Comparison of theories of anisotropy in transformer oil-based magnetic fluids

The external magnetic field in transformer oil-based magnetic fluids leads to the aggregation of magnetic nanoparticles and formation of clusters. These aggregations are the result of the interaction between the external magnetic field and the magnetic moments of the nanoparticles occurs. However, the temperature of magnetic fluids has also very important influence on the structural changes because the mechanism of thermal motion acts against the cluster creation. The acoustic spectroscopy was used to study the anisotropy of transformer oil-based magnetic fluids upon the effect of an external magnetic field and temperature. In present the anisotropy of the magnetic fluids can be described by two theories. Taketomi theory assumes the existence of spherical clusters. These clusters form long chains, aligned in a magnetic field direction. Shliomis in his theory supposed that only nanoparticles formed chains. A comparison of the experimental results with the predictions of the Taketomi theory allowed a determination of the cluster radius and the number density of the colloidal particles. The proportions of the acoustic wave energy used for excitation of the translational and rotational motion were determined

Clustering in ferronematics : The effect of magnetic collective ordering

Clustering of magnetic nanoparticles can dramatically change their collective magnetic properties, and it consequently may influence their performance in biomedical and technological applications. Owing to tailored surface modification of magnetic particles such composites represent stable systems. Here, we report ferronematic mixtures that contain anisotropic clusters of mesogen-hybridized cobalt ferrite nanoparticles dispersed in liquid crystal host studied by different experimental methods—magnetization measurements, small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and capacitance measurements. These measurements reveal non-monotonic dependencies of magnetization curves and the Fréedericksz transition on the magnetic nanoparticles concentration. This can be explained by the formation of clusters, whose structures were determined by SAXS measurements. Complementary to the magnetization measurements, SANS measurements of the samples were performed for different magnetic field strengths to obtain information on the orientation of the liquid crystal molecules. We demonstrated that such hybrid materials offer new avenues for tunable materials

Fréedericksz Transitions in 6CB Based Ferronematics—Effect of Magnetic Nanoparticles Size and Concentration

In this paper, results acquired from capacitance measurements performed on composites based on nematic liquid crystal 4-cyano-4′-hexylbiphenyl (6CB) and spherical iron oxide nanoparticles of various sizes are presented. Electric and magnetic Fréedericksz transitions, as well as structural transitions in combined electric and magnetic fields, were investigated. The obtained results showed the lowering of the threshold magnetic field with an increase in the volume concentration of nanoparticles. Estimations based on results obtained from measurements suggest soft anchoring between liquid crystal director and nanoparticles magnetization vector

Influence of X7GeS5I (X = Ag, Cu) Superionic Nanoparticles on Structural Changes in Nematic Liquid Crystal

The effect of two types of superionic nanoparticles; Cu7GeS5I and Ag7GeS5I, respectively on nematic liquid crystal (6CB) behavior under an external electric field is investigated. The response of both attenuation of surface acoustic waves propagating along with the substrate/liquid crystal interface and light transmission are used to study the structural changes induced by applied electric field. The increasing/decreasing regime as well as jumped change of applied field were used. The light transmission was investigated using linearly polarized laser beam (532 nm) propagating through the liquid crystal. Results obtained from both measurements for three different concentrations (0.01, 0.05 and 0.10 wt%) and in addition two different sizes of nanoparticles are compared and results, relying on structural changes, suspension stability and switching behavior are evaluated

The prebreakdown state of magnetic fluids at influence of electrical and magnetic field

The development of electric breakdown in magnetic fluids (MFs) has been analyzed. MFs have been consisted of magnetic particles (Fe3O4) of nanometric size, coated with oleic acid as a surfactant, dispersed in transformer oil. The electro-physical processes, which appear at action of the DC electric field and constant magnetic field on MFs, were observed. These processes have effect on electric breakdown. The especial attention was devoted to state in MFs before breakdown, during breakdown and post breakdown

Effect of Liquid Crystalline Host on Structural Changes in Magnetosomes Based Ferronematics

The effect of the liquid crystalline host on structural changes in magnetosomes based on ferronematics is studied using the surface acoustic wave (SAW) technique supported by some capacitance and light transmission measurements. The measurement of the attenuation response of SAW propagating along the interface between LC and the piezoelectric substrate is used to study processes of structural changes under magnetic field. The magnetosome nanoparticles of the same volume concentration were added to three different nematic LCs, 5CB, 6CB, and E7. Unlike to undoped LCs, the different responses of SAW attenuation under the influence of magnetic and electric fields in LCs doped with magnetosomes were observed due to characteristic structural changes. The decrease of the threshold field for doped LCs as compared with pure LCs and slight effects on structural changes were registered. The threshold magnetic fields of LCs and composites were determined from capacitance measurements, and the slight shift to lower values was registered for doped LCs. The shift of nematic-isotropic transition was registered from dependencies of SAW attenuation on temperature. The acoustic anisotropy measurement approved the previous supposition about the role of bulk viscosity in used SAW measurements. In addition, capacitance and light transmition investigations supported SAW results and pointed out conclusions about their magnetic field behavior. Obtained results are discussed and confronted with previous ones and coincide well with those observed using acoustic, optical, or dielectric techniques

Influence of Goethite Nanorods on Structural Changes and Transitions in Nematic Liquid Crystal E7

A composite ferronematic system based on the nematic liquid crystal E7, doped with lath-like goethite magnetic nanoparticles of volume concentrations 10−3, 5 × 10−4, and 10−5, was investigated. Both surface acoustic waves (SAWs) and the magneto-optical effect were used to study the influence of magnetic nanoparticles on ferronematic liquid crystals’ structural changes, focused above all on structural transitions. The responses of SAW attenuation and light transmission to external magnetic fields were investigated experimentally under linearly increasing/decreasing or jumped (time influence) magnetic fields, respectively. An investigation of temperature on structural changes was performed, as well. The experimental results validated the decrease in the threshold field of the ferronematic composites in comparison with the pure E7, as well as an increase in the transition temperature with the increasing volume fraction of nanoparticles. The effect of the nanoparticles’ concentration on both total structural changes and residual attenuations at the vanishing magnetic field was also registered. The light transmission measurements confirmed the effect of the concentration of goethite nanoparticles on the resultant magneto-optical behavior, concerning both its stability and switching time

Analýza I(t) závislosti počas štrukturalizácie magnetických nanočastíc v magnetickej kvapaline

The previous experiments were devoted to observation of structuralization processes in magnetic fluids in dependence on time at constant value of homogenous magnetic fields. The average value of clusters of magnetite particles reached stabilized length that depended on volume concentration of magnetite particles in magnetic fluids. This work is oriented on observation and explanation of anomaly on dependences of current intensity (I) on time (t) in magnetic fluids with help of observation of structuralization processes results at respecting of space charge. This anomaly was detected at upper concentration of magnetite particles (>1%) on I(t) dependences in given time region that characterizes the change of energetic conditions in magnetic fluids