纤维膜在膜蒸馏过程中的膜内毛细冷凝现象

膜蒸馏过程中的膜内冷凝现象是该技术在实际应用中不可忽视的一个重要问题。本文以疏水化改性处理的纤维膜作为研究对象,采用实验现象观察和理论分析并用的方法,研究了纤维膜在膜蒸馏过程发生膜内毛细管冷凝的影响因素包括膜蒸馏模块结构、蒸馏膜参数、热力学参数,探讨了毛细冷凝对膜蒸馏的影响。实验表明毛细冷凝可以在膜蒸馏过程中发生,但并不终止膜蒸馏过程,而是减小了传质系数。毛细冷凝容易发生在蒸馏膜厚度小、热料液温度高、冷却液温度低或气隙小的情况下。实验结果和理论分析表明,传质系数实验值的降低可作为膜孔内发生毛细冷凝的判定依据。通过简化蒸气在疏水多孔介质的传递过程,建立了膜蒸馏过程发生毛细冷凝时的传质平衡模型。     

Equilibrium capillary forces with atomic force microscopy

We present measurements of equilibrium forces resulting from capillary condensation. The results give access to the ultralow interfacial tensions between the capillary bridge and the coexisting bulk phase. We demonstrate this with solutions of associative polymers and an aqueous mixture of gelatin and dextran, with interfacial tensions around 10 microN/m. The equilibrium nature of the capillary forces is attributed to the combination of a low interfacial tension and a microscopic confinement geometry, based on nucleation and growth arguments.     

Metastability and nucleation in capillary condensation

This paper is devoted to thermally activated dynamics of capillary condensation. On the basis of a simple model we identify the critical nucleus involved in the transition mechanism and calculate the nucleation barrier from which we obtain information on the nucleation time. Close to the condensation point, the theory predicts extremely large energy barriers leading to strong metastabilities, long time dependencies, and large hysteresis in agreement with experimental observations in mesoporous media. The validity of the model is assessed using a numerical simulation of a time-dependent Ginzburg-Landau model for the confined system.     

Structure and phase transitions in a network-forming associating Lennard-Jones fluid in a slit-like pore: a density functional approach

A study is reported of adsorption of an associating Lennard-Jones fluid with four associative sites per molecule in a slit-like pore. The density distribution of particles in the pore and thermodynamics properties are evaluated by using a density functional method. It is found that at low temperatures the fluid exhibits a set of layering transitions, followed by capillary condensation. Transitions are localized by analysing the grand canonical potential. The density profiles of particles and the distribution of unbound and differently bonded particles demonstrate changes in the structure of the fluid in the pore along the phase coexistence. The critical temperature is lower for a confined fluid, compared with the bulk counterpart. However, an increase in the energy of association increases the critical temperature. The envelope of the capillary condensation is narrower than the bulk liquid-vapour phase diagram. The dependence of the solvation force on the energy of association and on the bulk density is discussed.     

Influence of capillary condensation on the near-critical solvation force

We argue that in a fluid, or magnet, confined by adsorbing walls which favor liquid, or the (+) phase, the solvation (Casimir) force in the vicinity of the critical point is strongly influenced by capillary condensation which occurs below the bulk critical temperature T(c). At T slightly below and above T(c), a small bulk field h<0, which favors gas, or the (-) phase, leads to residual condensation and a solvation force which is much more attractive (at the same large wall separation) than that found exactly at the critical point. Our predictions are supported by results obtained from density-matrix renormalization-group calculations in a two-dimensional Ising strip subject to identical surface fields.     

Capillary condensation in atomic scale friction: how water acts like a glue

We present atomic-scale friction force measurements that strongly suggest that the capillary condensation of water between a tungsten tip and a graphite surface leads to the formation of ice at room temperature. This phenomenon increases the friction force, introduces a short-term memory in the form of an elastic response against shearing, and allows us to "write" a temporary line of ice on a hydrophobic surface. Rearrangements of the condensate are shown to take place on a surprisingly slow time scale of seconds.     

Homogeneous nucleation and growth in iron-platinum vapour investigated by molecular dynamics simulation

Homogeneous nucleation and growth from binary metal vapour is investigated by molecular dynamics simulation. It is focused here mainly on the iron-platinum system with a mole fraction of 0.5. The simulations are started in the highly supersaturated vapour phase. Argon is added as carrier gas removing the heat of condensation from the forming clusters. The embedded atom method is employed for modelling of the force field of iron and platinum. The simulation runs are evaluated with respect to the nucleation rate, monomer temperature, monomer amount, and with respect to the size of the largest cluster in the system including possible pure metal clusters. It turns out that depending on the composition of the complete system pure platinum clusters with sizes up to 10 to 15 atoms are formed in addition to binary clusters. Due to the high temperature of these clusters iron atoms less likely condense at the beginning of the particle formation simulation. This leads to temporary difference in the temperatures of the platinum and the iron subsystems, which eventually approach each other when only binary clusters are present. In summary, the results obtained from the cluster statistics show that pure platinum nucleation and growth can take place to some extent within the binary system.     

Thermodynymic analysis of the mechanism of deep supercooling of tissue water in winter-hardy plants

The temperature at which ice grows through narrow, hydrophilic capillary is known to be depressed. Further, the nucleation temperature near a hydrophilic surface varies with the size, geometry and the properties of a particle. In this paper we show how these two effects are additive for the water that freezes on the wall of a capillary without the presence of pre-existing ice. The combined effect is a substantial lowering of nucleation temperature that could, if this analysis is correct, have important cryobiological consequences.     

方解石纳米孔隙内二氧化碳毛细凝聚的分子模拟

在二氧化碳地质封存、增产非常规油气以及孔隙材料表征测量方面,纳米孔隙中二氧化碳相态的准确预测具有重要意义。然而,由于纳米尺度下毛细力、分离压等作用力占据主导因素,流体在孔隙中的相行为与体相流体存在根本不同。已有实验和模拟表明,Kelvin毛细凝聚理论无法预测特征尺度10nm下的,孔隙内流体凝聚压力与体相饱和蒸气压的偏离程度。本文利用分子模拟方法,研究了孔径范围为0.83-8.0nm的方解石纳米孔隙中二氧化碳毛细凝聚。结果表明,微孔(小于2nm)中二氧化碳受吸附层影响,凝聚压力远低于体相饱和蒸气压,介孔(2?50nm)中二氧化碳受壁面影响,凝聚压力仍低于体相饱和蒸气压,且受孔径影响显著,并获得了纳米孔隙中二氧化碳相态随压力的变化规律。     

外力场对冷凝过程影响的分子动力学模拟研究

固壁表面发生的冷凝过程在工程应用中广泛存在,本文采用非平衡分子动力学模拟方法对高温蒸气在低温壁面上发生的冷凝过程进行了模拟研究.着重对该过程中的密度和局部温度等热力学参数分布进行了研究.同时,考察了两种气固温差条件下,壁面内、外法线方向上,不同强度的外力场对冷凝过程的影响,得到了结论:外力场是壁面内法线方向时促进冷凝进行,反之则阻碍冷凝进行,力场强度越大则影响效果越显著;当外力场一定时,气固温差越小,则外力场的相对影响程度越大.     

Kinetics and energy states of nanoclusters in the initial stage of homogeneous condensation at high supersaturation degrees

The condensation of metal vapor in an inert gas is studied by the molecular dynamics method. Two condensation regimes are investigated: with maintenance of partial pressure of the metal vapor and with a fixed number of metal atoms in the system. The main focus is the study of the cluster energy distribution over the degrees of freedom and mechanisms of the establishment of thermal equilibrium. It is shown that the internal temperature of a cluster considerably exceeds the buffer gas temperature and the thermal balance is established for a time considerably exceeding the nucleation time. It is found that, when the metal vapor concentration exceeds 0.1 of the argon concentration, the growth of clusters with the highest possible internal energy occurs, the condensation rate being determined only by the rate of heat removal from clusters.     

Design principles for broad-spectrum protein-crystal nucleants with nanoscale pits

Growing high-quality crystals is a bottleneck in the determination of protein structures by x-ray diffraction. Experiments find that materials with a disordered pitted surface seed the growth of protein crystals. Here we report computer simulations of rapid crystal nucleation in nanoscale pits. Nucleation is rapid, as the crystal forms in pits that have filled with liquid via capillary condensation. Surprisingly, we find that pits whose surfaces are rough are better than pits with crystalline surfaces; the roughness prevents the growing crystal from trying to conform to the pit surface and becoming strained.     

High Speed Visualization and Dynamic Measurement of Heat Driven Pumps

We provide the high speed flow visualization and dynamic measurement results for the U-shaped and the inverted U-shaped heat driven pumps. The U-shaped heat driven pumps at the high heating powers consist of a succession of tiny bubble nucleation, growth and coalescence process. Once the “larger” spherical bubble or the bubble slug forms, it expands quickly in both upstream and downstream directions. The increased pressure leads to the liquid discharge through the outlet check valve. When the advancing vapor/liquid interface reaches a higher position in the vertical discharge branch, the condensation heat transfer in the discharge branch shrinks the bubble slug, leading to the decreased pressure and initiating the open of the inlet check valve. Thus the fresh liquid can be sucked into the system. Heat driven pumps operating at the low heating powers display the similar process. However, two major differences are identified: (1) A full cycle includes a set of positive pressure pulses corresponding to a set of tiny bubble nucleation, growth and coalescence process in each substage. Only at the end of the cycle, an apparent negative pressure pulse is created. (2) For each substage in each cycle, when the newly formed bubble slug is chasing the ahead “old” bubble slug, the deformed liquid bridge is formed due to the gravity force effect. When the two bubble slugs are merging together, a wave vapor/liquid interface occurs along the bottom of the capillary tube. For the inverted U-shaped heat driven pumps, there are fewer positive pressure pulses included, corresponding to lesser number of new bubble nucleation, growth, and coalescence process. The bubble slug in the capillary tube is very standard with the smooth vapor/liquid interface. The cycle periods and the pumping flow rates are given versus the heating powers.     

Capillary force on a nanoscale tip in dip-pen nanolithography

Monte Carlo simulation has been used to characterize the capillary force due to the condensation of a liquid meniscus between a tip with a nanoscale asperity and a flat surface. To consider both hydrophobic and hydrophilic molecules coating the tip as a model of dip-pen nanolithography, tips with various wettabilities are studied. The capillary force due to the meniscus is calculated for various saturations (humidities). We have implemented a thermodynamic integration technique that can project the force into energetic and entropic contributions. In most cases, the force is mainly energetic in origin. At the snap-off separation where the meniscus disappears, the tip feels a significant entropic force at high saturation. Our calculation shows nonmonotonic behavior of the pull-off force as a function of saturation, which is in qualitative accord with experiments.     

Optical and structural investigation of a-plane GaN layers on r-plane sapphire with nucleation layer optimization

Nonpolar a-plane GaN epilayers are grown on several r-plane sapphire substrates by metal organic chemical vapour deposition using different nucleation layers:(A) a GaN nucleation layer deposited at low temperature(LT);(B) an AlN nucleation layer deposited at high temperature;or(C) an LT thin AlN nucleation layer with an AlN layer and an AlN/AlGaN superlattice both subsequently deposited at high temperature.The samples have been characterized by Xray diffraction(XRD),atomic force microscopy and photoluminescence.The GaN layers grown using nucleation layers B and C show narrower XRD rocking curves than that using nucleation layer A,indicating a reduction in crystal defect density.Furthermore,the GaN layer grown using nucleation layer C exhibits a surface morphology with triangular defect pits eliminated completely.The improved optical property,corresponding to the enhanced crystal quality,is also confirmed by temperature-dependent and excitation power-dependent photoluminescence measurements.     

Experimental evidence for ice formation at room temperature

The behavior of water under extreme confinement and, in particular, the lubrication properties under such conditions are subjects of long-standing controversy. Using a dedicated, high-resolution friction force microscope, scanning a sharp tungsten tip over a graphite surface, we demonstrate that water nucleating between the tip and the surface due to capillary condensation rapidly transforms into crystalline ice at room temperature. At ultralow scan speeds and modest relative humidities, we observe that the tip exhibits stick-slip motion with a period of 0.38+/-0.03 nm, very different from the graphite lattice. We interpret this as the consequence of the repeated sequence of shear-induced fracture and healing of the crystalline condensate. This phenomenon causes a significant increase of the friction force and introduces relaxation time scales of seconds for the rearrangements after shearing.     

纳米结构超疏水表面冷凝现象的三维格子玻尔兹曼方法模拟

采用三维多相流格子玻尔兹曼方法 (lattice Boltzmann method, LBM),对纳米结构超疏水表面液滴的冷凝行为进行模拟研究.通过Laplace定律和光滑表面的本征接触角理论对三维LBM模型进行定量验证.模拟分析了超疏水表面纳米阵列的几何尺寸和润湿性的局部不均匀性对冷凝液滴形核位置和最终润湿状态的影响规律.结果表明,较高的纳米阵列使液滴在纳米结构间隙的上部侧面和底部优先形核长大,通过采用上下不均匀的间隙可避免液滴在底部形核长大,而在上部侧面形核的冷凝液滴在生长过程中向上运动,其润湿状态由Wenzel态转变为Cassie态;较低的纳米阵列使液滴在纳米结构底部优先形核长大,液滴的最终润湿状态为Wenzel态;润湿性不均匀的纳米结构表面使液滴在阵列顶端亲水位置处优先形核长大,成为Cassie态.冷凝液滴在不同几何尺寸的纳米结构表面上的最终润湿状态的模拟结果与文献报道的实验结果符合良好.通过模拟还发现,冷凝液滴在生长过程中的运动行为与液滴统计平均作用力的变化有关.本文的LBM模拟再现了三维空间中液滴的形核、长大和润湿状态转变等物理现象.     

Capillary Interactions between a Probe Tip and a Nanoparticle

To understand capillary interactions between probe tips and nanoparticles under ambient conditions, a theoretical model of capillary forces between them is developed based on the geometric relations. It is found that the contribution of surface tension force to the total capillary force attains to similar order of magnitude as the capillary pressure force in many cases. It is also shown that the tip shape and the radial distance of the meniscus have great influence on the capillary force. The capillary force decreases with the increasing separation distances, and the variance of the contact angles may change the magnitudes of capillary forces several times at large radial distances. The applicability of the symmetric meniscus approximation is discussed.     

低压蒸汽滴状冷凝表面温度分布实验研究

本文利用自组装技术制备了铜基十八烷基硫醇疏水表面(SAM),通过红外热像仪分析了低压条件下液滴表面和换热表面的温度分布以及液滴脱落引起的温度分布演变。实验结果表明:低压蒸汽冷凝条件下,冷凝表面局部温度分布不均匀;单个液滴表面温度呈中心高边缘低的凸型分布;随着液滴半径的增加,液滴表面温度升高;相同操作压力下,随着过冷度的增加,液滴表面温度降低。在液滴脱落过程中,液滴表面温度逐渐升高,同时裸露出的换热表面局部过冷度增大,局部表面温度呈现出中心低周围高的凹型温度分布,随之恢复到液滴脱落前的温度。随着蒸汽压力降低,冷凝临界过冷度增加,导致裸露表面上具有更低温度的中心区域核化点密度高,最终加剧了整个换热表面液滴尺寸分布的非均匀程度。     

Wetting of a self-assembled 1,4-benzenedimethanethiol monolayer on gold and silver: An adhesion force spectroscopy study

Using atomic force microscopy we investigated how local capillary phenomena are affected by the deposition of a self-assembled 1,4-benzenedimethanethiol (BDMT) layer on epitaxially grown Au(1 1 1) and Ag(1 1 1) films. Force-distance curves monitored at varying relative humidity show clear differences in the adhesion forces on the different samples, which can be explained in terms of a change in the wetting behavior due to the presence of the molecules. Moreover, we found that not only the chemical structure of the molecules but also their orientation strongly influences the strength of the capillary forces. A detailed analysis of the measurements shows that condensation of water vapor on Au(1 1 1) films is drastically enhanced due to the vertically aligned BDMT molecules, while on Ag(1 1 1) water condensation is reduced due to a parallel molecule orientation.