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.

Self-assembly and rheology of dipolar colloids in simple shear studied using multi-particle collision dynamics.

Magnetic nanoparticles in a colloidal solution self-assemble in various aligned structures, which has a profound influence on the flow behavior. However, the precise role of the microstructure in the development of the rheological response has not been reliably quantified. We investigate the self-assembly of dipolar colloids in simple shear using hybrid molecular dynamics and multi-particle collision dynamics simulations with explicit coarse-grained hydrodynamics, conduct simulated rheometric studies and apply micromechanical models to produce master curves, showing evidence of the universality of the structural behavior governed by the competition between the bonding (dipolar) and erosive (thermal and/or hydrodynamic) stresses. The simulations display viscosity changes across several orders of magnitude in fair quantitative agreement with various literature sources, substantiating the universality of the approach, which seems to apply generally across vastly different length scales and a broad range of physical systems.

Numerical investigation of thermo-magneto-solutal flow of ferrocolloid through ordered and disordered permeable membranes.

We explore the mechanism of ferroparticle transfer in porous structures in the conditions of simultaneous action of the thermal gradient and the magnetic field. We show that when a ferrocolloid saturated porous matrix is placed in a homogeneous magnetic field the grains of the porous frame notably distort the uniformity of the internal field by creating sharp gradients in the vicinity of the interface. On the other hand, the application of the temperature gradient creates an imbalance of the ferroparticle concentration in the bulk of the porous structure due to colloidal thermophoresis. The combination of the imbalance of concentration of the magnetic nanoparticles and the internal gradients of the magnetic field creates a magnetic force and convective flow of solution through the porous structure. We report the results of the pore-scale numerical simulations of the ferrocolloid thermo-magneto-solutal flow in geometrically simple ordered and disordered permeable structures and membranes with different porosity.

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.

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.

Magnetically driven microconvective instability of optically induced concentration grating in ferrofluids.

In this paper, we consider a concentration grating of magnetic nanoparticles optically induced by thermodiffusion in a layer of ferrofluid in the presence of the external homogeneous magnetic field. The applied field is directed along the concentration gradient and leads to the appearance of the internal nonhomogeneous demagnetizing fields. When the system reaches equilibrium, the optical pumping is switched off, and the grating is allowed to relax. We carry out a stability analysis using the Galerkin approach and numerical simulations of the full system of equations to determine the growth rates and the mode amplitudes of the hydrodynamic and concentration perturbations during the relaxation stage.

Relaxation mechanisms of photoinduced periodic microstructures in ferrofluid layers.

We consider theoretically and numerically a periodic concentration grating induced in a layer of ferrofluid in the presence of the external magnetic field by nonuniform optical heating through photoabsorption. The stationary profiles of the periodic microstructures are governed by the equilibrium of the diffusive, thermodiffusive, and magnetic fluxes. The anisotropy of the diffusion coefficient and the magnetically driven microconvection contribute to the relaxation of these structures. The temperature-concentration coupling is shown to increase the initial effective diffusive relaxation rate by up to 50%. Microconvection dominates in the relaxation process even at small values of the control parameter and rapidly destroys the periodic part of the concentration grating. We describe this process in the weakly nonlinear regime by an approximate Galerkin model.