Showing papers on "Surface tension published in 1999"

Direct numerical simulation of free-surface and interfacial flow

Abstract: The numerical simulation of flows with interfaces and free-surface flows is a vast topic, with applications to domains as varied as environment, geophysics, engineering, and fundamental physics. In engineering, as well as in other disciplines, the study of liquid-gas interfaces is important in combustion problems with liquid and gas reagents. The formation of droplet clouds or sprays that subsequently burn in combustion chambers originates in interfacial instabilities, such as the Kelvin-Helmholtz instability. What can numerical simulations do to improve our understanding of these phenomena? The limitations of numerical techniques make it impossible to consider more than a few droplets or bubbles. They also force us to stay at low Reynolds or Weber numbers, which prevent us from finding a direct solution to the breakup problem. However, these methods are potentially important. First, the continuous improvement of computational power (or, what amounts to the same, the drop in megaflop price) continuously extends the range of affordable problems. Second, and more importantly, the phenomena we consider often happen on scales of space and time where experimental visualization is difficult or impossible. In such cases, numerical simulation may be a useful prod to the intuition of the physicist, the engineer, or the mathematician. A typical example of interfacial flow is the collision between two liquid droplets. Finding the flow involves the study not only of hydrodynamic fields in the air and water phases but also of the air-water interface. This latter part

A front-tracking algorithm for accurate representation of surface tension

Abstract: We present a front tracking algorithm for the solution of the 2D incompressible Navier-Stokes equations with interfaces and surface forces. More particularly, we focus our attention on the accurate description of the surface tension terms and the associated pressure jump. We consider the stationary Laplace solution for a bubble with surface tension. A careful treatment of the pressure gradient terms at the interface allows us to reduce the spurious currents to the machine precision. Good results are obtained for the oscillation of a capil- lary wave compared with the linear viscous theory. A classical test of Rayleigh-Taylor instability is presented.

On a three-dimensional volume tracking model of droplet impact

Abstract: A three-dimensional model has been developed of droplet impact onto asymmetric surface geometries. The model is based on RIPPLE, and combines a fixed-grid control volume discretization of the flow equations with a volume tracking algorithm to track the droplet free surface. Surface tension is modeled as a volume force acting on fluid near the free surface. Contact angles are applied as a boundary condition at the contact line. The results of two scenarios are presented, of the oblique impact of a 2 mm water droplet at 1 m/sec onto a 45° incline, and of a similar impact of a droplet onto a sharp edge. Photographs are presented of such impacts, against which the numerical results are compared. The contact angle boundary condition is applied in one of two ways. For the impact onto an incline, the temporal variation of contact angles at the leading and trailing edges of the droplet was measured from photographs. This data is applied as a boundary condition to the simulation, and an interpolation scheme propos...

Chemical properties handbook : physical, thermodynamic, environmental, transport, safety, and health related properties for organic and inorganic chemicals

Abstract: Critical properties and acentric factor heat capacity of gas heat capacity of liquid enthalpy of vapourization enthalpy of fusion vapour pressure density of liquid surface tension enthalpy of formation of gas Gibbs energy of formation of gas gas heat capacity for high temperature range solubility in water solubility in water containing salt-hydrocarbons solubility in water as a function temperature HenryAs law constant for compound in water adsorption on activated carbon soil sorption coefficient. Appendices: conversion tables CAS number and thermophysical properties computer program HenryAs law constant.

Orogenic displacement of protein from the air/water interface by competitive adsorption

Abstract: The displacement of proteins from an air/water interface by surfactant has been visualized by atomic force microscopy (AFM) through the imaging of Langmuir-Blodgett films formed on mica. Three different proteins were studied: beta-casein, a largely random coil protein, and two globular proteins, beta-lactoglobulin and alpha-lactalbumin. The proteins were displaced from both spread and coadsorbed films using the nonionic surfactant Tween 20. The combined use of AFM with studies of surface tension and surface rheology have revealed the mechanism of protein desorption from the air/water interface. The surfactant is found to adsorb at defects in the protein network and these nucleated sites then grow, compressing the protein network. At sufficiently high surface pressures the network fails, releasing proteins that then desorb from the interface. We have called this mechanism orogenic displacement. Stress propagation through beta-casein films is homogeneous resulting in the growth of circular surfactant domains. beta-Lactoglobulin and alpha-lactalbumin form stronger networks and stress propagation is restricted resulting in the growth of irregular (fractal) surfactant domains. The AFM images also provide direct evidence for the formation of elastic (gel-like) protein networks at the air/water interface. Copyright 1999 Academic Press.

Constant surface tension simulations of lipid bilayers: The sensitivity of surface areas and compressibilities

Abstract: Eight molecular dynamics simulations of a hydrated lipid bilayer have been carried out differing only in the applied surface tension, γ, defining the boundary conditions of the periodic cell. The calculated surface area per molecule and deuterium order parameter profile are found to depend strongly on γ. We present several methods to calculate the area compressibility modulus, KA, from the simulations. Equivalence between the constant area and constant surface tension ensembles is investigated by comparing the present simulations with earlier work from our laboratories and we find simulation results to depend much more strongly on the specified surface area or surface tension than on the ensemble employed.

Thermocapillary Pumping of Discrete Drops in Microfabricated Analysis Devices

Abstract: () A nonmechanical pumping mechanism, thermocapillary pumping TCP , is de- scribed for moing nanoliter- and picoliter-sized drops of liquid within microfabricated flow channels. In TCP, one end of a single drop is heated to create a surface tension difference between the ends of the drop. The induced surface tension difference causes a capillary pressure difference between the two drop ends and results in drop motion. TCP ¤elocities of up to 20 mmrmin were measured for seeral liquids at temperature differ- ences between 10 and 708C. An expression deeloped for TCPelocity yields predictions that agree with experimentalelocities within corresponding uncertainty limits. Seeral techniques for assisting TCP are also suggested when contact angle hysteresis, the major factor limiting TCPelocities, is too large. These techniques include using surface treat- ments to reduce the contact angle hysteresis, conerging channels to offset hysteresis, or an applied pressure to assist in moement.

Aberrant Aggregation Behavior in Cationic Gemini Surfactants Investigated by Surface Tension, Interfacial Tension, and Fluorescence Methods

Abstract: Bis(quaternary ammonium halide) surfactants (gemini surfactants) having, variously, diethyl ether, monohydroxypropyl, and dihydroxybutyl spacer groups have been investigated by surface tension, interfacial tension, and steady-state fluorescence techniques. The critical micelle concentration (cmc) and area per molecule (Amin) are shown to deviate from the expected patterns of behavior as the number of carbon atoms in the alkyl chain (n) increases beyond a certain maximum. This aberrant behavior is observed at the hydrocarbon/water as well as the aqueous/air interface. The unexpected values of the physicochemical parameters at long alkyl chain length have been interpreted on the basis of a concentration region in which submicellar or multilayer structures are forming. Fluorescence measurements provide confirmation of cmc values by an alternative technique. Comparison of the fluorescence emission maxima profiles of the gemini surfactants with those of their monoquaternary analogues demonstrates that there is...

Thermodynamics of Ionic Surfactant Adsorption with Account for the Counterion Binding: Effect of Salts of Various Valency

Abstract: The effect of counterion binding on the surface tension and surface potential of ionic surfactant solutions is accounted for theoretically. It turns out that no every couple of surfactant and counterion adsorption isotherms are thermodynamically compatible. To solve the problem, we develop a formalism which enables one to obtain the counterion adsorption isotherm corresponding to a given surfactant adsorption isotherm. Further, these adsorption isotherms are integrated to obtain the respective expression for the surface tension. The results are extended to the case when the solution contains ionic−nonionic surfactant mixtures and electrolytes of various valency. The integral, which takes into account the electrostatic interactions, is solved analytically for aqueous solutions containing 1:1, 2:1, 1:2, and 2:2 electrolytes. It is demonstrated that the derived equations can be applied to process experimental data for the surface tension as a function of the surfactant and salt concentrations. As a result on...

Geometric methods in the elastic theory of membranes in liquid crystal phases

Abstract: Contains a comprehensive description of the mechanical equilibrium and deformation of membranes as a surface problem in differential geometry. Following the pioneering work by W. Helfrich, the fluid membrane is seen as a nematic or smectic - a liquid crystal film and its elastic enegy form is deduced exactly from the curvature elastic theory of the liquid crystals. With surface variation, the minimization of the energy at the fixed osmotical pressure and surface tension gives a surface equation in geometry that involves potential interest in mathematics. This text presents the results of the investigation into the solution of the equation that have been carried out by the authors.

Rheology of polymer blends: linear model for viscoelastic emulsions

Abstract: Kerner's model for flow of composite elastic media is extended to an emulsion of viscoelastic phases with interfacial tension undergoing deformations of small amplitude. A privileged internal structure inside the suspended drops is discussed in terms of fluid circulation across the interface. It is shown that for usual drop radius and interfacial tension values of emulsions, the rheological behavior predicted by the model, with very simple expression for the complex shear modulus, is quantitatively similar to that predicted by Palierne's model. Predictions of the model are compared with experimental data obtained on a polystyrene/polyethylene blend sheared in a small-amplitude oscillatory mode.

Noncontact technique for measuring surface tension and viscosity of molten materials using high temperature electrostatic levitation

Abstract: A new, noncontact technique is described which entails simultaneous measurements of the surface tension and the dynamic viscosity of molten materials. In this technique, four steps were performed to achieve the results: (1) a small sample of material was levitated and melted in a high vacuum using a high temperature electrostatic levitator, (2) the resonant oscillation of the drop was induced by applying a low level ac electric field pulse at the drop of resonance frequency, (3) the transient signals which followed the pulses were recorded, and (4) both the surface tension and the viscosity were extracted from the signal. The validity of this technique was demonstrated using a molten tin and a zirconium sample. In zirconium, the measurements could be extended to undercooled states by as much as 300 K. This technique may be used for both molten metallic alloys and semiconductors.

Similarity solutions for van der Waals rupture of a thin film on a solid substrate

Abstract: Rupture of a thin viscous film on a solid substrate under a balance of destabilizing van der Waals pressure and stabilizing capillary pressure is shown to possess a countably infinite number of similarity solutions in each of which the horizontal lengthscale decreases like (tR−t)2/5 and the film thickness decreases like (tR−t)1/5, where tR−t is the time remaining before rupture. Only the self-similar solution corresponding to the least oscillatory curvature profile is observed in time-dependent numerical simulations of the governing partial differential equation. The numerical strategy employed to obtain the self-similar solutions is developed from far-field asymptotic analysis of the similarity equations.

On the gravitational displacement of three-dimensional fluid droplets from inclined solid surfaces

Abstract: The yield conditions for the gravitational displacement of three-dimensional fluid droplets from inclined solid surfaces are studied through a series of numerical computations. The study considers both sessile and pendant droplets and includes interfacial forces with constant surface tension. An extensive study is conducted, covering a wide range of Bond numbers Bd, angles of inclination β and advancing and receding contact angles, θA and θR. This study seeks the optimal shape of the contact line which yields the maximum displacing force (or BT ≡ Bd sin β) for which a droplet can adhere to the surface. The yield conditions BT are presented as functions of (Bd or β, θA, Δθ) where Δθ = θA − θR is the contact angle hysteresis. The solution of the optimization problem provides an upper bound for the yield condition for droplets on inclined solid surfaces. Additional contraints based on experimental observations are considered, and their effect on the yield condition is determined. The numerical solutions are based on the spectral boundary element method, incorporating a novel implementation of Newton's method for the determination of equilibrium free surfaces and an optimization algorithm which is combined with the Newton iteration to solve the nonlinear optimization problem. The numerical results are compared with asymptotic theories (Dussan V. & Chow 1983; Dussan V. 1985) and the useful range of these theories is identified. The normal component of the gravitational force BN ≡ Bd cos β was found to have a weak effect on the displacement of sessile droplets and a strong effect on the displacement of pendant droplets, with qualitatively different results for sessile and pendant droplets.

Using Light to Control Dynamic Surface Tensions of Aqueous Solutions of Water Soluble Surfactants

Abstract: We report the use of UV light to control the dynamic surface tensions of mixed surfactant systems containing sodium dodecyl sulfate, SDS, and 4,4‘-bis(trimethylammoniumhexyloxy)azobenzene bromide, BTHA. The light influences the dynamic surface tension of these solutions by driving the isomerization of the azobenzene moiety from cis to trans. By combining use of the du Nouy ring, maximum bubble pressure, and Wilhelmy plate methods to measure dynamic and equilibrium surface tensions, the dynamic surface tension of an illuminated aqueous solution of these surfactants is demonstrated to be up to 25 mN/m lower than the dynamic surface tension of a solution not previously exposed to UV light. In contrast, the equilibrium surface tensions of these solutions change by less than 2 mN/m upon illumination. Measurements of quasi-elastic and static light scattering from these mixed surfactant solutions support our hypothesis that illumination influences the dynamic surface tension through its effect on the state of ag...

Use of the pendant drop method to measure interfacial tension between molten polymers

Abstract: In this paper the pendant drop method to measure interfacial tension between molten polymers is reviewed. A typical pendant drop apparatus is presented. The algorithms used to infer interfacial tension from the geometrical profile of the pendant drop are described in details, in particular a new routine to evaluate correctly the value of the radius at the apex of the drop, necessary to the calculation of interfacial tension is presented. The method was evaluated for the possibility of measuring the interfacial tension between polyethylene and polystyrene. It is shown that the method is unsuitable for the measurement of interfacial tension between high density polyethylene and polystyrene due possibly to a too small difference of density between the two polymers. Values of interfacial tension between low density polyethylene (LDPE) and polystyrene (PS) as a function of the molecular weight of PS are presented. It was shown that the interfacial tension between LDPE and PS increased as a function of molecular weight of PS up to values of molecular weight of roughly 40,000 g/mol, value for which entanglements occur.

Neutron Reflectivity Studies of the Surface Excess of Gemini Surfactants at the Air−Water Interface

Abstract: Neutron reflection has been used to determine the surface coverage of a series of cationic gemini surfactants of the general formula [CmH2m+1N(CH3)2−(CH2)s−N(CH3)2CmH2m+1]Br2 designated Cn−Cm−Cn, where n and m denote the number of carbon atoms in the chains of the free alkyl chain and the spacer, respectively. The compounds studied were C12−C3−C12, C12−C4−C12, C12−C6−C12, C12−C12−C12 and C12−xylyl−C12, where the spacer is the xylene group, −CH2−φ−CH2, in the last of these compounds. By the use of partially deuterated forms of the surfactants in null reflecting water, neutron reflection was used to measure directly values of the area per molecule at the critical micelle concentration (cmc) of 66, 82, 95, 140, and 97 ± 3 A2, respectively. Comparison with the results from surface tension measurements shows that the appropriate prefactor P in the Gibbs equation, dγ = −PRTΓ d ln c, is approximately 2 for all except the compound with the xylyl spacer, for which P is about 3. A tentative explanation of the unexp...

Insoluble surfactants on a drop in an extensional flow: a generalization of the stagnated surface limit to deforming interfaces

Abstract: A drop in an axisymmetric extensional ow is studied using boundary integral methods to understand the effects of a monolayer-forming surfactant on a strongly deforming interface. Surfactants occupy area, so there is an upper bound to the surface concentration that can be adsorbed in a monolayer, Γ∞. The surface tension is a highly nonlinear function of the surface concentration Γ because of this upper bound. As a result, the mechanical response of the system varies strongly with Γ for realistic material parameters. In this work, an insoluble surfactant is considered in the limit where the drop and external fluid viscosities are equal.For Γ<Γ∞, surface convection sweeps surfactant toward the drop poles. When surface diffusion is negligible, once the stable drop shapes are attained, the interface can be divided into stagnant caps near the drop poles, where Γ is non-zero, and tangentially mobile regions near the drop equator, where the surface concentration is zero. This result is general for any axisymmetric fluid particle. For Γ near Γ∞, the stresses resisting accumulation are large in order to prevent the local concentration from reaching the upper bound. As a result, the surface is highly stressed tangentially while Γ departs only slightly from a uniform distribution. For this case, Γ is never zero, so the tangential surface velocity is zero for the steady drop shape.This observation that Γ dilutes nearly uniformly for high surface concentrations is used to derive a simplified form for the surface mass balance that applies in the limit of high surface concentration. The balance requires that the tangential flux should balance the local dilatation in order that the surface concentration profile will remain spatially uniform. Throughout the drop evolution, this equation yields results in agreement with the full solution for moderate deformations, and underscores the dominant mechanism at high deformation. The simplified balance reduces to the stagnant interface condition at steady state.Drop deformations vary non-monotonically with concentration; for Γ<Γ∞, the reduction of the surface tension near the poles leads to higher deformations than the clean interface case. For Γ near Γ∞, however, Γ dilutes nearly uniformly, resulting in higher mean surface tensions and smaller deformations. The drop contribution to the volume averaged stress tensor is also calculated and shown to vary non-monotonically with surface concentration.

Surface relaxation and stress of fcc metals: Cu, Ag, Au, Ni, Pd, Pt, Al and Pb

Abstract: The multilayer relaxation at (100), (110), (111), (210), (211), (310), (311) and (331) surfaces of the fcc metals Cu, Ag, Au, Ni, Pd, Pt, Al and Pb are calculated using the modified embedded atom method. The `anomalous' outward relaxation at Al (100), (111), Pt (111), and Cu (111) surfaces is described correctly. The relief of surface stress and tension on the relaxation is studied on (111), (100) and (110) surfaces. In general, the surface stress in the direction of the surface normal determines the relaxation direction except for the Al (110) surface. When the surface stress is negative, the surface relaxation is inward; otherwise, the relaxation is outward. An interesting result is that the surface tension does not always decrease after relaxation. The outward relaxation will induce the increase in surface tension while the inward relaxation induces the decrease in surface tension.