Showing papers on "Surface tension published in 1994"

Capillarity and Wetting of Carbon Nanotubes

Abstract: The wetting and capillarity of carbon nanotubes were studied in detail here. Nanotubes are not "super-straws," although they can be wet and filled by substances having low surface tension, such as sulfur, selenium, and cesium, with an upper limit to this tension less than 200 millinewtons per meter. This limit implies that typical pure metals will not be drawn into the inner cavity of nanotubes through capillarity, whereas water and organic solvents will. These results have important implications for the further use of carbon nanotubes in experiments on a nanometer scale.

Microemulsion microstructure and interfacial curvature

Abstract: The typical phase behavior of microemulsion systems undergoing phase inversion is briefly reviewed. As a model system H2O-n-octane-C12E5 is studied with various experimental techniques. The occurring microstructures are visualized by freeze fracture electron microscopy and the corresponding domain sizes are quantified by small-angle neutron scattering. From the variations of the domain sizes the mean and Gaussian curvatures of the interfacial film with temperature are determined. It is found that the mean interfacial curvatureH changes gradually and nearly linearly with temperature from positive (Winsor I) to negative (Winsor II), passing through zero for bicontinuous microemulsions where these contain exactly equal volume fractions of water and oil. There the interfacial tension between bulk water-and oil-rich phases passes through an extreme minimum. Quantitative knowledge of the curvatures permits the measurements of interfacial tensions between the bulk phases to be discussed in terms of the relative contributions of bending energy and entropy of dispersion.

Generation of microcellular polymeric foams using supercritical carbon dioxide. I: Effect of pressure and temperature on nucleation

Abstract: Supercritical carbon dioxide is known to swell and plasticize poly(methyl methacrylate), PMMA, dramatically. We have employed a pressure quench in a CO2-swollen PMMA sample to generate a microcellular core structure encased by a nonporous skin. Further, we have demonstrated that classical nucleation theory can be used to model the effects of saturation pressure, temperature, and time on the cell density of the porous materials, provided that the effects of the CO2-diluent on the surface tension of PMMA are adequately taken into account. This is because our system is in a homogeneous liquid state at our operating conditions because of the plasticization. Both model predictions and data indicate that cell density rises sharply at a saturation pressure of approximately 14 MPa (at 40°C), leveling out above 27 MPa. By contrast, the effect of temperature on cell density in the range 40°C to 80°C is minimal.

Instability and "pearling" states produced in tubular membranes by competition of curvature and tension.

Abstract: We investigate the stability of tubular fluid membranes by perturbing them with optical tweezers. A peristaltic instability appears, with wavelength on the order of the tube circumference, characterized by tautness and suppression of curvature fluctuations in the membrane. We interpret this in terms of a model that includes a surface tension term in the elastic energy, and describes a transition to stable, finite amplitude peristaltic states. At high amplitudes the experiment reveals new dynamic states of "pearls" interconnected via thin tubes along which they travel and aggregate.

Contribution of the surface free energy perturbation to protein-solvent interactions

Abstract: Surface tension measurements were carried out at 20 degrees C by a capillary drop-weight method on aqueous solutions of sodium glutamate (NaGlu), lysine hydrochloride (LysHCl), potassium aspartate (KAsp), arginine hydrochloride (ArgHCl), lysylglutamate (LysGlu), argininylglutamate (ArgGlu), guanidinium sulfate, trehalose, trimethylamine N-oxide (TMAO), dimethyl sulfoxide, 2-methyl-2,4-pentanediol (hexylene glycol), and poly(ethylene glycol)s of molecular weights 200, 400, 600, and 1000. All of the salts and the sugar increased the surface tension of water, while the last four compounds decreased it, with 2-methyl-2,4-pentanediol lowering it most effectively and TMAO being the least effective. The preferential hydration of bovine serum albumin (BSA) and lysozyme was measured in KAsp, ArgHCl, LysGlu, and ArgGlu. The high values of preferential hydration found in all cases, except for BSA in ArgHCl, suggest that they should stabilize protein structure, as had been found for lysine hydrochloride and monosodium glutamate [Arakawa, T., & Timasheff, S. N. (1984) J. Biol. Chem. 259, 4979-4986]. A correlation was found for both BSA and lysozyme in KAsp, NaGlu, LysHCl, ArgGlu, and LysGlu between the surface tension effect and the observed preferential interactions, indicating that the change in the surface free energy of the protein-containing cavity due to the surface tension increase for water by these amino acid salts contributes dominantly to the observed increase in the chemical potential of the protein by their addition. The lack of a correlation observed for BSA, but not lysozyme, in ArgHCl at low concentrations where preferential binding is close to zero suggests, however, that the surface tension effect is not the sole factor involved in the protein-solvent interactions in these amino acid salts. Binding of ArgHCl to BSA, probably through hydrogen bonds between the Arg guanidinium group and peptide bonds, was proposed to occur, the affinity of Arg+ being reduced by electrostatic repulsion when proteins carry a net positive charge, such as is the case with lysozyme. Since the four organic solvent additives also lead to protein preferential hydration, no correlation exists between their preferential interactions and the surface free energy perturbation. Therefore, in their case, the preferential hydration must be ascribed to other factors that overcome the preferential binding expected from the Gibbs adsorption isotherm. The surface tension results, however, are consistent with the binding of the organic solvents to proteins through hydrophobic interactions, explaining, at least in part, the observed concentration dependence of the interactions.

Solvation Free Energies Estimated from Macroscopic Continuum Theory: An Accuracy Assessment

Abstract: The solvation thermodynamics of a set of small organic molecules is studied using a simple continuum model. Transfer from vapor or cyclohexane to water is decomposed into three steps: first the atomic partial charges of the solute are removed, then it is transferred into aqueous solution, and then its partial charges are restored. The electrostatic steps are treated using continuum electrostatics; the hydrophobic transfer step is treated using atomic «solvation parameters» or «surface tensions». Vapor-to-water transfer free energies of 35 neutral compounds were estimated, including analogues of 17 amino acid side chains. Variants of the model were tested, using from one to three atom types, each with its own surface tension

The measurement of dynamic surface-tension by the maximum bubble pressure method

Abstract: The principle of maximum pressure in a bubble for measurements of dynamic surface tension is realized in a fully automatically operating apparatus. The set-up yields data in the time interval from 1 ms up to several seconds and can be temperature controlled from 5° to 80°C. Experimental data obtained for different surfactants and gelatine in water and/or water/glycerine mixtures at different temperatures are discussed. A direct comparison with results from oscillating jet and inclined plate experiments shows excellent agreement.

Molecular dynamics study of the curvature correction to the surface tension

Abstract: The first order curvature correction to the surface tension of a drop is given by Tolman’s length. Up until now, no accurate estimates of this length existed for realistic fluids. Recently Blokhuis and Bedeaux proposed a new relation that expresses Tolman’s length as an integral over the pair distribution function of a planar liquid–vapor interface. We have used this relation to obtain estimates of Tolman’s length from molecular dynamics simulations of a Lennard‐Jones liquid–vapor interface. We found it to be positive and small for a range of temperatures running from the triple‐point temperature to the critical temperature.

Rheology of oil-in-water emulsions

Abstract: The effect of interfacial tension on the steady-flow and dynamic viscoelastic behavior of emulsions are studied experimentally. At very low inter-facial tensions and low volume fractions, the viscosity decreases with increasing shear rate and becomes constant at high shear rates. The high-shear-rate Newtonian viscosity is not affected by interfacial tension, but the transition from pseudoplastic to Newtonian flow shifts to lower shear rates as the interfacial tension decreases. At an interfacial tension of 5 × 10–3 Nm−1, the viscosity decreases, passes through a minimum, and then increases as the shear rate is increased. The dilatant behavior may be attributed to elastic responses of interfaces during collision of drops. At high volume fractions, the emulsions show remarkable elasticity resulting from the interfacial energy associated with deformation of liquid films. The modulus and viscosity are proportional to interfacial tension and inversely proportional to drop size.

Hydrodynamic assist of dynamic wetting

Abstract: Dynamic wetting speeds are limited by the gross entrainment of air between the liquid and the moving substrate. We present experimental data for the curtaincoating method, in which liquid impinges at high speed on the substrate. We also show that air entrainment is strongly affected by macroscopic hydrodynamics and is subject to hysteresis. Using boundary-layer theory, a simple hydrodynamic model is developed for the flow field in the impingement zone away from the dynamic wetting line. The model approximately accounts for the shear thinning of polymer solutions and for the influence of surface tension. We apply the molecular kinetic theory of dynamic wetting, modified to account for hydrodynamic stress, to the immediate vicinity of the wetting line. The main result is a correlation for airentrainment data. The experimentally calibrated model predicts that the hydrodynamic assist of wetting is greatest when the dynamic wetting line is located beneath the impinging curtain. Flow visualization supports this physical picture.

Electrohydrodynamic stability of a slightly viscous jet

Abstract: Many electro-spraying devices raise to a high electric potential a pendant drop of weakly conducting fluid, which may adopt a conical shape from whose apex a thin, charged jet is emitted. Such a jet eventually breaks up into fine droplets, but often displays surprising longevity. This paper examines the stability of an incompressible cylindrical jet carrying surface charge in a tangential electric field, allowing for the finite rate of charge relaxation. The viscosity is assumed to be small so that the shear resulting from the tangential surface stress can be large, even for relatively small fields. This shear can suppress surface tension instabilities, but if too large, it excites electrical ones. For imperfect conductors, surface charge is redistributed by the rapid fluid reaction to variations in tangential stress as well as by conduction. Phase differences between the effects due to the tangential field and the surface charge lead to charge ‘over-relaxation’ instabilities, but the maximum growth rate can still be lower than in the absence of electric effects.

Nonlinear evolution equations for thin liquid films with insoluble surfactants

Abstract: The dynamics of a free‐liquid film with insoluble surfactants is followed until film rupture with a simple model based on three nonlinear evolution equations for the film thickness, the surfactants concentration and the tangential velocity of the fluid in the film. This model is derived asymptotically from the full Navier–Stokes equations for free films and incorporates the effect of van der Waals attraction, capillary forces and Marangoni forces due to gradients of surface tension. Different stability regimes are observed numerically for periodic and fixed boundary conditions and several initial conditions. Furthermore, the role of the relevant parameters (Hamaker constant, tension, Marangoni number) on the rupture time is assessed and comparison is made with the flow dynamics for a liquid film with insoluble surfactants on a solid substrate.

Density functional theory of crystal-fluid interfaces and surface melting

Abstract: The equilibrium structure of various crystal-fluid interfaces in hard-sphere and Lennard-Jones systems is investigated by a density functional approach based on a weighted density approximation which yields reliable bulk phase diagrams. The practically free minimization of the free energy is achieved. As a result the interfaces between the hard-sphere fluid and the fcc hard-sphere crystal are found to have a width of typically seven hard-sphere diameters. A comparison with previous constrained variational calculations demonstrates that free minimization is indispensable to obtain reliable values for the surface tension. In accordance with recent computer simulations we also find complete wetting of a hard structureless planar wall by the hard-sphere crystal at crystal-fluid coexistence. Finally, the fcc-crystal\char21{}gas interface of a Lennard-Jones system exhibits complete surface melting near the triple point for different surface orientations. The width of the interfacial quasiliquid layer depends significantly on temperature and on surface orientation.

Homogeneous Nucleation and the Temperature Dependence of the Crystal-Melt Interfacial Tension

Abstract: Publisher Summary This article presents an analysis of the crystal melt interfacial tension and its temperature dependence which emerged from application of classical nucleation theory to experiments on the kinetics of homogeneous nucleation of crystals in undercooled melts. The analysis shows that the temperature coefficients obtained from homogeneous nucleation experiments can be accounted for the entropy loss in the undercooled melts due to ordering near the crystal surface. Analysis of the equilibrium interface in the hard sphere system confirms that the interfacial entropy losses are sufficiently large to account entirely for the magnitude of the interfacial tension, because the enthalphic contribution in this system is by definition zero. The chapter also suggests that metallic and other simple liquids require remarkably deep undercoolings for the onset of measurable crystal nucleation. This behavior could be interpreted in terms of polytetrahedral models for melt structure.

Initial Interfacial Tension and Oil Uptake by Deep-fat Fried Foods

Abstract: Fundamental theory of surface chemistry was utilized to develop an equation for calculating initial interfacial tension between a product and liquid. The relationship provides a practical approach to overcome the complexity of quantitatively determining interfacial tension of foods. The range of interfacial tension between a restructured potato product and the frying medium was 0.01 to 5.23–10−3 N/m. This range was determined using various frying media or additives. A power relationship was found between the initial interfacial tension and uptake of oil or other frying media. A model of capillary rise in porous media was implemented to explain the linear relationship between uptake and contact angle.

Application of a thermodynamic database to the calculation of surface tension for iron‐base liquid alloys

Abstract: Thermodynamic models based on Butler's equation for surface tension of liquid alloys has been discussed. In alloys, in which activities of components deviate largely from Raoult's law, the calculated surface tensions are found to be affected by the selection of the ratio of the coordination number in the surface phase to that in the bulk phase. Then, the surface tension of liquid Fe-Al, Fe-Co, Fe-Cr, Fe-Mn, Fe-Mo, Fe-Ni, Fe-Si, Fe-Ti and Fe-W binary alloys and liquid Fe-Cr-Ni ternary alloys have been calculated from thermodynamic data in a database constructed by Kaufman. The calculated results reproduce the concentration dependence of the surface tension in those alloys reported so far, but their absolute values are dependent upon the selection of surface tension values for pure elements

Dynamic surface tension of surfactant TA: experiments and theory

Abstract: A bubble surfactometer was used to measure the surface tension of an aqueous suspension of surfactant TA as a function of bubble area over a range of cycling rates and surfactant bulk concentration...

A study on the components of surface free energy of quartz from contact angle measurements

Abstract: Measurements of the contact angle for water, glycerol, formamide, diiodomethane and 1,1,2,2-tetrabromoethane on a quartz surface were made. Using the results obtained, the “geometric mean” approach and long-range and acid-base interaction approach, the dispersion, non-dispersion, Lifshitz-van der Waals and acid-base components of the surface free energy of quartz were determined and compared with those determined in different ways. On the basis of the measurements and calculations it was found that the surface free energy of quartz depends largely on the amounts of silanol groups and physically adsorbed water molecules on its surface. It was also found that the two tested approaches to surface free energy of solids and liquids gave similar results, and it is suggested that the surface free energy of quartz results mainly from dispersion and hydrogen-bond intermolecular interactions.

On a nonlinear thermocapillary effect in thin liquid layers

Abstract: Dilute aqueous solutions of long alcohol chains were recently found to cause a quadratic dependence of surface tension on the temperature without affecting other bulk properties of the liquid: σ = σ0 + αQ(T − T0)2, αQ > 0. The impact of such Marangoni instability on the behaviour of a thin liquid layer is studied in this work. We derive an equation describing a nonlinear spatiotemporal evolution of a thin film. The behaviour of the perturbed film in the absence of gravity, critically depends on whether the temperature T0, yielding a minimal surface tension, is attained on the surface of the film. When this is the case, a qualitatively new behaviour is observed: perturbations of the film interface may evolve into continuous steady patterns that do not rupture. Otherwise, the observed patterns due to the linear and quadratic Marangoni effects are qualitatively similar and result in the rupture of the film into separate drops.

Thermocapillary migration of an attached drop on a solid surface

Abstract: Migration of a small drop of liquid, initially at rest on a level, solid surface, can be induced by means of thermocapillary forces. If a temperature gradient is imposed across the solid, it has the effect of diminishing the surface tension on the warmer side of the droplet. Consequently, migration manifests itself as the difference in surface tension preferentially draws the droplet toward the cooler region of the solid. The study describes the behavior of a droplet modeled as an infinitely long strip of finite width and arbitrary height profile, subject to a uniform temperature gradient imposed along the base. Lubrication theory is employed to determine the velocity and pressure fields within the drop, as well as the net migration velocity of the droplet as a whole. The role of the dynamic boundary condition in the vicinity of the contact lines (including the allowance for slip) on the migration velocity is highlighted.