Elmars Blums

Soret coefficient of nanoparticles in ferrofluids in the presence of a magnetic field

Experiments on a nonstationary separation of nanometer-sized Mn0.5Zn0.5Fe2O particles of hydrocarbon-based ferrocolloids in a flat vertical thermal diffusion column are performed. By using a modified separation theory which accounts for a one-dimensional mixed (thermal and concentration) convection in the column, the Soret coefficient of lyophilized nanoparticles from the separation curves are calculated. It is shown that in a zero magnetic field particles are transferring toward decreasing temperatures. The thermal diffusion ratio αT reaches a value αT≈+20. A significant influence of a uniform magnetic field B on particle separation is observed. If B is oriented along the temperature gradient ∇T, a strong decrease in thermal diffusion coefficient takes place whereas the transversal field B⊥∇T causes an intensification of particle thermophoretic transfer. Both effects qualitatively well agree with theoretical predictions based on a hydrodynamic theory of particle thermomagnetophoretic motion.

Thermal diffusion of magnetic nanoparticles in ferrocolloids: Experiments on particle separation in vertical columns

Abstract Experiments on nonstationary separation of nanometer-sized Fe 3 O 4 particles of hydrocarbon-based ferrocolloids in a flat vertical thermal diffusion column are performed. By using a modified separation theory which accounts for an one-dimensional mixed (thermal and concentration) convection in the column, the Soret coefficient of magnetic nanoparticles are calculated. It is shown that particles are transferred in the direction of decreasing temperature. The thermal diffusion ratio α T for magnetite particles suspended in tetradecane reaches the value α T ≈ − 20. Neithera dependence of α T on temperature nor any changes in particle-size distribution curves during the separation are observed.

Heat and mass transfer phenomena

This section deals with main problems of the heat and mass transfer in magnetic colloids. The analysis is mainly based on the general model given in the Chapter written by R. E. Rosensweig. Hydrodynamic and thermal problems are simplified considering incompressible liquids and neglecting the effects of polarization and electric conductivity as well as ignoring some other secondary effects that usually can be neglected in ferrofluid experiments. Contrarily, the analysis of mass transfer accounts for new sedimentation phenomena and cross effects of interrelated heat and mass transfer. Since the description given by Rosensweig is of general theoretical nature, while the present work mainly focusses on experimental problems, the various equations needed will not be cited from Rosensweig’s article but from the original literature since the transfer from the general model to the experimentally needed relations is often cumbersome and would exceed the frame of this presentation.

Ultrasonic cavitation induced water in vegetable oil emulsion droplets – A simple and easy technique to synthesize manganese zinc ferrite nanocrystals with improved magnetization

In the present investigation, synthesis of manganese zinc ferrite (Mn(0.5)Zn(0.5)Fe(2)O(4)) nanoparticles with narrow size distribution have been prepared using ultrasound assisted emulsion (consisting of rapeseed oil as an oil phase and aqueous solution of Mn(2+), Zn(2+) and Fe(2+) acetates) and evaporation processes. The as-prepared ferrite was nanocrystalline. In order to remove the small amount of oil present on the surface of the ferrite, it was subjected to heat treatment at 300 °C for 3h. Both the as-prepared and heat treated ferrites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM) and energy dispersion X-ray spectroscopy (EDS) techniques. As-prepared ferrite is of 20 nm, whereas the heat treated ferrite shows the size of 33 nm. In addition, magnetic properties of the as-prepared as well as the heat treated ferrites have also been carried out and the results of which show that the spontaneous magnetization (σ(s)) of the heat treated sample (24.1 emu/g) is significantly higher than that of the as-synthesized sample (1.81 emu/g). The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods; (b) usage of necessary additive components (stabilizers or surfactants, precipitants) and (c) calcination requirements. In addition, rapeseed oil as an oil phase has been used for the first time, replacing the toxic and troublesome organic nonpolar solvents. As a whole, this simple straightforward sonochemical approach results in more phase pure system with improved magnetization.

Thermodiffusion in magnetic fluids

Investigations were made to determine the Soret coefficient of magnetic particles in a ferrofluid under the influence of a magnetic field. This so-called magnetic Soret effect was theoretically predicted to be two to three orders of magnitude smaller than the conventional Soret effect. In contrast, former experiments have qualitatively shown that the magnetic Soret effect is much higher than the theoretical predictions. However, in those experiments the influence of buoyancy and magnetic driven convection disturbed the measurement significantly. Thus, it is still an open question how strong the magnetic Soret effect can be. Therefore, an experimental setup was developed which minimizes parasitic effects, simplifying the analysis of the experimental results. These results provide quantitative measures of the magnetic field dependence of the Soret effect in suspensions of magnetic nanoparticles. It is shown that the magnetic Soret effect can even be higher than the conventional one and that its strength as well as its direction depend on the magnetic field strength and its relative alignment to the temperature gradient in the fluid.

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 coeAcient ST of colloidal particles in ferrofluids from measurements of an unsteady particle separation in a flat thermodiAusion column. The column theory is modified taking into account MHD eAects of thermal convection aAected by a concentration buoyancy force. It is shown that the Hartmann eAect, 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 ST of surfacted particles in tetradecane based colloids are calculated. An anisotropy of the Soret eAect 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. # 2000 Elsevier Science Ltd. All rights reserved.

Mass transfer in nonisothermal ferrocolloids under the effect of a magnetic field

Abstract The paper presents a review of newest experimental results on particle thermal diffusion in hydrocarbon based ferrofluids. The measurements are performed using a vertical thermal diffusion column, the Soret coefficient is evaluated from particle separation curves employing a simple column theory which accounts for thermal convection in the vertical channel forced by a concentration buoyancy force arising during the thermophoretic transfer of heavy particles. High positive values of the Soret coefficient of lyophilized Fe 3 O 4 and Mn x Zn 1− x Fe 2 O 4 particles in surfacted tetradecane based colloids are measured. Experiments confirm the theoretically predicted anisotropy of particle thermophoretic mobility in the presence of a uniform magnetic field oriented parallel or perpendicular to the temperature gradient.

On the microconvective instability in optically induced gratings

In the present work, attention is paid to some significant effects (phenomena) in forced Rayleigh scattering (FRS) experiments with an applied magnetic field, ∇T∥H, which have not been analyzed before. In the work we deal with an idea that the experimentally observed effects are caused by microconvection [M. Igonin, Magnetohydrodynamics 37, 3 (2001); A. Cebers and M. Igonin, Magnetohydrodynamics 38, 265 (2002)]. The dimensionless numbers Cm and Rm, which are responsible for the onset of microconvection, are calculated from experiments as well as from the linear stability analysis in a Hele-Shaw approximation [M. Igonin, Magnetohydrodynamics 37, 3 (2001)]. The problem of the definition of the true values of diffusion and Soret coefficients from experiments and their field dependence, which are not affected by the assumed microconvection, is solved by original data processing. Visual observation of the FRS experiment provides additional information, applicable for a further analysis.

Magnetic Soret effect in a hydrocarbon based colloid containing surfacted Mn–Zn ferrite particles

Abstract The Soret effect has been investigated in a hydrocarbon based magnetic fluid containing surfacted Mn 0.5 Zn 0.5 Fe 2 O 4 nanoparticles with a magnetic volume concentration of 2.3%. The magnetic fluid fills up a vertical diffusion column which consists of a flat vertical channel between two walls and two reservoirs at the ends of the channel. One wall of the channel is being heated while the another one is cooled, maintaining a temperature gradient over the channels width that leads to separation of particles due to the Soret effect as well as the convective flow in the channel. The combination of the two mentioned effects brings up a measurable change of the concentration of magnetic particles in both reservoirs. The results have been obtained for uniform magnetic fields, aligned parallelly and normally to the temperature gradient respectively, by measuring the concentration of magnetic particles in the reservoirs. Obtained results are in good qualitative agreement with thermodiffusion column theory, published earlier (E. Blums, J. Magn. Magn. Mater. 149 (1995) 111).

Diffusive and thermodiffusive transfer of magnetic nanoparticles in porous media

Experimental results on mass transfer within a thin porous layer saturated with ferrofluid are outlined in this paper. From the analysis of particle concentration distribution across the layer it is shown that both the mass diffusion and the Soret coefficients of nanoparticles are remarkably less than those measured in free fluid. The particle transport coefficient changes due to an external uniform magnetic field qualitatively well agree with the predictions of existing theoretical research. The magnetic field that is oriented transversely to the porous layer causes an increase in the diffusion coefficient and a decrease in the Soret coefficient whilst the longitudinal field causes a reduction of the mass diffusion and an intensification of the particle thermodiffusion.Graphical abstract