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We present a lattice model based on two n→0 spin vectors, capable of treating the thermodynamics of living networks in micellar solutions at any surfactant concentration. We establish an isomorphism between the coupling constants in the two spin vector Hamiltonian and the surfactant energies involved in the micellar situation. Solving this Hamiltonian in the mean-field approximation allows one to calculate osmotic pressure, aggregation number, free end and cross-link densities at any surfactant concentration. We derive a phase diagram, including changes in topology such as the transition between spheres and rods and between saturated and unsaturated networks. A phase separation can be found between a saturated network and a dilute solution composed of long flexible micelles or a saturated network and a solution of spherical micelles.

We discuss extensive sets of experimental data including static and dynamic light scattering, ultrasonic velocity measurements and high and low frequency dielectric relaxation, taken in the vicinity the critical point of a ternary microemulsion system. Upon approaching the critical point we observed a slowing down of the dielectric relaxation time and of the first cumulant of the time-dependent droplet density correlation function C2(t) which shows a non-exponential behavior at long time. These features can be well acccounted for by assuming that the microemulsion system is made of polydispersed transient fractal aggregates having a fractal dimension df=2.5 and a polydispersity index τ=2.2.

We discuss the static and dynamic critical behavior of binary solutions of water with some alkyl-oxyethylene-glycol monoether (CiEj) and of di 2 ethylhexyl sodium sulfosuccinate (AOT)-water-decane ternary microemulsion systems. Extensive sets of measurements of the scattered light intensity, of the order-parameter fluctuations time correlation function, and of the shear viscosity, have been performed for different systems. In most of the cases our data sets show systematic deviations from the behavior usually observed for molecular binary fluids. We show that generalized hydrodynamics cannot account for these deviations. A modified version of the Sorensen et al. (1976) dynamical droplet model has been worked out. This model which assumes that the physical clusters formed close to the critical point can be treated like percolating aggregates, and which includes the finite size of the monomers, very well accounts for all our experimental results.

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

A new analytical concept is applied to the kinetics of the shrinking process of poly(N-isopropylacrylamide) (PNIPA) gels. When PNIPA gels are put into hot water above the critical temperature, two-step shrinking is observed and the secondary shrinking of gels is fitted well by a stretched exponential function. The exponent β characterizing the stretched exponential is always higher than one, although there are few analytical concepts for the stretched exponential function with β>1. As a new interpretation for this function, we propose a superposition of step (Heaviside) function and a new distribution function of characteristic time is deduced.

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.

We investigate the effect of shear flow on a lamellar-sponge phase-separating fluid when subjected to shear flow. We show the existence of two different steady states (droplets and ribbons structures) whose nature does not depend on the way to reach the two-phase unstable region of the phase diagram (temperature quench or stirring). The transition between ribbons and droplets is shear thickening and its nature strongly depends on what dynamical variable is imposed. If the stress is fixed, flow visualization shows the existence of shear bands at the transition, characteristic of coexistence in the cell between ribbons and droplets. In this shear-banding region, the viscosity oscillates. When the shear rate is fixed, no shear bands are observed. Instead, the transition exhibits a hysteretic behavior leading to a structural bi-stability of the phase-separating fluid under flow.

The formation of closed-compact multilamellar vesicles (referred to in the literature as the "onion texture") obtained upon shearing lamellar phases is studied using small-angle light scattering and cross-polarized microscopy. By varying the shear rate gamma;, the gap cell D, and the smectic distance d, we show that: (i) the formation of this structure occurs homogeneously in the cell at a well-defined wave vector q(i), via a strain-controlled process, and (ii) the value of q(i) varies as (dgamma;/D)(1/3). These results strongly suggest that formation of multilamellar vesicles may be monitored by an undulation (buckling) instability of the membranes, as expected from theory.

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.

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 7rst 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 7tted to power laws. The absolute values of the new exponents we derived from experimental data are nearly equal, and they are very close to � =0 :33 characterising the shape of the coexistence curve. The exponent characterising the in7nite frequency permittivity is very close to 0.04 relevant to the diverging shear viscosity. For dense microemulsions, two well de7ned relaxation domains have been identi7ed 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

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.

We show the existence under shear flow of steady states in a two-phase region of a brine-surfactant system in which lyotropic dilute lamellar (non-Newtonian) and sponge (Newtonian) phases are coexisting. At high shear rates and low sponge phase-volume fractions, we report on the existence of a dynamic transition corresponding to the formation of a colloidal crystal of multilamellar vesicles (or "onions") immersed in the sponge matrix. As the sponge phase-volume fraction increases, this transition exhibits a hysteresis loop leading to a structural bistability of the two-phase flow. Contrary to single phase lamellar systems where it is always 100%, the onion volume fraction can be monitored continuously from 0 to 100 %.

One of the source terms of Z-pinch experiments is the gas puff density profile. In order to characterize the gas puff, we have used two interferometrical methods and performed some numerical simulations. The merits of both optical techniques are presented in terms of sensitivity, accuracy, and full time recording. Hence, one technique has been chosen to characterize the gas puff. The computation fluid dynamics (CFD) code (ARES) has been used to simulate the gas flow with the aim of testing its performances. Comparing experimental and numerical data shows off the taking into account of gas viscosity in computations. Given these consistent results, the nozzle geometries can be optimized in order to obtain specific Z-pinch gas puffs and check the computation with the interferometric method. Results obtained with a cylindrical nozzle are presented herein.

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...

The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behaviour in the vicinity of room temperature. The phase diagram in the temperature-volume-fraction (of the dispersed phase) plant: exhibits a lower consolute critical point at about 40 degrees C and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to the high-volume-fraction side at progressively lower temperatures. This phase behaviour can be understood in terms of a system of polydispersed spherical water droplets, each coated by a monolayer of AOT, dispersed in a continuum of oil. These droplets interact with each other via a hard-core plus a short-range attractive interaction, the strength of which increases with temperature. We show that Baxter's sticky-sphere model can account for the phase behaviour, including the percolation line, quantitatively provided that the stickiness parameter is a suitable function of temperature. We use the structure factors measured by small-angle neutron scattering (SANS) below the critical temperature to determine this functional dependence. We also investigate the dynamics of droplets, below and approaching the critical and percolation points, 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 can also be interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system.

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 study the static electric conductivity in water-in-oil microemulsion systems both in the droplet phase and in the vicinity of a percolation transition in the non-percolating region. We discuss the mechanisms of conduction in the two regimes. In particular, we interpret the behavior of conductivity far from percolation in terms of charge fluctuations and close to percolation in terms of collection of relaxation times connected to the presence of a polydisperse set of independent fractal clusters.

We study water-in-oil microemulsion systems in the droplet phase and in the vicinity of a percolation transition in the non-percolating region. We focus on the electrical conductivity and permittivity, quantities that show large variations when approaching the percolation threshold. The accepted model for the interpretation of the increasing conductivity - very large compared to that of the bathing oil phase - is related to clustering of the microemulsion droplets and migration of charges within the aggregates. Power laws have been used to interpret the behaviour of the static dielectric properties and scaling functions proposed for the frequency-dependent conductivity and permittivity. We review some relevant experiments in this field and the proposed interpretations, and formulate a phenomenological model of conduction. It is based on the physical picture of cluster formation due to attractive interactions among the constituent water droplets, anomalous diffusion in the bulk of fractal aggregates and polydispersity of the clusters. The model gives quantitative expressions for both conductivity and permittivity over the entire frequency range of the percolative relaxation phenomena, including the static behaviour. A closed expression is derived for the scaling function of a scaling variable which involves frequency, the cut-off cluster size and the parameters of the bulk components. The results are also expressed in the time domain in terms of the polarization time correlation function. The latter exhibits a rather interesting behaviour, since it gradually evolves from an exponential decay to a power-law decay and to a stretched exponential as time increases. The time-scales of the different stages are obtained from the typical decay times of the single droplet and the largest cluster. We have analysed many different sets of data obtained for different microemulsion systems as functions of the composition of the dispersed phase, the temperature and the frequency of the applied field, with a very good agreement with the model in all cases.