Ionic Magnetic Fluids Research Articles

Zero-field birefringence in magnetic fluids: Temperature, particle size, and concentration dependence

The temperature, particle size, and particle concentration dependence of the zero-field birefringence were investigated using γ-Fe2O3 and MnFe2O4 ionic magnetic fluids in the range of 290–350 K. Upon heating a sample from below its characteristic temperature Tc, which depends upon the particle size and particle concentration, the zero-field birefringence goes critically down to zero. The experimental data are successfully explained when the following two points are considered in the model describing magnetic birefringence in magnetic fluids. First, dimers rather than monomers are responsible for the zero-field birefringence. Second, thermal disruption of the dimer structure follows a critical behavior. A theory for the zero-field birefringence is developed using classical statistics to evaluate the orientational order tensor component Szz. Finally, the zero birefringence signal at zero field, as found in magnetic fluids containing surfactant on the particle surface, is discussed within the dimer model.

Thermophoretic separation of ultrafine particles in ferrofluids in thermal diffusion column under the effect of an MHD convection

The aim of the paper is to evaluate the Soret coefficient S T of colloidal particles in ferrofluids from measurements of an unsteady particle separation in a flat thermodiffusion column. The column theory is modified taking into account MHD effects of thermal convection affected by a concentration buoyancy force. It is shown that the Hartmann effect, not only in hydrocarbon based colloids but also in ionic magnetic fluids of relatively high electric conductivity, does not influence significantly the particle separation dynamics. From measurements, positive values of S T of surfacted particles in tetradecane based colloids are calculated. An anisotropy of the Soret effect in the presence of a uniform magnetic field is experimentally established. The obtained results agree qualitatively well with the authors hydrodynamic theory of particle thermomagnetophoresis.

Time dependence of the magnetic grain concentration and secondary grain aggregation in ferronematic lyotropic liquid crystals subjected to magnetic field gradients.

The dynamical behavior of lyotropic nematic liquid crystals doped with ionic and surfacted magnetic fluids (ferronematics) is studied using a linear optical technique. The response of these mesophases to a combination of a static and a pulsed magnetic field is investigated by measuring the relaxation times as a function of the pulse width. A reversible modification of the magnetic grain concentration in the bulk of the samples and a secondary aggregation process due to the presence of a field gradient introduced by the pulsed field is discussed.

Characterization of field-induced needle-like structures in ionic and water-based magnetic fluids

An attempt has been made to characterize the field-induced needle-like structures (secondary cluster SC) in ionic and water-based magnetic fluids (MFs) using Rayleigh light scattering technique. This technique has the advantage of detecting objects smaller than the resolution limit of the optical column. From the observations, it was found that though water is the solvent in both ionic and water-based magnetic fluids, the cluster characteristics under zero and applied magnetic field, their growth and disintegration were different.

Biological effects of magnetic fluids: toxicity studies

Toxicity of ionic and citrate-based magnetic fluids administrated intraperitoneally to mice was investigated through cytogenetic analysis, evaluation of mitotic index and morphological and cytometric alterations. Both magnetic fluid samples cause severe inflammatory reactions, being very toxic and thus not biocompatible. Peritoneal cells and tissues studies may provide a useful strategy to investigate the in vivo biological effects of magnetic nanoparticles.

Raman study of ionic water-based copper and zinc ferrite magnetic fluids

Raman spectroscopy is used to investigate the OH-bending and OH-stretching modes of chemisorbed OH-groups in copper-ferrite and zinc-ferrite water-based magnetic fluid samples. Room-temperature Raman spectra were taken from diluted magnetic fluid samples and compared with the spectrum taken from liquid water. The suppression of the symmetric OH-stretching Raman modes from the ionic magnetic fluid sample spectra is discussed in terms of the replacement of a hydrogen atom from the water molecule by the nanoparticle surface. Moreover, changes on the area ratio between hydrogen bonded and nonhydrogen bonded OH-Raman peaks in the ionic magnetic fluid samples, as compared to the OH-Raman peaks from liquid water, are discussed in terms of changes on the hydrogen bond strength.

X-ray image analysis and ultramicroscope study of magnetic fluids used as working liquid in heat transfer experiments

Boiling characteristics of water-based and ionic magnetic fluid have been visualized using x-ray microfocus. During magnetic fluid boiling, large vapor bubbles nucleated around the vicinity of the heating surface and particles that came into contact with the heating element formed agglomerates large enough to be seen in real time x-ray images. This caused irreversible damage to the magnetic fluid heated to temperatures higher than 373 K. However, application of the magnetic field showed improvements in heat transfer characteristics and ionic magnetic fluid performed better over the water-based magnetic fluid. Ultramicroscope observations of ionic and water-based magnetic fluids heated only up to 353 K in vacuum for ten consecutive days and were found stable. However, ionic magnetic fluid heated in a vacuum glass cell at 373 K burst suggesting the accumulation of noncondense gas.

Toxic effects of ionic magnetic fluids in mice

Toxicity of ionic and tartrate-based magnetic fluids administered intraperitoneally to mice was investigated through morphological and cytometric alterations and cytogenetic analysis. Both magnetic fluids cause cellular death, mutagenicity and severe inflammatory reactions, being very toxic and thus not biocompatible. Peritoneal cell and tissue studies may provide a useful strategy to investigate the in vivo biological effects of magnetic nanoparticles.

Doped ionic magnetic fluids: A resonance characterization

Electron paramagnetic resonance is used to investigate ionic cobalt ferrite-based magnetic fluids. Two magnetic fluid samples having particles with different average radius of 6.8 nm and 7.8 nm were doped with Cu2+ at different concentrations. Changes on the resonance line shape of the Cu2+-ion in the aqueous phase was analyzed taking into account the particle polydispersity, the contribution of the dipolar field due to the surrounding magnetic nanoparticles, and the electrostatic interaction between the surface charged magnetic nanoparticles and the Cu2+-probe. The particle polydispersity parameters we found from the resonance line shape analysis are in excellent agreement with the parameters obtained from electron microscopy. The Cu2+ spatial distribution function around the magnetic nanoparticles gives very reasonable values for both, the particle–particle distance and the ionic layer thickness at the particle surface.

Surface-induced ordering in ionic and surfacted magnetic fluids

The wetting of a glass surface by ionic and surfacted ferrofluids is discussed. Optical experiments are used to investigate the existence of an ordering in a thin ferrofluid layer stabilized by the glass surfaces. The experimental results indicate that the ferrofluid wetting on a glass surface having an optical axis orients the magnetic grains perpendicularly to the glass optical axis.

Osmotic Equilibrium between an Ionic Magnetic Fluid and an Electrostatic Lamellar Phase

We describe the preparation of a “stiff” ferrosmectic. The domain of composition for which charged magnetic nanoparticles are included inside the aqueous layers of an Lα structure is narrow. This result is explained in terms of osmotic pressure equilibrium. Identification of biphasic regions allows the construction of the water corner of the phase diagram.

Preparation and properties of monodisperse magnetic fluids

A ‘size sorting’ method performed on ionic magnetic fluids, constituted by polydisperse anionic particles dispersed in water at pH 7 allows ‘monodisperse’ samples to be obtained. Here results on phase transitions observed in these samples with decreasing temperature, application of a magnetic field or increasing ionic strength are presented.

Ionic strength effect on magnetic fluids: A resonance study

Magnetic resonance data showing the ionic strength effect in ionic water-based magnetic fluids is reported for the first time. Changes in the effective radii of the suspended particles is considered as the main source for magnetic resonance changes. The effective radius is analyzed as a function of the ionic strength taking into account the electrostatic screening effect. A simple particle charge-discharge mechanism due to changes in ionic strength of the solution is proposed.