The study conducts an experimental analysis of the response of diluted magnetic suspensions with nano- and micro-sized particles structured in an external magnetic field to a quasi-static shear strain applied perpendicular to the direction of the external magnetic field. Magnetic rheometry is used as the research method. For microparticle-based composites, X-ray computed microtomography is used to evaluate qualitatively the morphology of particle aggregates induced in the suspension. It is found that a structured magnetic suspension based on nanoparticles with a volume concentration of around 1–2 ∼ 0.1

Magneto-active elastomers (MAEs) are advanced composite materials consisting of a soft elastomeric matrix embedded with magnetic micro-inclusions. These materials exhibit complex multiscale response that presents significant challenges for modeling and analysis. In the absence of an external magnetic field, the mechanical behavior of MAEs can be approximated as that of a rubber matrix reinforced with rigid filler particles. However, under an applied magnetic field, the magneto-mechanical coupling arises from magnetic interactions among the embedded particles. These interactions induce macroscopic deformations of the elastomer and lead to microstructural rearrangements, such as the formation of particle columns aligned with the field. To characterize the mechanical behavior, a transversely isotropic Neo-Hookean model is employed, capturing the anisotropic elastic response of the material. The magnetic behavior is modeled using a dipolar mean-field approach, which accounts for interactions between magnetized particles under an external field. Furthermore, an additional term is introduced governing the microstructural evolution caused by the application of a magnetic field. Several forms of this term are systematically evaluated to identify the most effective framework for capturing microstructural dynamics. The proposed model enhances our understanding of the interplay between microstructural evolution and the reinforcement of mechanical stiffness of the MAEs caused by the application of magnetic field, thereby providing critical insights into their behavior and guiding the development of predictive tools for these multifunctional materials.

This study investigates the experimental conditions necessary to observe a non-monotonic dependence of shear stress on quasi-static shear strain in structured magnetorheological (MR) fluids of a known composition. The research varies several key parameters, including the concentration and size of magnetic particles, the working gap between rheometer plates, the strength of the external magnetic field, and the material properties of the plates. Rheometric measurements are complemented by microstructural observations of low-concentration MR fluid specimens using X-ray computed microtomography. The results demonstrate that non-monotonic shear stress-strain behavior is characteristic of diluted MR suspensions with linear chain-like particle aggregates. Crucially, this phenomenon occurs only when using rheometer plates with magnetic microparticles embedded in their surface, and the microparticles must be of a size comparable to or larger than the transverse dimensions of the chain aggregates. Conversely, monotonic shear stress increases are observed with non-magnetic or entirely magnetic plates, irrespective of the MR fluid composition. These findings highlight the critical role of particle-structure interactions with the measuring geometry and provide insights into operating conditions for applications requiring precise control of static yield stress. The results also offer valuable experimental data to refine and validate models of MR fluid behavior under quasi-static shear.

The magnetic properties of elastomers based on silicone matrix and iron microparticles assembled in linear chain-like aggregates of different length are experimentally investigated. For this purpose, elastomer samples with a low concentration of magnetic filler are structured in a magnetic field of various strength. The influence of the particle aggregates morphological characteristics on the macroscopic magnetic response of the samples is revealed. Moreover, the influence of the angle between the direction of particle aggregates, that is, the field applied in the process of crosslinking the polymer matrix, and the direction of the field applied during magnetic measurements on the macroscopic magnetic properties of the composite is taken into account. Magnetic measurements are supported by the evaluation of the real microstructure of the samples using X-ray computed microtomography and corresponding digital image processing methods.

The planar motion behavior of the mobile robot utilizing two multipole magnetized functional elements is studied experimentally. The beam-shaped functional elements are synthesized from a special magnetoactive elastomer (MAE), filled with both hard and soft magnetic particles of micrometer size, and feature bristle pads protruding on their underside. The robot locomotion is controlled by the actuation of two cylindrical coils, which generate alternating magnetic fields causing the functional beams to vibrate. The field-induced bending vibrations of the beams lead to the robot movement across a plane due to the cyclic interplay of inertial and friction forces. The motion capture analysis reveals that the coil actuation frequencies control the robot velocity and the movement direction, showing the skid-steer behavior. The pronounced resonant dependency of the velocity is observed, when the robot is driven at equal actuation frequencies. The frequency combinations required to perform straight-line movement are identified. The research highlights the potential of MAEs to design advanced actuators and soft robotics applications with simple actuation.

The paper reports on a composite based on a polydimethylsiloxane matrix filled with liquid metal and magnetic hard microparticles. The effect of the concentration of such a complex filler on the elasticity and relative permittivity of the composite has been investigated. Inclusions of liquid metal provide improved permittivity compared to the unfilled matrix, while the stiffening effect is not as high as when the matrix is filled with solid particles alone. On the other hand, magnetic hard fillers allow the functionalisation of the composite in terms of its magnetic properties. It is also shown that changing the residual magnetisation of the composite has no significant effect on the elasticity and relative permittivity.

Magnetoactive elastomers are able to significantly change their material properties in a controlled manner under magnetic field stimulation. These smart materials provide solutions for many application systems, including magnetic-field-actuated mobile robots and sensors with an adjustable operation range. The work presented here studies multipole magnetized magnetoactive elastomers with the aim of utilizing them as functional elements of vibration-driven locomotion robots. The bi-directional locomotion of the elastomeric bristle-bot is achieved in two prototype designs with a single magnetic actuation. The motion principle is based on cyclic interplay of asymmetric friction and inertia forces caused by magnetic-field-induced bending vibrations of the magnetized elastomer. The locomotion behavior of both prototypes shows a strong non-monotonic dependency of the advancing speed on the magnetic actuation frequency.

The effect of repetitive quasi-static magnetisation of a magnetorheological elastomer on its magnetic response is discussed. Typical components of this material, namely soft silicone rubber and carbonyl iron powder, are used to produce magnetically sensitive composite samples. The composite specimens are examined by vibrating sample magnetometry. The influence of the elasticity of the composite matrix on the change in magnetic differential susceptibility of the material as a function of the number of repeated magnetisation cycles is evaluated. The soft matrix elastomers are characterised by the presence of maxima in the initial part of the differential susceptibility curves. The sample magnetisation values corresponding to such maxima vary with the number of magnetisation cycles. In addition, the initial magnetic susceptibility curves of soft samples obtained at different polarity of the applied field are not symmetrical. Symmetry is obtained by performing several subsequent magnetisation cycles. The differential susceptibility curves are affected by both reversible and irreversible processes related to particle mobility in the matrix. The magnetic response of composite samples with rigid matrices is more similar to that of classical soft magnetic materials.

This chapter gives a review of the current progress on investigations of the microstructure of magnetorheological elastomers using X-ray computed microtomography. The basic physical principles, the equipment used and an overview of results on both the global microstructure of magnetorheological elastomers of diverse compositions and their microstructural analysis on single particles basis are presented.

Soft actuators are deformable materials that change their dimensions or shape in response to external stimuli. Among the various stimuli, remote magnetic fields are one of the most attractive forms of actuation, due to their ease of use, fast response, and safety in biological systems. Composites of magnetic particles with polymer matrices are the most common materials for magnetic soft actuators. In this paper, we demonstrate the fabrication and actuation of magnetic shape-memory materials based on hydrogels containing field-structured magnetic particles. These actuators are formed by placing the pregel dispersion into a mold of the desired on-field shape and exposing it to a homogeneous magnetic field until the gel point is reached. At this point, the material may be removed from the mold and fully gelled in the desired off-field shape. The resultant magnetic shape-memory material then transitions between these two shapes when it is subjected to successive cycles of a homogeneous magnetic field, acting as a large deformation actuator. For actuators that are planar in the off-field state, this can result in significant bending to return to the on-field state. In addition, it is possible to make shape-memory materials that twist under the application of a magnetic field. For these torsional actuators, both experimental and theoretical results are given.

Measuring the magnetic susceptibility of fluids is an expensive and challenging task. Thus, this paper presents a simple and low-cost technique based on one solenoid. The coil is plunged into a ferrofluid. By knowing the system response from simulation, the magnetic susceptibility can be derived by measuring the coil's inductance. In this paper, the system response was assessed, and the susceptibility was measured in the range of 0.005 to 0.054 for six particle concentrations from 1 to 14 mg Fe per ml. The obtained results were also validated with a reference using a vibrating sample magnetometer.

Dielectric elastomer actuators (DEAs) have been proposed as a promising technology for developing soft robotics and stretchable electronics due to their large actuation. Among available fabrication techniques, inkjet printing is a digital, mask-free, material-saving, and fast technology, making it versatile and appealing for fabricating DEA electrodes. However, there is still a lack of suitable materials for inkjet-printed electrodes. In this study, multiple carbon black (CB) inks were developed and tested as DEA electrodes inkjet-printed on acrylic membranes (VHB). Triethylene glycol monomethyl ether (TGME) and chlorobenzene (CLB) were selected to disperse CB. The inks' stability, particle size, surface tension, viscosity, electrical resistance, and printability were characterized. The DEA with Ink-TGME/CLB (mixture solvent) electrodes obtained 80.63% area strain, a new benchmark for the DEA actuation with CB powder electrodes on VHB. The novelty of this work involves the disclosure of a new ink recipe (TGME/CLB/CB) for inkjet printing that can obtain stable drop formations with a small nozzle (17 × 17 μm), high resolution (∼25 μm, approaching the limit of drop-on-demand inkjet printing), and the largest area strain of DEAs under similar conditions, distinguishing this contribution from the previous works, which is important for the fabrication and miniaturization of DEA-based soft and stretchable electronics.

The transition dynamics in silicon rubber based isotropic magnetorheological (MR) elastomers in terms of the normal force induced by an external homogeneous magnetic field is experimentally addressed. The primary goal was to evaluate dynamic performances of the MR elastic isotropic composite using a transparently presented measuring system with known characteristics in contrast to few previous studies on the topic. It was found that an increase in the magnetic field leads to an increase in the induced force and a decrease in the response time of the MR elastomer. At the same time, both the use of coarse particles as magnetic filler and a significant reduction in the stiffness of the polymer matrix reduce the response time of the MR elastomer under study. The analysis carried out takes into account the dynamics of the electromagnetic coil and the eddy currents induced in the magnet circuit. The shortest response times obtained for various MR elastomer samples are in the range of 27–72 ms for the maximal used magnetic field with an induction of 230 mT. These times correspond to the fastest previously reported ones for MR elastomers and MR elastomer based systems. In addition, the obtained results indicate the presence of different mechanisms responsible for the measured magnetodeformational effect observed in MR elastomers.

Creation of and the following research on systems featuring elastomer filled with a magnetic disperse material with good electroconductive properties have been a continuation of the development of magnetorheological fluids with the goal of finding compositions exhibiting a stronger magnetorheological effect. More profound investigations have revealed that composite materials of the given type also exhibit other significant features such as magnetodeformation, magnetostriction, field-induced shape memory, and piezomagnetoresistance, for which reason they are frequently classified as ‘magnetoactive elastomers’. Within the frames of this work, investigations of relationships between the electroconductive and dielectric properties of the polymer composite and external magnetic fields have been done. As has been shown by the experiments, changing the external magnetic field from zero to 330 mT causes the best samples to improve their conductive properties by six orders of magnitude. At the same time, the capacitance measured along with the resistance increases by a factor of 30. Reproducible and less subjected to the emergence of runouts at frequencies of 1 kHz and higher, the capacitance-based data offer hope that such elastomers may be good candidates for being employed as sensors. In order to make the sample-dependent results comparable, the capacitances are interpreted as the dielectric permeabilities. It should be noted though that this approach is strictly formal and the mechanism of the phenomenon observed still awaits its scrupulous study.

We present results of experimental and theoretical study of the effect of external magnetic field on elastic properties of dense soft magnetic polymers filled with micron-sized magnetizable particles. The samples were cured without a magnetic field, thus with isotropic internal morphology of the particles disposition. Experiments demonstrate that under quite moderate magnetic fields the shear elastic modulus of the studied composites increased more than two orders of magnitudes. Decrease in the modulus with the value of the global shear deformation is detected. We explain these effects by adhesive isotropic agglomeration of the particles, at the stage of the composite polymerization. Under magnetic field, these agglomerates are magnetized and aggregate into chains, oriented in the field direction. The chain length is determined by competition between magnetic attraction of the agglomerates and the host medium elastic resistance to their translocation. Theoretical results of the proposed model quantitatively reproduce data's of measurements of the composite elastic modulus.

The paper gives an overview of tunable elastic magnetic composites based on silicon rubbermatrix highly filled with amagnetic soft and hard filler. Themagnetic soft phase, which is represented by iron microparticles, allows active control of the physical properties of the composites, while the magnetically hard phase (e.g. neodymium–iron–boron alloy microparticles) is mainly responsible for passive adjustment of the composite. The control is performed by the application of an external magnetic field in situ, and passive adjustment is performed by means of premagnetization in order to change material remanent magnetization, i.e. the initial state. The potential and limits of active control and passive tuning of these composites in terms of their magneto-mechanical behavior are presented and discussed.

First-order reversal curve (FORC) analysis allows one to investigate composite magnetic materials by decomposing the magnetic response of a whole sample into individual responses of the elementary objects comprising the sample. In this work, we apply this technique to analysing silicone elastomer composites reinforced with ferromagnetic microparticles possessing low intrinsic coercivity. Even though the material of such particles does not demonstrate significant magnetic hysteresis, the soft matrix of the elastomers allows for the translational mobility of the particles and enables their magnetomechanical hysteresis which renders into a wasp-waisted major magnetization loop of the whole sample. It is demonstrated that the FORC diagrams of the composites contain characteristic wing features arising from the collective hysteretic magnetization of the magnetically soft (MS) particles. The influence of the matrix elasticity and particle concentration on the shape of the wing feature is investigated, and an approach to interpreting experimental FORC diagrams of the MS magnetoactive elastomers is proposed. The experimental data are in qualitative agreement with the results of the simulation of the particle magnetization process obtained using a model comprised of two MS particles embedded in an elastic environment.

In this work we study the magnetization of magnetoactive elastomers (MAE) in which the interface between the matrix and magnetic particles is unstable and allows for slipping of the particles against the wall of their elastomer cavities. The estimate of the maximal angle at which each particle can decline its axis from the initial position is made based on cyclic measurement of several consecutive hysteresis loops at different maximal magnetic fields. A model of magnetization of magnetically hard multigrain particles in an elastic environment with allowance for their possible slipping is proposed. Results of modelling is in fair agreement with the experimental data obtained on MAEs whose polymeric matrix is made of polydimethylsiloxane and the magnetic filler is NdFeB spherical particles.

The study experimentally examines the quasi-static shear deformation of a magnetorheological (MR) fluid structured in an external magnetic field. Experiments are carried out using a rheometer with a plate–plate configuration. The working surfaces of the measuring geometry are modified to demonstrate their influence on the response of the field-structured MR fluid. The simplest possible suspension of microparticles of carbonyl iron in mineral oil without using surfactants or any modifiers is used. The difference in results obtained for structured MR fluid with different concentrations of magnetic particles using different modifications of the surface is demonstrated. The results are intended to motivate more intensive research on the issue and further more in-depth theoretical analysis of static elastic properties of structured MR fluids. Certain related critical issues are briefly highlighted.

We successfully integrated dielectric elastomer switches (DESs) with dielectric elastomer actuators (DEAs) on one flexible dielectric elastomer (DE) membrane via spray painting. The actuator can generate an in-plane compression in the direction of DES, leading to a dramatic reduction in the electric resistance of the switch; while the electric resistance changes back to its initial value when the actuator is off. Therefore, the resultant component exhibits piezoresistive properties and can be further used as DE inverters. This entirely soft inverter can be potentially applied to develop biomimetic robotics in terms of driving, controlling, monitoring, sensing, and self-regulation.