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International audience; Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide - EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at Φ = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m-1) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.

We focus here on the relationship between the physicochemical properties of electrostatically stabilized colloidal dispersions and the nanoparticles/solvent interface. Dispersions of maghemite (γ-Fe2O3) in polar solvents, here water and dimethyl sulfoxide (DMSO), are prepared with a new process that enables tuning easily this interface. Departing from the point of zero charge (PZC), the nanoparticles (NPs) are charged in a controlled way by adding acid or base. This pathway enables control of the surface state of the nanoparticles, i.e., the NP’s charge and the nature of the counterions, as well as the amount of free electrolyte in the dispersion. Stable dispersions are obtained because of electrostatic repulsion, in water and in DMSO, with electrolyte concentrations up to 20–40 mM. Small angle X-ray scattering (SAXS) and dynamic light scattering (DLS) techniques are here applied to concentrated dispersions in order to understand the nanostructure and quantify the interparticle interactions. Specific ioni...

The influence of the magnetic field on the Seebeck coefficient (Se) was investigated in dilute magnetic nanofluids (ferrofluids) composed of maghemite magnetic nanoparticles dispersed in dimethyl-sulfoxide (DMSO). A 25% increase in the Se value was found when the external magnetic field was applied perpendicularly to the temperature gradient, reminiscent of an increase in the Soret coefficient (ST, concentration gradient) observed in the same fluids. In-depth analysis of experimental data, however, revealed that different mechanisms are responsible for the observed magneto-thermoelectric and -thermodiffusive phenomena. Possible physical and physico-chemical origins leading to the enhancement of the fluids' Seebeck coefficient are discussed.

Under a temperature gradient, the direction of thermodiffusion of charged γ-Fe2O3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either a positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient ST is measured in stationary conditions together with the mass-diffusion coefficient Dm using a forced Rayleigh scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of Dm with NP volume fraction Φ. The Φ-dependence of ST is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transfer ŝNP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively, with ST0 and with ST0 are then probed under an applied field H[combining right harpoon above], and an anisotropy of Dm and of ST is induced while the in-field system remains monophasic. Whatever the H[combining right harpoon above]-direction (parallel or perpendicular to the gradients and ), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.

Thermodiffusion properties at room temperature of colloidal dispersions of hydroxyl-coated nanoparticles (NPs) are probed in water, in dimethyl sulfoxide (DMSO) and in mixtures of water and DMSO at various proportions of water, [Formula: see text]. In these polar solvents, the positive NPs superficial charge imparts the systems with a strong electrostatic interparticle repulsion, slightly decreasing from water to DMSO, which is here probed by Small Angle Neutron Scattering and Dynamic Light Scattering. However if submitted to a gradient of temperature, the NPs dispersed in water with ClO4- counterions present a thermophilic behavior, the same NPs dispersed in DMSO with the same counterions present a thermophobic behavior. Mass diffusion coefficient [Formula: see text] and Ludwig-Soret coefficient [Formula: see text] are measured as a function of NP volume fraction [Formula: see text] at various [Formula: see text]. The [Formula: see text]-dependence of [Formula: see text] is analyzed in terms of thermoelectric and thermophoretic contributions as a function of [Formula: see text]. Using two different models for evaluating the Eastman entropy of transfer of the co- and counterions in the mixtures, the single-particle thermophoretic contribution (the NP's Eastman entropy of transfer) is deduced. It is found to evolve from negative in water to positive in DMSO. It is close to zero on a large range of [Formula: see text] values, meaning that in this [Formula: see text]-range [Formula: see text] largely depends on the thermoelectric effect of free co- and counterions.

International audience; Under a temperature gradient, the direction of thermodiffusion of charged g Fe 2 O 3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient S T is measured in stationary conditions together with the mass-diffusion coefficient D m using a Forced Rayleigh Scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of D m with NP's volume fraction F. The F-dependence of S T is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transferŜ NP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively with S T > 0 and with S T < 0 are then probed under an applied fieldH, an anisotropy of D m and of S T is induced while the in-field system remains monophasic. Whatever theH-direction (parallel or perpendicular to the gradients-T and-F), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.

International audience; Thermodiffusion of different ferrite nanoparticles (NPs), ∼10 nm in diameter, is explored in tailor-made aqueous dispersions stabilized by electrostatic interparticle interactions. In the dispersions, electrosteric repulsion is the dominant force, which is tuned by an osmotic-stress technique, i.e. controlling of osmotic pressure Π, pH and ionic strength. It is then possible to map Π and the NPs’ osmotic compressibility χ in the dispersion with a Carnahan–Starling formalism of effective hard spheres (larger than the NPs’ core). The NPs are here dispersed with two different surface ionic species, either at pH ∼ 2 or 7, leading to a surface charge, either positive or negative. Their Ludwig–Soret $S_{T}$ coefficient together with their mass diffusion $D_{m}$ coefficient are determined experimentally by forced Rayleigh scattering. All probed NPs display a thermophilic behavior ($S_{T}$ < 0) regardless of the ionic species used to cover the surface. We determine the NPs’ Eastman entropy of transfer and the Seebeck (thermoelectric) contribution to the measured Ludwig–Soret coefficient in these ionic dispersions. The NPs’ Eastman entropy of transfer $Ŝ_{NP}$ is interpreted through the electrostatic and hydration contributions of the ionic shell surrounding the NPs.

The aim of the present study is to propose a new reproducible method for preparing colloidal dispersions of electrostatically charged nanoparticles (NPs) in polar solvents with different kinds of counter-ions. Maghemite NPs are here dispersed in solvents of different dielectric constant, namely water, dimethylsulfoxide (DMSO) and an ionic liquid, ethylammonium nitrate (EAN). If the existence of a NP superficial charge happens to be necessary for the colloidal stability of the dispersions in these three solvents, the standard DLVO theory cannot be used any more to describe the colloidal stability in EAN. The structure of the dispersions and the strength of the interparticle repulsion are investigated by small angle X-ray scattering measurements, in association with Ludwig–Soret coefficient determinations. Specificities, associated to the nature of the counter-ions are identified in this work on the colloidal stability, on the interparticle repulsion and on the Ludwig–Soret coefficient.

International audience; The Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle’s entropy of transfer as high as 82 meV K$_{−1}$. Such a property may be used to improve the thermoelectric coefficients in liquid thermocells.

Diffusion processes in a magnetic colloid are studied by a forced Rayleigh scattering technique under an applied static magnetic field. A periodic spatial modulation of the particle concentration (transient grating) is induced in the colloid with three different field geometries, B being either parallel or perpendicular to the grating direction. The value of the translational diffusion coefficient of the particles is given by the transient grating relaxation time. It depends on the magnetic field strength and on the field geometry. A theoretical model based on a mean field approximation taking magnetic interactions of particles under a field into account is given which agrees with experimental results.

A spatial modulation of colloidal concentration of nanoscopic magnetic particles is written in a ferrofluid solution inside the fringes of a forced Rayleigh scattering device. The relaxation time of this transient grating is inversely proportional to the square of the characteristic wave vector and thus to the cooperative diffusion coefficient of particles. If an external magnetic field is applied in the plane of the fringes and perpendicularly to these ones, the experimental diffusion coefficient increases, due to local magnetic field gradients in the spatially inhomogeneous magnetic medium.

Reported is on the inhibition of second harmonic generation in optical fibers by preirradiation at 532 nm. It is demonstrated that preirradiation at 532 nm induces a significant decrease of second harmonic generation efficiency, writing speed and time-integrated input energy of saturation. This phenomenon can be compared to the erasing process and is found to be in accordance with the predictions of the anisotropic photoinduced model.

Reported are experimental results concerning harmonic generation efficiency and speed of fiber preparation in erbium3+ -doped optical fibers of various ion concentrations. The maximum saturated value of the second-harmonic generation efficiency is found to be independent of pulse duration, pump peak intensity and doping of the fiber. The time-integrated input energy required to reach the saturated level is shown to vary significantly with pump peak power. Holographic grating build-up time is shown to be significantly reduced in Er-doped fibers. The results are compared to the spatially anisotropic ionisation model of Anderson et al.

We report experimental results concerning self-organised holographic grating length for second harmonic generation in optical fiber. By a cut-back method of the fiber and a comparison between different laser regimes we point out the limiting factors to its extension. The grating length is found to be independent of the pulse duration. We have established experimental evidence of the limitation of the grating length by self- and cross-phase modulations effects, spectral width of the pulses and central frequency fluctuation of the laser. With a stable Nd:YAG injected laser delivering transform-limited pulses we obtain for the first time to our knowledge, holographic grating lengths above 60 cm.

We report on the spectroscopic characterization of infrared (1064 nm) to uv-visible up-converted emission in multimode Tm3+-doped fluorozirconate fibers. Different excitation pathways were clearly observed for the up-converted fluorescence, which varied according to the incident pump power.

A dynamical freezing, which is the analogous of a glass transition, is observed at large concentrations in aqueous dispersions of maghemite nanoparticles. We study experimentally the structure and the dynamical properties of two dense ferroglass-formers in two very distinct states of interparticle interaction, either strongly repulsive or attractive. The static structure of the magnetic colloidal dispersions is probed by means of Small Angle X-ray Scattering both in and without the presence of an external magnetic field. Translational dynamics of the repulsive glassformer are investigated by X ray Photon Correlation Spectroscopy. Slow dynamics and aging properties, which both become anisotropic under an applied field, are here investigated.

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 ST and the thermal diffusion coefficient DT of magnetic colloids (ferrofluids). It has been theoretically predicted that the thermal diffusion coefficient DT 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 here from the particle/solvent interaction and determines its sign. Interparticle interactions have an influence on ST in the moderate concentration range where virial like expansions are possible. In this range (Φ ≤ 0.10) and within the experimental error bar, 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. KEYWORD: Ferrofluids, Forced Rayleigh Scattering, thermal diffusion, Soret effect, friction, dynamical arrest.

Experiments and Brownian dynamics simulations have been coupled in order to better understand structural and dynamical properties of ferrofluids, especially the role of the magnetic dipolar interaction. The ferrofluid used is a 'home-made' well defined suspension, the experimental characteristics of which are introduced in the modelled system. In this system, the determination of the structure using simulations and small angle neutron scattering (SANS) experiments shows no sign of chaining in the suspensions, both without and with a magnetic field. Nevertheless, on the scale of the interparticle distance, the structure is strongly anisotropic. This is at contrast with the weak anisotropy of the translational diffusion coefficient under magnetic field on the same scale. Moreover, on the macroscopic scale, both structure and translational dynamics are strongly anisotropic. Also the rotational diffusion is strongly modified if determined without field or after a weak orientation of the particles with a weak field. These results all emphasize the role of the collective phenomena associated with the dipolar interaction.

The structure and dynamics of concentrated aqueous magnetic fluids with tunable interparticle interaction are investigated by small angle neutron scattering and forced Rayleigh scattering. The structure factor and the collective diffusion coefficient are determined. An effect of the interparticle interaction on the diffusion coefficient is observed and related to thermodynamic data with a good agreement.

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).

International audience; The sign of the Soret coeff. ST of a large set of ionic magnetic colloids (ionic ferrofluids (IFFs)) and surfacted magnetic colloids (surfacted ferrofluids (SFFs)) is detd. using forced Rayleigh scattering and the single-beam Z-scan techniques. Different samples were investigated: acid and alk. colloids with different values of pH; colloids with different concns. of magnetic grains; colloids with grains of different typical diams.; colloids with magnetic grains with different coating natures; colloids with different non-polar and polar liq. carriers. Our results indicate that the sign of ST depends on the sign of the surface charge of grains in IFFs. In the case of water-based SFFs, the thermodiffusive behavior is opposite to that of IFFs; i.e., grains coated with a cationic surfactant behave like neg. charged IFF (alk.) grains and grains coated with an anionic surfactant behave like pos. 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 liq. carrier itself is not the only determinant factor, except apparently in the case of non-polar fluids, where only ST > 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 behavior of these complex fluids.

This paper shows how forced Rayleigh scattering can be used as an experimental tool for studying thermodiffusion (Soret effect). The systems investigated are magnetic colloids of different types. A framework including thermodiffusion and dielectrophoresis is described in which the evolutions of temperature and of colloid concentration are clearly distinguished. The framework is then shown to account for experiments on steady-state concentration gratings coupled with transient temperature ones, and the parameters are determined therefrom. Dielectrophoretic forces are found to be negligible. Studying different types of magnetic colloids with various dilution rates shows that the sign of the Soret effect is controlled by the nature of the particle coating made up of electrostatic charges or of surfactant, and that its mechanism is located at the nanoparticle core-solvent interface.

We show that an optical second harmonic (SH) is generated in a magnetic colloid if a static magnetic field which breaks the fluid isotropy is applied. We propose a statistical model in which all the magnetic nanoparticles are supposed to be identical with a nonzero complex second-order polarizability tensor bound to their magnetic moment. These grains align under a static magnetic field according to Boltzmann's statistics. The nonlinear second-order macroscopic electric susceptibility tensor, ruled by a Langevin-like model, is found to be zero without any applied magnetic field and to be an increasing function of its strength. The nonlinear susceptibility tensor of the colloidal solution exhibits an axial ${\mathit{C}}_{\mathrm{\ensuremath{\infty}}}$ (and not ${\mathit{C}}_{\mathrm{\ensuremath{\infty}}\mathit{v}}$) symmetry around the magnetic field. The measurements of the generated SH intensity as a function of the applied field strength, in all the independent input and output electric polarization directions, are in perfect agreement with our model and confirm the expected ${\mathit{C}}_{\mathrm{\ensuremath{\infty}}}$ symmetry. Under oblique incidence and without any applied magnetic field, the surface SH is generated by particles adsorbed on the glass cell walls and orientated normally to the interfaces. The complex values of the nonzero elements of the second-order polarizability tensor (leading to the observed SH wave ellipticity) are determined with a high precision, as well as that of the particle magnetic moment. The magnetic size found for the nanoparticle is in accordance with those given by other methods and could imply quantum confinement effects. A correlation between the ferrite particle atomic structure and the magnetic moment is found to be responsible of the noncentrosymmetry and of the chirality of the colloid under applied magnetic field. \textcopyright{} 1996 The American Physical Society.

The stable operation of a mode-locked dye laser synchronously pumped by the second harmonic of an Nd:YAG laser produced in an Nd codoped germanosilicate optical fiber is reported. The optical fiber preparation technique, which results in a second harmonic conversion efficiency of 2 percent, is described. This optical fiber SHG conversion efficiency is the highest reported to date using a continuous-wave mode-locked laser.

Ethylammonium nitrate (ionic liquid) based ferrofluids with citrate-coated nanoparticles and Na + counterions were synthesized for a wide range of nanoparticle (NP) volume fractions ( &Phi, ) of up to 16%. Detailed structural analyses on these fluids were performed using magneto-optical birefringence and small angle X-ray scattering (SAXS) methods. Furthermore, the thermophoretic and thermodiffusive properties (Soret coefficient S T and diffusion coefficient D m ) were explored by forced Rayleigh scattering experiments as a function of T and &Phi, They were compared to the thermoelectric potential (Seebeck coefficient, Se) properties induced in these fluids. The results were analyzed using a modified theoretical model on S T and Se adapted from an existing model developed for dispersions in more standard polar media which allows the determination of the Eastman entropy of transfer ( S ^ NP ) and the effective charge ( Z 0 e f f ) of the nanoparticles.