Effect Of Capillary Waves Research Articles

Resolving the microscopic hydrodynamics at the moving contact line

New analysis techniques allow removing the smearing effect of capillary waves from molecular dynamics simulation data. The attained resolution is unprecedented and allows resolving the hydrodynamic fields down to the molecular size. Matching the continuum description obtained with finite elements allows distinguishing dissipation at the liquid-solid interface from that at the contact line.

Optical coherence viscometry.

We report a technique, named optical coherence viscometry (OCV), to measure the viscosity of Newtonian fluids in a noncontact manner. According to linear wave theory with small amplitudes, capillary waves are associated with fluid mechanical properties. To perform this measurement and avoid the overdamped effects of capillary waves in viscous fluids, transient acoustic radiation force was applied to generate capillary waves. Within a very limited field-of-view using optical coherence tomography, wave motion acquired in the time domain was analyzed using Fourier methods to study the wave velocity dispersion and attenuation relationships for capillary waves, which can reduce the fluid quantity drastically into tissue culture scale. We measure the viscosities of water, water–glycerol solutions with three concentrations, and biological plasma using the proposed OCV and compare the experimental results to theoretical calculations. OCV is sensitive to wave perturbations and can be a promising technique for measuring the viscosity of biological fluids and could be applied in future applications for measurements for lipid membranes in cell biology and tissue engineering investigation.

A complete symmetric solution for trichromatic tri-directional surface capillary-gravity waves with uniform currents in water of finite depth

Making a basic description of a typical characteristic of multichromatic multidirectional ocean surface waves and taking into account widespread wave-current interactions and the rich effects of capillary waves, a complete symmetric solution of trichromatic tri-directional waves in water of finite depth is presented, leading to a sufficient inclusion of and a centralized reflection of the existing monochromatic and multichromatic multidirectional wave theories.

Interfacial layering and capillary roughness in immiscible liquids

The capillary roughness and the atomic density profiles of extended interfaces between immiscible liquids are determined as a function of the interface area by using molecular dynamics and Lennard-Jones (12-6) potentials. We found that with increasing area, the interface roughness diverges logarithmically, thus fitting the theoretical mean-field prediction. In systems small enough for the interfacial roughness not to blur the structural details, atomic density profiles across the fluid interface are layered with correlation length in the range of molecular correlations in liquids. On increasing the system size, the amplitude of the thermally excited position fluctuations of the interface increases, thus causing layering to rapidly vanish, if density profiles are computed without special care. In this work, we present and validate a simple method, operating in the direct space, for extracting from molecular dynamics trajectories the "intrinsic" structure of a fluid interface that is the local density profile of the interface cleaned from capillary wave effects. Estimated values of interfacial properties such as the tension, the intrinsic width, and the lower wavelength limit of position fluctuations are in agreement with results collected from the literature.

Stability and interactions of nanocolloids at fluid interfaces: effects of capillary waves andline tensions

We analyze the effective potential for nanoparticles trapped at a fluid interface within asimple model which incorporates surface and line tensions as well as a thermal average overinterface fluctuations (capillary waves). For a single colloid, a reduced steepness of thepotential well hindering movements out of the interface plane compared to rigid interfacemodels is observed, and an instability of the capillary wave partition sum in the case ofnegative line tensions is pointed out. For two colloids, averaging over the capillary wavesleads to an effective Casimir-type interaction which is long ranged, power-like in theinverse distance, but whose power sensitively depends on possible restrictions ofthe colloid degrees of freedom. A nonzero line tension leads to changes in themagnitude but not in the functional form of the effective potential asymptotics.

Water/Hydrocarbon Interfaces: Effect of Hydrocarbon Branching on Single-Molecule Relaxation

Water/hydrocarbon interfaces are studied using molecular dynamics simulations in order to understand the effect of hydrocarbon branching on the dynamics of the system at and away from the interface. A recently proposed procedure for studying the intrinsic structure of the interface in such systems is utilized, and dynamics are probed in the usual laboratory frame as well as the intrinsic frame. The use of these two frames of reference leads to insight into the effect of capillary waves at the interface on dynamics. The systems were partitioned into zones with a width of 5 A, and a number of quantities of dynamical relevance, namely, the residence times, mean squared displacements, the velocity auto correlation functions, and orientational time correlations for molecules of both phases, were calculated in the laboratory and intrinsic frames at and away from the interface. For the aqueous phase, translational motion is found to be (a) diffusive at long times and not anomalous as in proteins or micelles, (b) faster at the interface than in the bulk, and (c) faster upon reduction of the effect of capillary waves. The rotational motion of water is (a) more anisotropic at the interface than in the bulk and (b) dependent on the orientation of the covalent O-H bond with respect to the plane of the interface. The effect of hydrocarbon branching on aqueous dynamics was found to be small, a result similar to the effect on the interfacial water structure. The hydrocarbon phase shows a larger variation for all dynamical probes, a trend consistent with their interfacial structure.

Interfacial profiles of miscible poly(4‐trimethylsilylstyrene)/polyisoprene bilayer films

AbstractCompositional profiles of bilayer films in the direction normal to the interfaces have been investigated by neutron reflectivity measurements and analyzed with mean field theory. The bilayer films were prepared with poly(4‐trimethylsilylstyrene) (PTMSS) and polyisoprene (PI), which constitute a miscible polymer pair and whose blends show phase separation at the lower critical solution temperature (LCST) by heating. Because we can accurately control the degree of polymerization of component polymers and can adjust the Flory–Huggins interaction parameter, χ, with the temperature, T, according to the relationship χ = 0.027–9.5/T, the phase behavior and the interfacial structure of PTMSS and PI are predictable by mean field theory. When the bilayer films of PTMSS and PI were set at 90 °C, which is a temperature below the LCST, diffusion at the interface was observed, and the original interface disappeared in several hours; this supports the idea that the polymer pair is miscible. No clear interfaces were identified below the LCST, whereas broad interfaces, compared with that of the strong segregation pairs, were observed above the LCST. The compositions of each layer are consistent with that of the coexisting phase in the polymer blends, and the interfacial widths agree well with the theoretical prediction considering the effect of capillary waves. In addition, all annealed films have a thin surface layer of PTMSS corresponding to surface segregation induced by the lower surface energy of PTMSS (with respect to that of PI). Thus, the interfacial profiles of PTMSS/PI bilayer films have been totally prospected in the framework of mean field theory. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1486–1494, 2005

Further refinements to models for the bulk‐skin sea surface temperature difference

The bulk‐skin sea surface temperature difference, ΔT, has an important impact on both satellite remote sensing and air‐sea interaction studies. Extensive in situ data from diverse environmental conditions are used to explore the dependence of ΔT on environmental conditions and to evaluate and refine models for ΔT during nighttime. Data from 10 different cruises covering large ranges of latitudes and seasons in both the Atlantic and Pacific Oceans were accumulated and processed consistently. Comparison of the measured ΔT values with the environmental parameters generally confirmed an observed tendency for ΔT to approach a near‐constant value as the wind speed increases but also demonstrated a dependence of the residual variability on the net heat flux. To adequately predict ΔT, it is necessary to use a model that incorporates a dependence on all these parameters. Existing models for ΔT are all shown to be unable to fully reproduce the observed variability under all conditions. To improve the ability to predict ΔT, a new surface renewal‐type model was developed incorporating four additional physical processes. New expressions are proposed for renewal timescales, accounting for microscale and large‐scale wave breaking, the effects of capillary waves, and a saturated shear regime. To incorporate the simultaneous effects of multiple physical processes within the sampling region and period, the modeled ΔT is computed as a weighted combination of the contributions of the relevant regimes. An initial specification of the weights, derived from the observed ΔT values and environmental parameters, demonstrates that a suitable combination of the timescales can reproduce the majority of the observed variability in ΔT. A second, objective determination of the weights, using conjugate gradient minimization, produces slightly poorer results but demonstrates that the model can be used in a prognostic manner to significantly improve upon the existing predictions of ΔT.

Incorporating Molecular Scale Structure into the van der Waals Theory of the Liquid−Vapor Interface

We have developed a new and general theory of nonuniform fluids that naturally incorporates molecular scale information into the classical van der Waals theory of slowly varying interfaces. Here the theory is applied to the liquid-vapor interface of a Lennard-Jones fluid. The method combines a molecular field treatment of the effects of unbalanced attractive forces with a locally optimal use of linear response theory to approximate fluid structure by that of the associated (hard sphere like) reference fluid. Our approach avoids many of the conceptual problems that arise in the classical theory and shows why capillary wave effects are not included in the theory. The general theory and a simplified version gives results for the interface profile and surface tension for states with different temperatures and potential energy cutoffs that compare very favorably with simulation data.

Polymer thin films and surfaces: possible effects of capillary waves.

It is discussed how the proximity of a free surface or mobile interface may affect the strain relaxation behavior in a viscoelastic material, such as a polymer melt. The eigenmodes of a viscoelastic film are thus derived, and applied in an attempt to explain the experimentally observed substantial shift of the glass transition temperature of sufficiently thin polymer films with respect to the bulk. Based on the idea that the polymer freezes due to memory effects in the material, and exploiting results from mode-coupling theory, the experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. The model is finally applied discussing the possibility of polymer surface melting. A surface molten layer is predicted to exist, with a thickness diverging as the inverse of the reduced temperature. A simple model of thin polymer film freezing emerges which accounts for all features observed experimentally so far.

Commentary on “Polymer thin films and surfaces: Possible effects of capillary waves” by S. Herminghaus. I

Commentary on “Polymer thin films and surfaces: Possible effects of capillary waves” by S. Herminghaus. I

Commentary on “Polymer thin films and surfaces: Possible effects of capillary waves” by S. Herminghaus. II

Commentary on “Polymer thin films and surfaces: Possible effects of capillary waves” by S. Herminghaus. II

Molecular dynamics simulations of the liquid–vapor interface of a molten salt. II. Finite size effects and comparison to experiment

An examination of the finite system size effects on the calculated interfacial properties of the molten salt KI is described, as an extension of the work described in the preceding paper [J. Chem. Phys. 115, 8603 (2001)]. It is shown that, for system sizes beyond ∼1000 ions, the finite size effects are accounted for by the predictions of capillary wave (CW) theory, as in van der Waals liquids. Hence, there do not appear to be additional long-range effects arising from the Coulombic interactions. Techniques used to avoid truncation effects in the Coulombic and dispersion interactions appear to have eliminated system size dependent artefacts from these sources. The scaling behavior predicted from CW theory is used to predict the bulk surface tension for the chosen interaction potential, which is then compared with experimental values, and very good agreement found. Examination of the interfacial layers show that the local structure remains strongly charge ordered, but the surface is highly roughened by clustering and, on longer length-scales, by the effect of capillary waves.

Capillary-wave effects at critical wetting in type-I superconductors

We discuss the effect of fluctuations of the superconductor-normal (SC/N) interface on the (short-range) critical wetting transition in type-I superconductors. Functional renormalization of a standard effective interface Hamiltonian shows that the fluctuation regimes found for short-range critical wetting in conventional fluid systems appear in superconductors with slight modifications. Because the fluctuation parameter omega approximately 1/(1-sqrt[2]kappa) depends on the Ginzburg-Landau parameter kappa, strong fluctuation effects would be expected in the limit kappa-->1/sqrt[2]. However, the capillary-wave spectrum of the SC/N interface has an unusual form due to a relevant magnetic field contribution which suppresses long wavelength fluctuations, invalidating conclusions drawn from the standard effective interface Hamiltonian, and validating the results of mean-field theory.

Effect of capillary waves on the double layer capacitance of the interface between two immiscible electrolytes

The effect of capillary waves on the double layer capacitance of the interface between two immiscible electrolytes has been studied. We have found that a corrugation of the interface due to capillary waves leads to an increase in the capacitance, as compared with the Gouy–Chapman result for a flat interface. The theory offers a concept of a roughness function, R̃, which determines the deviation of capacitance from the Gouy–Chapman result. R̃ is a function of Debye lengths of the two immiscible electrolytes. Analytical expression for the roughness function has been derived which shows a strong dependence of R̃ on the interfacial tension and the dielectric constants of contacting solutions. Predictions amenable to experimental tests have been discussed.

The role of visualization in clarifying interfacial heat and mass transfer mechanisms

Mechanisms governing heat and mass transfer at air-water interfaces may be studied experimentally and by mean of Direct Numerical Simulations (DNS). Flow visualizations play a central role in unraveling the mechanisms that govern these transfer rates. In particular visualizations show that the flow is organized in large structures. These are sweeps, high-speed (relative to the interface velocity) fluid traveling toward the interface, and ejections, low speed fluid moving away from the interface region. It is the frequency with which these large flow structures refresh the interface that controls mass transfer. On the liquid side, flow fluctuations in the near-interface region are relatively unimpeded, so fluid poor in solute can be transported by sweeps to the interface, and then pick up solute through diffusion before being carried away again. On the gas side, flow fluctuations in the near-interface region are strongly impeded, so mass transfer is controlled by sweeps and ejections, i.e., any events with significant interface-normal velocity. The frequency, with which these large flow structures are generated, can be computed from the DNS. Simple parameterizations, based on the mechanisms discussed above, can be developed and appear to predict mass transfer velocities in excellent agreement with experimental and numerical results. The parameterizations capture the effect of capillary waves.

A Flux Parameterization Including the Effects of Capillary Waves and Sea State

Abstract An air–sea interaction model that includes turbulent transport due to capillary waves (surface ripples) is developed. The model differs from others in that the physical premises are applicable to low wind speeds (10-m wind speed, U10 < 5 m s−1) as well as higher wind speeds. Another new feature of the model is an anisotropic roughness length, which allows a crosswind component of the stress to be modeled. The influence of the angle between the mean wind direction and the mean direction of wave propagation is included in the anisotropic roughness length. Most models are not accurate at low wind speeds and tend to underestimate fluxes in low wind speed regions such as the tropical oceans. Improvements over previous models are incorporated in the momentum roughness length parameterization. In addition, the dimensionless constant in the relationship between the capillary wave component of momentum roughness length and friction velocity is reevaluated using both wave tank data and field data. The new ...

Surface structure of liquid metals and the effect of capillary waves: X-ray studies on liquid indium

We report x-ray reflectivity (XR) and small angle off-specular diffuse scattering (DS) measurements from the surface of liquid Indium close to its melting point of $156^\circ$C. From the XR measurements we extract the surface structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surface structure factor, thermally excited capillary waves and the experimental resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvolved from the thermally excited height fluctuations. The resulting local electron density profile displays exponentially decaying surface induced layering similar to that previously reported for Ga and Hg. We compare the details of the local electron density profiles of liquid In, which is a nearly free electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles have comparable amplitudes in both metals, but surface layering decays over a length scale of $3.5\pm 0.6$ \AA for In and $5.5\pm 0.4$ \AA for Ga. Upon controlled exposure to oxygen, no oxide monolayer is formed on the liquid In surface, unlike the passivating film formed on liquid Gallium.

Gas transport across an air/water interface populated with capillary waves

An experimental study of gas transport across an air/water interface, populated by a field of standing capillary waves is presented. The experiments were conducted in a small tank containing distilled water, enriched with carbon dioxide. The capillary waves were of the Faraday type, generated by providing a small vertical vibration to the water tank. The frequency of excitation was varied from 200 to 400 Hz, giving wavelengths from 3.62 to 2.26 mm (linear estimate). The gas transport rate across the interface increased by almost two orders of magnitude as the wave slope was increased from zero to slightly above 0.2 m/m. A unique aspect of these experiments is that capillary waves were isolated from the obfuscating effects of turbulence, aerosol generation, and other phenomena typically present in wind/wave tunnel experiments. Consequently the large enhancement in gas transfer was due to the effects of capillary waves alone, demonstrating their importance in gas exchange processes. The maximum mass transfer coefficients obtained in these experiments are not achieved in typical wind/wave tunnel experiments below wind speeds of 10 m/s.

Capillary waves and air-sea gas transfer

The effects of capillary waves are considered on the transfer of gas into (or out of) solution through a gas-liquid interface. The bulk liquid is assumed to be otherwise motionless in the analysis of a preliminary problem; in this problem, a concentration boundary layer is developed as a consequence of a first-order chemical reaction that is assumed to deplete the dissolved gas in the liquid. The reaction rate determines the asymptotic thickness of the concentration boundary layer. It is shown that gas transfer through the concentration boundary layer is most enhanced by the presence of capillary waves when there is vigorous removal of dissolved gases by chemical reaction - i.e. when the reaction is fast and the boundary layer is thin. The results of this theory are then measured against gas transfer through a turbulent, sheared interface in the context of a surface renewal model. Here it is the exchange, from time to time, of fluid between the interface and the bulk that leads to the development of a thin concentration boundary layer when the bulk fluid is not saturated with dissolved gas. Capillary waves are shown to thicken the concentration boundary layer at the interface and to increase the rate of gas transfer.