Effect Of Surface Tension Gradient Research Articles

Unveiling the Interactions between Water Molecule Clusters and Conical Structures via Molecular Dynamics Simulations.

Water scarcity presents a pressing global challenge, necessitating innovative solutions, such as the collection of water from the air using conical structures. However, current research primarily focuses on mist collection rather than on nanoscale clusters of water molecules. Under standard atmospheric conditions, water vapor predominantly exists as imperceptible clusters. Therefore, it is crucial to investigate the interactions between these water molecule clusters and conical structures, particularly regarding whether the conical shape induces Laplace pressure difference on the adhering cluster formations. To gain deeper insights and determine optimal droplet collection structures, we conducted molecular dynamics simulations to investigate interactions between water molecule clusters and conical structures. Our investigations focused on studying the interactions between conical structures and water molecule clusters with varying densities, as well as the impact of surface energies on the collection of water by these conical structures. Notably, our simulations unveiled the significant roles played by van der Waals forces and Laplace pressure in the process of collecting water molecule clusters. Furthermore, our simulations revealed that Janus conical structures, featuring two distinct surface energy regions, played a crucial role in promoting the aggregation of water molecules, resulting in the formation of larger droplets. This aggregation was driven by surface tension gradients, which arise from the contrasting wetting properties in different regions of the Janus structure. As a consequence, under the influence of gravitational forces, these larger droplets could eventually detach from the structure. Through the combined effects of surface tension gradients and gravitational forces, Janus conical structures offer a promising avenue for enhancing the collection efficiency of water from the air. Our research sheds light on the fundamental mechanisms governing water molecule cluster-based water collection and provides valuable insights for the design of more efficient and effective water collection systems.

Effects of the surface tension gradient and viscosity on coalescence-induced droplet jumping on superamphiphobic surfaces

With the development of superhydrophobic surface preparation technology, coalescence-induced droplet jumping shows broad application prospects in the fields of enhanced condensation heat transfer and self-cleaning. In this work, the coalescence-induced jumping process of heterogeneous and homogeneous droplets on superamphiphobic surfaces was studied by using glycerol–water mixtures with different glycerol volume fractions. The results showed that the surface tension gradient of heterogeneous droplets will lead to asymmetric deformation of droplets, asymmetric distribution of internal pressure of droplets, as well as decrease in the energy conversion efficiency and the vertical departure velocity. Our study also revealed that the effects of surface tension gradient and viscosity on droplet jumping are different in the two regions. When the glycerol volume fraction is less than 40%, the droplet velocity and energy conversion are dominated by the surface tension gradient, and the vertical departure velocity and the energy conversion efficiency of homogeneous droplets are larger. When the glycerol volume fraction is greater than 40%, the droplet velocity and energy conversion are dominated by the surface tension gradient and viscosity together, and the vertical departure velocity and the energy conversion efficiency of heterogeneous droplets are larger.

Distribution control and formation mechanism of gas inclusions in directionally solidified Al2O3-Er3Al5O12-ZrO2 ternary eutectic ceramic by laser floating zone melting

Distribution control and formation mechanism of gas inclusions formed in directionally solidified Al2O3-Er3Al5O12-ZrO2 eutectic ceramic rods are explored during laser floating zone melting. In atmospheric environment, highly-dense bubble-free eutectic rods are well fabricated at low solidification rate (<25 μm/s). Gas inclusions form intermittently when the solidification rate is in the range of 25−50 μm/s, but produce continuously at higher solidification rates (100−200 μm/s). The gas inclusions exhibit an elongated finger-like pattern along the growth direction, which of the maximum value of diameter first increases and then decreases with increasing the solidification rate. Meanwhile, the volume fraction of gas inclusions increased gradually with the solidification rate. Based on the effect of surface tension gradient, heterogeneous nucleation of gas bubbles is evaluated to be the primary formation mechanism of gas inclusions.

Marangoni effect on the motion of a droplet covered with insoluble surfactant in a square microchannel

Despite its significance in various applications, e.g., droplet microfluidics and chemical enhanced oil recovery, the motion of surfactant-laden droplets in non-circular microchannels remains an unsolved fundamental problem. To facilitate studies in this area, we present a systematic investigation on the motion of a droplet covered with an insoluble surfactant in a square microchannel. This work is realized via our three-dimensional front-tracking finite-difference model with integration of the convection-diffusion equation for surfactant transport on a deforming drop surface. Our results indicate significant effects of the surfactant on steady-state characteristics of droplet motion, especially the droplet-induced additional pressure loss in the channel. More particularly, the surfactant-induced reduction in drop surface tension remarkably lowers the additional pressure loss, but this effect can be fully counteracted by the effect of surface tension gradient induced Marangoni stress (i.e., to enlarge the additional pressure loss). The increasing effect of the Marangoni stress is primarily determined by two surfactant-related dimensionless parameters, i.e., the surface Peclet number and the elasticity number. The additional pressure loss significantly increases with either of them increasing. Besides, the Marangoni effect on the droplet-induced additional pressure loss also strongly depends on three other independent parameters, i.e., it is inhibited by decreasing the size ratio of the drop to the channel, increasing the viscosity ratio of the drop to the surrounding fluid, or increasing the capillary number. Finally, we discuss the mechanism of the Marangoni effect on drop motion via analyzing the distributions of the surfactant concentration and drop surface velocity on a three-dimensional drop surface.

Marangoni Effects on Forced Convection of Power Law Fluids in Thin Film over an Unsteady Horizontal Stretching Surface with Heat Source

Objectives: In this paper, the effect of surface tension gradient on the transport phenomenon and temperature differences within a power-law liquid film on an unsteady horizontal stretching sheet with heat source is been investigated. Method: The flow of a fluid film and the heat transfer from the subsequent stretching surface is been found by applying similarity transformation. The numerical computation of the problem is done with aid of Maple. The system of governing non-linear partial differential equation is converted to a set of nonlinear ordinary differential equation using local similarity transformations, then solved numerically applying the Runge Kutta Felhberg-45 method. The effect of power law index n, Prandtl number Pr, the unsteadiness parameter S, Space dependent heat source parameter α 1 , temperature dependent heat source parameter β 1 , and film thickness parameter β(=η) on heat transfer and flow are studied and graphically presented. And it was found that the surface tension gradient and nonuniform heating source have tremendous impact on controlling the rate of convective heat transfer near the boundary layer region.

Influence of surface tension gradient on liquid circulation time in a draft tube airlift reactor

This paper investigates the impact of surface tension gradient on liquid circulation time in a draft tube airlift reactor using dilute alcohol solutions. The experimental work proves the existence of three bubble regimes (Regime I: no bubbles in the downcomer; Regime II: a stagnant swarm of bubbles in the downcomer; Regime III: circulation of bubbles through the reactor) in the downcomer. A paired sample t-test also confirms the presence of these hydrodynamic regimes. The liquid circulation time results can be predicted from a proposed correlation which has two independent variables: surface tension gradient in dilute alcohol solutions as a variable representing the physical property of liquid phase, and superficial gas velocity as a hydrodynamic variable. The statistical evaluation quantifies the influence of each independent variable through its elimination from the proposed correlation. Additionally, the second approach compares the rate of change in the liquid circulation time with respect to the surface tension gradient and the superficial gas velocity. It was found that the superficial gas velocity was more significant variable than the surface tension gradient for all three hydrodynamic regimes. The influence of the surface tension gradient on the liquid circulation time was insignificant at lower superficial gas velocity due to a negligible amount of bubbles in the reactor. However, the opposite was true at higher superficial gas velocity. It is evident that in the draft tube airlift reactor environment, the effect of surface tension gradient on liquid circulation time largely depends on superficial gas velocity.

Influences of deposited metal material parameters on weld pool geometry during shield metal arc welding

Weld pool geometry performs an important function in improving weld quality during welding. In the present work, the influences of deposited metal material parameters, including surface tension gradient, magnetic permittivity, and thermal expansion coefficient, on weld pool geometry were investigated. Fusion length (FL) and fusion ratio (FR) were proposed to analyze weld pool evolution quantitatively. Results reveal that surface tension gradient exerts the most significant effects on weld pool geometry. The effects of magnetic permittivity are less extensive than those of surface tension gradient, and thermal expansion coefficient exerts only slight influences on weld pool geometry. Considering the effects of surface tension gradient, magnetic permittivity, and thermal expansion coefficient, a classification method for deposited metals was proposed. Based on the characteristics of the weld pool geometry, heat and mass transfer processes in the weld pool, and application conditions, various types of deposited metals were analyzed. Comparison of the simulated and experimental results showed that the error ranges of FR and FL are 3.66–4.53% and 1.03–2.83%, respectively.

Suppression of Thermocapillary Effect in Evaporating Thin Film of Micro Heat Pipes

This paper presents a theoretical study on the flow mechanism of different types of working fluids incorporated with Marangoni effect in a microelectronics cooling device. It is known that surface tension gradient effect or thermocapillary effect can be induced by temperature gradient which leads to the thermocapillary flow. By adding a small quantity of alcohol into the pure working fluid, the characteristics of surface tension can be altered without changing other thermo physical properties of the working fluid. A theoretical model is employed to focus on the suppression of thermocapillary effect in evaporating thin liquid film. The study reveals the fluid flow mechanism of a working fluid can be altered with thermocapillary effect. Thermal performance of microelectronics cooling devices can also be enhanced by utilizing aqueous solution as the working fluid.

3D finite element simulation of TIG weld pool

The aim of this paper is to propose a three-dimensional weld pool model for the moving gas tungsten arc welding (GTAW) process, in order to understand the main factors that limit the weld quality and improve the productivity, especially with respect to the welding speed. Simulation is a very powerful tool to help in understanding the physical phenomena in the weld process. A 3D finite element model of heat and fluid flow in weld pool considering free surface of the pool and traveling speed has been developed for the GTAW process. Cast3M software is used to compute all the governing equations. The free surface of the weld pool is calculated by minimizing the total surface energy. The combined effects of surface tension gradient, buoyancy force, arc pressure, arc drag force to drive the fluid flow is included in our model. The deformation of the weld pool surface and the welding speed affect fluid flow, heat flow and thus temperature gradients and molten pool dimensions. Welding trials study is presented to compare our numerical results with macrograph of the molten pool.

Numerical Analysis of Surface Tension Gradient Effect on the Behavior of Gas Bubbles at the Solid/Liquid Interface of Steel

Numerical Analysis of Surface Tension Gradient Effect on the Behavior of Gas Bubbles at the Solid/Liquid Interface of Steel

Effect of surface tension gradient on gas hold-up enhancement in aqueous solutions of electrolytes

The effect of addition of electrolytes on gas hold-up of air/water system was investigated experimentally in a laboratory scale bubble column. The experiments were carried out with four electrolytes, namely, NaCl, MgSO 4·7H 2O, Na 2SO 4 and CaCl 2·2H 2O and the concentrations of the solutions were varied from 0 to 0.3 mol/l. Enhancement of gas hold-up was observed for all four electrolytes at concentrations less than 0.1 mol/l. With the increase in concentration, the gas hold-up showed two different trends; in Na 2SO 4 and CaCl 2·2H 2O solutions, gas hold-up formed a sharp peak after the enhancement and leveled off at a value somewhat higher than that in water, whereas in NaCl and MgSO 4·7H 2O solutions, gas hold-up leveled off immediately after the enhancement without forming any peak. Experiments were also conducted to measure the surface tensions of the solutions with special focus in the low concentration region. A strong relation between the gas hold-up enhancement and the change of surface tension with the addition of electrolyte was found. It was also observed that the concentration at which maximum value of C( dσ/ dC) 2 i.e. ( concentration × surface tension gradient with respect to concentration 2) is obtained corresponds to the concentration at which maximum gas hold-up enhancement occurs.

Two-Dimensional Self-Assembly in Diblock Copolymers

Diblock copolymers confined to a two-dimensional surface may produce uniform features of macromolecular dimensions (approximately 10-100 nm). We present a mathematical model for nanoscale pattern formation in such polymers that captures the dynamic evolution of a solution of poly(styrene)-b-poly(ethylene oxide), PS-b-PEO, in solvent at an air-water interface. The model has no fitting parameters and incorporates the effects of surface tension gradients, entanglement or vitrification, and diffusion. The resultant morphologies are quantitatively compared with experimental data.

Forces acting on water droplets falling in oil under the influence of an electric field: numerical predictions versus experimental observations

The combination of an electric field and a moderate turbulent flow is a promising technique for separating stable water–oil emulsions. Field-induced charges on the water droplets will cause adjacent droplets to align with the field and attract each other. The present work describes the forces that influence the kinematics of droplets falling in oil when exposed to an electric field. Mathematical models for these forces are presented and discussed with respect to a possible implementation in a multi-droplet Lagrangian framework. The droplet motion is mainly due to buoyancy, drag, film-drainage, and dipole–dipole forces. Attention is paid to internal circulations, non-ideal dipoles, and the effects of surface tension gradients. Experiments are performed to observe the behavior of a droplet falling onto a stationary one. The droplet is exposed to an electric field parallel to the direction of the droplet motion. The behavior of two falling water droplets exposed to an electric field perpendicular to the direction of their motion is also investigated until droplet coalescence. The droplet motion is recorded with a high-speed CMOS camera. The optical observations are compared with the results from numerical simulations where the governing equations for the droplet motion are solved by the RK45 (Runge Kutta) Fehlberg method with step-size control and low tolerances. Results, using different models, are compared and discussed in detail. A framework is otlined to describe the kinematics of both a falling rigid spherical particle and a fluid droplet under the influence of an electric field.

Effect of Surface Tension Gradient on Froth Stabilization and Tray Efficiency

The effect of the surface tension gradient on froth stabilization and distillation tray efficiency is investigated experimentally in an Oldershaw column. Six binary mixtures with a wide range of physical properties are selected for this study. A point efficiency model is proposed, which takes into account the surface tension gradient effect that gives rise to froth stabilization in gas–liquid dispersion. The model satisfactorily predicts the trend and magnitude of point efficiency with respect to concentration for different systems, especially for the surface tension positive systems where enhanced point efficiency accompanied by a sharp decline at the high concentration region is observed.

Critical Heat Flux Prediction for Pool Boiling of Binary Mixtures

The model of binary mixtures boiling crisis presented here develops the previous approach of the author to the problem. The boiling crisis is considered to be a result of irreversible increase of the area of dry spots at the heated surface. Very small dry spots always exist as nucleation sites in nucleate boiling and very high peak heat fluxes have been observed at their boundaries. In the near-critical regime the dry spots become larger and the heat flux averaged over a dry spot becomes comparable to the average heat flux at the heated wall. The equality of these heat fluxes is assumed to be the boiling crisis condition. As the liquid flow rate to the dry spot boundary is assumed to be controlled by the capillary pressure gradient, an additional effect of surface tension gradient in mixture boiling is naturally involved in the analysis. A simple correction factor to the earlier developed equation for critical heat flux in pure component boiling is derived. The resulting equation is shown to be in rather good agreement with the available experimental data on critical heat flux for boiling of binary mixtures.

Effect of nonionic surfactant on wetting behavior of an evaporating drop under a reduced pressure environment

The evaporation of sessile drops at reduced pressure is investigated. The evaporation of water droplets on aluminum and PTFE surfaces at reduced pressure was compared. It was found that water droplets on an aluminum surface exhibit a ‘depinning jump’ at subatmospheric pressures. This is when a pinned droplet suddenly depins, with an increase in contact angle and a simultaneous decrease in the base width. The evaporation of sessile water droplets with a nonionic surfactant (Triton X-100) added to an aluminum surface was then studied. The initial contact angle exhibited a minimum at 0.001 wt% Triton X-100. A maximum in the evaporation rate was also observed at the same concentration. Droplets with low surfactant concentrations are found to exhibit the ‘depinning jump.’ It is thought that the local concentration of the surfactant causes a gradient of surface tension. The balance at the contact angle is dictated by complex phenomena, including surfactant diffusion and adsorption processes at interfaces. Due to the strong evaporation near the triple line, an accumulation of the surfactant will lead to a surface tension gradient along the interface. The gradient of surface tension will influence the wetting behavior (Marangoni effect). At low surfactant concentrations the contact line depins under the strong effect of surface tension gradient that develops spontaneously over the droplet interface due to surfactant accumulation near the triple line. The maximum evaporation rate corresponds to a minimum contact angle for a pinned droplet.

Pore network simulations of drying of capillary porous media. Influence of thermal gradients

A pore network model of drying with heat transfer is developed. The model is applied to study the influence of surface tension gradients induced by thermal gradients on the phase distribution within a capillary porous medium. The numerical simulations show that surface tension gradients can lead to invasion percolation in a destabilizing gradient (IPDG) patterns or invasion percolation in a stabilizing gradient patterns depending on the sign of thermal gradient. The surface tension gradient effect is shown to be significant for sufficiently weakly disordered porous media. The results are summarized on a phase diagram delineating the various patterns that can be expected as functions of thermal gradient and disorder parameter. This diagram is pertinent to situations where occupation probability gradients induced by viscous or gravity effects are negligible. The results also indicate the possibility of a somewhat paradoxical convective drying situation when thermal gradients and disorder are such that a IPDG pattern develops. In this case, contrary to more conventional situations, it may be much more efficient to blow an air colder than the porous medium initial temperature.

Characterisation of hydraulic jumps/falls with surface tension variations in thin film flows

Abstract The effects of surface tension gradients in flowing thin films are studied. Thermodynamic viability of inverted hydraulic jump-like waves is characterised through a dimensionless parameter β arising from the control volume analysis of hydraulic jumps. The factor β is the ratio of effective gravity forces upstream to downstream when the upstream and downstream surface tensions differ. Further, the linear stability theory is numerically solved to determine critical stability due to the surface tension gradients along the free surface. The surface tension is assumed to be changing on a longer scale than the depth. This assumption allows the treatment of the spatial dependency of surface tension parametrically. Approximate analytical solutions to the perturbation system of first order in wave number give a relationship between the parameters which determines the bifurcating value of Reynolds number, Re b . This bifurcation Reynolds number explicitly depends on the initial pressure gradient imposed along the flow direction. Surface tension gradients tend to destabilise the flow. For a surface tension gradient of 1%, Re b drops about 33% from its value in the absence of surface tension gradients. The numerical results are in accordance with analytical results for limiting cases of long waves and small inertia, respectively.

Pore network simulation of evaporation of a binary liquid from a capillary porous medium

A pore-network model of evaporation of a binary liquid mixture into a ternary gas phase is developed. The model is applied to study the influence of surface tension gradients induced by composition variations of the liquid on the phase distribution within a capillary porous medium. Numerical simulations based on the proposed model show that the surface tension gradients lead to the accumulation of liquid near the open edge of the network. This surface tension gradient effect is only significant for weakly disordered porous media.

Modeling Draining Flow in Mobile and Immobile Soap Films

A mathematical model is constructed to describe the two-dimensional flow in a vertical soap film that is draining under gravity. An asymptotic analysis is employed that uses the long-wave or “lubrication” approximation. The modeling results in three coupled partial differential equations that include a number of dimensionless input parameters. The equations are solved numerically. The three functions calculated, as they vary in space and time, are the film thickness, the surface concentration of an assumed insoluble surfactant, and the slip or surface velocity. The film is assumed to be supported by “wire frame” elements at both the top and the bottom; thus the liquid area and the total surfactant are conserved in the simulation. A two-term “disjoining” pressure is included in the model that allows the development of thin, stable, i.e., “black,” films. While the model uses a simplified picture of the relevant physics, it appears to capture observed soap film shape evolution over a large range of surfactant concentrations. The model predicts that, depending on the amount of surfactant that is present, the film profile will pass through several distinct phases. These are (i) rapid initial draining with surfactant transport, (ii) slower draining with an almost immobile interface due to the surface tension gradient effect, and (iii) eventual formation of black spots at various locations on the film. This work is relevant to basic questions concerning surfactant efficacy, as well as to specific questions concerning film and foam draining due to gravity. Prospects for extension to three-dimensional soap film flows are also considered.