高温熔体表面张力测量方法的进展

高温熔体表面现象在冶金、化工、熔盐和材料科学等领域十分普遍。测量高温熔体表面张力有着重要的意义。本文评述了高温熔体表面张力的测量原理和方法。并简述了当前关于高温熔体表面现象以及相关的微重力科学的研究进展。     

Determination of solid surface tension from particle-substrate pull-off forces measured with the atomic force microscope

Atomic force microscopy (AFM) is capable of solid surface characterization at the microscopic and submicroscopic scales. It can also be used for the determination of surface tension of solids (gamma) from pull-off force (F) measurements, followed by analysis of the measured F values using contact mechanics theoretical models. Although a majority of the literature gamma results was obtained using either Johnson-Kendall-Roberts (JKR) or Derjaguin-Muller-Toporov (DMT) models, re-analysis of the published experimental data presented in this paper indicates that these models are regularly misused. Additional complication in determination of gamma values using the AFM technique is that the measured pull-off forces have poor reproducibility. Reproducible and meaningful F values can be obtained with strict control over AFM experimental conditions during the pull-off force measurements (low humidity level, controlled and known loads) for high quality substrates and probes (surfaces should be free of heterogeneity, roughness, and contamination). Any probe or substrate imperfections complicate the interpretation of experimental results and often reduce the quality of the generated data. In this review, surface imperfection in terms of roughness and heterogeneity that influence the pull-off force are analyzed based upon the contact mechanics models. Simple correlations are proposed that could guide in selection and preparation of AFM probes and substrates for gamma determination and selection of loading conditions during the pull-off force measurements. Finally, the possibility of AFM measurements of solid surface tension using materials with rough surfaces is discussed.     

Modelling interfacial properties of binary and ternary liquid mixtures of tetrahydrofuran, 2-propanol and 2,2,4-trimethylpentane

The present modelling study has been dedicated to determining the interfacial properties of binary and ternary liquid mixtures made up of tetrahydrofuran, 2-propanol and 2,2,4-trimethylpentane. The variation of the temperature is from 288 to 308 K. By using both UNIFAC activity model and the fugacity model based on the cubic plus association (CPA) equation of state (EOS), a model based on the equality of chemical potentials in the liquid and the surface layer is utilised to describe the liquid–vapour interface of these liquid mixtures. The surface tension, composition and density are simultaneously predicted. The results of this model show that experimental surface tension data are in a good agreement with the predicted ones. The model using CPA EOS and molar volume has a better performance than the one uses the UNIFAC activity model.     

Development of a new methodology to study drop shape and surface tension in electric fields

Development of a new methodology for the study of both shape and surface tension of conducting drops in an electric field is presented. This methodology, called axisymmetric drop shape analysis-electric fields (ADSA-EF), generates numerical drop profiles in an electrostatic field, for a given surface tension. Then, it calculates the true value of the surface tension by matching theoretical profiles to the shape of experimental drops, using the surface tension as an adjustable parameter. ADSA-EF can be employed to simulate and study drop shapes in the electric field and to determine its effect on liquid surface tension. The method can also be used to measure surface tension in microgravity, where current drop-shape techniques are not applicable. The axisymmetric shape of the drop is the only assumption made in the development of ADSA-EF. The new scheme is applicable when both gravity and electrostatic forces are present. Preliminary measurements using ADSA-EF suggest that the surface tension of water increases by about 2% when an electric field with the magnitude of 10(6) V/m is applied.     

A novel method to study particle–air/liquid surface interactions

A novel method to study the dynamics of hydrophilic solid particle interactions with the gas/aqueous surface has been developed; the gas/liquid surface oscillation was monitored using light reflection microscopy. Two modes of gas/liquid surface oscillation are observed: a high mode of oscillation during the first 0.1 s and a low mode of oscillation during the last 0.4 s. It has been shown that the two modes of oscillation are related with the two-stage process of particle–gas/liquid interactions; during the first stage, the particle and gas/liquid surface interact via a long-range hydrodynamic interaction. The liquid film between the particle and the gas/liquid surface decreases its thickness. During the second stage, the particle is adhered (via liquid film) to the air/aqueous surface, and the air/aqueous surface oscillates with the particle. Effects of particle size and surface tension on the frequency of oscillation of the gas/liquid surface were also studied. Two theoretical models are used to predict the high frequency and low frequency of the air/aqueous surface oscillation.     

Synthesis and properties of cationic surfactants with tuned hydrophylicity

A series of pyridinium-based cationic surfactants has been synthesised and their amphiphilic properties have been studied by conductivity and surface tension measurements. The modification of the substitution pattern on the pyridinium ring by hydrophobic moieties (methyl vs. hydrogen and presence or not of condensed benzene ring) gave the opportunity to investigate structure–activity relationships. Characterization by conductivity and surface tension measurements shed light on the behaviour at the air/water interface and in the micellar environment. In particular, the tendency to form ion pairs at very low concentration was evidenced for all the surfactants substituted on the ring, but not for the simple pyridinium ones. The formation of ion pairs affects both the conductivity and the surface tension plots, showing that a series of steps is involved during the adsorption to the air/water surface. An attempt was made to qualify the single steps in the adsorption at the surface layer. Those steps were attributed to different chemical species (free surfactant ions or ion pairs) and to different arrangements of the surfactant. This work also represents a contribution of investigation at very low surfactant concentrations and high surface tension values.     

Dynamic surface tension measurements with submillisecond resolution using a capillary-jet instability technique

An oscillating capillary jet method is implemented to measure surface tension of aqueous nonionic surfactant solutions as a function of surface age from the jet orifice. The experimental technique captures the evolution of jet swells and necks continuously along the jet propagation axis and is used in conjunction with an existing linear, axisymmetric, constant-property model to determine surface tension of liquids. The method is first validated using deionized water and isopropyl alcohol (constant surface tension test fluids) and a procedure is described to identify the optimum wavelength from the breakup point, which produces the smallest error in surface tension measurements. Dynamic surface tension data of concentrated aqueous Tergitol NP-8 surfactant solutions is then presented. The measurements are performed over a spatial length of approximately 1.5 wavelengths, a span corresponding to 0.6-4.2 ms time window from the jet orifice. Submillisecond surface age measurements are made possible by decreasing the jet diameter. Increased surfactant concentrations make the liquid jet more stable and allow measurements at higher surface ages. The correlation of Hua and Rosen fits well the dynamic surface tension data, which includes submillisecond surface ages. Finally, the time required for surface tension to reach equilibrium levels is estimated using a simple adsorption kinetics theory of surfactant molecules on the liquid/air interface.     

液体的表面吸附:表面相厚度和热效应的理论研究

本文定量研究了6种纯液体的表面张力与温度的关系,进一步预测了这些液体从体相到表面相的相变过程中所放出的热量,并提出放热的根源是分子在表面相更有序的排列引起了熵的减少。本文还研究了CaCl_2和K_2CO_3水溶液的表面张力随浓度的变化关系,理论模拟结果与实验数据非常一致;同时,在给定β值的情况下,还对16种强电解质溶液的表面层厚度进行了估算,对所揭示的溶液表面层增厚现象给出了理论解释。     

Surface Tension of the Liquid Hg-In Alloys

Abstract

The measurements of the surface tension of the liquid Hg-In alloys were made by means of a maximum drop pressure method. The surface tension increases monotonously with increasing the In concentration. It is thermodynamically shown that the composition of the Hg atoms adsorbed on the surface is larger than that in the bulk. Experimental results are compared with calculated results due to various model theories; in particular the hard sphere model with a density-dependent cohesive potential is found to be in qualitative agreement with experimental results of both surface tensions and its temperature coefficients.     

Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: an experimental investigation

Sessile liquid drops are predicted to deform an elastic surface onto which they are placed because of the combined action of the liquid surface tension at the periphery of the drop and the capillary pressure inside the drop. Here, we show for the first time the in situ experimental confirmation of the effect of capillary pressure on this deformation. We demonstrate micrometer-scale deformations made possible by using a low Young's modulus material as an elastic surface. The experimental profiles of the deformed surfaces fit well the theoretical predictions for surfaces with a Young's modulus between 25 and 340 kPa.     

Wetting criteria for the applicability of membrane distillation

Membrane distillation can only be applied on liquid mixtures which do not wet a microporous hydrophobic membrane. Solutions of inorganic material in water have such high values of surface tension (γ_L⩾72x10⁻³ N/m) that the non-wetting condition is fulfilled for a number of hydrophobic membranes. As soon as organic solutes are present in the solution, the surface tensionγ_L will be lowered, and if the concentration of organic material becomes too high, wetting of the membrane will occur. By means of theoretical considerations a critical solute concentration or surface tension at which a homogeneous smooth material will be wetted (gq < 90/deg) can be calculated. For a (micro)porous membranes no such theoretical relation can be derived. Therefore, a simple experimental method is described to measure the maximum allowable concentration for a (micro)porous membrane. On the basis of these measurements, the maximum allowable concentration under process conditions can be determined.     

Surface tension of four binary systems containing (1-ethyl-3-methyl imidazolium alkyl sulphate ionic liquid + water or + ethanol)

In this work, we present surface tension experimental measurements for eight binary systems containing water or ethanol and an ionic liquid (IL) of the 1-ethyl-3-methyl imidazolium alkyl sulphate family, being the alkyl chain of the anion: ethyl, butyl, hexyl and octyl. Measurements were performed at the temperature of 25.0 °C and atmospheric pressure. All four ILs are completely miscible with water and ethanol, but for a concentration range of the octyl sulphate IL aqueous system the mixture jellifies, and so it is not possible to measure its surface tension. These measurements allow us to study the influence of the anion size on the surface tension for the pure IL compounds, and the role of the two different solvents in the surface tension behaviour. Thus, we observe that it is completely different when mixed with water or with ethanol, as also happens in other mixtures with different ionic liquids. From the experimental data, we extract surface tension deviations using the most popular definition. The calculated deviations for the ethanol based system are fitted using the Redlich–Kister equation and a novel one previously reported by us. Furthermore, we have also calculated the reduced surface pressure for the aqueous mixtures, which is fitted with good agreement using a theoretical equation obtained from the Bahe–Varela pseudo-lattice model.     

过渡金属离子液体EMIFeCl3的性质研究

在干燥高纯氩气氛的手套箱内, 直接将摩尔比为1∶1的高纯无水FeCl3与氯化1-甲基-3-乙基咪唑(EMIC)混合, 得到棕色透明的离子液体EMIFeCl4. 在293.15~343.15 K温度范围内测定了该离子液体的密度和表面张力. 利用Glasser经验方程和空隙模型研究了EMIFeCl4的性质, 并与离子液体EMIAlCl4进行比较, 指出空隙模型具有一定的合理性.     

Status of the three-phase line tension: a review

From a thermodynamic point of view, the concept of line tension is on solid ground; it is well recognized and defined. There is a notion in the literature that there is little consensus with regards to magnitude or sign of line tension. Partly, inappropriate comparisons of line tension values may have contributed to the current uncertainty, especially some between theoretical and experimental values. A wide range of reported line tension values may not necessarily be a result of conflicting findings, but it may reflect the diversity of systems studied. However, differences among similar approaches and systems observed are sometimes caused by one or several of the following factors: difficulties in sample preparation, poor experimental techniques, non-equilibrium, or conceptual and theoretical difficulties and oversimplifications. It is important to clearly establish the magnitude and sign of line tension as it may determine its relevance to technological applications such as microfluidic systems.

This review has examined a wide range of recent and past studies on the subject of line tension. The review classifies line tension studies into theoretical and experimental; it further groups the studies into liquid–liquid–vapor and solid–liquid–vapor systems for each category. The review provides a comparison between similar studies in an attempt to clarify the current status of line tension research. The majority of theoretical line tension studies (excluding the near wetting studies) have estimates for line tension of about 10⁻¹⁰ N; however, for special cases, values as high as 10⁻⁶ N are also reported. There is less consensus regarding the sign for the line tension as theoreticians often assert that line tension can have either a positive or a negative sign. In experimental studies of liquid–liquid–vapor systems the majority of studies reported positive values for line tension; for film studies, the magnitude of line tension reported was between 10⁻⁹ and 10⁻⁷ N, whereas for studies of liquid lens values as high as 10⁻⁶ N were also reported. The clear majority of studies for solid–liquid–vapor systems reported a positive sign for line tension. In studies involving particles, line tension values ranged from 10⁻⁹ to 10⁻⁶ N. Studies using drop size dependence of contact angles reported values mostly in the span of 10⁻⁹–10⁻⁶ N; however, a clear majority of the reported values fell in the higher end of the above range. The special topic of line tension near wetting was also studied. While the interface displacement model appears to have brought about consensus with respect to certain aspects of line tension behavior near wetting, this model maybe still too phenomenological to be satisfactory from a more fundamental standing. Experimentalists have just begun studying line tension for systems near wetting, this is encouraging, but more comprehensive studies are needed.     

Calculating the surface tension between a flat solid and a liquid: a theoretical and computer simulation study of three topologically different methods

We discuss three topologically different methods for calculating the surface tension between a flat solid and a liquid from theoretical and computer simulation viewpoints. The first method, commonly used in experiments, measures the contact angle at which a static droplet of liquid rests on a solid surface. We present a new analysis algorithm for this method and explore the effects of line tension on the contact angle. The second method, commonly used computer simulations, uses the pressure tensor through the virial in a system where a thick, infinitely extended slab of liquid rests on a solid surface. The third method, which is original to this paper and is closest to the thermodynamic definition of surface tension, applies to a spherical solid in contact with liquid in which the flat solid is recovered by extrapolating the sphere radius to infinity. We find that the second and third methods agree with each other, while the first method systematically underestimates surface tension values.     

Thermophysical properties of pure 1-ethyl-3-methylimidazolium methylsulphate and its binary mixtures with alcohols

The density and surface tension of 1-ethyl-3-methylimidazolium methylsulphate, [EMIM][CH₃SO₄] ionic liquid have been measured from (283.15 to 333.15) K. The coefficient of thermal expansion was calculated from the experimental density results using an empirical correlation for T = (283.15-338.15) K. Molecular volume and standard entropies of [EMIM][CH₃SO₄] ionic liquid were obtained from the experimental density values. The surface properties, critical temperature and enthalpy of vaporization were also discussed. Density and surface tension have been measured over the whole composition range for [EMIM][CH₃SO₄] with alcohols (methanol, ethanol, 1-butanol) binary systems at 298.15 K and atmospheric pressure. Excess molar volumes and surface tension deviations for the binary systems have been calculated and were fitted to a Redlich-Kister equation to determine the fitting parameters and the root mean square deviations.     

Effect of temperature and pressure on surface tension of polystyrene in supercritical carbon dioxide

The surface tension of polymers in a supercritical fluid is one of the most important physicochemical parameters in many engineering processes, such as microcellular foaming where the surface tension between a polymer melt and a fluid is a principal factor in determining cell nucleation and growth. This paper presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendent drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and good qualitative agreement with experiment is obtained. The physical mechanisms for three main experimental trends are explained by using SCFT, and none of the explanations quantitatively depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the slope of surface tension change with temperature is dependent on pressure.     

Nonequilibrium surface tension for mixed adsorption kinetics

Experimental nonequilibrium surface tension measurements of 1–9 nonanediol solutions obtained by the oscillating-jet method have been interpreted in terms of our theoretical predictions derived for a mixed-controlled adsorption kinetics of the surfactant. The surface tension values have been calculated from the Szyszkowski equation using the Langmuir model of surfactant adsorption. Our theoretical results, obtained by a numerical solution of the adsorption equations, agree well with experimental data giving a value of the kinetics Szyszkowski constant very similar to the thermodynamic equilibrium value determined from experimental measurements of the static surface tension of 1–9 nonanediol solutions of various concentration. The approximate kinetic equation derived by P. Joos, G. Bleys, and G. Petre (J. Chim. Phys.79, 387 (1982)) for purely barrier-controlled adsorption proved to be less accurate.     

Wetting Study of Hydrophobic Membranes via Liquid Entry Pressure Measurements with Aqueous Alcohol Solutions

A new concept of liquid entry pressure measurements is applied to study the hydrophobicity of microporous membranes for aqueous alcohol solutions. The effects of alcohol concentration, type of alcohol, and temperature on liquid entry pressure of the membrane have been studied. Two theoretical equations for the determination of membrane pore size have been proposed. The former equation was developed taking into account the deviation from the Laplace–Young equation due to the membrane structure by means of the structure angle. The latter equation was established considering only the range of alcohol concentration in which the dispersion component of liquid surface tension remains practically constant. Hydrophobicity has been expressed in terms of wetting surface tension, γ_L^w. Based on these measurements, the maximum concentration before the spontaneous wetting occurs would be predicted.