A. Bourdon

Investigation of the sign of the Soret coefficient in different ionic and surfacted magnetic colloids using forced Rayleigh scattering and single-beam Z -scan techniques

The sign of the Soret coefficient S T of a large set of ionic magnetic colloids (ionic ferrofluids (IFFs)) and surfacted magnetic colloids (surfacted ferrofluids (SFFs)) is determined using forced Rayleigh scattering and the single-beam Z -scan techniques. Different samples were investigated: acid and alkaline colloids with different values of pH; colloids with different concentrations of magnetic grains; colloids with grains of different typical diameters; colloids with magnetic grains with different coating natures; colloids with different non-polar and polar liquid carriers. Our results indicate that the sign of S T depends on the sign of the surface charge of grains in IFFs. In the case of water-based SFFs, the thermodiffusive behaviour is opposite to that of IFFs; that is, grains coated with a cationic surfactant behave like negatively charged IFF (alkaline) grains and grains coated with an anionic surfactant behave like positively charged IFF (acid) grains. SFFs with grains coated with non-ionic surfactants dispersed in non-polar fluid carriers behave like SFFs with grains coated with a cationic surfactant. The nature of the liquid carrier itself is not the only determinant factor, except apparently in the case of non-polar fluids, where only S T > 0 is found. These results cannot be explained with the available theories and it is highly probable that different mechanisms are present in the thermodiffusive behaviour of these complex fluids.

Diffusion and thermodiffusion studies in ferrofluids with a new two-dimensional forced rayleigh-scattering technique

In this paper, we present a new simple two-dimensional forced Rayleigh-scattering (FRS) experimental set-up for determination of the nanoparticle-diffusion coefficient (DM) and the Soret coefficient (ST) in colloids. For this purpose, we give a two-timescale model for the evolutions of temperature and colloid concentration (similar to that given for a former one-dimensional FRS method) and a complete description of the signals diffracted by a squared-lattice grating. Both transport properties in ferrofluids (magnetic colloids) determined with this new set-up are in good agreement with those found with samples already studied using the one-dimensional technique. This work is completed by studying new samples. Experimental results we obtained confirm and make clearer the following: (i) the strong Soret effect in ferrofluids has a nanoparticle origin and (ii) furthermore, this origin lies in the immediate surroundings of the nanoparticles (ionic or surfacted coating and dispersion liquid).

Wetting of glass surfaces by ionic magnetic fluids: Effect of the concentration and the pH of the solution and of the size of the magnetic grains on a surface stabilized birefringent layer

The wetting of glass surfaces by ionic magnetic fluids (ferrofluids) is studied, using an improved birefringence technique, as a function of the concentration and size of the colloidal grains, and the pH of the solution. The problem is considered as a phase transition induced by the surface field on a thin ferrofluid layer which is in contact with the glass walls. The induced optical phase shift increases with the grain concentration. The penetration length in the bulk, induced by the ordered surface ferrofluid layer, calculated assuming that the order parameter at the surface is independent of grain concentration, is of the order of 2–4 grain diameters. The grain size, in the range from 2.5 to 9.5 nm, and the solution pH do not seem to be relevant parameters for the induced phase shift.

Surface stabilized nematic phase in a magnetic fluid

Abstract The wetting of glass surfaces by magnetic fluids (ferrofluids) is treated theoretically as a wall-induced orientational nematic ordering. The nematic coherence length of the surface stabilized ordered phase is assumed to be about a few magnetic grain sizes. In this framework the surface order parameter is evaluated to 0.3.

Experimental determination of the Soret coefficient of ionic ferrofluids Influence of the volume fraction and the ionic strength

Abstract Forced Rayleigh scattering (FRS) is a powerful technique for investigating heat and mass transfers in colloids. In the present work, we determine the Soret coefficient S T and the thermal diffusion coefficient D T of magnetic colloids (ferrofluids). It has been theoretically predicted that the thermal diffusion coefficient D T of colloids depends both on the particle–solvent interfacial interaction and on the interactions between the colloidal particles. In order to understand the microscopic behavior of the Soret effect in these ionic magnetic colloids, experiments are performed on aqueous samples of various volume fractions Φ and ionic strengths. The dominant effect on the Soret coefficient comes from the particle–solvent interaction and determines its sign. Interparticle interactions have an influence on S T in the moderate concentration range where virial-like expansions are possible. In this range (Φ ≤ 0.10) and within the experimental error bars, the thermal diffusion does not depend on the ionic strength of the dispersion, and the Φ-dependence of the friction coefficient is comparable to that of hard spheres. At larger concentrations, the thermal diffusion drastically decreases as the colloid approaches its dynamical glass transition.

Water dynamics in ionic magnetic colloids studied by 1H nuclear magnetic resonance

Abstract In a previous nuclear magnetic resonance (NMR) study we observed that the NMR spectra of water in both surfacted and ionic ferrofluids are asymmetric and several orders of magnitude wider than the one of pure water. It has been proposed that this effect is produced by extremely strong magnetic field gradients in the intergrain volume and/or by surface interactions between the carrier liquid molecules and the grains surface. In the case of aqueous ionic ferrofluids the latter possibility should be interpreted as electric interactions between water (polar) molecules and the charges in the grain surface. In this work we study a series of ionic and surfacted ferrofluids prepared at different magnetic grain concentrations and sizes, and with different surface charge densities. Our experiments clearly show that the sign and the density of the electric charge on the magnetic grains have no influence on NMR spectra. On the other hand, spectral widths increase with the magnetic grain concentration.