Variation Of Surface Tension Research Articles

Oscillatory Marangoni flows with inertia

When the surface of a liquid has a non-uniform distribution of a surfactant that lowers surface tension, the resulting variation in surface tension drives a flow that spreads the surfactant towards a uniform distribution. We study the spreading dynamics of an insoluble and non-diffusing surfactant on an initially motionless liquid. We derive solutions for the evolution over time of sinusoidal variations in surfactant concentration with a small initial amplitude relative to the average concentration. In this limit, the coupled flow and surfactant transport equations are linear. In contrast to exponential decay when the inertia of the flow is negligible, the solution for unsteady Stokes flow exhibits oscillations when inertia is sufficient to spread the surfactant beyond a uniform distribution. This oscillatory behaviour exhibits two properties that distinguish it from that of a simple harmonic oscillator: the amplitude changes sign at most three times, and the decay at late times follows a power law with an exponent of $-3/2$. As the surface oscillates, the structure of the subsurface flow alternates between one and two rows of counter-rotating vortices, starting with one row and ending with two during the late-time monotonic decay. We also examine numerically the evolution of the surfactant distribution when the system is nonlinear due to a large initial amplitude.

Experimental investigation on flame spread over diesel fuel near sea level and at high altitude

Flame spread over a petroleum-based diesel fuel was experimentally examined at two different altitude conditions (Lhasa plateau 3658m, and Hefei Plain, 50m). Based on the test results, the flame appearance, velocity of flame spread and temperature profile near oil surface were comprehensively characterized. Two types of flame are observed during the flame spread: a blue precursor flame and a yellow main flame. The main flame possesses a flame shape under diffusion combustion, whereas the precursor flame belongs to premixed combustion. The liquid temperature initiating flame propagation over diesel fuel is lower in Lhasa, indicating that oils can more easily catch fire at a higher altitude. For a wide range of fuel temperatures, the velocity of flame spread at low-pressure region is considerably larger than that near sea level. The intrinsic reasons are predicted by variations in flashpoint and surface tension of liquid fuel with altitude. Theoretical analyses confirm that the flashpoint of liquid fuel decreases with altitude while the surface tension increases. Moreover, the measured temperature distributions normal to the oil surface reveal that the oil surface temperature as well as the liquid temperature inside the subsurface convection flow is larger at high altitude, while the time interval between the flame leading edge and the subsurface convection flow front is smaller.

Engineering Surface Energy and Nanostructure of Microporous Films for Expanded Membrane Distillation Applications.

We investigated the factors that determine surface omniphobicity of microporous membranes and evaluated the potential application of these membranes in desalination of low surface tension wastewaters by membrane distillation (MD). Specifically, the effects of surface morphology and surface energy on membrane surface omniphobicity were systematically investigated by evaluating wetting resistance to low surface tension liquids. Single and multilevel re-entrant structures were achieved by using cylindrical glass fibers as a membrane substrate and grafting silica nanoparticles (SiNPs) on the fibers. Surface energy of the membrane was tuned by functionalizing the fiber substrate with fluoroalkylsilane (FAS) having two different lengths of fluoroalkyl chains. Results show that surface omniphobicity of the modified fibrous membrane increased with higher level of re-entrant structure and with lower surface energy. The secondary re-entrant structure achieved by SiNP coating on the cylindrical fibers was found to play a critical role in enhancing the surface omniphobicity. Membranes coated with SiNPs and chemically modified by the FAS with a longer fluoroalkyl chain (or lower surface energy) exhibited excellent surface omniphobicity and showed wetting resistance to low surface tension liquids such as ethanol (22.1 mN m(-1)). We further evaluated performance of the membranes in desalination of saline feed solutions with varying surface tensions by membrane distillation (MD). The engineered membranes exhibited stable MD performance with low surface tension feed waters, demonstrating the potential application omniphobic membranes in desalinating complex, high salinity industrial wastewaters.

An experimental study of buoyancy-Marangoni convection in confined and volatile binary fluids

Marangoni convection in a volatile liquid film subject to a horizontal temperature gradient confined in a rectangular cavity is a basic problem of heat transfer and fluid mechanics with applications in evaporative cooling. The Marangoni stresses that drive this flow are due to surface tension gradients at the liquid–vapor interface. The thermocapillary stresses due to variations in surface tension with temperature for the most part drive the liquid away from hot regions. A volatile binary-fluid mixture can, however, be designed so that the solutocapillary stresses due to variations in surface tension with concentration drive the liquid instead towards hot regions, and hence oppose thermocapillary stresses. In thicker liquid layers, the flow is also affected by buoyancy. This work details an experimental study of buoyancy-Marangoni convection in a ∼0.31cm deep layer of methanol–water (MeOH–H2O) in a sealed rectangular cuvette driven by a temperature difference of ∼6°C over a horizontal distance of 4.9cm. Particle pathline visualizations and particle-image velocimetry (PIV) were used to study this flow and determine how the liquid composition, quantified in terms of the MeOH concentration CM, and noncondensables (i.e., air), quantified by the concentration of air in the gas space above the liquid ca, affect the flow. Solutocapillary effects are strong enough to drive the liquid near the free surface towards the heated end over the entire horizontal extent of the liquid layer at low ca (i.e., ca<6%), suggesting that binary-fluid coolants could significantly reduce film dryout. This flow, driven by thermocapillarity, solutocapillarity and buoyancy, is classified into four distinct flow regimes that are summarized in a cavs. CM flow regime map.

Dilational Viscoelasticity of Adsorption Layers Measured by Drop and Bubble Profile Analysis: Reason for Different Results.

The dilational viscoelasticity of adsorption layer was measured at different frequencies of drop and bubble surface area oscillations for aqueous C12EO5 solutions. The modulus values obtained by the two experimental protocols are the same for Π < 15 mN/m, while for higher surface pressures the values from drop experiments exceed those from bubble profile analysis. The nature of this phenomenon was studied using stress deformation experiments. At high surfactant concentrations the magnitude of surface tension variations is essentially higher for drops as compared with bubbles, leading to an increased viscoelasticity modulus for oscillating drops. The observed effects are analyzed quantitatively using a diffusion controlled exchange of matter model. The viscoelasticity moduli for a number of surfactants (different CnEOm and Tritons, C13DMPO, and SDS) are reported, and it is shown that the discrepancies between the data obtained by the two methods for many surfactants agree well with the predictions made here.

Understanding contact angle hysteresis on an ambient solid surface.

We report a systematic study of contact angle hysteresis (CAH) with direct measurement of the capillary force acting on a contact line formed on the surface of a long glass fiber intersecting a liquid-air interface. The glass fiber of diameter 1-2μm and length 100-200μm is glued onto the front end of a rectangular cantilever beam, which is used for atomic force microscopy. From the measured hysteresis loop of the capillary force for 28 different liquids with varying surface tensions and contact angles, we find a universal behavior of the unbalanced capillary force in the advancing and receding directions and the spring constant of a stretched meniscus by the glass fiber. Measurements of the capillary force and its fluctuations suggest that CAH on an ambient solid surface is caused primarily by two types of coexisting and spatially intertwined defects with opposite natures. The contact line is primarily pinned by the relatively nonwetting (repulsive) defects in the advancing direction and by the relatively wetting (attractive) defects in the receding direction. Based on the experimental observations, we propose a "composite model" of CAH and relevant scaling laws, which explain the basic features of the measured hysteresis force loops.

Transition from Micelle to Vesicle of a Novel Sugar-Based Surfactant Containing Trisiloxane

Abstract Vesicles form spontaneously in aqueous solution of a novel sugar-based surfactant containing trisiloxane [Si(3)N-GHA]. The transition was monitored among the monomer, micelle and vesicle through the variation of surface tension, accompanied by surface tension measurements. The vesicles of a trisiloxane containing sugar-based surfactant have been characterized by dynamic light scattering (DLS) and negative-stained transmission electron microscopy (TEM). The vesicle aggregation can be explained by the critical packing parameter derived from an analysis of chemical structure.

A semi-empirical equation for describing the surface tension of aqueous organic liquid mixtures

A thermodynamic analysis is made for the dependence on composition of the surface tension showing that the latter is a mechanical thermodynamic property which is linearly related to the reciprocal of each component standard molar surface area; and that there is no simple mixing formula linking the ideal surface tension to pure-component surface tensions.The ability of five current empirical equations to correlate the surface tension of the ideal and real binary systems water–ethanol or propan-2-ol or ethanenitrile or 1,4-dioxane at T = 298 K is examined. Polynomial equations are not able to describe the general, overall trend of surface tension variation across the entire composition range, where equations containing a hyperbolic term perform much better. This is the case of the proposed new equation in which two of its parameters can be estimated theoretically.

Bubble entrapment and escape from sprayed paint films

Abstract A model paint consisting of resin dissolved in butanol was sprayed onto horizontal glass substrates to form films varying from 100 μm to 450 μm in thickness. The substrates were photographed at 5 s intervals and image analysis software used to measure the number, diameter and velocity of air bubbles trapped in the paint layer. Painted substrates were weighed to determine the rate of solvent evaporation. Bubbles escaped from the paint in 100–900 s, the time increasing with paint thickness. The Sauter mean diameter of bubbles in films less than 300 μm in thickness decreased with time because larger bubbles escaped faster than small bubbles, while the mean diameter of bubbles in a 450 μm thick layer increased due to bubble coalescence. Bubble velocities due to movement of the liquid increased with paint thickness and reached 30 μm/s. Bubbles were observed to escape from both upward and downwards facing surfaces. Concentration gradients due to solvent evaporation in a paint film create surface tension variations that drive Marangoni flows, which bring bubbles to the paint surface. An analytical one-dimensional model of solvent diffusion was used to calculate solvent concentration variations in the paint film and the Marangoni number.

Simulation of a suspended droplet under evaporation with Marangoni effects

We investigate the Marangoni effects in a hexane droplet under evaporation and close to its critical point. A lattice Boltzmann model is used to perform 3D numerical simulations. In a first case, the droplet is placed in its own vapor and a temperature gradient is imposed. The droplet locomotion through the domain is observed, where the temperature differences across the surface is proportional to the droplet velocity and the Marangoni effect is confirmed. The droplet is then set under a forced convection condition. The results show that the Marangoni stresses play a major role in maintaining the internal circulation when the superheated vapor temperature is increased. Surprisingly, surface tension variations along the interface due to temperature change may affect heat transfer and internal circulation even for low Weber number. Other results and considerations regarding the droplet surface are also discussed.

Confinement Correction to Mercury Intrusion Capillary Pressure of Shale Nanopores.

We optimized potential parameters in a molecular dynamics model to reproduce the experimental contact angle of a macroscopic mercury droplet on graphite. With the tuned potential, we studied the effects of pore size, geometry, and temperature on the wetting of mercury droplets confined in organic-rich shale nanopores. The contact angle of mercury in a circular pore increases exponentially as pore size decreases. In conjunction with the curvature-dependent surface tension of liquid droplets predicted from a theoretical model, we proposed a technique to correct the common interpretation procedure of mercury intrusion capillary pressure (MICP) measurement for nanoporous material such as shale. Considering the variation of contact angle and surface tension with pore size improves the agreement between MICP and adsorption-derived pore size distribution, especially for pores having a radius smaller than 5 nm. The relative error produced in ignoring these effects could be as high as 44%—samples that contain smaller pores deviate more. We also explored the impacts of pore size and temperature on the surface tension and contact angle of water/vapor and oil/gas systems, by which the capillary pressure of water/oil/gas in shale can be obtained from MICP. This information is fundamental to understanding multiphase flow behavior in shale systems.

Analysis of the wetting state of super-repellent fabrics with liquids of varying surface tension

A super-repellent fabric was fabricated using oxygen plasma etching and plasma enhanced chemical vapor deposition and the influence of dual roughness on wettability in micro and nano-scale structures was analysed.

Orange peel formation due to surface tension-driven flows within drying paint films

Paint films with uniform thicknesses ranging from 150 to 820 μm were applied on stainless steel substrates using a model paint consisting of a resin dissolved in butanol and the test samples were cured in a natural convection oven at a temperature of 140°C. Photographs of the paint surface were taken during drying, and the weight loss was measured. Cellular structures appeared on the paint surface, induced by surface tension-driven flows due to solvent concentration variations. For thin films (<500 µm), the patterns disappeared in a few minutes and the dried paint surface was smooth, while for thicker paint films, wave-like structures remained on the hardened paint layer, creating an uneven surface. An analytical solution of the mass-diffusion equation was used to model solvent evaporation from the paint film and to calculate the concentration gradient and surface tension variations in the paint films. In thin films, all the solvent was depleted, and surface tension gradients disappeared before curing was complete, allowing the surface to become smooth. In thicker films, concentration gradients that drove cellular flows persisted until the paint dried, leaving orange peel on the surface.

Thermohydrodynamics of boiling in binary compressible fluids.

We numerically study the thermohydrodynamics of boiling for a CO(2) + ethanol mixture on lyophilic and lyophobic surfaces in both closed and open systems, based on a diffuse interface model for a two-component system. The corresponding wetting boundary conditions for an isothermal system are proposed and verified in this paper. New phenomena due to the addition of another component, mainly the preferential evaporation of the more volatile component, are observed. In the open system and the closed system, the physical process shows very different characteristics. In the open system, except for the movement of the contact line, the qualitative features are rather similar for lyophobic and lyophilic surfaces. In the closed system, the vortices that are observed on a lyophobic surface are not seen on a lyophilic surface. More sophisticated wetting boundary conditions for nonisothermal, two-component systems might need to be further developed, taking into account the variations of density, temperature, and surface tension near the wall, while numerical results show that the boundary conditions proposed here also work well even in boiling, where the temperature is nonuniform.

A three-dimensional pseudo-potential-based lattice Boltzmann model for multiphase flows with large density ratio and variable surface tension

In this paper, we present a three-dimensional pseudo-potential-based lattice Boltzmann (LB) model with an improved forcing scheme for multiphase flows. The Chapman–Enskog multiscale analysis shows that the proposed forcing scheme allows the lattice Boltzmann equation to recover the three-dimensional hydrodynamical equations with additional terms that correspond to the mechanical stability condition and surface tension. Validations of the present LB model with Maxwell construction, Laplace’s law and oscillation dynamics demonstrate that the model enables the density ratio to be as large as 700 in static and quasi-static cases while maintaining variable surface tension. Finally, the application of the model to simulation of the droplet motion in a microchannel shows that the model allows the analysis of important effects, including droplet surface tension, channel surface wettability, and channel surface roughness.

Experiment and simulation of the shrinkage of falling film upon direct contact with vapor

An experiment is conducted to study the shrinkage of falling film in countercurrent direct contact with saturated vapor. The experiment indicates that falling film shrinks along its width. The degree of shrinkage decreases with increasing liquid inlet temperature and flow rate. To study the details of the phenomenon, a three-dimensional multiphase computational fluid dynamics (CFD) model based on the method of volume of fluid (VOF) is developed. The model considers the variation of surface tension with temperature. The results of the simulation are consistent with the experimental data. Simulations reveal the temperature, velocity and tension surface profiles. The modified Marangoni number is compared to illustrate the influence of surface tension and demonstrates that the surface tension plays an important role in this phenomenon and the shrinkage of the film causes the heat transfer to decrease. This model is expected to be applied to the design of the industrial equipment with falling film.

Noncontact atomization of droplets using an aerial ultrasonic source with two vibrating plates

For use in mass spectrometry, we investigated the noncontact atomization of droplets using a rectangular transverse vibrating plate ultrasonic source. To determine the atomization properties of the ultrasonic source, we examined the sound pressure distribution of the standing wave acoustic field formed and observed the behavior of the atomized particles in the acoustic field. We determined the relationship between sound pressure and the conditions and location where atomization occurs with the variations in droplet surface tension and viscosity using three different compounds: water, ethanol, and glycerin. Furthermore, we clarifies the distribution of particle diameters in atomized water.

Cusps and cuspidal edges at fluid interfaces: Existence and application.

One of the intriguing questions in fluid dynamics is on the interrelation between dynamic singularities in the solutions of fluid dynamic equations - unboundedness of the velocity field in an appropriate norm - and the geometric ones - divergence of curvature at fluid interfaces. The present work focuses on two generic interfacial singularities - genuine cusps and cuspidal edges - found here in both two and three dimensions thus establishing a relation between real fluid interfaces and geometric singularity theory. The key finding is the necessary condition for the existence of geometric singularities, which is a variation of surface tension. It is also established here that the dynamic and geometric singularities entail each other only in the case of three-dimensional cusps. Explicit asymptotic solutions for the flow field and interface shape near steady-state singularities at fluid interfaces are developed as well. The practical motivation for the present study comes from the fundamental role interfacial singularities play in sustaining self-driven conversion of chemical into mechanical energy.

Numerical study of droplet evaporation in coupled high-temperature and electrostatic fields

The evaporation of a sessile water droplet under the coupled electrostatic and high-temperature fields is studied numerically. The leaky dielectric model and boiling point evaporation model are used for calculating the electric force and heat mass transfer. The free surface is captured using the volume of fluid method accounting for the variable surface tension and the transition of physical properties across the interface. The flow behaviors and temperature evolutions in different applied fields are predicted. It shows that in the coupled fields, the external electrostatic field restrains the flow inside the droplet and keeps a steady circulation. The flow velocity is reduced due to the interaction between electric body force and the force caused by temperature gradient. The heat transfer from air into the droplet is reduced by the lower flow velocity. The evaporation rate of the droplet in the high-temperature field is decreased.

Applicability of Cassie–Baxter equation for superhydrophobic fluoropolymer–silica composite films

In this study, surface topographies and wettabilities of flat and composite rough films of perfluoro-styrene (TM/S) random copolymers with silica particles were investigated. The water and oil repellencies of flat TM/S copolymer and rough silica-TM/S composite surfaces were studied with varying perfluoro and silica contents. Increase in silica content resulted in an increase in water contact angle values; water contact angle values increased from 113° up to 170°, and the resultant surfaces were showing extremely superhydrophobic behavior depending on their silica contents. However a decrease in hexadecane contact angles from 61° to 25° depending on both silica %wt and perfluoro content of the TM/S copolymer was seen. Applicability of Cassie–Baxter equation was also investigated for TM/S copolymer films with silica particles. The results showed that Cassie–Baxter equation cannot be used for superhydrophobic surfaces, however, can be applied to oleophilic surfaces with caution. The oil repellencies of TM/S flat films in the terms of contact angle hysteresis was also compared with perfluoroethyl alkyl methacrylate–methyl methacrylate copolymer (TM/MMA) by using oil drops with varying surface tensions between 20.9mN/m and 26.9mN/m. The surface oleophobicities were changed according to the characteristics of the functional groups of MMA and S. The dependency of work of adhesion results of TM/S flat surfaces on wt.% of perfluoroalkyl content was also examined.