Dmitry Borin

Structural control of elastic moduli in ferrogels and the importance of non-affine deformations

One of the central appealing properties of magnetic gels and elastomers is that their elastic moduli can reversibly be adjusted from outside by applying magnetic fields. The impact of the internal magnetic particle distribution on this effect has been outlined and analyzed theoretically. In most cases, however, affine sample deformations are studied and often regular particle arrangements are considered. Here we challenge these two major simplifications by a systematic approach using a minimal dipole-spring model. Starting from different regular lattices, we take into account increasingly randomized structures, until we finally investigate an irregular texture taken from a real experimental sample. On the one hand, we find that the elastic tunability qualitatively depends on the structural properties, here in two spatial dimensions. On the other hand, we demonstrate that the assumption of affine deformations leads to increasingly erroneous results the more realistic the particle distribution becomes. Understanding the consequences of the assumptions made in the modeling process is important on our way to support an improved design of these fascinating materials.

Tunable dynamic response of magnetic gels: impact of structural properties and magnetic fields.

Ferrogels and magnetic elastomers feature mechanical properties that can be reversibly tuned from outside through magnetic fields. Here we concentrate on the question of how their dynamic response can be adjusted. The influence of three factors on the dynamic behavior is demonstrated using appropriate minimal models: first, the orientational memory imprinted into one class of the materials during their synthesis; second, the structural arrangement of the magnetic particles in the materials; and third, the strength of an external magnetic field. To illustrate the latter point, structural data are extracted from a real experimental sample and analyzed. Understanding how internal structural properties and external influences impact the dominant dynamical properties helps to design materials that optimize the requested behavior.

First-order reversal curve analysis of magnetoactive elastomers

The first magnetization loop and the first stress–strain cycle of magnetoactive elastomers (MAEs) in a magnetic field differ considerably from the following loops and cycles, possibly due to the internal restructuring of the magnetic filler particles and the matrix polymer chains. In the present study, the irreversible magnetization processes during the first magnetization of MAEs with different filler compositions and tensile moduli of the matrix are studied by first-order reversal curve (FORC) measurements. For MAEs with mixed magnetic NdFeB/Fe fillers the FORC distributions and magnetization distributions of the first major loop reveal a complex irreversible magnetization behavior at interaction fields Hu 600 kA m−1.

Magnetorheological effect of magneto-active elastomers containing large particles

The magnetorheological effect of elastomer composites containing a mixture of large (50-80 μm) and small (3-5 μm) particles has been experimentally examined. The data shows that elasticity in the range of small deformations (1%) for a magnetic field strength of 290 mT increased by two order of magnitude. This effect can be explained with the presence of the large particles in the structure of the composite assisting the aggregation effect. Due to the strong increase of the interparticle interaction compared with the restoring elastic forces, the presence of the large particles leads to the observed steep increase of Youngs modulus.

A comparison between micro- and macro-structure of magnetoactive composites

A possibility to determine the microstructure of magneto active composites using micro-computed tomography (μ-CT) [1] was investigated in this work. The main interest of the current study has been an observation of the magnetic field dependent shift of individual particles from their initial positions inside an elastomeric matrix. For this purpose a μ-CT system has been combined with a sample holder coupled with two permanent magnets, enabling the investigation of the micro-structure under influence of an external homogeneous field. In the experimental investigations samples based on carbonyl iron particles have been used. The particles have been dispersed in a polymeric matrix and the polymer has been created in the presence of a magnetic field driving structure formation of the particles. After the characterization of the sample in its initial state, i.e. without external stimuli, it has been subjected to the magnetic field and its internal structure has been once more studied by μ-CT. As a result a comparison of the macroscopic change of the sample structure and the particle displacement could be undertaken.

Thermomagnetic convection influenced by the magnetoviscous effect

The mechanism of thermomagnetic convection in ferrofluids which is introduced by a magnetic field is well understood. To describe this kind of convection, it is assumed that the material properties, e. g. the viscosity η are independent from the magnetic field. However under certain circumstances ferrofluids show a magnetoviscous effect that means the increase of η in the presence of a magnetic field. Both effects have been well investigated but independently from each other. In this paper it is experimentally shown how the magnetoviscous effect and therefore the increase of η influence the behaviour of the thermomagnetic convection.

Stress relaxation in a ferrofluid with clustered nanoparticles

The formation of structures in a ferrofluid by an applied magnetic field causes various changes in the rheological behaviour of the ferrofluid. A ferrofluid based on clustered iron nanoparticles was investigated. We experimentally and theoretically consider stress relaxation in the ferrofluid under the influence of a magnetic field, when the flow is suddenly interrupted. It is shown that the residual stress observed in the fluid after the relaxation is correlated with the measured and theoretically predicted magnetic field-induced yield stress. Furthermore, we have shown that the total macroscopic stress in the ferrofluid after the flow is interrupted is defined by the presence of both linear chains and dense, drop-like bulk aggregates. The proposed theoretical approach is consistent with the experimentally observed behaviour, despite a number of simplifications which have been made in the formulation of the model. Thus, the obtained results contribute a lot to the understanding of the complex, magnetic field-induced rheological properties of magnetic colloids near the yield stress point.

Electrorheological fluids and magnetorheological suspensions

The 11th International Conference on Electrorheological Fluids and Magnetorheological Suspensions (ERMR08) was organized between 25–29 August 2008 at the Technische Universitat Dresden in Germany. The conferences scientific program focused on the synthesis, characterization and application of fluids which can be controlled by external electric or magnetic fields. Within this framework electrorheological fluids, magnetorheological suspensions, ferrofluids and—for the first time—magnetorheological elastomers, which have attracted great interest in recent years, formed the class of materials discussed during the conference. With more than 180 participants from 24 different countries and 85 oral presentations in non-parallel sessions, including 8 plenary talks and 81 posters, it has been the largest ERMR meeting held during the last decade. This growth is at least partly driven by the synthesis of improved fluids and elastomers, which open new possibilities for applications as well as in basic research characterizing the flow and properties of such materials under the influence of external magnetic and electric fields. The general proceedings of ERMR08 include 116 peer-reviewed papers covering all conference topics. They were published in Journal of Physics: Conference Series volume 149 in 2009. The present special issue of Journal of Physics: Condensed Matter contains six selected papers dealing with basic research problems which will be of general interest to the condensed matter community. We hope that this selection will provide an insight into the fascination of soft matter controlled by external fields.

Positioning magnetorheological actuator

In this work we consider a construction of a positioning magnetorheological actuator based on bellow units, as well as dynamical model, which include such elements as a magnetically hysteresis, pressure loses in hydraulic system, nonlinearity of rheological behaviour of working fluid. Two operating modes of positioning actuator are taken into account and transients are presented. Dynamical modelling shows possibility for the improvement of a real control system and ensure of submicron precision of positioning with millisecond time of response.

RHEOLOGY OF NOVEL FERROFLUIDS

The progress in the synthesis of new magnetic nanoparticles and agglomerates stimulates the development of novel ferrofluids with enhanced rheological properties. In the current work ferrofluids based on Co-nanoplatelets and clustered iron oxide nanoparticles have been considered. Steady-shear experiments and yield stress measurements of these ferrofluids have been performed using rotational rheometry.