Photonic Hall Effect in ferrofluids: Theory and Experiments

An experimental and theoretical study on the Photonic Hall Effect (PHE) in liquid and gelled samples of ferrofluids is presented. The ferrofluids are aqueous colloidal suspensions of Fe(_{2})CoO(_{4}) particles, which can be considered as anisotropic and absorbing Rayleigh scatterers. The PHE is found to be produced by the orientation of the magnetic moments of the particles, as is also the case for the Faraday effect. The dependence of the PHE with respect to the concentration of the scatterers, the magnetic field and the polarization of the incident light is measured in liquid and in gelled samples and is compared to a simple model based on the use of a scattering matrix and the single scattering approximation.Comment: 20 pages, 11 figures, submitte

Kinetics of bond formation in crosslinked gelatin gels

In chemical crosslinking of gelatin solutions, two different time scales affect the kinetics of the gel formation in the experiments. We complement the experimental study with Monte Carlo numerical simulations of a lattice model. This approach shows that the two characteristic time scales are related to the formation of single bonds crosslinker-chain and of bridges between chains. In particular their ratio turns out to control the kinetics of the gel formation. We discuss the effect of the concentration of chains. Finally our results suggest that, by varying the probability of forming bridges as an independent parameter, one can finely tune the kinetics of the gelation via the ratio of the two characteristic times.Comment: 8 pages, 9 figures, revised versio

Gel to glass transformation in base catalysed silica

The effects of heat-treatments on silica xerogels prepared from base catalyzed hydrolysis and condensation of TMOS are studied by low frequency Raman scattering, scanning electron microscopy, small angle X-ray scattering and BET measurements. Raman scattering in the range 3-700 cm-1 reveals a low-frequency band that arises from the discrete particulate character of base-catalysed gels at very small scale. A continuous evolution of the structure inside particles is observed during the thermal treatment and the Raman spectra is similar to the fused silica one after treatment at 1050°C. Electron microscopy observations of the nanostructure of the xerogels correlate with the particle Raman band maximum. Complementary SAXS and BET measurements allows us to propose a model for the gel-to-glass transformation

TEXTURAL AND STRUCTURAL STUDIES OF AEROGELS BY RAMAN SCATTERING AND THERMOPOROMETRY

Les résultats expérimentaux obtenus par diffusion Raman basse fréquence et thermoporométrie sur la structure d'aérogels préparés en milieu basique sont présentés. Les dimensions des unités texturales -ou billes- et des mesopores sont déterminés. Les résultats sont en accord avec une structure fractale uniquement dans les unités texturales.Experimental results on the structure of base-catalysed aerogels obtained from low frequency Raman scattering and thermoporometry are presented. The size of textural units or beads and of mesopores are determined. These results are in agreement with a fractal structure only in the textural units

A SAXS STUDY OF SILICA AEROGELS

La diffusion des Rayons X aux petits angles et la microscopie électronique en transmission, sont utilisées pour étudier la fractalité d'aérogels de silice. On a montré l'existence dans des aérogels obtenus en milieu basique, d'unités structurales dont les dimensions dépendent de la densité de l'aérogel. L'extension du domaine fractal est limitée à la taille des particules constituant le squelette de l'aérogelX-ray small angle scattering (SAXS) and transmission electron microscopy (TEM) are used to characterise the fractal properties of base catalysed silical aerogels : observation of change over points in the X-rays scattering curves confirms the existence of structural units, like building blocks or beads forming the network. The fractal behavior exists only in the beads or building blocks