Showing papers on "Surface tension published in 2008"

Robust omniphobic surfaces

Abstract: Superhydrophobic surfaces display water contact angles greater than 150° in conjunction with low contact angle hysteresis. Microscopic pockets of air trapped beneath the water droplets placed on these surfaces lead to a composite solid-liquid-air interface in thermodynamic equilibrium. Previous experimental and theoretical studies suggest that it may not be possible to form similar fully-equilibrated, composite interfaces with drops of liquids, such as alkanes or alcohols, that possess significantly lower surface tension than water (γlv = 72.1 mN/m). In this work we develop surfaces possessing re-entrant texture that can support strongly metastable composite solid-liquid-air interfaces, even with very low surface tension liquids such as pentane (γlv = 15.7 mN/m). Furthermore, we propose four design parameters that predict the measured contact angles for a liquid droplet on a textured surface, as well as the robustness of the composite interface, based on the properties of the solid surface and the contacting liquid. These design parameters allow us to produce two different families of re-entrant surfaces— randomly-deposited electrospun fiber mats and precisely fabricated microhoodoo surfaces—that can each support a robust composite interface with essentially any liquid. These omniphobic surfaces display contact angles greater than 150° and low contact angle hysteresis with both polar and nonpolar liquids possessing a wide range of surface tensions.

Liquid-vapor phase-change phenomena : an introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment

Abstract: Pt. 1. Thermodynamic and mechanical aspects of interfacial phenomena and phase transitions -- 1. Liquid-vapor interfacial region - A nanoscale perspective -- 1.1. Molecular perspective on liquid-vapor transitions -- 1.2. Interfacial region - Molecular theories of capillarity -- 1.3. Nanoscale features of the interfacial region -- 1.4. Molecular dynamics simulation studies of interfacial region thermophysics -- 2. Liquid-vapor interface - a macroscopic treatment -- 2.1. Thermodynamic analysis of interfacial tension effects -- 2.2. Determination of interface shapes at equilibrium -- 2.3. Temperature and surfactant effects on interfacial tension -- 2.4. Surface tension in mixtures -- 2.5. Near critical point behavior -- 2.6. Effects of interfacial tension gradients -- 3. Wetting phenomena and contact angles -- 3.1. Equilibrium contact angles on smooth surfaces -- 3.2. Wettability, cohesion, and adhesion -- 3.3. Effect of liquid surface tension on contact angle -- 3.4. Adsorption --^

Decompressing emulsion droplets favors coalescence.

Abstract: The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely, the coalescence of two droplets which collide. We demonstrate a counterintuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connection of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation. Finally, we note that the fusion mechanism is responsible for a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.

Geometry and a priori estimates for free boundary problems of the Euler's equation

Abstract: In this paper we derive estimates to the free boundary problem for the Euler equation with surface tension, and without surface tension provided the Rayleigh-Taylor sign condition holds. We prove that as the surface tension tends to 0, when the Rayleigh-Taylor condition is satisfied, solutions converge to the Euler flow with zero surface tension. © 2007 Wiley Periodicals, Inc.

The influence of surface mechanics on diffusion induced stresses within spherical nanoparticles

Abstract: We examine the effects of surface tension and surface modulus on diffusion-induced stresses within spherical nanoparticles. We show that both the magnitude and distribution of stresses can be significantly affected by surface mechanics if the particle diameter is in the nanometer range. In particular, a tensile state of stress may be significantly reduced in magnitude or even be reverted to a state of compressive stress with decreasing particle radius. This reduction in tensile stress may be responsible for the observed resilience to fracture and decrepitation of nanoparticles used in various industrial applications.

The molecular mechanism of lipid monolayer collapse.

Abstract: Lipid monolayers at an air-water interface can be compressed laterally and reach high surface density. Beyond a certain threshold, they become unstable and collapse. Lipid monolayer collapse plays an important role in the regulation of surface tension at the air-liquid interface in the lungs. Although the structures of lipid aggregates formed upon collapse can be characterized experimentally, the mechanism leading to these structures is not fully understood. We investigate the molecular mechanism of monolayer collapse using molecular dynamics simulations. Upon lateral compression, the collapse begins with buckling of the monolayer, followed by folding of the buckle into a bilayer in the water phase. Folding leads to an increase in the monolayer surface tension, which reaches the equilibrium spreading value. Immediately after their formation, the bilayer folds have a flat semielliptical shape, in agreement with theoretical predictions. The folds undergo further transformation and form either flat circular bilayers or vesicles. The transformation pathway depends on macroscopic parameters of the system: the bending modulus, the line tension at the monolayer-bilayer connection, and the line tension at the bilayer perimeter. These parameters are determined by the system composition and temperature. Coexistence of the monolayer with lipid aggregates is favorable at lower tensions of the monolayer-bilayer connection. Transformation into a vesicle reduces the energy of the fold perimeter and is facilitated for softer bilayers, e.g., those with a higher content of unsaturated lipids, or at higher temperatures.

Thermodynamic Origin of Hofmeister Ion Effects

Abstract: Quantitative interpretation and prediction of Hofmeister ion effects on protein processes, including folding and crystallization, have been elusive goals of a century of research. Here, a quantitative thermodynamic analysis, developed to treat noncoulombic interactions of solutes with biopolymer surface and recently extended to analyze the effects of Hofmeister salts on the surface tension of water, is applied to literature solubility data for small hydrocarbons and model peptides. This analysis allows us to obtain a minimum estimate of the hydration b1 (H2O A−2), of hydrocarbon surface and partition coefficients Kp, characterizing the distribution of salts and salt ions between this hydration water and bulk water. Assuming that Na+ and SO42− ions of Na2SO4 (the salt giving the largest reduction in hydrocarbon solubility as well as the largest increase in surface tension) are fully excluded from the hydration water at hydrocarbon surface, we obtain the same b1 as for air-water surface (∼0.18 H2O A−2). Ran...

Surface Tension and Density of Pure Ionic Liquids and Some Binary Mixtures with 1-Propanol and 1-Butanol

Abstract: The pendant drop method has been used for measuring the surface tension of the pure ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium ethylsulfate, and 1-butyl-3-methylimidazolium octylsulfate in the range of (278 to 333) K. In addition, densities and thermal expansion coefficients of these ionic liquids are presented. The values of surface tension lie between (25 and 48) mN·m−1 and decrease with temperature. Surface tension σ of the mixtures of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide + 1-propanol and + 1-butanol have also been determined as a function of the mole fraction at 298 K. Since the pendant drop method requires precise density measurements of the mixtures, densities were also measured.

Coarse-grained molecular modeling of non-ionic surfactant self-assembly

Abstract: A coarse-grained (CG) molecular model for nonionic surfactants is presented. The transferability and versatility are demonstrated by applying the model to the bulk aqueous solution as well as to the air–water and oil–water interfacial systems. The model is designed to reproduce several key properties including surface/interfacial tension, bulk density, compressibility, hydration/transfer free energy as well as distribution functions obtained by all-atom molecular dynamics simulations. The CG surfactants are demonstrated to spontaneously organize into micelles, and either hexagonal or lamellar bulk structures, respectively, at the corresponding concentration and thermodynamic conditions for those phases. Moreover, even a hexagonal-to-lamellar phase transformation is attainable using the present CG model. The experimental surface pressure–area (π–A) curve for a representative surfactant monolayer at the air–water interface is well reproduced. Micelle budding is observed from an overly compressed monolayer; a phenomenon that is likely to be a reasonable finding because the hydration/transfer free energy is a fitted parameter in the model. The correct molecular partitioning together with a relatively rapid relaxation of the CG model system towards equilibrium enables the computation of self-assembled surfactant behavior at the mesoscale regime.

Surface Tensions for the 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids

Abstract: This work addresses surface tension measurements of eight imidazolium-based ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide, Tf2N, common anion, and their dependence with temperature, from (293 to 353) K, at atmospheric pressure. The set of selected ionic liquids was chosen to provide a detailed and comprehensive study of the influence of the cation alkyl chain length on the surface tensions of the ionic liquids based on the Tf2N anion. It is shown that, unlike other ionic liquids, the surface tensions of the Tf2N family do not have a linear decrease with the chain length. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension, and their values indicate the importance of the surface ordering in ionic liquids. The use of the Guggenheim and Eotvos correlations for the estimation of the critical temperatures of ionic liquids is discussed, and the applicability of the Stefan rule to ionic liquids is analyzed.

Contact angle hysteresis on regular pillar-like hydrophobic surfaces.

Abstract: A series of pillar-like patterned silicon wafers with different pillar sizes and spacing are fabricated by photolithography and further modified by a self-assembled fluorosilanated monolayer. The dynamic contact angles of water on these surfaces are carefully measured and found to be consistent with the theoretical predictions of the Cassie model and the Wenzel model. When a water drop is at the Wenzel state, its contact angle hysteresis increases along with an increase in the surface roughness. While the surface roughness is further raised beyond its transition roughness (from the Wenzel state to the Cassie state), the contact angle hysteresis (or receding contact angle) discontinuously drops (or jumps) to a lower (or higher) value. When a water drop is at the Cassie state, its contact angle hysteresis strongly depends on the solid fraction and has nothing to do with the surface roughness. Even for a superhydrophobic surface, the contact angle hysteresis may still exhibit a value as high as 41 degrees for the solid fraction of 0.563.

On the free boundary problem of three‐dimensional incompressible Euler equations

Abstract: In this paper, we consider in three dimensions the motion of a general inviscid, incompressible fluid with a free interface that separates the fluid region from the vacuum. We assume that the fluid region is below the vacuum and that there is no surface tension on the free surface. Then we prove the local well-posedness of the free boundary problem in Sobolev space provided that there is no self-intersection point on the initial surface and under the stability assumption that with ξ being restricted to the initial surface. © 2007 Wiley Periodicals, Inc.

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

Abstract: 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.

Temperature dependence of interfacial properties and viscosity of nanofluids for droplet-based microfluidics

Abstract: Interfacial tension and viscosity of a liquid play an important role in microfluidic systems. In this study, temperature dependence of surface tension, interfacial tension and viscosity of a nanofluid are investigated for its applicability in droplet-based microfluidics. Experimental results show that nanofluids having TiO2 nanoparticles of 15?nm diameter in deionized water exhibit substantially smaller surface tension and oil-based interfacial tension than those of the base fluid (i.e. deionized water). These surface and interfacial tensions of this nanofluid were found to decrease almost linearly with increasing temperature. The Brownian motion of nanoparticles in the base fluid was identified as a possible mechanism for reduced surface and interfacial tensions of the nanofluid. The measured effective viscosity of the nanofluid was found to be insignificantly higher than that of the base fluid and to decrease with increasing fluid temperature. The dependence on the temperature of the droplet formation at the T-junction of a microfluidic device is also studied and the nanofluid shows larger droplet size compared with its base fluid.

Pressure tensor calculation in a class of nonideal gas lattice Boltzmann models.

Abstract: In nonideal gas lattice Boltzmann (LB) models, obtaining the correct form of the pressure tensor is essential in determining many of the statistical mechanical properties such as the surface tension and the density profile. Here we outline a general approach for calculating the pressure tensor in LB models with interactions beyond nearest neighbors. The statistical mechanical properties calculated from such a pressure tensor are shown to agree very well with those measured from numerical experiments. Comparisons with alternative theories are also made.

Molecular interactions of cationic and anionic surfactants in mixed monolayers and aggregates.

Abstract: The properties of anionic-rich and cationic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were investigated with conductometry and surface tension measurements and by determining the surfactant NMR self-diffusion coefficients. The critical aggregate concentration (CAC), surface tension reduction effectiveness(γCAC), surface excess(Γmax), and mean molecular surface area (Amin) were determined from plots of the surface tension (γ) as a function of the total surfactant concentration. The compositions of the adsorbed films (Z) and aggregates (χ) were estimated by using regular solution theory, and then the interaction parameters in the aggregates (β) and the adsorbed film phases (βσ) were calculated. The results showed that the synergism between the surfactants enhances the formation of mixed aggregates and reduces the surface tension. Further, the nature and strength of the interaction between the surfactants in the mixtures were obtained by calculating the values of...

Organic el element

Abstract: PROBLEM TO BE SOLVED: To obtain an organic EL element in which a light-emitting material of a low molecular material is applied by an ink jet method. SOLUTION: A low molecular light-emitting material and a non-light-emitting high molecular compound having a low surface tension are used for the light-emitting layer of the organic EL element. The high molecular compound has a surface tension of ≤35 mN/m, and the high molecular compound extracted from the light-emitting layer does not involve absorption in which a chemical shift value (δ value) of nuclear magnetic resonance spectrum of protons is ≥6.5 ppm. COPYRIGHT: (C)2009,JPO&INPIT

Subcritical to supercritical mixing

Abstract: Liquid jet injection into a quiescent gaseous environment has been experimentally and analytically studied covering subcritical to supercritical conditions. The focus was placed on the influence that the surrounding gas pressure and temperature have on the jet breakup. Under subcritical conditions, the surrounding gas inertia and surface tension forces controlled this process with ligament formation from which the material broke off and the drops, later, formed. Decreasing surface tension influenced the jet surface behavior under transcritical conditions: Ligament formation was significantly reduced under these conditions with only occasional drop formation. Further increasing the pressure and temperature led to supercritical breakup modes. This manifested through a smoothening of the liquid-gas interface. Ligament formation was not observed under supercritical conditions; this indicated that surface tension did not play any role in the supercritical jet breakup. The experimental technique, using planar laser induced fluorescence, revealed important core jet structures previously undetected. A linear jet stability analysis was, then, performed to gain physical insight into the jet breakup mechanisms. The results showed good correlation with experimental results under subcritical mixing but failed to predict the transcritical and supercritical regimes.

Height functions for applying contact angles to 2D VOF simulations

Abstract: Recent work on a consistent representation of surface tension and on the accurate computation of interface curvature has extended the applicability of the volume-of-fluid, or VOF, method to surface tension-driven phenomena. We have extended these methodologies to interfaces near solid surfaces; specifically, we show how height functions can be used to enforce a contact angle boundary condition at a contact line, for the full range of contact angles. As such, this work may be viewed as following up on the methodology of Renardy et al. (J. Comput. Phys. 2001; 171:243–263). This approach yields accurate values of curvature and the surface tension force at contact lines, values that converge with mesh refinement. The efficacy of the methodology is demonstrated using 2D examples of static fluid configurations in equilibrium and dynamic contact line phenomena that approach known equilibrium configurations. Copyright © 2007 John Wiley & Sons, Ltd.