Dispersions of magnetic nanoparticles in the mixture ethyleneglycol-choline chloride: The role of solvent association

Effect of an excess of surfactant on thermophoresis, mass diffusion and viscosity in an oily surfactant-stabilized ferrofluid

The effect of an excess of surfactant on the thermophoresis of a sterically stabilized ferrofluid is investigated experimentally by forced Rayleigh scattering (FRS). The experiments are performed with a stable magnetic fluid sample to which controlled amounts of surfactant are added. A decrease in the thermally induced transport of magnetic nanoparticles is observed while increasing the temperature T. The positive Soret coefficient [Formula: see text] decreases by adding 2 vol% of surfactant at room temperature. As shown by FRS relaxation, this decreasing is mainly associated with a reduction of the interaction between the carrier fluid and individual nanoparticles. No significant effect of extra surfactant on the sign of [Formula: see text] is observed at higher T's (up to [Formula: see text]C). Dynamic light scattering at room temperature reveals the presence of a small amount of clusters/aggregates in the samples, which are hardly detectable by FRS relaxation. The presence of these small clusters/aggregates is confirmed by a rheological probing of the fluid properties. Whatever T, a small amount of added surfactant first causes a decrease of the ferrofluid viscosity, associated with a 10% decreasing of the flow activation energy. Further on, viscosity and activation energy both recover at higher excess surfactant concentrations. These results are analyzed in terms of saturation of the surfactant layer, concentration of free surfactant chains and heat of transport of the nanoparticles.

Free ions in kerosene-based ferrofluid detected by impedance spectroscopy

The influence of the free ions on the electric response of cells filled with kerosene-based ferrofluids in the low-frequency region is explored. The experimental investigations have been performed on cells limited by different types of electrodes, with the same kind of ferrofluid, by means of the impedance spectroscopy technique. The electrodes considered in our study are made of titanium, platinum, gold, brass and surgical steel. The analysis of the spectra of the real and imaginary parts of the electric impedance of the cell data has been done by means of a simplified version of the Poisson-Nernst-Planck model, in which only the carriers of a given sign are mobile. The agreement between the theoretical predictions and the experimental data is rather good on the whole frequency range. From the analysis of the data in the low-frequency range, dominated by the properties of the electrodes, we discovered that only gold electrodes behave in a manner different from the other electrodes. From the best fit of the experimental data the free-ions density is determined as well as their diffusion coefficient in kerosene. The estimated dielectric constant of the kerosene is in good agreement with the values reported in the literature. In the framework of our model, the surface conductivity of the electrodes has been also determined.