Showing papers on "Surface tension published in 2007"

Designing Superoleophobic Surfaces

Abstract: Understanding the complementary roles of surface energy and roughness on natural nonwetting surfaces has led to the development of a number of biomimetic superhydrophobic surfaces, which exhibit apparent contact angles with water greater than 150 degrees and low contact angle hysteresis. However, superoleophobic surfaces-those that display contact angles greater than 150 degrees with organic liquids having appreciably lower surface tensions than that of water-are extremely rare. Calculations suggest that creating such a surface would require a surface energy lower than that of any known material. We show how a third factor, re-entrant surface curvature, in conjunction with chemical composition and roughened texture, can be used to design surfaces that display extreme resistance to wetting from a number of liquids with low surface tension, including alkanes such as decane and octane.

Calculation of interfacial tension in polymer systems

Abstract: We propose an equation, based on “reciprocal” mean and force additivity, for calculating the interfacial tension between polymers or between a polymer and an ordinary liquid: where γ12 is the interfacial tension; γi the surface tension; γ and γ the dispersion and polar components of γi, respectively. This equation is shown to predict accurately the interfacial tension between polymers or between a polymer and an ordinary liquid. Fowkes' equation or Fowkes' equation with a geometric-mean polar term 2(γiPγ2p)1/2 is not applicable to polarlpolar systems. The interfacial tension arises mainly from disparity in the polarities of the two phases. The above equation can also be used to calculate the surface tension and polarity of polymers or organic solids from contact angle data.

Surface tension of the most popular models of water by using the test-area simulation method.

Abstract: We consider the calculation of the surface tension from simulations of several models of water, such as the traditional TIP3P, SPC, SPC/E, and TIP4P models, and the new generation of TIP4P-like models including the TIP4P/Ew, TIP4P/Ice, and TIP4P/2005. We employ a thermodynamic route proposed by Gloor et al. [J. Chem. Phys. 123, 134703 (2005)] to determine the surface tension that involves the estimate of the change in free energy associated with a small change in the interfacial area at constant volume. The values of the surface tension computed from this test-area method are found to be fully consistent with those obtained from the standard mechanical route, which is based on the evaluation of the components of the pressure tensor. We find that most models do not reproduce quantitatively the experimental values of the surface tension of water. The best description of the surface tension is given by those models that provide a better description of the vapor-liquid coexistence curve. The values of the surface tension for the SPC/E and TIP4P/Ew models are found to be in reasonably good agreement with the experimental values. From the present investigation, we conclude that the TIP4P/2005 model is able to accurately describe the surface tension of water over the whole range of temperatures from the triple point to the critical temperature. We also conclude that the test area is an appropriate methodological choice for the calculation of the surface tension not only for simple fluids, but also for complex molecular polar fluids, as is the case of water.

Influence of substrate conductivity on circulation reversal in evaporating drops.

Abstract: Nonuniform evaporation from sessile droplets induces radial convection within the drop, which produces the well-known ‘‘coffee-ring’’ effect. The evaporation also induces a gradient in temperature and consequently a gradient in surface tension, generating a Marangoni flow. Here we investigate theoretically and experimentally the thermal Marangoni flow and establish criteria to gauge its influence. An asymptotic analysis indicates that the direction of the flow depends on the relative thermal � ;

Surface adsorption and micelle formation of surface active ionic liquids in aqueous solution.

Abstract: Aqueous solutions of three kinds of surface active ionic liquids composed of the 1-alkyl-3-methylimidazolium cation have been investigated by means of surface tension and electrical conductivity measurements at room temperature (298 K). The surface tension measurements provided a series of parameters, including critical micelle concentration (cmc), surface tension at the cmc (γcmc), adsorption efficiency (pC20), and effectiveness of surface tension reduction (Πcmc). In addition, with application of the Gibbs adsorption isotherm, maximum surface excess concentration (Γmax) and minimum surface area/molecule (Amin) at the air−water interface were estimated. The effect of sodium halides, NaCl, NaBr, and NaI, on the surface activity was also investigated. It was found that both the pC20 and the Πcmc were rather larger than those reported for traditional ionic surfactants and the cmc values were somewhat lower than those for typical cationic surfactants, alkyltrimethylammonium bromides, and comparable to typica...

Effects of surface elasticity and residual surface tension on the natural frequency of microbeams

Abstract: Surface effects often play a significant role in the physical properties of micro- and nanosized materials and structures. In this letter, the authors presented a theoretical model directed towards investigation of the effects of both surface elasticity and residual surface tension on the natural frequency of microbeams. A thin surface layer was introduced on the upper and lower surfaces to rationalize the near-surface material properties that are different from the bulk material. An explicit solution is derived for the natural frequency of microbeams with surface effects. This study might be helpful for the design of microbeam-based sensors and some related measurement techniques.

Surface tensions of imidazolium based ionic liquids: anion, cation, temperature and water effect.

Abstract: This work addresses the experimental measurements of the surface tension of eight imidazolium based ionic liquids (ILs) and their dependence with the temperature (288–353 K) and water content. The set of selected ionic liquids was chosen to provide a comprehensive study of the influence of the cation alkyl chain length, the number of cation substitutions and the anion on the properties under study. The influence of water content in the surface tension was studied for several ILs as a function of the temperature as well as a function of water mole fraction, for the most hydrophobic IL investigated, [omim][PF6], and one of the more hygroscopic IL, [bmim][PF6]. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension values.

Hofmeister salt effects on surface tension arise from partitioning of anions and cations between bulk water and the air-water interface.

Abstract: We apply a recently developed surface-bulk partitioning model to interpret the effects of individual Hofmeister cations and anions on the surface tension of water. The most surface-excluded salt (Na2SO4) provides a minimum estimate for the number of water molecules per unit area of the surface region of 0.2 H2O A-2. This corresponds to a lower bound thickness of the surface region of ∼6 A, which we assume is a property of this region and not of the salt investigated. At salt concentrations ≲1 m, single-ion partition coefficients Kp,i, defined relative to Kp,Na+ = Kp,SO42− = 0, are found to be independent of bulk salt concentration and additive for different salt ions. Semiquantitative agreement with surface-sensitive spectroscopy data and molecular dynamics simulations is attained. In most cases, the rank orders of Kp,i for both anions and cations follow the conventional Hofmeister series, qualitative rankings of ions based on their effects on protein processes (folding, precipitation, assembly). Most ani...

Density and Surface Tension Measurements of Imidazolium-, Quaternary Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids: Data and Correlations

Abstract: Presented are experimental densities and surface tensions of imidazolium-, quaternary ammonium-, and phosphonium-based room-temperature ionic liquids (RTILs) in the temperature range of (298 to 350) K. Densities of the RTILs decrease slightly with temperature in the studied range (298 to 333) K. At 298 K, the densities of the phosphonium-based RTILs ranged from (0.88 to 1.05) g·cm-3, while those of the ammonium-based RTILs ranged from (1.08 to 1.37) g·cm-3. The volume expansivities of phosphonium and ammonium RTILs at 1 atm and 298 K are in the range of (5.5 to 6.5)·10-4 K-1. Surface tension and temperature relationships were established using the van der Waals−Guggenheim equation, σ = Es(1 − T/Tc)n, where n ≈ 1, for the RTILs studied. The Macleod−Sugden−Wright (MSW) equation was used to correlate surface tension with the respective molar volume of the various RTILs. The developed equation can accurately correlate surface tension for this grouping of RTILs with a maximum estimated error of 0.15 % within t...

Marangoni flow in an evaporating water droplet

Abstract: Marangoni effect has been observed in many liquids, but its existence in pure water is still a debated problem. In the present work, the Marangoni flow in evaporating water droplets has been observed by using fluorescent nanoparticles. Flow patterns indicate that a stagnation point where the surface flow, the surface tension gradient, and the surface temperature gradient change their directions exists at the droplet surface. The deduced nonmonotonic variation of the droplet surface temperature, which is different from that in some previous works, is explained by a heat transfer model considering the adsorbed thin film of the evaporating liquid droplet.

Control and measurement of the phase behavior of aqueous solutions using microfluidics

Abstract: A microfluidic device denoted the Phase Chip has been designed to measure and manipulate the phase diagram of multicomponent fluid mixtures. The Phase Chip exploits the permeation of water through poly(dimethylsiloxane) (PDMS) in order to controllably vary the concentration of solutes in aqueous nanoliter volume microdrops stored in wells. The permeation of water in the Phase Chip is modeled using the diffusion equation, and good agreement between experiment and theory is obtained. The Phase Chip operates by first creating drops of the water/solute mixture whose composition varies sequentially. Next, drops are transported down channels and guided into storage wells using surface tension forces. Finally, the solute concentration of each stored drop is simultaneously varied and measured. Two applications of the Phase Chip are presented. First, the phase diagram of a polymer/salt mixture is measured on-chip and validated off-chip, and second, protein crystallization rates are enhanced through the manipulation of the kinetics of nucleation and growth.

Design of a superhydrophobic surface using woven structures.

Abstract: The relationship between surface tension and roughness is reviewed. The Cassie-Baxter model is restated in its original form, which better describes the most general cases of surface roughness. Using mechanical and chemical surface modification of nylon 6,6 woven fabric, an artificial superhydrophobic surface was prepared. A plain woven fabric mimicking the Lotus leaf was created by further grafting 1H,1H-perfluorooctylamine or octadecylamine to poly(acrylic acid) chains which had previously been grafted onto a nylon 6,6 woven fabric surface. Water contact angles as high as 168 degrees were achieved. Good agreement between the predictions based on the original Cassie-Baxter model and experiments was obtained. The version of the Cassie-Baxter model in current use could not be applied to this problem since the surface area fractions in this form is valid only when the liquid is in contact with a flat, porous surface. The angle at which a water droplet rolls off the surface has also been used to define a superhydrophobic surface. It is shown that the roll-off angle is highly dependent on droplet size. The roll-off angles of these superhydrophobic surfaces were less than 5 degrees when a 0.5 mL water droplet was applied.

A note on the effective slip properties for microchannel flows with ultrahydrophobic surfaces

Abstract: A type of superhydrophobic surface consists of a solid plane boundary with an array of grooves which, due to the effect of surface tension, prevent a complete wetting of the wall. The effect is greatest when the grooves are aligned with the flow. The pressure difference between the liquid and the gas in the grooves causes a curvature of the liquid surface resisted by surface tension. The effects of this surface deformation are studied in this paper. The corrections to the effective slip length produced by the curvature are analyzed theoretically and a comparison with available data and related mathematical models is presented.

Physical Properties of 1-Butyl-3-methylimidazolium Methyl Sulfate as a Function of Temperature

Abstract: Density, speed of sound, refractive index, dynamic viscosity, and surface tension measurements of 1-butyl-3-methylimidazolium methyl sulfate have been made as a function of temperature. The synthesis of the ionic liquid is given. The low viscosity of the ionic liquid suggests its use as a solvent in the extraction process for the separation of azeotropic mixtures. The thermal expansion coefficient of the ionic liquid was calculated from the density, and the results are discussed. An analysis of the influence of the alkyl chain length of the cation on the density was performed by comparison with recently published values.

Line Tension at Fluid Membrane Domain Boundaries Measured by Micropipette Aspiration

Abstract: Line tension is a determinant of fluid phase domain formation kinetics and morphology in lipid bilayer membranes, which are models for biological membrane heterogeneity. We describe the first direct measurement of this line tension by micropipette aspiration. Our data are analyzed with a model that does not rely on independently measured (and composition dependent) secondary parameters, such as bending stiffness or membrane viscosities. Line tension is strongly composition dependent and decreases towards a critical consolute point in a quasiternary room temperature phase diagram.

The size-dependent bending elastic properties of nanobeams with surface effects

Abstract: A theoretical model is presented to investigate the size-dependent bending elastic properties of a nanobeam with the influence of the surface relaxation and the surface tension taken into consideration. The surface layer and its thickness of a nanostructure are defined unambiguously. A three-dimensional (3D) crystal model for a nanofilm with n layers of relaxed atoms is investigated. The four nonzero elastic constants of the nanofilm are derived, and then the Young's modulus for simple tension is obtained. Using the relation of energy equilibrium, the size-dependent effective elastic modulus and effective flexural rigidity of a nanobeam with two kinds of cross sections are derived, and their dependence on the surface relaxation and the surface tension is analysed.

Polymer brushes in solvents of variable quality: molecular dynamics simulations using explicit solvent.

Abstract: The structure and thermodynamic properties of a system of end-grafted flexible polymer chains grafted to a flat substrate and exposed to a solvent of variable quality are studied by molecular dynamics methods. The macromolecules are described by a coarse-grained bead-spring model, and the solvent molecules by pointlike particles, assuming Lennard-Jones-type interactions between pairs of monomers (ϵpp), solvent molecules (ϵss), and solvent monomer (ϵps), respectively. Varying the grafting density σg and some of these energy parameters, we obtain density profiles of solvent particles and monomers, study structural properties of the chain (gyration radius components, bond orientational parameters, etc.), and examine also the profile of the lateral pressure P‖(z), keeping in the simulation the normal pressure P⊥ constant. From these data, the reduction of the surface tension between solvent and wall as a function of the grafting density of the brush has been obtained. Further results include the stretching fo...

Size-Dependent Elastic State of Ellipsoidal Nano-Inclusions Incorporating Surface∕Interface Tension

Abstract: The determination of elastic states of an embedded inclusion is of considerable importance in a wide variety of physical problems. In the classical elasticity context this problem was first solved rigorously by 1. The latter work, both with and without modifications, has been employed to tackle a diverse set of problems: Localized thermal heating, residual strains, dislocationinduced plastic strains, phase transformations, overall or effective elastic, plastic and viscoplastic properties of composites, damage in heterogeneous materials, quantum dots, interconnect reliability, microstructural evolution, to name a few. The classical solution of an embedded inclusion neglects the presence of surface or interface energies and indeed, the effects of those are negligible except in the size range of tens of nanometers, where one contends with a significant surface-to-volume ratio. Clearly, the influence of surface/interface energies only extends to the nanoscale regime, as illustrated by various mechanical and optoelectronic applications

Surface modification of wood flour and its effect on the properties of pp/wood composites

Abstract: The surface of wood flour used as reinforcement in PP/wood composites was successfully modified by benzylation in NaOH solution of 20 wt% concentration at 105 °C. The time of the reaction was changed between 5 and 360 min in several steps. The progress of modification was followed by the measurement of weight increase and by diffuse reflectance infrared spectroscopy (DRIFT). The structure of the wood was characterized by X-ray diffraction (XRD) and its surface tension was determined by inverse gas chromatography (IGC). PP composites containing 20 wt% filler were prepared from a PP block copolymer and the modified wood flour. The mechanical behavior of the composites was characterized by tensile testing. The majority of the active hydroxyl groups at the surface were replaced by benzyl groups in about 2 h under the conditions used. Further increase in reaction time did not influence the properties of the filler. Both the structure of the wood flour and its surface tension changed as an effect of modification. The reduction of surface tension led to significant changes in all interactions between the wood flour and other substances resulting in a considerable decrease of water absorption, which is the major benefit of this modification. All measured mechanical properties of the composites decreased slightly with increasing degree of modification. A detailed analysis of the results proved that the dominating micromechanical deformation process of these PP/wood composites is debonding, which is further facilitated by the decrease in the surface tension of the filler. Chemical modification of wood flour slightly improved processability and the surface appearance of the composites prepared with them and considerably decreased the water absorption of these latter.

Short-term dynamics of a density interface following an impact

Abstract: A tube filled with a perfectly wetting liquid falls axially by its own weight. In its gravity free reference frame, the liquid interface deforms by surface tension into a hemispherical shape. At the impact of the tube on a rigid floor, the interface curvature reverses violently, forming a concentrated jet. If the contact angle at the tube wall is such that the interface is flat, the liquid rebounds as a whole with the tube, with no deformation. We analyze this phenomenon using an impulse pressure description, providing an exact description of the initial liquid velocity field at the impact, supported by high speed image velocimetry measurements. This initial dynamics is insensitive to liquid surface tension and viscosity.