Conformational changes of a single magnetic particle string within gels

CHARACTERIZATION OF RHEOLOGICAL PROPERTIES OF NOVEL MAGNETORHEOLOGICAL FLUIDS

Introduction. When heating with high-frequency currents (HFCs) at high speeds, more significant strengthening effects can be observed compared to using machine generators. Therefore, hardening at high frequencies is more efficient. However, the increase in the generator frequency results in a decrease in the depth of penetration of eddy currents and an increased unevenness of heating across the cross-section. The application of a constant external magnetic field during HFC hardening can increase the depth of eddy current penetration and create more uniform heating. Unfortunately, there is not enough information available on the effect of the external magnetic field on HFC heating processes and phase transformations in steel. Currently, there are no quantitative estimates for the impact of an external magnetic field on changes in the kinetics of electric heating and the penetration depth of eddy currents. In connection with the above, the aim of this paper is to investigate changes in the kinetics of high-frequency heating of iron-carbon alloys when an external constant magnetic field is applied and, and, based on this, to consider the potential for technological applications. Materials and Methods. Theoretical assessment of the influence of an external magnetic field on the change in the kinetics of electric heating and the penetration depth of eddy currents is based on the general theory of induction heating kinetics. An experimental study of the influence of a magnetic field on the kinetics of high-frequency current heating was performed on samples of 45 steel, pearlitic gray (SCh30), and ferritic malleable cast iron (KCh30-6). The temperature distribution over the cross-section of ferromagnetic materials during induction heating with an external magnetic field has been studied using special samples of iron, 45 steel, and SCh30 gray pearlitic cast iron. Electric tempering processes have been investigated on samples of U8A steel using a vacuum tube generator (heating temperature — 450℃, heating rate — 750℃/s). Changes in austenite grain size after high-speed heating with external magnetization have been examined on samples of reduced-hardenability 55PP steel. To study the processes of thermal treatment in a magnetic field during experiments involving heating samples using high-frequency currents, a specially designed electromagnet was created to apply an external constant magnetic field. Results. Theoretical curves were constructed for heating conditions with and without an external constant magnetic field. Experimental data on the effect of an external constant magnetic field on induction heating in the surface layer of various materials were summarized in kinetic diagrams. Evidence that the observed changes were due to increased depth of penetration of eddy currents came from experiments on cylindrical samples of 45 steel with different wall thicknesses. Kinetic curves were provided for estimating the temperature field (at 6 points at different depths) during high-frequency current heating with and without external magnetization. The paper presents experimental data on the micro-hardness distribution across the cross-section of a U8 steel sample after quenching, quenching and electric tempering, quenching and electric tempering with external magnetization, and quenching and bulk tempering. It also includes the results of the study of the austenite grain size of 55PP steel after high-speed heating with external magnetization and conventional (slow) deep heating. Discussion. The application of a high-intensity external constant magnetic field during the first quasi-stationary process resulted in a decrease in the rate of induction heating of the ferromagnetic material and an increase in the depth of its uniform heating. However, above the Curie point, the effect of the magnetic field was negligible due to the low magnetic susceptibility of the material, and the heating rate remained unchanged as if there was no field present. In addition, due to the insignificant difference in the values of magnetic permeability below and above the Curie point during heating in the field, the thermal curve did not exhibit the characteristic inflection typical of kinetic curves observed during the transition of the surface layer to a paramagnetic state. Experiments with electric tempering have demonstrated that by applying an external field, it was possible to temper a material to the desired depth and it could be done on a single high-frequency current setup. The size of the austenite grains after high-speed heating with magnetization was reduced compared to conventional deep heating of steel with low hardenability, eliminating the issue of induction heating for low-hardenability steel. Conclusion. The results of the study demonstrated that the use of an external magnetic field enabled the achievement of strengthening effects during heating at higher frequencies, thereby eliminating the drawbacks of such heating methods.

A new modified model for the rheological properties of magnetorheological fluids based on different magnetic field

Magneto-rheological (MR) fluids, as smart fluids, are suspensions of micron-sized magnetic particles, such as iron particles or cobalt, in a silicone or hydraulic oil carrier fluid, which can change from the liquid state to a semi-solid state in several milliseconds in the external magnetic field. In this paper, the produced chain, which will give rise to the MR effect, is investigated by the magnetic force between two particles, the relationship between the produced shear torque, and magnetic particles are proposed. A set of testing systems, including the plate-on-plate MR fluid shearing test rig, is built to study the percentage of magnetic particles in MR fluid. The magnetic field in the gap, before and after the gap is added with MR fluid, is investigated by measuring and calculating. The results validate the effect of the content of magnetic particles in MR fluid on the shear performance.

To harness light-matter interactions at the nano-/micro-scale, better tools for control must be developed. Here, it is shown that by applying an external electric and/or magnetic field, ablation of Si and glass under ultra-short (sub-1 ps) laser pulse irradiation can be controlled via the Lorentz force , where is velocity of charge e, is the applied electrical bias and is the magnetic flux density. The external electric E-field was applied during laser ablation using suspended micro-electrodes above a glass substrate with an air gap for the incident laser beam. The counter-facing Al-electrodes on Si surface were used to study debris formation patterns on Si. Debris was deposited preferentially towards the negative electrode in the case of glass and Si ablation. Also, an external magnetic field was applied during laser ablation of Si in different geometries and is shown to affect ripple formation. Chemical analysis of ablated areas with and without a magnetic field showed strong chemical differences, revealed by synchrotron near-edge X-ray absorption fine structure (NEXAFS) measurements. Harnessing the vectorial nature of the Lorentz force widens application potential of surface modifications and debris formation in external E-/B-fields, with potential applications in mass and charge spectroscopes.

A new system was developed for measuring AC current distributions in high T/sub c/ superconducting tapes under an external AC magnetic field. In our measurement the current distributions are indirectly found from the results of the self-field distributions measured by an array of pickup coils located around the tape. For improved measurement accuracy under the external magnetic field, a compensation coil was used to cancel the component of the external magnetic field from the signal of pickup coil. In order to confirm the validity of this system, measurement of current distributions in a test conductor composed of insulated copper wires was carried out under an external AC magnetic field. Using this system, we measured the current distributions in multifilamentary Ag alloy sheathed Bi-2223 tape as a function of the angle of the external AC magnetic field at 77 K. Further, we investigated the characteristics of the current distributions for the case of a simultaneous application of AC transport current and transverse AC magnetic field. On the basis of the obtained results the practicality of our measurement system is discussed.

The boundary element method was applied to study the motion of magnetic particles in fluid flow under the action of external nonuniform magnetic field. The derived formulation combines the velocity-vorticity resolved Navier---Stokes equations with the Lagrange based particle tracking model, where the one-way coupling with fluid phase was considered. The derived algorithm was used to test a possible design of high gradient magnetic separation in a narrow channel by computing particles trajectories in channel flow under the influence of hydrodynamic and magnetic forces. Magnetic field gradient was obtained by magnetization wires placed outside of the channel. Simulations with varying external magnetic field and flow rate were preformed in order to asses the collection efficiency of the proposed device. We found that the collection efficiency decreases linearly with increasing flow rate. Also, the collection efficiency was found to increase with magnetic field strength only up a saturation point. Furthermore, we found that high collection efficiently is not feasible at high flow velocity and/or at weak magnetic field. Recommendation for optimal choice of external magnetic field and flow rate is discussed.

We investigate collision of two atoms in an external magnetic field and in the field of laser radiation with formation of Fano-Feshbach resonances. At one-photon resonance of laser radiation with two discrete vibrational states of molecule the dressed states are formed (Autler-Townes effect) which form Fano-Feshbach resonances in interaction with the external magnetic field. In addition, the lower molecular vibrational state is coupled with the continuum of the elastic channel via also LICS (laser-induced continuum structure) forming laser-induced resonance. We obtain cross-sections of elastic and inelastic resonant scattering and expression for the scattering length depending on the external magnetic and laser radiation fields.

This article deals with magnetohydrodynamic (MHD) flows of a thin rotating layer of astrophysical plasma in external magnetic field. We use the shallow water approximation to describe thin rotating plasma layer with a free surface in a vertical external magnetic field. The MHD shallow water equations with external vertical magnetic field are revised by supplementing them with the equations that are consequences of the magnetic field divergence-free conditions and reveal the existence of third component of the magnetic field in such approximation providing its relation with the horizontal magnetic field. It is shown that the presence of a vertical magnetic field significantly changes the dynamics of the wave processes in astrophysical plasma compared to the neutral fluid and plasma layer in a toroidal magnetic field. The equations for the nonlinear wave packets interactions are derived using the asymptotic multiscale method. The equations for three magneto-Poincare waves interactions, for three magnetostrophic waves interactions, for the interactions of two magneto-Poincare waves and for one magnetostrophic wave and two magnetostrophic wave and one magneto-Poincare wave interactions are obtained. The existence of parametric decay and parametric amplifications is predicted. We found following four types of parametric decay instabilities: magneto-Poincare wave decays into two magneto-Poincare waves, magnetostrophic wave decays into two magnetostrophic waves, magneto-Poincare wave decays into one magneto-Poincare wave and one magnetostrophic wave, magnetostrophic wave decays into one magnetostrophic wave and one magneto-Poincare wave. Following mechanisms of parametric amplifications are found: parametric amplification of magneto-Poincare waves, parametric amplification of magnetostrophic waves, magneto-Poincare wave amplification in magnetostrophic wave presence and magnetostrophic wave amplification in magneto-Poincare wave presence. The instabilities growth rates are obtained respectively.

BSCCO/Ag tapes are being developed for electrical power applications at liquid nitrogen temperatures. In these applications, conductors are exposed to an alternating magnetic field and fed simultaneously with an alternating transport current. In this contribution the influence of an external alternating magnetic field on the transport current loss of a single tape is studied experimentally. Special attention is paid to the loss as a function of the orientation of the external magnetic field. The transport current loss in a DC external magnetic field can be described well by the decrease of the critical current due to the magnetic field. In the case of an AC external magnetic field this is only a minor effect. Effects responsible for the increase of the transport current loss in external AC magnetic field are investigated with respect to their angle dependency. A model based on a 1 dimensional modelling of the conductor and an average critical current density during the magnetic field cycle is presented. This model describes the measured results reasonably well below the critical current.

Magnetorheological (MR) fluids have rheological properties, such as the viscosity and yield stress that can be altered by an external magnetic field. The design of novel devices utilizing the MR fluid behavior in multi-degree of freedom applications require three dimensional models characterizing the coupling of magnetic behavior to mechanical behavior in MR fluids. A 3-D MR fluid model based on multiscale kinetic theory is presented. The kinetic theory-based model relates macroscale MR fluid behavior to a first-principle description of magnetomechanical coupling at the microscale. A constitutive relation is also proposed that accounts for the various forces transmitted through the fluid. This model accounts for the viscous drag on the spherical particles as well as Brownian forces. Interparticle forces due to magnetization and external magnetic forces applied to ferrous particles are considered. The tunable rheological properties of the MR fluids are studied using a MR rheological instrument. High and low viscosity carrier fluids along with small and large carbonyl iron particles are used to make and study the behavior of four different MR fluids. Experiments measuring steady, and dynamic oscillatory shear response under a range of magnetic field strengths are performed. The rheological properties of the MR fluid samples are investigated and compared to the proposed kinetic theory-based model. The storage (G') and loss (G") moduli of the MR fluids are studied as well.

The combination of polymers with nano- or microsized solid materials displays novel and often enhancedproperties compared to the traditional materials. They can open up possibilities for new technologicalapplications. Materials whose physical properties can be varied by application of magnetic fields belongto a specific class of smart materials. The broad family of magnetic field-controllable soft materialsincludes ferrofluids, magneto-rheological fluids, magnetic gels, and magnetic elastomers. The magneticgels and elastomers (magnetoelasts) represent a new type of composite and consist of small magneticparticles, usually in the nanometer to micron range, dispersed in a highly elastic polymeric matrix.The magnetic particles can be incorporated into the elastic body either randomly or in ordered structure.If a uniform magnetic field is applied to the reactive mixture during the cross-linking process, particlechains form and become locked into the elastomer. The resulting composites exhibit anisotropic properties.Combination of magnetic and elastic properties leads to a number of striking phenomena that areexhibited in response to impressed magnetic fields. The magnetic particles couple the shape and the elasticmodulus with the external magnetic field. Giant deformational effects, high elasticity, anisotropic elasticand swelling properties, and quick response to magnetic fields open new opportunities for using such materialsfor various applications. Since the magnetic fields are convenient stimuli from the point of signal control,the magnetoelasts are promising smart materials in engineering due to their real-time controllable elasticproperties.More recently, increasing interest has been devoted to exploration of multiresponsive magnetic polymers,which exhibit sensitivity to several external stimuli. Micro- and nanospheres that combine both magnetic,temperature, and pH sensitivity were also elaborated and studied. These new results provide novel possibilitiesfor preparation of more complex magnetic field-responsive materials like membranes with on/off switchingcontrol.In this article, we review recent advances in mechanical and swelling behavior of magnetic field-responsivesoft materials, including flexible polymer networks and gels.Anisotropic elastomersFerrogelsMagnetic compositesMagnetic latexesStress–strain dependenceTemporary reinforcementVibration and shock absorber

Fabrication and characterization of cobalt ferrite magnetic hydrogel combined with static magnetic field as a potential bio-composite for bone tissue engineering

Magnetorheological (MR) fluids are suspensions of magnetizable micrometer particles in a liquid. The rheological properties of the fluids can be changed rapidly, reversibly and repeatedly from that of a liquid to that of a solid when an external magnetic field is applied. This tunability of the fluids comes from the microscopic structure of the particles. Different microscopic structures are observed, quantified and correlated with rheological properties. The yield stress and viscosity of MR fluids depends on the particle size and volume fraction, sample cell geometry, magnetic field strength and application rate. The application of MR fluids in cancer therapy is discussed. In-vitro experiments show the blood flow can be blocked leading to a tumor by injecting a dilute MR fluid in a blood vessel and placing a magnet on top of the tumor. Without blood supply, tumor necrosis occurs shortly.

Magnetic particles have been being adopted in various areas, ranging from engineering to biomedical fields such as magnetorheological fluids, separation of magnetically tagged DNA, drug delivery and identification of biological species. The particles in a fluid domain can be controlled by an external magnetic field. The magnetized particles also interact between themselves, revealing interesting characteristics. Many theoretical and computational studies have been presented to analyze these characteristics. However, since most of them are based on the point-dipole model, the mutual interaction between the particles has not been taken into account with a high degree of accuracy. In this paper, we propose a new analysis method for particle interaction based on full magnetic field calculation. The particle system is modeled using the finite element method for magnetic field analysis. This is coupled with the level set method that can effectively capture moving geometry of ferromagnetic particles. Numerical results of two test problems of particle chaining and magnetic separation validate this approach for the system dynamics of magnetic particles.