxidized natural rubber (ENR-50) is used as a base material with carbonyl iron particles. Sucrose Acetate Isobutyrate (SAIB) ester is added to the formulation to improve the viscosity and enhance the MRE properties. The isotropic MRE is fabricated using two roll mill and a compression mould. Various tests comprise mechanical, morphology, thermal and magnetic tests were conducted for MRE characterization purpose. The results showed that the addition of SAIB on the MRE had reduced 53% of viscosity in the rubber matrix compared to non-ester based MRE. Dispersion of magnetic particles is improved by the addition of ester as observed through Field Emission Scanning Electron Microscope (FESEM). Additionally, the thermal stability was also improved. Tensile strength of MRE consisting SAIB ester achieved maximum strength of 12.3 MPa and an elongation of 620% compared to non-ester based MRE. Â

Influence of Magnetite Dispersion on Tensile Properties

Nanofibers have high cell affinity due to their fine structure and surface roughness, and are expected to be used as biomaterials. In particular, magnetic nanofibers containing magnetic particles are expected to be used for magnetically induced drug delivery systems and hyperthermia. However, due to the aggregation of the magnetic particles contained in the nanofibers, there is a problem that the aggregation location becomes a starting point of fracture and causes a decrease in tensile strength. In this study, to improve the dispersibility of magnetic particles in Magnetite/PLA nanofiber nonwoven fabrics for suppressing the decrease in tensile strength, magnetite is subjected to surface treatment with oleic acid or stearic acid and ultrasonic agitation. Magnetite/PLA nanofiber nonwoven fabric was prepared by the electrospinning method, and dispersion of magnetite in PLA nanofiber nonwoven fabric and tensile strength were evaluated. Magnetite dispersion was improved by the surface treatment and increasing the ultrasonic agitation time. In particular, by performing the stearic acid treatment and prolonging the ultrasonic agitation time, the magnetite dispersion tended to be improved. This treated Magnetite/PLA nanofiber nonwoven fabric showed higher tensile strength.

Anisotropic magnetic hydrogels: design, structure and mechanical properties.

Anisotropy is an intrinsic feature of most of the human tissues (e.g. muscle, skin or cartilage). Because of this, there has been an intense effort in the search of methods for the induction of permanent anisotropy in hydrogels intended for biomedical applications. The dispersion of magnetic particles or beads in the hydrogel precursor solution prior to cross-linking, in combination with applied magnetic fields, which gives rise to columnar structures, is one of the most recently proposed approaches for this goal. We have gone even further and, in this paper, we show that it is possible to use magnetic particles as actuators for the alignment of the polymer chains in order to obtain anisotropic hydrogels. Furthermore, we characterize the microstructural arrangement and mechanical properties of the resulting hydrogels. This article is part of a theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Magnetorheology of alginate ferrogels

Magnetorheological (MR) effect is a phenomenon typical of suspensions of magnetizable particles in a liquid carrier, characterized by strong changes of their mechanical properties in response to applied magnetic fields. Its origin is on the migration of magnetized particles and their aggregation into chain-like structures. However, for ferrogels, consisting of dispersions of magnetic particles in a polymer matrix, migration of particles is hindered by the elastic forces of the polymer network, preventing from strong MR effect. Interestingly, in this manuscript, we demonstrate that strong MR effect in robustly cross-linked polymer ferrogels is still possible. Experimental results showed enhancement of the storage modulus of more than one order of magnitude for alginate ferrogels containing less than about 10 vol% of iron particles under moderate magnetic fields. The differential feature of these ferrogels is that, instead of individual particles, the disperse phase consisted of large clusters of iron microparticles homogeneously distributed within the polymer networks. These clusters of magnetic particles were formed at the stage of the preparation of the ferrogels and their presence within the polymer networks had two main consequences. First, the volume fraction of clusters was considerably larger than this of individual particles, resulting in a larger effective volume fraction of solids. Second, since the force of magnetic attraction between magnetic bodies is roughly proportional to the cube of the body size, the existence of such clusters favored inter-cluster interaction under a magnetic field and the appearance of strong MR effect. On this basis, we demonstrated by theoretical modeling that the strong MR effect displayed by the alginate ferrogels of the present work can be quantitatively explained by assuming the existence of large, roughly spherical particle aggregates formed at the stage of the preparation of the ferrogels. Our theoretical model provides a reasonable quantitative prediction of the experimental results.