Showing papers on "Surface tension published in 1993"

Prediction of Polymer Properties

Abstract: Topological method for structure-property correlations volumetric properties thermodynamic properties cohesive energy and solubility parameter transition and relaxation temperatures surface tension and interfacial tension optical properties electrical properties magnetic properties mechanical properties properties of polymers in dilute solutions sheer viscosity thermal conductivity and thermal diffusivity transport of small penetrant molecules.

Review of data for the surface tension of pure metals

Abstract: A literature survey of the experimentally determined values for the surface tension γ of molten pure metals has been carried out. It is intended to provide an assessed data base for use with a mathematical model currently being developed at the National Physical Laboratory, UK, to predict the surface tension of molten metal alloys.

Linear viscoelastic behavior of some incompatible polymer blends in the melt. Interpretation of data with a model of emulsion of viscoelastic liquids

Abstract: The linear viscoelastic behavior of two-phase polymer blends in the melt has been analyzed by an emulsion model, with takes into account the viscoelasticity of the phases. In this paper, we have studied two types of blends: PS/PMMA and PDMS/POE-DO. For PS/PMMA blends, the model leads to values of relaxation times and secondary plateau modulus in accordance with experimental data. This supports the assumption that long-time relaxation mechanisms are due to geometrical relaxation of the droplets of the dispersed phase. For a quantitative comparison, further experiments have been carried out on PDMS/POE-DO blends for which the distribution of size of the dispersed POE-DO inclusions may be easily determined. The data confirm the validity of the model and show that dynamic shear measurements can be used as a method to determine the interfacial tension between two polymer melts. © 1993, American Chemical Society. All rights reserved.

A Molecular Dynamics Study of the Decane/Water Interface

Abstract: Several molecular dynamics simulations on the interface between liquid decane and liquid water have been performed with the purpose to study the sensitivity of surface properties to the van der Waals parameters. The models used consisted of 50 decane molecules and 389 water molecules in a two-phase system. By changing the van der Waals parameters, i.e., varying the Lennard-Jones parameters e and σ between the united CH2 and CH3 atoms of the decane molecules and the O atom of the water molecules, sharper interfaces were obtained. The excess free energy of water in decane and the surface tension between water and decane are presented for the van der Waals parameters used. From comparison to experimental values for the solubility of water in decane and the surface tension, the best parameters within the tested sets are when using the SPC/E water model: eO-CH3 = 0.849 kJ/mol, eO-CH2 = 0.706 kJ/mol, and σO-C = 0.344 nm. When using the SPC water model, the best parameters are eO-CH3 = 0.637 kJ/mol, eO-CH2 = 0.529 kJ/mol, and σO-C = 0.344 nm. We also evaluated differences between the interfacial and bulk liquids. At the interface, water showed an orientational preference, whereas the decane molecules were more laterally oriented with respect to the interface.

Effect of surface roughness on the response of thickness-shear mode resonators in liquids

Abstract: The effect of surface microstructure on the response of thickness-shear mode resonators in contact with liquids has been examined. Resonators were fabricated with different degrees of random surface roughness by polishing AT-cut quartz crystals with various abrasive particle sizes and then depositing conformal Cr/Au electrodes. The electrical response of liquid-contacted resonators was measured over a range of frequencies near resonance and fit to an equivalent-circuit model. A method is described for determining the complex shear mechanical impedance (the ratio of shear stress to particle velocity at the solid/liquid interface) from the equivalent-circuit elements. This impedance is sensitive to the influence of surface microstructure on the solid/liquid interaction. The surface mechanical impedance was measured while surface roughness, contacting liquid properties, device operating frequency, and liquid contact angle were varied. Results show that, for roughness features much less than the liquid decay length, the surface may be considered hydrodynamically smooth and the responses depend only on the density-viscosity product. As features become comparable to or larger than the decay length, new mechanisms, including liquid trapping and compressional wave generation, arise for energy storage and power dissipation. 53 refs., 14 figs., 3 tabs.

Modeling of the deformation of a liquid droplet impinging upon a flat surface

Abstract: This article presents a theoretical study of the deformation of a spherical liquid droplet impinging upon a flat surface. The study accounts for the presence of surface tension during the spreading process. The theoretical model is solved numerically utilizing deforming finite elements and grid generation to simulate accurately the large deformations, as well as the domain nonuniformities characteristic of the spreading process. The results document the effects of impact velocity, droplet diameter, surface tension, and material properties on the fluid dynamics of the deforming droplet. Two liquids with markedly different thermophysical properties, water and liquid tin, are utilized in the numerical simulations because of their relevance in the industrial processes of spray cooling and spray deposition, respectively. The occurrence of droplet recoiling and mass accumulation around the splat periphery are standout features of the numerical simulations and yield a nonmonotonic dependence of the maximum splat radius on time.

A Lattice Boltzmann Model for Multi-phase Fluid Flows

Abstract: We develop a lattice Boltzmann equation method for simulating multi-phase immiscible fluid flows with variation of density and viscosity, based on the model proposed by Gunstensen {\em et al} for two-component immiscible fluids. The numerical measurements of surface tension and viscosity agree well with theoretical predictions. Several basic numerical tests, including spinodal decomposition, two-phase fluid flows in two-dimensional channels and two-phase viscous fingering, are shown in agreement of experiments and analytical solutions.

Adsorption of Dodecyl Sulfate Surfactants with Monovalent Metal Counterions at the Air-Water Interface Studied by Neutron Reflection and Surface Tension

Abstract: Neutron reflection and surface tension have been used to study the adsorption at the air-water interface of the dodecyl sulfates of the alkali metals (MDS). The critical micelle concentration (cmc) at 33°C was found to vary from 8.5 × 10 -3 M for LiDS to 5.9 × 10 -3 M for CsDS, in agreement with the values in the literature. Neutron reflection was used to measure the adsorption of LiDS, NaDS, and CsDS at their cmc's at a temperature of 25°C for LiDS and NaDS, and 33°C for CsDS. The area per molecule( A cmc ) was found to be 50, 45, and 38 ± A 2 respectively. Surface tension measurements were made for LiDS, NaDS, KDS, RbDS, and CsDS at varying temperatures and A cmc has also been calculated using the Gibbs equation. The values corresponding to the neutron reflection measurements are 51, 48, and 39 ± 3 A 2 when the counterions are Li + , Na + , and Cs + , respectively. The effect of temperature on A cmc was found to be smaller than the change of counterion. The variation of the surface tension with temperature was used to determine the thermodynamic parameters Δ G ads , Δ H ads , and Δ S ads and Δ G mic , Δ H mic , and Δ S mic for adsorption and micellization respectively. The most noteworthy feature is that an increase in temperature decreases the surface tension of LiDS but increases it for all the other species. The structure of the adsorbed layer was determined by neutron reflection. Two methods of analysis of the data were used. One is the optical matrix method which fits a single structural model to the reflectivity profiles from a set of isotopic species at a given concentration, and the other by a more direct approach based on the kinematic theory. Both methods give identical structures for all three systems. The thickness of the layers at their corresponding cmc's are 18 A for LiDS, 18 A for NaDS, and 19 ± 1 A for CsDS and the number of water molecules associated with each surfactant head group region is respectively eight, seven, and four. The distance from the center of the alkyl chain distribution to that of the solvent is found to be 6.5 A for LiDS, 7.5 A for NaDS, and 8.0 ± 1 A for CsDS. The extent to which the alkyl chain is immersed in water decreases with the area per molecule and is about 35% for LiDS and below 30% for CsDS at their cmc's.

The spreading of heat or soluble surfactant along a thin liquid film

Abstract: The spreading of a localized distribution of surfactant on a thin viscous film is considered, in the situation in which the surfactant is soluble in the bulk layer and the boundary beneath the fluid is impermeable to surfactant. The surfactant distribution is controlled by advection and diffusion both at the surface of the film, where the surfactant forms a monolayer, and in the bulk. The bulk and surface surfactant concentrations are related by linearized sorption kinetics. The surfactant diffuses rapidly across the thin fluid layer, and lubrication theory is used to derive evolution equations for the film height and the surface and cross‐sectionally averaged bulk surfactant concentrations. A special case of the governing equations describes the Marangoni flow induced by a locally hot region of the layer. It is shown that in comparison to the spreading of insoluble surfactant, transient desorption of surfactant from the monolayer to the bulk causes the spreading rate to diminish, although once the bulk and surface concentrations are locally in equilibrium, film deformations are more severe, with a sharp pulse in the film height created just upstream of the leading edge of the surfactant distribution.

Surface crystallization of liquid normal-alkanes.

Abstract: X-ray scattering and surface tension measurements reveal the formation of a crystalline monolayer on the surface of liquid n-alkanes at about 3 \ifmmode^\circ\else\textdegree\fi{}C above the bulk solidification temperature. The molecules in the monolayer are hexagonally packed and oriented normal to the surface. The single solid monolayer persists down to the bulk solidification temperature, thus exhibiting a very limited partial wetting.

Splat-quench solidification: estimating the maximum spreading of a droplet impacting a solid surface

Abstract: This study investigates the mechanisms that contribute to determining the maximum spreading of a liquid droplet impacting a solid surface in connection with splat-quench solidification. This paper defines two domains, the viscous dissipation domain and the surface tension domain, which are characterized by the Weber and the Reynolds numbers, and that are discriminated by the principal mechanism responsible for arresting the splat spreading. This paper illustrates the importance of correctly determining the equilibrium contact angle (a surface tension characteristic that quantifies the wetting of the substrate) for predicting the maximum spreading of the splat. Conditions under which solidification of the splat would or would not be expected to contribute to terminating the spreading of the splat are considered. However, our a priori assumption is that the effect of solidification on the spreading of a droplet, superheated at impact, is secondary compared to the effects of viscous dissipation and surface tension.

Drying of Granular Ceramic Films: I, Effect of Processing Variables on Cracking Behavior

Abstract: Drying of binder-free granular ceramic films was studied to identify processing variables which affect their cracking behavior. Films were prepared from electrostatically stabilized suspensions of α-alumina in water. A critical cracking thickness (CCT) was determined, above which films would spontaneously crack during drying. The effects of particle size, liquid surface tension, drying rate, dispersion stability, and sedimentation time were evaluated by a statistical design methodology. The CCT for films prepared on glass substrates was used as a measure of the effect of each variable on cracking. The statistically significant variables were particle size, dispersion stability, and sedimentation time. The effect of substrate constraint was also studied by producing films on a Teflon substrate and a pool of liquid Hg. The observations were consistent with a capillary formed tensile stress acting on the entire film rather than differential stress generated by a moisture gradient over the film thickness.

Surface Tension Measurements of Surface Freezing in Liquid Normal Alkanes

Abstract: Surface tension measurements reveal surface freezing in liquid n-alkanes. A solid monolayer of molecules is found to exist up to 30°C above the bulk freezing point. This surface phase exists only for carbon numbers 14 n ≤ 50. The measured carbon number and temperature dependence of the surface tension is interpreted within a simple thermodynamical model based on known bulk latent heat data and surface energy considerations. The vanishing of the surface phase for n ≤ 14 is a possible transition from surface freezing to surface melting behavior.

The stress driven instability in elastic crystals: Mathematical models and physical manifestations

Abstract: Equilibrium equations and stability conditions for the simple deformable elastic body are derived by means of considering a minimum of the static energy principle. The energy is supposed to be sum of the volume (elastic) and the surface terms. The ability to change relative positions of different material particles is taken into account, and appropriate natural definitions of the first and second variations of the energy are introduced and calculated explicitly. Considering the case of negligible magnitude of the surface tension, we establish that an equilibrium state of a nonhydrostatically stressed simple elastic body (of any physically reasonable elastic energy potential and of any symmetry) possessing any small smooth part of free surface is always unstable with respect to relative transfer of the material particles along the surface. Surface tension suppresses the mentioned instability with respect to sufficiently short disturbances of the boundary surface and thus can probably provide local smoothness of the equilibrium shape of the crystal. We derive explicit formulas for critical wavelength for the simplest models of the internal and surface energies and for the simplest equilibrium configurations. We also formulate the simplest problem of mathematical physics, revealing peculiarities and difficulties of the problem of equilibrium shape of elastic crystals, and discuss possible manifestations of the above-mentioned instability in the problems of crystal growth, materials science, fracture, physical chemistry, and low-temperature physics.

Theoretical investigation of phenomena caused by heterogeneous surface tension in solids

Abstract: From the analysis of the mechanical equilibrium of an interface between two different media (the generalized Laplace equation) it follows that, in addition to the discontinuity of the normal stress, there exists a discontinuity of the tangential stress across this interface due to the surface tension gradient. Using the solution of the corresponding problem for a system “elastic solid-elastic surface” the generalization of the Cassie equation for the contact angle on the heterogeneous surface is obtained and the level of stress arising from the interaction of surface-active melt with metal is estimated.

A critical study of the simulation of the liquid-vapour interface of a Lennard-Jones fluid

Abstract: Despite the fact that the surface tension for a Lennard-Jones fluid has been simulated many times in the past, there is some considerable disagreement between the results. This paper calculates the surface tension and density profiles for the liquid-vapour interface of a Lennard-Jones fluid using molecular dynamics (MD) simulation techniques for a variety of system sizes, film thicknesses, interfacial areas, interatomic potential cut-offs, and temperatures. The results are compared with previous work in order to resolve some of the discrepancies of the past work. Combining this work with some reliable results from the past, the minimum system size, film thickness, and equilibration time necessary for the accurate description of the surface tension was determined. Using simulation results calculated for computationally-economic values of the potential cut-off, the surface tension was extrapolated to the full potential value using a tail correction and the results compared to simulations performed with long...

Compatibilizing agents in polymer blends : interfacial tension, phase morphology, and mechanical properties

Abstract: The interfacial tension, phase morphology, and phase growth was determined for four polymer blend systems: polyethylene/polystyrene, polyethylene/polyamide-6, polystyrene/polyamide-6, and polystyrene/poly(ethylene terephthalate). Generally, high interfacial tension correlates with coarse phase morphology and rapid phase coalescence. The addition of various potential compatibilizing agents to these binary blend systems results in lowered interfacial tension, finer and stabilized phase morphologies. The characteristics of different compatibilizing agents were compared for several of the blend systems. We also look at the influences of compatibilizing agents on mechanical properties of the blend systems. Some compatibilizing agents are able to produce substantial improvements in ultimate properties.

A function of lung surfactant protein SP-B

Abstract: The primary function of lung surfactant is to form monolayers at the alveolar interface capable of lowering the normal surface tension to near zero. To accomplish this process, the surfactant must be capable of maintaining a coherent, tightly packed monolayer that avoids collapse during expiration. The positively charged amino-terminal peptide SP-B1-25 of lung surfactant-specific protein SP-B increases the collapse pressure of an important component of lung surfactant, palmitic acid (PA), to nearly 70 millinewtons per meter. This alteration of the PA isotherms removes the driving force for "squeeze-out" of the fatty acids from the primarily dipalmitoylphosphatidylcholine monolayers of lung surfactant. An uncharged mutant of SP-B1-25 induced little change in the isotherms, suggesting that a specific charge interaction between the cationic peptide and the anionic lipid is responsible for the stabilization. The effect of SP-B1-25 on fatty acid isotherms is remarkably similar to that of simple poly-cations, suggesting that such polymers might be useful as components of replacement surfactants for the treatment of respiratory distress syndrome.

Role of pulmonary surfactant in airway closure: a computational study

Abstract: A numerical model that simulates airway closure by liquid bridging during expiration has been developed. The effects of both surfactant and time-varying geometry have been included; the model determines the liquid layer flow resulting from a surface tension (Rayleigh) instability, and the computation traces the film's development to closure, yielding pressure, velocity, surface shape, and surfactant concentration distributions. It is found that surfactant is effective in retarding or eliminating liquid bridging through the reduction of the mean surface tension and the action of surface tension gradients. The former effect is also critical in minimizing the magnitude of the negative pressure in the liquid layer and thus presumably in reducing the tendency for airway compliant collapse.

Energetical and Force Approaches to the Capillary Interactions between Particles Attached to a Liquid-Fluid Interface

Abstract: Theoretical expressions are derived for calculating the capillary meniscus interactions between two vertical cylinders or two spheres partially immersed in a liquid layer. In general, the radii and the contact angles of the two particles can be different. Two alternative approaches are followed. The first one consists in calculation of capillary interaction energy, which is then differentiated to get the force. In the second approach the force is determined directly by integrating the pressure and the interfacial tension. A very good coincidence between the numerical results of these two independent approaches is established. The results can contribute to a better understanding of phenomena like surface aggregation, surface coagulation, and formation of two-dimensional arrays at interfaces.

Molecular weight dependence and end-group effects on the surface tension of poly(dimethylsiloxane)

Abstract: The surface tensions of amine-, hydroxyl-, and methyl-terminated poly(dimethylsiloxane) (PDMS) oligomers with molecular weights ranging from 1000 to 75 000 are measured by pendant drop tensiometry. The surface tension increases with molecular weight for the methyl-terminated polymers, decreases with molecular weight for the amine-terminated polymers, and is nearly independent of molecular weight for the hydroxyl-terminated polymers. This behavior is attributed to differences in surface tension of the end groups and the repeat unit of the chain (σ ∞-σ e ); the methyl end group has a lower surface energy than the PDMS backbone whereas the surface energy of the amine group exceeds that of PDMS

Second Harmonic Generation Studies of Adsorption at a Liquid-Liquid Electrochemical Interface

Abstract: oo22-36~~/~~/209 ~-04a~~o~oo/o optical response time, and theability toeasilydiscriminatebetween surface species and species in the adjacent buIk media, SHG has been employed extensively at a wide variety of surfaces10J)-M In this paper we report the first application of surface SHG measurements to the study of an electrochemical liquid-liquid interface Resonant SHG measurements are used to monitor the adsorption of the surfactant molecule 2-(n-octadecylamino)naphthalene-6-sulfonate (ONS) at a water/ 1 ,2-dichloroethane interface The relative surface coverage of the anionic surfactant molecule as a function of applied potential is determined from the surface SHG signal The results of these SHG experiments are used to provide new information complementary to that obtained from measurements of the interfacial tension of the ITIES At a pH of 9, anionic ONS adsorption is found to mr at all potentials positive of the potential of zero charge (pzc) The adsorption of ONS can be described by a Frumkin isotherm with a free energy of adsorption that increases with the applied potential and with the surface coverage of ONS At a pH of 3, the combination of SHG and interfacial tension measurements is used todetermine that both the anionic and the zwitterionic forms of ONS are present at the water-dichloroethane interface At potentialsverypositiveofthepzc, theanionicONSat theinterface undergoes a field-dependent protonation of its amino group to form the zwitterion

The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on porosity formation

Abstract: Modification of the eutectic Si in Al-Si foundry alloys by adding strontium or sodium is, unfortunately, accompanied by an increase of porosity in the casting. In an attempt to understand the nature of this problem, this study used a sessile-drop method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity occur-rence. The addition of 0.01 wt pct Sr and 0.005 wt pct Na to A356 alloy decreases the surface tension of the liquid by about 19 and 10 pct, respectively, and may increase the volume shrink-age by about 12 pct. These changes to surface tension and volumetric shrinkage promote the early formation of the pores during solidification and give the availability of a longer period of growth prior to complete solidification, resulting in a larger pore size. The effect of surface tension on the pores is more significant than volumetric shrinkage. Although the predicted pore diameter increases with lower surface tension or higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in porosity that is associated with modification.

Liquid–vapor interface of water–methanol mixture. I. Computer simulation

Abstract: Results of molecular dynamics computer simulation are presented for liquid–vapor interface of water–methanol mixture of various compositions at room temperature. The composition dependence of calculated surface tension is typical of aqueous solutions of organic compounds. The outermost surface layer is saturated with methanol even at low bulk concentrations of methanol. The density profile of each component seems oscillatory at some compositions.

A surface tension method for determining binding constants for cyclodextrin inclusion complexes of ionic surfactants

Abstract: A new method has been developed for determining binding constants of complexes of cyclodextrins with surface-active compounds, including water-soluble ionic surfactants. The technique requires measuring the change in surface tension caused by addition of a cyclodextrin (CD) to aqueous solutions of the surfactant; the experimental results lead directly to inferred values of the thermodynamic activity of the surfactant. Surface tension results are reported for three different surfactants sodium dodecyl sulfate (SDS), cetylpyridinium chloride (CPC), and cetyltrimethylammonium bromide (CTAB) in the presence and in the absence of added [beta]-CD. Data for CPC have been obtained at surfactant concentrations below and above the critical micelle concentration. Correlations between surface tension and surfactant activity are expressed by the Szyszkowski equation, which subsumes the Langmuir adsorption model and the Gibbs equation. It is observed that the surface tension increases monotonically as [beta]-cyclodextrin is added to ionic surfactant solutions. At concentrations of CD well in excess of the surfactant concentration, the surface tension approaches that of pure water, indicating that neither the surfactant-CD complexes nor CD itself are surface active. Binding constants are inferred from a model that incorporates the parameters of the Szyszkowski equation and mass action constants relating to the formation of micellesmore » from monomers of the surfactant and the counterion. Evidence is given that two molecules of CD can complex the C-16 hydrocarbon chain of the cetyl surfactants. 30 refs., 5 figs., 1 tab.« less

Wetting Properties of Thin Liquid Polyethylene Propylene Films

Abstract: Polyethylene propylene films of various molecular weights, which would normally wet native-oxide-covered Si surfaces, were observed to dewet the surface when the film thickness became less than the polymer radius of gyration. These films could be made to wet either by increasing the annealing temperature or by chemically modifying the surface. The results are shown to be consistent with an expression for the spreading parameter that incorporates a stretching free energy term for the polymer chains. Measurements of the diffusion constant of the polymer on the silica surface indicated that an activation energy was required to desorb the polymer segments from the surface before diffusion could occur.