Some characteristics of low-speed streaks under sheared air-water interfaces

The characteristics of low-speed fluid streaks occurring under sheared air-water interfaces were examined by means of hydrogen bubble visualization technique. A critical shear condition under which the streaky structure first appears was determined to be u(tau) approximate to 0.19 cm/s. The mean spanwise streak spacing increases with distance from the water surface owing to merging and bursting processes, and a linear relationship describing variation of non-dimensional spacing <(+)over bar> versus y(+) was found essentially independent of shear stress on the interface. Values of <(+)over bar>, however, are remarkably smaller than their counterparts in the near-wall region of turbulent boundary layers. Though low-speed streaks occur randomly in time and space, the streak spacing exhibits a lognormal probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that <(+)over bar> takes rather smaller values than that in wall turbulence is briefly discussed.

An experimental study on turbulent coherent structures near a sheared air-water interface

The turbulence structures near a sheared air-water interface were experimentally investigated with the hydrogen bubble visualization technique. Surface shear was imposed by an airflow over the water flow which was kept free from surface waves. Results show that the wind shear has the main influence on coherent structures under air-water interfaces. Low- and high- speed streaks form in the region close to the interface as a result of the imposed shear stress. When a certain airflow velocity is reached, "turbulent spots" appear randomly at low-speed streaks with some characteristics of hairpin vortices. At even higher shear rates, the flow near the interface is dominated primarily by intermittent bursting events. The coherent structures observed neat sheared air-water interfaces show qualitative similarities with those occurring in near-wall turbulence. However, a few distinctive phenomena were also observed, including the fluctuating thickness of the instantaneous boundary layer and vertical vortices in bursting processes, which appear to be associated with the characteristics of air-water interfaces.

Modelling water adsorption on Au(210) surfaces: II. Monte Carlo simulations

Canonical Monte Carlo simulations for the Au(210)/H(2)O interface, using a force field recently proposed by us, are reported. The results exhibit the main features normally observed in simulations of water molecules in contact with different noble metal surfaces. The calculations also assess the influence of the surface topography on the structural aspects of the adsorbed water and on the distribution of the water molecules in the direction normal to the metal surface plane. The adsorption process is preferential at sites in the first layer of the metal. The analysis of the density profiles and dipole moment distributions points to two predominant orientations. Most of the molecules are adsorbed with the molecular plane parallel to surface, while others adsorb with one of the O-H bonds parallel to the surface and the other bond pointing towards the bulk liquid phase. There is also evidence of hydrogen bond formation between the first and second solvent layers at the interface. (c) 2007 Elsevier B.V. All rights reserved.

Dodecyltrimethylammonium chloride adsorption at the silica/water interface studied by Sum Frequency Generation

La génération des fréquences somme (SFG), une technique spectroscopique spécifique aux interfaces, a été utilisée pour caractériser les changements de la structure macromoléculaire du surfactant cationique chlorure de dodécyltriméthylammonium (DTAC) à l’interface silice/eau dans une plage de pH variant entre 3 et 11. Les conditions expérimentales ont été choisies pour imiter les conditions les plus communes trouvées pendant les opérations de récupération assistée du pétrole. Particulièrement, la silice a été étudiée, car elle est un des composantes des surfaces minérales des réservoirs de grès, et l’adsorption du surfactant a été étudiée avec une force ionique pertinente pour les fluides de la fracturation hydraulique. Les spectres SFG ont présenté des pics détectables avec une amplitude croissante dans la région des étirements des groupes méthylène et méthyle lorsque le pH est diminué jusqu’à 3 ou augmenté jusqu’à 11, ce qui suggère des changements de la structure des agrégats de surfactant à l’interface silice/eau à une concentration de DTAC au-delà de la concentration micellaire critique. De plus, des changements dans l’intensité SFG ont été observés pour le spectre de l’eau quand la concentration de DTAC augmente de 0,2 à 50 mM dans les conditions acide, neutre et alcaline. À pH 3, près du point de charge zéro de la surface de silice, l’excès de charge positive en raison de l’adsorption du surfactant cationique crée un champ électrostatique qui oriente les molécules d’eau à l’interface. À pH 7 et 11, ce qui sont des valeurs au-dessus du point de charge zéro de la surface de silice, le champ électrostatique négatif à l’interface silice/eau diminue par un ordre de grandeur avec l’adsorption du surfactant comme résultat de la compensation de la charge négative à la surface par la charge positive du DTAC. Les résultats SFG ont été corrélés avec des mesures de l’angle de contact et de la tension interfaciale à pH 3, 7 et 11.

Dodecyltrimethylammonium chloride adsorption at the silica/water interface studied by Sum Frequency Generation

La génération des fréquences somme (SFG), une technique spectroscopique spécifique aux interfaces, a été utilisée pour caractériser les changements de la structure macromoléculaire du surfactant cationique chlorure de dodécyltriméthylammonium (DTAC) à l’interface silice/eau dans une plage de pH variant entre 3 et 11. Les conditions expérimentales ont été choisies pour imiter les conditions les plus communes trouvées pendant les opérations de récupération assistée du pétrole. Particulièrement, la silice a été étudiée, car elle est un des composantes des surfaces minérales des réservoirs de grès, et l’adsorption du surfactant a été étudiée avec une force ionique pertinente pour les fluides de la fracturation hydraulique. Les spectres SFG ont présenté des pics détectables avec une amplitude croissante dans la région des étirements des groupes méthylène et méthyle lorsque le pH est diminué jusqu’à 3 ou augmenté jusqu’à 11, ce qui suggère des changements de la structure des agrégats de surfactant à l’interface silice/eau à une concentration de DTAC au-delà de la concentration micellaire critique. De plus, des changements dans l’intensité SFG ont été observés pour le spectre de l’eau quand la concentration de DTAC augmente de 0,2 à 50 mM dans les conditions acide, neutre et alcaline. À pH 3, près du point de charge zéro de la surface de silice, l’excès de charge positive en raison de l’adsorption du surfactant cationique crée un champ électrostatique qui oriente les molécules d’eau à l’interface. À pH 7 et 11, ce qui sont des valeurs au-dessus du point de charge zéro de la surface de silice, le champ électrostatique négatif à l’interface silice/eau diminue par un ordre de grandeur avec l’adsorption du surfactant comme résultat de la compensation de la charge négative à la surface par la charge positive du DTAC. Les résultats SFG ont été corrélés avec des mesures de l’angle de contact et de la tension interfaciale à pH 3, 7 et 11.

Viscoelastic Properties of Nanocrystalline Films of Semiconducting Chalcogenides at Liquid/Liquid Interface

The interfacial shear rheological properties of a continuous single-crystalline film of CuS and a 3D particulate gel of CdS nanoparticles (3−5 nm in diameter) formed at toluene−water interfaces have been studied. The ultrathin films (50 nm in thickness) are formed in situ in the shear cell through a reaction at the toluene−water interface between a metal−organic compound in the organic layer and an appropriate reagent for sulfidation in the aqueous layer. Linear viscoelastic spectra of the nanofilms reveal solid-like rheological behavior with the storage modulus higher than the loss modulus over the range of angular frequencies probed. Large strain amplitude sweep measurements on the CdS nanofilms formed at different reactant concentrations suggest that they form a weakly flocculated gel. Under steady shear, the films exhibit a yield stress, followed by a steady shear thinning at high shear rates. The viscoelastic and flow behavior of these films that are in common with those of many 3D “soft” materials like gels, foams, and concentrated colloidal suspensions can be described by the “soft” glassy rheology model.

剪切水气界面下湍流特性的实验测量

通过实验研究了受气流剪切但无明显波动的水气界面下的湍流特性。当界面剪切较强(‰≥O．20cn∥s)时，水面边界层中平均流速、速度脉动强度和Reynolds切应力的分布形状与固壁湍流相似，预示剪切水气界面和固壁附近的湍流相干结构是相类似的。另一方面，水面湍流也表现出不同于固壁湍流的细节特征。与自由面湍流不同的是，在靠近剪切水气界面的流场中流向和垂向速度脉动同时受到抑制。

城市土-水界面污染流条件下典型污染物生态毒理效应及其机理

人工合成麝香作为一种新型污染物，其生态行为及生态效应已经越来越受到关注。城市土-水界面污染流是除农业面源污染以外第二大面源污染，其成分复杂，采用常规的理化监测方法很难对其生态安全性加以评价。本研究在水生生态毒理实验基础之上，利用城市标识污染物-重金属镉(Cd)、铅（Pb）、佳乐麝香（HHCB）和城市土壤模拟城市土-水界面污染流，并以斑马鱼、草鱼为对象，以超氧化物岐化酶（SOD），过氧化物酶（POD），过氧化氢酶（CAT），丙二醛（MDA），可溶性蛋白（SP）为生物标志物，对其急性和亚急性生态毒理过程进行模拟研究，并通过对联合毒性实验的开展，系统研究了Cd、Pb和HHCB的单一与联合毒性效应、机理和过程。主要结论如下： HHCB和Cd均为中等毒性污染物，Cd和HHCB对斑马鱼的96h-LC50值分别为30.33和4.45 mg/L；在HHCB和Cd的等毒性复合急性实验中，联合作用类型随暴露时间的延长从联合变为拮抗，HHCB是复合污染中的主要污染物。 对水体中总浓度0.95-15.16 mg/L 的Cd和0.14-2.22mg/L的HHCB7天急性毒性机理实验显示，暴露于单一Cd污染斑马鱼体内抗氧化酶活性升高，暴露4天后，其体内MDA含量升高；而在HHCB胁迫2-4天后抗氧化酶活性和MDA含量均降低；联合作用中Cd在暴露的前1-4天对抗氧化酶的变化起主要作用，而之后HHCB开始起主导作用；斑马鱼急性胁迫下，抗氧化系统的诱导是斑马鱼对Cd胁迫的最初反应之一，而对于受HHCB胁迫的斑马鱼，蛋白含量的降低相对于抗氧化系统来说是斑马鱼受到损害的更重要原因。 对水体中总浓度0.005-0.05mg/L的Cd、0.5-4mg/L的Pb和0.01-0.2mg/L的HHCB对草鱼不同器官的14天亚急性毒性机理研究显示：肝脏受污染物的毒性作用最为显著，体内生物标志物也最为敏感；在Cd和Pb胁迫下，肝脏和肾脏内抗氧化酶随浓度的升高被显著抑制，而HHCB对各器官SOD和CAT活性产生诱导作用； MDA在重金属胁迫下其含量变化均不明显，SP含量轻微降低，而在HHCB胁迫下却分别呈现显著的升高和降低的趋势，所以MDA和SP含量可以作为检测HHCB的生物标志物；复合作用下HHCB的加入能使草鱼各器官中抗氧化酶的活性有所升高， MDA含量显著大幅度升高，SP含量显著大幅度降低，HHCB和Cd表现出很强的协同作用，而且HHCB在复合污染中起主导作用。

土-水界面污染流的生态毒理过程模拟研究

土-水界面污染流污染物成分复杂，采用常规的理化监测方法，很难对其生态安全性加以评价。本研究在水生生态毒理实验基础之上，利用重金属Cd、Zn和有机农药甲胺磷（MAP）模拟流经受重金属和农药污染土壤的土-水界面污染流，并以草鱼（Ctenopharyngodon idellus）为刘"象，以SOD，PoD为生物标志物，对其生态毒理过程进行模拟研究，并应用广义联合效应理论，系统研究了Cd、Zn、MAP的单一与联合毒性效应、机理以及土一水界面污染流对Cd、Zn、MAP的单一与联合毒性效应的影响。主要结论如下：1．Cd、Zn、MAP联合作用与污染暴露的时间有关；2．在含cd、zn、MAP土一水界面污染流中，Cd、Zn、MAP三者之间在毒理过程中的交互作用关系具有方向性，可以用广义联合效应来解释；3．土一水界面污染流增大了毒理过程中对草鱼的胁迫强度；4．土一水界面污染流能改变Cd、Zn、MAP在毒理过程中联合作用的类型；5．土一水界面污染流的毒理过程中，总的联合效应更多的依赖各因子的浓度水平组合。

Mass transfer kinetics of yttrium(III) using a constant interfacial cell with laminar flow. Part II. Extraction with Cyanex 272

The yttrium(III) extraction kinetics and mechanism with bis-(2,4,4-trimethyl-pentyl) phosphinic acid (Cyanex 272, HA) dissolved in heptane have been investigated by constant interfacial cell with laminar flow. The data has been analyzed in terms of pseudo-first order constants. Studies on the effects of stirring rate, temperature, acidity in aqueous phase, and extractant concentration on the extraction rate show that the extraction regime is dependent on the extraction conditions. The plot of interfacial area on the rate has shown a linear relationship. This fact together with the strong surface activity of Cyanex 272 at heptane-water interfaces has made the interface the most probable location for the chemical reactions. The forward, reverse rate equations and extraction rate constant for the yttrium extraction with Cyanex 272 have been obtained under the experimental conditions. The rate-determining step has been also predicted from interfacial reaction models. The predictions have been found to be in good agreement with the rate equations obtained from experimental data, confirming the basic assumption that the chemical reaction is located at the liquid-liquid interface.

Kinetics of Y(III) stripping for the system Y/HCl/Cyanex 272-P507/heptane with constant interfacial area cell with laminar flow

Kinetics and mechanism of stripping of yttrium(III) previously extracted by mixtures of bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272, HA), and 2-ethylhexyl phosphonic acid mono-2-ethylhexl ester (P507, HB) dissolved in heptane have been investigated by constant interfacial-area cell by laminar flow. The corresponding equilibrium stripping equation and equilibrium constant were obtained. The studies of effects of the stirring rate and temperature on the stripping rate show that the stripping regime is dependent on the stripping conditions. The plot of interfacial area on the rate has shown a linear relationship. This fact together with the strong surface activity of mixtures of Cyanex 272 and P507 at heptane-water interfaces makes the interface the most probable locale for the chemical reactions. The stripping rate constant is obtained, and the value is compared with that of the system with Cyanex 272 and P507 alone. It is concluded that the stripping ability with the mixtures is easier than that of P507 due to lower the activation energy of the mixtures. The stripping rate equation has also been obtained, and the rate-determining steps are the two-step interfacial chemical reactions as predicted from interfacial reaction models.

Mass-transfer kinetics of ytterbium extraction with sec-nonylphenoxy acetic acid

The extraction kinetics of ytterbium with sec-nonylphenoxy acetic acid (CA-100) in heptane have been investigated using a constant interfacial area cell with laminar flow. The influence of stirring speed and temperature on the rate indicated that the extraction rate was controlled by the experiment conditions. The plot of interfacial area on the rate showed a linear relationship. This fact together with the low solubility in water and strong surface activity of CA-100 at heptane-water interfaces made the interface the most probable locale for the chemical reactions. The influences of extractant concentration and hydrogen ion concentration on the extraction rate were investigated, and the forward and reverse rate equations for the ytterbium extraction with CA-100 were also obtained. Based on the experimental data, the apparent forward extraction rate constant was calculated. Interfacial reaction models were proposed that agree well with the rate equations obtained from experimental data.

Study of the mechanism for controlled crystallization of BaF2 under two kinds of monolayers

Controlled crystallization of BaF2 under two different kinds of monolayers, octadecylamine [CH3(CH2)(17)NH2] and hexadecanol [CH3(CH2)(14)CH2OH], has been studied by using x-ray diffraction (XRD) and scanning electron microscope. It was found that the monolayer headgroup, the degree of ionization of the headgroup, etc., had a complicated effect on the selectivity of monolayers for crystal and on the morphology and orientation of crystals grown under the compressed monolayers. At pH = 7.0, XRD analysis showed that (100)-oriented BaF2 crystals were formed under the octadecylamine monolayer, while several kinds of crystals were found under the hexadecanol monolayer. In comparison, at pH = 8.5, both (100)-oriented BaF2 and (111)-oriented Ba(NO3)(2) crystals were obtained under the monolayer of octadecylamine. However, crystals formed under hexadecanol monolayer consist of BaF2, Ba(NO3)(2), etc. The detailed mechanism for crystallization was discussed in terms of the specific interaction and lattice matching between the monolayer headgroup and the nucleating species.