Surface Tension Gradient Research Articles

Variation of Surface Tension of Liquid Metal with Fluorides in Tungsten Inert Gas Welding

Real-time measuring and obtaining surface tension of liquid metal during the arc welding process is critical for studying the behavior of the weld pool, such as Marangoni flow and flow velocity. The dynamic variation of surface tension of weld pool during tungsten inert gas welding process with and without fluoride flux was measured by weld pool oscillation sensing system, and the effect of fluoride flux on surface tension and pool behavior was analyzed. The results show that the surface tension gradient of liquid metal can convert from a negative to a positive value in a critical arc time. The flow velocity of liquid metal and critical arc time significantly increased and decreased, respectively, with fluoride flux. The variation of surface tension, flow velocity, and critical arc time with fluoride flux was induced by arc temperature increasing.

Investigation of Churn and Annular Flow Transition Boundaries in Vertical Upward Gas-Water Two-Phase Flow

ABSTRACT Flow patterns in two-phase flow in vertical upward pipes are crucial for predicting phase distribution, flow transitions, and stability. Existing studies on churn to annular flow transitions primarily focus on flow rates, physical parameters, pressure, and temperature, often overlooking pipe diameter. This study addresses this gap using a multiphase pipe flow experimental setup to examine transitions under varying pipe diameters, gas flow rates, and liquid flow rates. Results show that with a constant liquid flow rate, increasing pipe diameter requires a higher gas flow rate to form annular flow, leading to greater pressure gradients and smoother fluctuations. Conversely, with a constant gas flow rate, larger pipe diameters decrease the liquid flow rate needed for annular flow, resulting in smaller pressure gradients and smoother fluctuations. Smaller pipe diameters make annular flow formation easier, with smaller pressure gradients but larger fluctuations. A novel churn-annular flow transition boundary model was developed, considering factors like gas shear force, surface tension, gravity, viscous force, and pressure gradient. Evaluated with 804 experimental data sets, the new model’s prediction accuracy exceeds 98.19%, outperforming traditional models with 53.92%-86.27% accuracy, enhancing applicability across varying diameters.

A non-contact thermocapillary driving system at the gas-liquid interface

Non-contact driving technology is widely utilized in various fields due to its advantages of being non-contact, wear-free, and low noise. Thermocapillary driving is an effective approach for non-contact driving at gas-liquid interfaces. When a temperature gradient exists at the gas-liquid interface, it generates a surface tension gradient, which drives the movement of micro-objects at the interface. This research proposes a system that utilizes an array of thermoelectric coolers (TECs) as a heat source, which changes the local temperature at the gas-liquid interface and generates surface tension gradients for driving the movement of interface objects. Experimental results demonstrate that foam particles with a diameter of 0.5 mm can achieve a maximum moving speed of 2.1 mm/s. Furthermore, the system can control multiple micro-objects at the gas-liquid interface for self-assembly. We have also developed a miniature biomimetic water strider robot, this system can drive the robot to perform linear and turning movements at the gas-liquid interface. This system provides a novel approach for non-contact driving of gas-liquid interfaces.

Reduction-Induced Self-Propelled Oscillatory Motion of Perylenediimides on Water.

The emergence of macroscopic self-propelled oscillatory motion based on molecular design has attracted continual attention in relation to autonomous systems in living organisms. Herein, a series of perylenediimides (PDIs) with various imide side chains was prepared to explore the impact of molecular design and alignment on the self-propelled motion at the air-water interface. When placed on an aqueous solution containing a reductant, a solid disk of neutral PDI was reduced to form the water-soluble, surface-active PDI dianion species, which induces a surface tension gradient in the vicinity of the disk for self-propelled motion. We found that centimeter-scale oscillatory motion could be elicited by controlling the supply rate of PDI dianion species through the reductant concentration and the structure of the imide side chains. Furthermore, we found that the onset and speed of the self-propelled motion could be changed by the crystallinity of PDI at the water surface. This design principle using π-conjugated molecules and their self-assemblies could advance self-propelled, non-equilibrium systems powered by chemical energy.

Reduction‐Induced Self‐Propelled Oscillatory Motion of Perylenediimides on Water

AbstractThe emergence of macroscopic self‐propelled oscillatory motion based on molecular design has attracted continual attention in relation to autonomous systems in living organisms. Herein, a series of perylenediimides (PDIs) with various imide side chains was prepared to explore the impact of molecular design and alignment on the self‐propelled motion at the air‐water interface. When placed on an aqueous solution containing a reductant, a solid disk of neutral PDI was reduced to form the water‐soluble, surface‐active PDI dianion species, which induces a surface tension gradient in the vicinity of the disk for self‐propelled motion. We found that centimeter‐scale oscillatory motion could be elicited by controlling the supply rate of PDI dianion species through the reductant concentration and the structure of the imide side chains. Furthermore, we found that the onset and speed of the self‐propelled motion could be changed by the crystallinity of PDI at the water surface. This design principle using π‐conjugated molecules and their self‐assemblies could advance self‐propelled, non‐equilibrium systems powered by chemical energy.

Effect of concentration of aqueous alcohol solution and layer thickness on free convection due to local heating

This paper investigates the effect of point heating, ethyl alcohol concentration, and layer thickness on the convective behavior of the solution. Laser heating enhances heat and mass transfer at low energy costs. For the first time, the research determines boundaries of the transition to the chaotic behavior of the velocity field at a change in several parameters: the initial alcohol concentration and the initial thickness of the liquid layer. Simple estimated ratios are obtained to determine the critical value of the Bond number in a binary solution. The solution behavior is predominantly influenced by the solute Rayleigh and Marangoni numbers. A small alcohol concentration (about 10%) is shown to have a stabilizing effect on the velocity field. In the course of evolution, several flow patterns change sequentially, which is associated with changes in the gradient of surface tension, solution concentration and layer thickness with evaporation time.

Dynamics of a droplet on the surfactant-infested free surface of another liquid

The dynamics of liquid droplets surfing over the surfactant-infested free surface of another liquid have been explored experimentally. We analyze the motion of oil droplets that has been initiated through the creation of a surface tension gradient resulting from the deposition of a drop of surfactant at the water surface contained in the petri dish. The experiments reveal that the location of surfactant deposition with respect to the droplet position influences its motion. Due to the presence of a surface tension gradient, the footprint area of the droplet reduces and its shape changes. We have studied the temporal variation in the velocity (|vx|) of the droplets in relation to their proximity to a wall. Based on the evolution of droplet shape and change in droplet velocity, the drop dynamics can be experimentally divided into four distinct zones. Results indicate that in zone-1, |vx| grows with t as |vx|≈tn, where n is between 0.8 and 1.0. The scaling argument shows that in this zone, the surface tension force dominates the drag force, and thereby, |vx| of the droplets increases linearly with t expressed as |vx|∝t. The experimental investigation and the scaling law exhibit a reasonable agreement. In zone-2, |vx| remains more or less constant, as it is postulated that the surface tension force balances the drag force. In zone-3, a decrease in surface tension force results in a deceleration of the droplets. In zone-4, the deceleration becomes more prominent as the droplet approaches the petri dish wall.

Marangoni-driven pattern formation in an absorbing binary mixture

We investigate the evolution of convective instability in a LiBr–water binary mixture, driven by thermal and solutal Marangoni stresses, through numerical simulations. A small perturbation in absorption at the surface disturbs the equilibrium, generating surface tension gradients that drive Marangoni flows. To isolate and better understand the interplay between the Marangoni effect and absorption, we extended the previous study by investigating the LiBr–water binary system in the absence of gravity. For the first time, we observed the formation of a stable rim in an absorbing binary mixture, which underwent a slight contraction followed by rapid Marangoni spreading. This behavior shows similarities with the flow patterns seen in the “coffee-ring” and Marangoni spreading phenomena in evaporating binary mixtures. On the subsurface, convective motion breaks into several vortices, accompanied by the formation of plumes with reduced mass fraction. As the system evolves, the symmetry of the flow pattern around the cell center breaks down. The absence of buoyancy-driven forces eliminates a key counterforce to Marangoni flows, transforming the previously ordered patterns into non-periodic oscillations, followed by the development of non-stationary but regular patterns. These results complement our earlier findings [P. F. Arroiabe et al., Phys. Fluids 36, 022119 (2024)] where gravitational forces obscured these phenomena.

Biomimetic Transparent Slippery Surface for the Locomotion of Photocontrol Droplets and Bubbles.

Directed transportation and collection of liquids and bubbles play a vital role in the survival of ecosystems. Among them, the optical response control is widely used in the fields of microfluidic chips and chemical synthesis because of its high remote operation and fast response speed. However, due to poor light transmission, the development direction of traditional near-infrared (NIR) absorbing materials in the field of visualization is limited, and there are few reports of manufacturing an operating platform that can realize the directional movement of droplets/bubbles on a single platform. Here, a transparent photo-responsive PBFS platform is prepared for droplet and bubble manipulation by coating the etched glass substrate with Prussian blue (PB) nanocubes. When near-infrared (NIR) irradiation on the PBFS platform, PB nanocubes trigger heat production by photothermal means, due to the action of Marangoni force, the surface tension on the left and right sides of the droplets and bubbles is not uniform, forming a surface tension gradient, thereby driving the movement of the droplets and bubbles. The control platform has good application potential in the field of microchemical reaction and biomedical engineering and brings new solutions to the field of transparent photothermal materials.

Axisymmetric Hertzian contact problem accounting for surface tension and strain gradient elasticity

In this paper, we investigate the axisymmetric Hertzian contact problem at micro-/nanoscale. The deformation of material bulk is described by a simplified theory of strain gradient elasticity, and the influence of surface tension is integrated based on the surface elasticity theory. Using the Mindlin's potential function method and double Fourier integral transform, the normal surface displacement induced by a concentrated force is derived in a closed form. Following this, the contact between a rigid sphere and an elastic half-space is formulated in terms of singular integral equation, which is numerically solved by applying the Gauss-Chebyshev method. The results indicate that the distribution of contact pressure is distinctly different from that in classical elasticity theory. The indented substrate tends to perform stiffer due to the effects of surface tension and strain gradient elasticity. When the contact radius is comparable with the material length parameter, the indentation force (depth) can be ten (three) times of that given by classical Hertz theory.

A comprehensive study on active mixer performance using liquid metal droplets: an experimental approach

In this experimental study, we delve into the performance of an active mixer that utilizes liquid metal droplets. Our mixer configuration follows an innovative Y-type design, where two liquid metal-based pumping mechanisms operate within the minichannel inlet branches. By applying alternating current voltage to both sides of the liquid metal, we induce a surface tension gradient, propelling the fluid from the reservoirs toward the minichannel. Notably, the two entering liquids are distinctly colored, allowing us to quantify mixing using a Mixing Index (MI) definition and image processing techniques. Our investigation centers on four key variables governing mixer performance: applied voltage, liquid metal droplet diameter, input angle between the branches, and minichannel width. Given the experimental nature of our research, we employ a central composite design method within the framework of design of experiments (DoE). From our obtained experimental results, we establish correlations between MI and the other variables. Our examination of the four mentioned parameters revealed that liquid metal droplet diameter exerts the most significant influence on the mixing index. Following this, in descending order of impact, we find applied voltage, input angle, and channel width. In the optimal dimensions of our mixer, we achieve a remarkable maximum MI of 0.867.

Menthyl acetate powered self-propelled Janus sponge Marangoni motors with self-maintaining surface tension gradients and active mixing

HypothesisSmall scale Marangoni motors, which self-generate motion by inducing surface tension gradients on water interfaces through release of surface-active “fuels”, have recently been proposed as self-powered mixing devices for low volume fluids. Such devices however, often show self-limiting lifespans due to the rapid saturation of surface-active agents. A potential solution to this is the use volatile surface-active agents which do not persist in their environment. Here we investigate menthyl acetate (MA) as a safe, inexpensive and non-persistent fuel for Marangoni motors. ExperimentsMA was loaded asymmetrically into millimeter scale silicone sponges. Menthyl acetate reacts slowly with water to produce the volatile surface-active menthol, which induces surface tension gradients across the sponge to drive motion by the Marangoni effect. Videos were taken and trajectories determined by custom software. Mixing was assessed by the ability of Marangoni motors to homogenize milliliter scale aqueous solutions containing colloidal sediments. FindingsMarangoni motors, loaded with asymmetric “Janus” distributions of menthyl acetate show velocities and rotational speeds up to 30 mm s−1 and 500 RPM respectively, with their functional lifetimes scaling linearly with fuel volume. We show these devices are capable of enhanced mixing of solutions at orders of magnitude greater rates than diffusion alone.

Positional Information-Based Organization of Surfactant Droplet Swarms Emerging from Competition Between Local and Global Marangoni Effects.

Positional information is key for particles to adapt their behavior based on their position in external concentration gradients, and thereby self-organize into complex patterns. Here, position-dependent behavior of floating surfactant droplets that self-organize in a pH gradient is demonstrated, using the Marangoni effect to translate gradients of surface-active molecules into motion. First,fields of surfactant microliter-droplets are generated, in which droplets floating on water drive local, outbound Marangoni flows upon dissolution of surfactantand concomitantly grow myelin filaments. Next, a competing surfactant based on a hydrolysable amide is introduced, which is more surface active than the myelin surfactant and thereby inhibits the local Marangoni flows and myelin growth from the droplets. Upon introducing a pH gradient, the amide surfactant hydrolyses in the acidic region, so that the local Marangoni flows and myelin growth are reestablished. The resulting combination of local and global surface tension gradients produces a region of myelin-growing droplets and a region where myelin growth is suppressed, separated by a wave front of closely packed droplets, of which the position can be controlled by the pH gradient. Thereby, it is shown how "French flag"-patterns, in synthetic settings typically emerging from reaction-diffusion systems, can also be established via surfactant droplet systems.

Interplay of phoresis and self-phoresis in active particles: Transport properties, phoretic, and self-phoretic coefficients.

Self-propelled synthetic particles have attracted scientific interest due to their potential applications as nanomotors in drug delivery and their insight into bacterial taxis. Research on their dynamics has focused on understanding phoresis and self-phoresis in catalytic Janus particles at both the nano- and microscale. This study explores the combined effects of self-diffusiophoresis and self-thermophoresis induced by exothermic chemical reactions on the surface of active particles moving in non-electrolyte media. We examine how these phoretic phenomena interact, influenced by the coupling between chemical reactions, heat generation, and the concentration and temperature fields at the particle interface. Using a theoretical framework based on the induction of surface tension gradients at the particle interface, we analyze the phoretic dynamics, quantifying parameters such as effective diffusivities, transport coefficients, and, most importantly, phoretic coefficients. Our findings provide insights into the conditions that dictate coupled or independent phoretic behaviors, with implications for drug delivery and nanomotor applications, enabling customized transport processes at the nanoscale.

Thermal improvement of the porous system through numerical solution of nanofluid under the existence of activation energy and Lorentz force

Background: The important physical phenomenon under the action of microgravity is called Marangoni convection. This convection occurs due to the surface tension gradient of the interface, which has various applications, such as crystal growth melt. Objective: This research aims to explore the effects of heat radiation, heat generation, viscous dissipation, and activation energy on the Marangoni flow of nanofluids. The development of the mathematical model takes into account the activation energy and uniform liquid properties. The Darcy–Forchheimer model is also used to emphasize the influence of porous media parameters. Method: The numerical algorithm of the shooting method based on Newton’s Raphson method is developed in MATLAB and used to find the numerical solution of the obtained equations. Findings: The temperature field is enhanced by thermophoresis parameters, Brownian motion parameters, Schmitt number, and magnetic number, but decreases in the range where the Marangoni ratio increases. The Nusselt and Sherwood numbers are increased and decreased via the Marangoni ratio parameter, respectively. Research gap: The numerical solution of the Marangoni flow of nanofluids in a porous medium under the effects of heat radiation, heat generation, viscous dissipation, and activation energy was not discussed before.

Multiple shape factor effects of nanofluids on marangoni mixed convection flow through porous medium

This study explores the effects of multiple shape factors on nanofluids within Marangoni mixed convection flows through porous media. Nanofluids, containing nanoparticles in a base fluid, show unique thermal properties, making them valuable in cooling systems, energy storage, and medical devices. Marangoni mixed convection, driven by surface tension gradients, adds complexity to fluid dynamics and heat transfer in porous media. Analytical and numerical analyses examine how various particle geometries, nanoparticle volume fractions, and porous medium configurations influence transport phenomena. Entropy analysis calculates thermodynamic irreversibilities. Findings reveal relationships between shape factors, heat transfer rates, velocity profiles, temperature distribution, and entropy generation rates, offering insights for optimizing nanofluid-based systems. The study also investigates magnetic forces, heat source/sink effects, and viscous dissipation on thermal energy transfer in Graphene oxide/water and Molybdenum disulfide/water nanofluids. Using similarity transformations, Partial Differential Equations are converted into Ordinary Differential Equations, solved with HAM and NDSolver by using the Wolfram tools. Results graphically depict velocity, temperature, entropy, and skin friction values, providing practical insights for engineering applications.

Nature of instability in flow-driven porous anodic oxide.

Self-organized porous anodic oxide films are formed by the electrochemical oxidation of reactive metal aluminum in acidic solutions in which the oxide is soluble. Recently, viscous flow models have shown using linear stability analysis that the instability results from a trade-off between the destabilizing effect of viscous flow of oxide and the stabilizing effect of oxide formation, which provides the wavelength selection mechanism for pattern formation. Anion adsorption on surface growth sites causes nonuniform compressive stress at the oxide-solution interface, which drives the flow. This anodic instability is analogous to the classical Marangoni instability induced by surface tension gradients. In this work, nature of the instability beyond the stability threshold is determined using a weakly nonlinear analysis. For the growth of well-developed pores beyond the threshold, a subcritical nature of the instability is essential. However, our weakly nonlinear analysis shows that the solutions emerging from neutral stability are supercritical in nature at all wavenumbers for the practical range of anodizing control parameters investigated. We also determine the region where the model is Hadamard stable, a necessary condition for well-posedness.

Experimental, analytical, and numerical quantification of the Marangoni effect in static refractory finger test

This study investigated the local corrosion of alumina and magnesia refractory in CaO–Al2O3–SiO2–MgO slag due to the Marangoni effect, which is a key factor for localized wear in different industrial processes, using experimental, analytical, and numerical approaches. The objective is to explore the Marangoni effect using computational fluid dynamics simulation aiming to provide insights into the dominant slag flow patterns influencing refractory corrosion and to determine whether the observed corrosion groove can be attributed solely to the Marangoni effect. Static finger tests were conducted at temperatures of 1500 and 1550 °C employing a continuous wear testing device. A two-dimensional axisymmetrical section model of the test assembly was created, incorporating concentration-dependent surface tension gradients and surface tension forces to replicate the Marangoni flow. As the surface-tension simulation required time steps in the order of 10−5 s, modeling up to the experimental time scale cannot be realized. Consequently, the simulation outcomes were extrapolated to anticipate the groove radii of the experimental corrosion steps. Corrosion rates were derived from measurements and analytical methodologies. Established analytical equations for corrosion under surface-tension-flows were adapted to enhance the accuracy of corrosion rate estimation. However, the analytical considerations yielded poor estimates. Meanwhile, the employed simulation model successfully generated plausible predictions of the Marangoni flow. Drawing from the extrapolated simulation outcomes, the projected groove radii exhibited a close correspondence to the measured values, demonstrating a relative error of approximately 3 %, 11 %, and 15 % for the magnesia system at 1500 °C and alumina systems at 1500 and 1550 °C, respectively. Taking into account the uncertainties inherent in the methodological approach, the disparities in the alumina values suggest the involvement of mechanisms beyond the Marangoni effect in the localized wear. Consequently, this study effectively clarifies the impact of the Marangoni effect on the local wear of the examined material systems.

Flow analysis of temperature-dependent variable viscosity Phan Thien Tanner fluid thin film over a horizontally moving heated plate

Comprehending the temperature-dependent variable viscosity Phan Thien Tanner fluid film flow over a horizontal heated plate with surface tension is essential to enhancing coating and lubrication process prediction models. This paper accords with the flow analysis of a temperature-dependent variable viscosity Phan Thien Tanner fluid film over a horizontal heated plate. The flow over a heated plate occurs as a result of the plate’s motion and the surface tension gradient. The Adomian decomposition method is utilized to solve a system of linear and nonlinear ordinary differential equations, resulting in series-form solutions. The series-form expressions for flow variables such as velocity, temperature, volume flow rate, and surface tension are derived. Moreover, the positions of stationary points are computed using MATHEMATICA. The analysis delineated that when the inverse capillary number, variable viscosity parameter, Deborah number, elongation parameter, and Brinkman number increase, the stationary points move closer to the heated plate. The temperature also rises with an increase in these parameters. The temperature rises with increasing viscous dissipation while it lowers with increasing thermal diffusion. When the Deborah number is high, the Phan Thien Tanner fluid behaves like a solid and the flow is only driven by the motion of the plate. A comparison between Newtonian and Phan Thien Tanner fluids is made for velocity, temperature, and stationary points as a special case.

One-step construction of light-responsive intelligent foam by the Hoffmeister effect and 2D black phosphorus: High stability and on-demand degradation

Stimulus-responsive liquid foams have gained much attention for use in various industrial applications. However, it remains challenging to construct such systems with integrated functionality of easy preparation, high stability, high foaming ability, and rapid on-demand degradation. Herein, by combining the Hofmeister effect and nanotechnology, a promising ultrastable and photoresponsive liquid foam was prepared that had a lifetime of several months and could be destroyed on demand in a few minutes. Specifically, the system was prepared by simply mixing a gelatine solution containing black phosphorus nanosheets (BPNs) and kosmotropic anions in the Hofmeister series with air in one step using only two syringes, and there were no chemical modifications or crosslinking agents required. The kosmotropic anions induced stronger hydrophobic interactions, bundling within molecular chains, and blockage of foam drainage channels, which significantly improved the foaming ability and the lifetime and mechanical properties of the foam. Moreover, rational structure design realized a promising on-demand degradation mechanism via a cascading “light trigger–heat generation–Marangoni flow generation” process occurring on the bubble surfaces. On this basis, the BPNs converted light into thermal energy, which induced Marangoni flow driven by surface tension gradients along the gas‒liquid interfaces, and the bubble film ruptured within seconds upon light illumination. The designed stimulus‒response systems combined stable, fast and repeatable processes without sacrificing the foaming abilities, thus providing a general way to control the stabilities of foams, bubbles and films.