AbstmcL The three-component ionic microemulsion system consisting o f ~o~/water/decane shows an U~USUBI phase behaviour in the vicinity of room temperature The phase diagram in the temperaturevolume-fra~tio" (of the dispened phase) plane exhibits a lower coiisolute critical point at about 40 'C and 10% t'olume fraction. A percolorion line, stmtmg from the vicinity of the critical point. cuts across the plane. extending to the high-volume-fraction side at progrecsively lower temperatures. This phase behaviour can be understood in t e r m o f a system o f polydispersed spherical water droplets, each coated by a monolayer of AOT, dispersed in a continuum of oil. These droplets interact with each other via n hard-core plus a shon-range attractive interaction. the strength of which increases with temperature. We rhow thal Baxter's sticky-sphere model CM account for the phase behaviour, including the percolation line, quantitatively provided th3t the stickiness pmmeter 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 3150 invstigate the dynamics of droplets. below and approaching the critical and pcrcolalion points, by dynamic light scattering. The first cumulant and time evoluuon of the droplet density correlation function can be quantitatively calculared by assuming the existence o f polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena obselved in an extensive set of measurements o f electrical conductivity and permittivity close to percolalion can also be inrerpreted through the same cluster-forming mechanism, which reproduces the most relevant features o f the frequency-dependent complex dielectric constant o f this system.

No abstract

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.

An extensive set of measurements of low-frequency conductivity cr of a three-component microemulsion system, AOT-water-decane, as a function of temperature and volume fraction of the dispersed phase has been made, a can be calculated at low 0 by means of a charge-fluctuation model, while it can be interpreted for higher values of (p in terms of power-law behavior. The percolation locus in the (p-T plane has been determined starting from the vicinity of the lower consolute point up to 0 -0.65. This line can be successfully interpreted in terms of a modified version of the analytical theory of percolation given by Xu and Stell.PACS numbers: 82.70.KjThe AOT(surfactant)-water-decane(oil) three-component microemulsion (AOT denotes sodium di-2-ethylhexylsulfosuccinate) is an ideal system for studying electrical-conductivity percolation phenomena since throughout the isotropic one-phase region it forms a water-in-oil microemulsion. A water-in-oil microemulsion can be effectively considered as a heterogeneous two-component system made up of conducting spherical droplets of water, coated by a monolayer of surfactant molecules, immersed in a continuous nonconducting oil medium. From a liquid-state-physics point of view a water-in-oil microemulsion, can be considered as a collection of conducting hard spheres interacting among themselves via an attractive potential. There has been a number of experimental'"^ and theoretical^''^ studies of electrical-permittivity and -conductivity percolation phenomena in the literature in this AOT-based model microemulsion system over the last few years. In particular, we have recently experimentally established the validity of dynamic scaling for the dielectric relaxation phenomena.^ As far as the static conductivity percolation is concerned, the picture that emerges from the experiments is as follows: The low-frequency conductivity a (below 100 kHz) has a power-law behavior with exponents which are different below and above the percolation threshold. At a given volume fraction of the microemulsion droplets 0, a temperature threshold Tp exists above which there is a dramatic increase in the conductivity. On the other hand, at a given temperature, there exists a volume-fraction threshold 0^ above which the system has high conductivity. Kim and Huang^ showed that 0p and Tp are uniquely related to each other. The locus of percolation points in the r-0 plane, which we shall simply call the percolation line, seems to originate from the vicinity of the cloud-point curve (the critical temperature is about 40 °C and the critical volume fraction about 0.