Percolation phenomenon of calcium bis(2-ethylhexyl) sulfosuccinate water-in-oil microemulsions by conductivity and dielectric spectroscopy measurements

The sodium counterion (Na+) of the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) surfactant was exchanged with calcium Ca2+ to investigate the counterion charge effect on the structure of water in normal decane microemulsions. Ohmic conductivity and dielectric permittivity measurements were performed on samples at constant water to surfactant mole ratio [water]/[Ca(AOT)(2)]=26.6. Increasing the volume fraction of the dispersed phase phi, a percolation phenomenon was observed at the constant temperature of 25 degrees C. The percolation threshold was found at phi approximately 15% by Ohmic conductivity and static dielectric permittivity studied as a function of phi, and by the frequency dependence of the complex permittivity. Critical exponents typical of the static percolation mechanism (formation of bicontinuous microemulsions) were found below and above threshold. The comparison of these results obtained for the two different counterions, Ca2+ and Na+, in AOT surfactant water in normal decane microemulsions allows detection of an important difference. The percolation below threshold is dynamic for the sodium-based microemulsions, accounting for the formation of clusters of droplets, whereas calcium-based microemulsions show a static percolation. For this system, the coalescence of droplets begins to occur below threshold at phi approximately 12%.     

The 38 K transition in TTF-TCNQ viewed as a percolation phenomenon

The d.c. conductivity, static dielectric constant and thermopower of TTF-TCNQ have been calculated for temperatures below 54 K within a self-consistent effective medium approximation. TTF-TCNQ is assumed to consist of small regions which are semiconducting and others which are highly conducting. The conducting regions have a large negative dielectric constant, a large d.c. conductivity and a negative thermopower; the semiconducting portions have a large positive dielectric constant, an activated d.c. conductivity and a large positive thermopower. The volume fraction of conductor increases from zero at T = 0 to unity at 54 K. The model is the first to reproduce the observed metal-insulator transition (e.g. the d.c. conductivity anomaly) near 38 K. The dielectric constant is calculated to rise initially with temperature and then become negative as the temperature increases above 38 K. The thermopower changes sign below 54 K and increases very rapidly at 38 K.     

Cluster description of water-in-oil microemulsions near the critical and percolation points

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. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Diffusion limited cluster aggregation with irreversible slippery bonds

Irreversible diffusion limited cluster aggregation (DLCA) of hard spheres was simulated using Brownian cluster dynamics. Bound spheres were allowed to move freely within a specified range, but no bond breaking was allowed. The structure and size distribution of the clusters was investigated before gelation. The pair correlation function and the static structure factor of the gels were determined as a function of the volume fraction and time. Slippery bonds led to local densification of the clusters and the gels, with a certain degree of order. At low volume fractions densification of the clusters occurred during their growth, but at higher volume fractions it occurred mainly after gelation. At very low volume fractions, the large-scale structure (fractal dimension), size distribution and growth kinetics of the clusters was found to be close to that known for DLCA with rigid bonds. Restructuring of the gels continued for long times, indicating that aging processes in systems with strong attraction do not necessarily involve bond breaking. The mean-square displacement of particles in the gels was determined. It is shown to be highly heterogeneous and to increase with decreasing volume fraction.     

导电球简立方格子有效介电常数的计算

基于MaxwellWagner模型,推导出有效介电常数关于颗粒体积比的一系列正幂项解析表达式,计算导电球颗粒悬浮于介质或导电液体中构成的简立方格子结构的有效介电常数.数值计算结果表明:在高频条件下(0.1～1.0kHz),有效介电常数主要由导电球颗粒与液体的介电常数比决定,而在低频或直流条件下,导电球颗粒与液体电导率比则起主要作用;有效介电常数的虚数部分有时会很大,即电流变液中的电致损耗有时会相当强,在设计高性能的电流变液时,体系的电导率效应不能忽略.     

杂质对硬球系统团簇成核临界条件的影响

通过CS状态方程及熵极大原理在多元胶体系统研究了杂质对硬球系统团簇成核临界条件的影响. 极少量元素或杂质的小胶球对系统成核的临界条件的影响也是不可忽略的: 成核的临界体积分量显著地降低;此外,研究还发现杂质对成核的临界尺寸影响较少.     

An association theory for the formation of ion oligomers and its application to 1-1 electrolytes

A theory for modelling electrolyte solutions which includes the formation of ion clusters of different size has been developed in the framework of the primitive model. In primitive models the solvent is described as a dielectric continuum and the solvent–solute interactions are neglected. For the dielectric constant the value of the pure solvent has been used. The ion cluster distribution is calculated from the mass action law. The association constants are related to integrals over the cluster distribution functions which are calculated with the Kirkwood superposition approximation from low density pair distribution functions. The ion clusters are defined by a certain distance which rules if two ions belong to the same cluster. This so-called Bjerrum distance is chosen according to fundamental investigations of the structure of ion cluster. All ion clusters are modelled as hard spheres. For the free ions and charged clusters the mean spherical approximation expression for the Coulomb interaction is added. The co-volumes of the ion clusters have been taken from the investigation of the ion cluster structures, and are consistent with the definition of an ion cluster chosen here.     

Critical conditions of entropy-driving nucleation in binary colloidal system

In a binary colloidal hard-sphere system, due to the entropic depletion interactions, large spheres may be pushed to pack into clusters. If a cluster is taken as a new kind of component, then the binary colloidal system turns to be a ternary one and it therefore can be dealt in the framework of a multi-component system. It is known that, under certain conditions, a cluster can grow larger and larger and turn into a nucleation, even a kind of phase transition will take place. In this paper, the critical conditions, including the critical size of the nucleating cluster and the corresponding volume fraction of the colloidal system, were determined by analyzing entropy variation through the multi-component of Carnahan–Starling (CS) state equation and the principle of entropy maximum. The results obtained in this way are in good agreement with that of the experiments. In addition, the results also show that, when the critical volume fraction of the system increases from 0.18 to 0.19, the cluster’s density ?$?$ will change from 0.74 to 0.58. Except for the size ratio of large to small spheres, the critical size of the nucleating cluster is almost independent of its density and the volume fraction of the system.     

导电高聚物的微波吸收机理的研究

本文以“导电孤岛”物理模型阐明界面极化率对导电高聚物电损耗的影响。通过物理模型的数学解析表明,导电高聚物中的“导电孤岛”是导电高聚物的介电损耗的主要来源,并发现“导电孤岛”所引起的介电损耗依赖于“导电孤岛”本身的电导率、介电常数以及它的体积分数。“导电孤岛”物理模型所揭示的介电特性与实验中所观祭到的微波吸收率和导电高聚物的室温电导率有关的实验事实相符合。 关键词：     

Effective dielectric constant of dilute polydisperse suspensions of spheres

We study the effective dielectric constant of a dilute, polydisperse suspension of spheres embedded in a uniform background. We consider a frequency region where the dipole polarizability of individual spheres exhibits a resonance. We evaluate the effective dielectric constant to second order in the volume fraction employing the dipole approximation, which in previous work has been shown to be applicable in resonance. We apply our results to suspensions of spheres with identical and uniform dielectric constant, assuming a log-normal distribution of sphere radii.     

Ultrasound emulsification: effect of ultrasonic and physicochemical properties on dispersed phase volume and droplet size

Ultrasonic emulsification of oil and water was carried out and the effect of irradiation time, irradiation power and physicochemical properties of oil on the dispersed phase volume and dispersed phase droplet size has been studied. The increase in the irradiation time increases the dispersed phase volume while decreases the dispersed phase droplets size. With an increase in the ultrasonic irradiation power, there is an increase in the fraction of volume of the dispersed phase while the droplet size of the dispersed phase decreases. The fractional volume of the dispersed phase increases for the case of groundnut oil-water system while it is low for paraffin (heavy) oil-water system. The droplet size of soyabean oil dispersed in water is found to be small while that of paraffin (heavy) oil is found to be large. These variations could be explained on the basis of varying physicochemical properties of the system, i.e., viscosity of oil and the interfacial tension. During the ultrasonic emulsification, coalescence phenomenon which is only marginal, has been observed, which can be attributed to the collision of small droplets when the droplet concentration increases beyond a certain number and the acoustic streaming strength increases.     

Depletion forces and packing models

In binary colloidal suspensions, due to the depletion forces, large spheres tend to pack together so as to make entropy maximum. Based on the complexities of depletion forces and phase behaviors in binary mixtures, two packing models, named nucleation and non-nucleation packing, are proposed in this Letter. Numerical results show that, when the volume fraction is small, the nucleation is possible. As a result, the large spheres tend to pack into a larger and larger cluster. However, when the volume fraction is very large in a way, the hard-spheres will proceed in the non-nucleation packing fashion. The numerical results also show that differences between the two packing models decrease with the decrease of volume fraction.     

Systematic studies of tunneling magnetoresistance in granular films made from well-defined Co clusters

The tunneling magnetoresistance (TMR) of granular films prepared by the co-deposition of well-defined ferromagnetic Co clusters and insulating inert-gas matrix atoms has been studied as function of matrix atoms (Kr, Xe), cluster volume fraction, temperature (4–40 K) and cluster size (4.2–5.2 nm). Tunneling samples with resistivities that differ by about five orders of magnitude have a TMR that is found to be independent of matrix and cluster volume fraction, i.e. is independent of both tunneling barrier height and width. All samples show a characteristic TMR(T)-dependence that can be explained by a model which takes into account that magnetic moments at the cluster surface are becoming misaligned with increasing temperature. The fraction of misaligned moments at a fixed temperature is increasing with decreasing clusters size.     

Electrical field on non-ceramic insulators and its relation to contact angles for constant volume droplets

The presence of water droplets on the surface of insulators provides intensification of the electric field, which are caused by the non-uniform voltage distribution and dielectric properties of the different materials. The degree of Electric field (E-field) intensification at the triple joint, the region where water, the non-ceramic dielectric and the air are in contact, must be carefully analyzed, because under certain conditions, breakdown voltages can be reached, sometimes, under very little or no contamination at all, which can cause the insulator wettability, tracking, flashover and degradation. In this paper, variations of the electrical field over two polymer compounds widely used in the insulator manufacturing industry, Silicone Rubber (SIR) and High Density Polyethylene (HDPE), are analyzed on a 2D geometry with respect to contact angles of constant volume water droplets. The Electrical field is also computed as a function of relative water droplets distances.     

Multipolar corrections to the Clausius-Mossotti formula for the effective dielectric constant of a polydisperse suspension of spheres

We study multipolar corrections to the Clausius-Mossotti formula for the effective dielectric constant of a polydisperse suspension of spheres with identical uniform dielectric constant embedded in a uniform background. The first few moments of the spectral density occurring in the Bergman representation are evaluated exactly to second order in the volume fraction. We present numerical results for a log-normal distribution of sphere radii.     

Spectral broadening in suspensions of metallic spheres

We study the effective dielectric constant of dilute suspensions of metallic spheres in a transparent background, e.g. glass or water. In the low density limit the effective dielectric constant as a function of frequency exhibits a strong absorption peak corresponding to the dipolar surface plasmon of isolated spheres. We investigate how the absorption peak is modified at higher density due to electrostatic interactions between spheres. We find appreciable deviations from the Clausius-Mossotti formula which may be seen most clearly in a Cole-Cole plot of the effective dielectric constant.     

聚苯乙烯相对分子量对薄壁聚苯乙烯空心微球质量的影响北大核心CSCD

激光惯性约束聚变(ICF)作为探索受控核聚变的有效途径,有望获得清洁无污染的能源,而薄壁聚苯乙烯(PS)空心微球是ICF物理实验中亟需的一类微球。针对薄壁空心微球因径厚比(直径/壁厚)增大导致其在干燥、使用中易开裂的问题,研究了PS原料对薄壁微球质量的影响,探讨了其影响机制。结果表明:当油相PS质量分数为4%时,随着油相粘度增加,W1/O/W2复合乳粒稳定性逐渐提高;当油相质量分数不低于8%时,复合乳粒稳定性良好。PS原料对微球表面粗糙度影响较小,微球球形度和壁厚均匀性随初始油相粘度的增大而降低,在干燥过程中微球开裂率随原料力学性能提高而减小。在外水相中引入氟苯(FB)液滴,延缓固化速率,可减小油相粘度增加对微球球形度和壁厚均匀性的不利影响。     

结合有效介质理论的乳状液太赫兹光谱分析

原油乳状液对原油的长距离输运具有重要影响,乳状液的油、水状态及相互作用机制还需新理论和新方法获得新认识,基于有效介质理论,本文研究了原油乳状液太赫兹光谱响应特征.通过太赫兹时域光谱系统测试得到了含水率为0～28％的原油乳状液的太赫兹时域光谱,结合傅里叶变换计算了吸收系数和介电常数等光学参数,同一频率下吸收系数等光学参数...     

Giant dielectric constant in the one-dimensional semiconductor Meφ3As (TCNQ)2

We have measured a large highly anisotropic dielectric constant in methyltriphenylarsonium ditetracyanoquinodimethanide as a function of frequency and temperature. We present a model based upon finite barriers in the conducting strands. Magnetic resonance and electrical conductivity are used in testing the model; good agreement is obtained.