L. Letamendia

Dynamic Light Scattering Study of Concentrated W/O Microemulsions

Light beating spectroscopy experiments were performed on concentrated W/O microemulsions located in the vicinity of the gel phase on the phase diagram. New results are reported: the most interesting feature is the non-exponential behaviour of the self-correlation function g (1)(K, τ) of the scattered field. The initial decay rate of g (1)(K, τ) is not directly proportional to K 2; whereas the long time decay is found fairly exponential.

Relaxation phenomena in AOT-water-decane critical and dense microemulsions

We report on extensive measurements of the low and high frequencies sound velocity and sound absorption in AOT-water-decane microemulsions deduced from ultrasonic and, for the first time as far as the absorption is concerned, from Brillouin scattering experiments. New experimental results on dielectric relaxation are also reported. Our results, which include data taken for critical as well as dense microemulsions, show new interesting relaxation phenomena. The relaxation frequencies deduced from very high frequency acoustical measurements are in good agreement with new high frequency dielectric relaxation measurements. We show that along the critical isochore, sound dispersion, relaxation frequency, and static dielectric permittivity can be accurately fitted to power laws. The absolute values of the new exponents we derived from experimental data are nearly equal, and they are very close to β=0.33 characterising the shape of the coexistence curve. The exponent characterising the infinite frequency permittivity is very close to 0.04 relevant to the diverging shear viscosity. For dense microemulsions, two well defined relaxation domains have been identified and the temperature variations of the sound absorption and the zero frequency dielectric permittivity bear striking similarities. We also show that the relaxation frequency of the slow relaxation process is almost independent of temperature and volume fraction and so cannot be attributed to percolation phenomena, whereas it can more likely be attributed to an intrinsic relaxation process probably connected to membrane fluctuations.

Low frequency depolarized light scattering in the VH geometry from quinoline

Low frequency depolarized light scattering spectra in the VH geometry are measured in liquid and supercooled quinoline from 37 to −35 °C and compared with the theory of Gershon and Oppenheim. The spectra are separated into contributions of different modes which can be dissipative and/or vibrating according to Gershon and Oppenheim’s results, and it is shown that a central dip does not necessarily mean dissipative modes, e.g., at 2 °C. Shear viscosities deduced from the spectra are close to values measured macroscopically. Orientational relaxation times obtained are exponential vs 1/T over the temperature range measured, and the coupling parameter between the orientational and the linear momentum densities is about proportional to the viscosity width.

VH spectra from liquid and supercooled salol

Abstract VH spectra of light scattered by salol have been measured over a wide temperature range. A hydrodynamic model using two relaxing processes fits accurately the line shapes in the full viscosity range.

Flow Properties of Multilamellar Droplets in AOT/Brine/Glycerol Mixtures

The properties of lamellar solutions formed in a quaternary mixture made of brine, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and glycerol have been studied under shear flow. As in AOT, brine pseudo-ternary system, the lamellar solutions exhibit a structural transition above a threshold shear rate, and form quasi-monodispersed multilamellar droplets. In this regime, the solutions are shear thinning. Both the viscosity and the droplet size scale with the shear rate with exponents respectively equal to -0.7 and -0.4. This last value is smaller than the one, 0.5, reported in the ternary systems. Tentative explanations of our experimental results are given.

Local motions of polystyrene chain in semi-concentrated polymer solutions

Abstract In the 1980s, the mechanism of local motions of polymer chain was discussed theoretically and experimentally by many researchers. Finally, the intermittent motion model was proposed with respect to the measurements of emission anisotropy decay curves of a chromophore incorporated into a polymer main chain in dilute polymer solutions. The relationship between orientational auto-correlation function type based on power law and measured decay curves was discussed in these reports. The local motion of anthryl groups end-capped polystyrene (APSA) was measured in various polystyrene/di-n-butyl phthalate mixtures by using the fluorescence depolarization method. The intermittent motion model for local motions of polymer segments is modified in order to apply to both dilute and semi-concentrated polymer solutions. The model predicts that the orientational auto-correlation function will be given by the KWW function when the segment–solvent collision is effective on the relaxation process. On the other hand, the hyperbolic function will be observed when the segment–segment collision becomes effective. It was found that measured decay curves in dilute and concentrated polymer solutions were well represented by the KWW function and the hyperbolic function, respectively. The result implies the fact that modified intermittent motion model is in good agreement with the experimental data.

Some thermodynamic properties of liquid xenon

Abstract Using new sound velocity measurements and accurate PVT data, we have calculated the thermodynamical properties of liquid xenon from the triple point to fifteen degrees below the critical temperature. Within the experimental errors, the calculated quantities agree with the theoretical values and allow us to test the significant structure theory.

Comment on light scattering studies of disparate mass gas mixtures: Helium-Xenon

In a recent paper (Physica A 141 (1987) 211), H.M. Schaink and G.H. Wegdam give Rayleigh-Brillouin measurements on Xe-He mixtures at moderate pressures. This comment is to give some clarifications about (a) remarks given in that paper about a previous paper partially devoted to the same mixture (L. Letamendia, J.P. Chabrat, G. Nouchi, C. Vaucamps, J. Rouch and S.-H. Chen, Phys. Rev. A 24 (1981) 1574) and (b) the situation of their new results in regard to the conclusions of previous studies.

Critical perturbation of Xe dynamics at high pressure

We present xenon dynamics experimental results obtained by light scattering techniques along the isotherm T 1 = 293.26 K for pressures up to 130. atm. We find two differents behaviours for the fluids properties. The sound speed and the Landau-Placzek intensity ratio follows the hydrodynamic theory of Mountain provided that the numerical simulation were effectued with reliable thermodynamic data. The sound absorption and the thermal relaxation are perturbated by the proximity of the critical point in a pressure range 40 atm < P < 90 atm but outside this domain that dynamics follows the hydrodynamical model.