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10 pages, 2 tables, 6 figures., The formation of liposomes with low polydispersity index by application of ultrasounds was investigated considering methodology specifications such as sonication time and sonication power. Phosphatidylcholine (PC) liposomes were formed by the evaporation–hydration method. The vesicles were sonicated using several sonication conditions. The liposomes were then characterized by dynamic light scattering (DLS) and freeze-fracture electron microscopy (FFEM). Correlation functions from DLS were treated by cumulants method and GENDIST to obtain the mean radius and polydispersity index. These calculations allowed to fix an optimal sonication time (3000 s) and a useful interval of ultrasound power between 39 and 91 W. DLS and FFEM results confirmed that vesicle size, lamellarity and the polydispersity index decreased with the increase of sonication power. Thus, we propose a systematic method to form liposomes in which the physical characteristics of the vesicles may be controlled as a function of sonication time and power, The authors wish to thank the expert technical assistance of Dr. Carmen López Iglesias. This work was supported by funds from CICYT (MAT2001-1188-C02-02) and the financial support from the Grant FP-2001-1568

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.

18 pages, 9 figures, 3 tables.-- Printed version published Dec 1, 2003., Miniemulsions have been formed in quaternary systems water/hexanol/sodium dodecyl sulfate/decane by dilution of a microemulsion with an excess of water. We have investigated systematically the effect of composition variables in the droplet size and Ostwald Ripening rate. This droplet size has been investigated by using dynamic light scattering of samples submitted to further dilution in water. According to the dynamic light scattering results, the initial droplet size depends on the initial microemulsion water content, the larger the initial water concentration, the smaller the initial droplet size. This is probably related to the structure of the initial phase. The rate of Ostwald Ripening depends on the final surfactant concentration as expected for differences in interfacial tension. At high surfactant and dispersed phase contents, instability due to flocculation has been observed. Other experiments in which a fifth component (a more hydrophobic oil) was added to slow down Ostwald Ripening showed an initial droplet size increasing comparable to the systems without additive but after a relatively long time of approximately 2 h, a decrease was observed., Financial support from CIRIT (Generalitat de Catalunya) projects PIC 99-15 and ITT-CTP98-13 and MCYT MAT2001-1188-CO2-02.

We have studied by optical microscopy, small-angle light scattering, and rheology, the behavior under shear flow of a phase-separated lamellar-sponge (Lα − L3) ternary mixture. We observe in the Lα-rich region (ΦLα > 80%) the existence of a Newtonian assembly made of closed compact monodisperse lamellar droplets immersed in the sponge phase. Contrary to the classical onion glassy texture obtained upon shearing Lα phases, the droplet size scales herein as −1, the inverse of the shear rate. This result is in good agreement with Taylor's picture. Above a critical shear rate, c, the droplets organize to form a single colloidal crystal whose lattice size varies as −1/3. To the memory of Tess Melissa P.

We describe a new type of low-cost easy-to-use viscometer based on the temperature elevation in a liquid under shear flow. After calibration, this instrument can be used to measure the apparent steady state viscosity for both Newtonian and non-Newtonian liquids with no yield stress. We compute the rise in temperature due to viscous dissipation in a Couette cell and compare it to experimental results for different fluids. We show that the variation of the temperature with shear rate can be used to characterize the rheological behaviour of viscous fluids and to evaluate their viscosity in a large domain, from typically a few cP up to more than 10 P, with an accuracy of about ±5%. In contrast to simple viscometers, non-Newtonian fluids can be studied with this apparatus. We give experimental results for Newtonian and non-Newtonian liquids and show that they are very similar to those given in the literature by using much more sophisticated instruments.

Dynamic light scattering experiments on lyotropic lamellar phases of brine and surfactant subjected to a flow field have been realized. The obtained results reveal that the relaxation times measured depend strongly on the velocity of the flow. This dependence is indicative of an increase of the effective elasticity modulus K and a decrease of the effective compressibility modulus (-)B of the lamellar phase with the flow velocity. This leads to the conclusion that the shear can induce a suppression of the undulation fluctuations of the bilayers of the lamellar phase. Our results show also that the rigidity of the membranes decreases as the salt concentration of the sample increases.

We report the effect of shear flow on a phase-separated system composed of lyotropic lamellar (L(alpha)) and sponge (L3) phases in a mixture of brine, surfactant, and cosurfactant. Optical microscopy, small-angle light, and x-ray scattering measurements are consistent with the existence of a steady state made of multilamellar ribbon-like structures aligned in the flow direction. At high shear rates, these ribbon-like structures become unstable and break up into monodisperse droplets resulting in a shear-thickening transition.

The results of measurements of the velocity fluctuations of a thin column of water streaming down a wire show that the dynamics of this process is turbulent at high injection flux and is characterized by a Kolmogorov like spectrum for the energy density. This density E(k), proportional to the power spectrum of the longitudinal velocity, scales as with reminiscent of Kolmogorov turbulence in three-dimensions. It is suggested that this system can be modeled as a quasi one-dimensional compressible system where the compressibility stems from the instability of the column to thickness fluctuations.

The electromagnetic scattering of rotating blades is investigated both theoretically and experimentally. We have developed an analytical method based on physical optics (PO) and the method of equivalent currents (MEC), in conjunction with the quasi-stationary method. We have predicted the bistatic field scattered by a multiple skew-plated rotating fan. Comparisons with a numerical calculation based on the method of moments and with measurements have given very satisfactory results. A comparison with previously published results has also been carried out. Physical interpretations are given both in time and frequency domains. Our analytical model correctly predicts the spreading and magnitude of the frequency response as a function of the scatterer's skew angles, its rotation frequency, and the directions of incidence and scattering.

International audience; no abstract

We report on the self-aggregation patterns of sodium deoxycholate investigated successively by small-angle neutron scattering, quasi-elastic light scattering, viscosity, electrical conductivity, and time domain reflectometry as a function of pH, temperature, and detergent concentration. Experimental results reveal a rodlike association which seems to be strongly dependent on the pH and ionic strength. The radius and length of such conformations compare reasonably well with the dimensions of helical models and rods previously observed by small-angle X-ray scattering and electronic spin resonance. We also show the existence of two critical micellar concentrations. The lower one, corresponding to small primary micelles, is pH and temperature independent. The second one, which is strongly dependent on pH and temperature, leads to the formation of larger aggregates and in some cases to gelation. A sharp phase transition between solution and gel was observed at pH 7.5. The growth of the aggregate occurs along t...

A modified version of the dynamical droplet model, originally derived by Sorensen et al. [Phys. Rev. A 13, 1593 (1976)], is used to explain experimental results on alkyl-oxyethylene-glycol monoether (${\mathrm{C}}_{\mathit{i}}$${\mathit{E}}_{\mathit{j}}$)--water critical micellar solutions. The model assumes that the physical clusters formed close to the critical point can be treated much like percolating clusters with a fractal dimension ${\mathit{d}}_{\mathit{f}}$=2.49 and a polydispersity exponent \ensuremath{\tau}=2.21. For ${\mathrm{C}}_{6}$${\mathit{E}}_{3\mathrm{\ensuremath{-}}}$${\mathrm{H}}_{2}$O and ${\mathrm{C}}_{10}$${\mathit{E}}_{4\mathrm{\ensuremath{-}}}$${\mathrm{H}}_{2}$O critical mixtures, the modified version of the dynamical droplet model provides results in very good agreement with the experimental determinations of the scattered intensity, the turbidity, and the order parameter relaxation rate, when using as input parameters the three-dimensionsal universal Ising values of the critical exponents and the proper sizes of the individual scattering micelles. Static and dynamical background effects can be explained by the finite size of the monomers, which is explicitly taken into account by the model.

The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behavior in the vicinity of room temperature. The phase diagram in the temperature-volume fraction (of the dispersed phase) plane exhibits a lower consolute critical point at about 40 degrees centigrades and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to high volume fraction side at progressively lower temperatures. In this paper we review the evidence that allows to interpret the phase behavior of our system in terms of interacting spherical droplets. We also investigate the dynamics of droplets, below and approaching the critical point by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation is also reviewed and interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system.

We report on the extension of the droplet model to account for static and dynamic light scattering studies of a ternary water-in-oil microemulsion close to its critical point. This model, which assumes that the critical microemulsion droplets are forming transient polydisperse fractal aggregates, reproduces well the usual features of critical phenomena and is also able to account, in a very natural way, for the dynamical background effects evidenced experimentaly.