Surface Tension Gradient Research Articles

Numerical investigation of molten pool dimension, temperature field and melting flow during pulsed laser welding of Ti-6Al-4V alloy sheets with different thicknesses

In this paper, a transient numerical model of pulsed laser welding is performed to predict the weld depth and width, temperature distribution, and melt flow. The material was made of Ti-6Al-4V with different thicknesses of laser beam energy, and an asymmetric temperature field was obtained. Three different thermal models were used to consider the laser beam energy. The results showed that the combination of Gaussian surface heat flux and heat source provides an accurate prediction of the dimensions and shape of the molten pool. The Marangoni flow due to the surface tension gradient at the surface of the molten pool had the greatest effect on the liquid metal flow. The analysis of the temperature field and dimensions of the weld bead was performed by changing the laser parameters such as welding speed and average power. The results showed that the heat affected zone and molten pool are directed toward the thin sheet. By increasing the movement speed of the laser beam, the molten pool became conical and did not penetrate completely. Also, increasing the average power has the greatest effect on the dimensions of the molten pool, temperature distribution, and liquid metal flow. Increasing the average power from 180 to 240 W increases the maximum temperature in the center of the laser beam by ∼1000 °C.

Optical Manipulation of Liquids by Thermal Marangoni Flow along the Air-Water Interfaces of a Superhydrophobic Surface.

The control of liquid motion on the micrometer scale is important for many liquid transport and biomedical applications. An efficient way to trigger liquid motion is by introducing surface tension gradients on free liquid interfaces leading to the Marangoni effect. However, a pronounced Marangoni-driven flow generally only occurs at a liquid–air or liquid–liquid interface but not at solid–liquid interfaces. Using superhydrophobic surfaces, the liquid phase stays in the Cassie state (where liquid is only in contact with the tips of the rough surface structure and air is enclosed in the indentations of the roughness) and hence provides the necessary liquid–air interface to trigger evident Marangoni flows. We use light to asymmetrically heat this interface and thereby control liquid motion near superhydrophobic surfaces. By laser scanning confocal microscopy, we determine the velocity distribution evolving through optical excitation. We show that Marangoni flow can be induced optically at structured, air-entrapping superhydrophobic surfaces. Furthermore, by comparison with numerical modeling, we demonstrate that in addition to the Marangoni flow, buoyancy-driven flow occurs. This effect has so far been neglected in similar approaches and models of thermocapillary driven flow at superhydrophobic surfaces. Our work yields insight into the physics of Marangoni flow and can help in designing new contactless, light-driven liquid transport systems, e.g., for liquid pumping or in microfluidic devices.

Effect of surfactants on jet break-up in drop-on-demand inkjet printing

Jet formation and break-up in inkjet printing has been studied and understood mainly for pure liquids. Questions remain as to the role of surfactants on the inkjet printing process at the microsecond timescale. Here, numerical and experimental results demonstrating the effects of surfactants on jet break-up and drop formation at the scales relevant to drop-on-demand inkjet printing are presented. The rapid expansion of the free surface during the fast jetting process results in a depletion of surfactants along the air–liquid interface, resulting in surface tension gradients. During ejection, surfactants are concentrated toward the head of the droplet, while the trailing ligament is found to be almost devoid of surfactants. As a consequence, the initial evolution and pinch-off of the jet from the nozzle are found to be very similar to that of pure water, even though the equilibrium surface tension of the surfactant solution is lower by a factor of two. However, particularly for strong surfactants, Marangoni forces arising from surface tension gradients between the head drop and the ligament are found to delay and can even prevent the break-up of the main drop from the ligament thereby inhibiting the formation of satellite drops.

Silicone Nanofilament Support Layers in an Open-Channel System for the Fast Reduction of Para-Nitrophenol.

Chemical vapor phase deposition was used to create hydrophobic nanostructured surfaces on glass slides. Subsequently, hydrophilic channels were created by sputtering a metal catalyst on the channels while masking the outside. The surface tension gradient between the hydrophilic surface in the channels and the outside hydrophobicity formed the open-channel system. The reduction of para-nitrophenol (PNP) was studied on these devices. When compared to nanostructure-free reference systems, the created nanostructures, namely, silicone nanofilaments (SNFs) and nano-bagels, had superior catalytic performance (73% and 66% conversion to 55% at 0.5 µL/s flow rate using 20 nm platinum) and wall integrity; therefore, they could be readily used multiple times. The created nanostructures were stable under the reaction conditions, as observed with scanning electron microscopy. Transition electron microscopy studies of platinum-modified SNFs revealed that the catalyst is present as nanoparticles ranging up to 13 nm in size. By changing the target in the sputter coating unit, molybdenum, gold, nickel and copper were evaluated for their catalytic efficiency. The relative order was platinum < gold = molybdenum < nickel < copper. The decomposition of sodium borohydride (NaBH4) by platinum as a concurrent reaction to the para-nitrophenol reduction terminates the reaction before completion, despite a large excess of reducing agent. Gold had the same catalytic rate as molybdenum, while nickel was two times and copper about four times faster than gold. In all cases, there was a clear improvement in catalysis of silicone nanofilaments compared to a flat reference system.

Passive enhancement of ammonia-water absorption by the addition of surfactants

Surface-active agents or surfactants have the potential to substantially enhance heat and mass transfer in ammonia-water absorption by reducing the surface tension of the working fluid. The enhancement is caused by interfacial turbulence at the vapor-liquid interface that results from surface tension gradients. A surfactant selection criterion is developed based on the plateau value of surface tension, critical concentration, and the critical Marangoni number required to initiate interfacial turbulence. Based on this criterion, surface-active agents, and their ideal concentrations for the enhancement of ammonia-water absorption are recommended. The preferred additives are found to be 500 PPM of 1-octanol or 2-ethyl-1-hexanol. A heat and mass transfer model is developed to predict the performance of a falling-film absorber due to the addition of surfactants at conditions representative of an absorption heat pump. The model indicates that the overall conductance of the absorber is improved by ~30% by the addition of surfactants. The results from this work can guide intensification of various coupled heat and mass transfer processes using surfactants.

Achieving Stable Patterns in Multicomponent Polymer Thin Films Using Marangoni and van der Waals Forces.

Liquid-air interfaces can be deformed by surface-tension gradients to create topography, a phenomenon useful for polymer film patterning. A recently developed method creates these gradients by photochemically patterning a solid polymer film. Heating the film to the liquid state leads to flow driven by the patterned surface-tension gradients, but capillary leveling and diffusion of surface-active species facilitate eventual dissipation of the topography. However, experiments demonstrate that using blends of high- and low-molar-mass polymers can considerably delay the decay in topography. To gain insight into this observation, we develop a model based on lubrication theory that yields coupled nonlinear partial differential equations describing how the film height and species concentrations evolve with time and space. Incorporation of a nonmonotonic disjoining pressure is found to significantly increase the lifetime of topographical features, making the model predictions qualitatively consistent with experiments. A parametric study reveals the key variables controlling the kinetics of film deformation and provides guidelines for photochemically induced Marangoni patterning of polymer films.

Alternate Route Selection of Self-Propelled Filter Papers Impregnated with Camphor for Two-Branched Water Channels.

The route selection of self-propelled filter papers impregnated with camphor for two-branched water channels was investigated. The two-branched water channel was composed of a stem channel and two branch channels, and the branch channels were connected to the stem channel at a junction. When a single camphor paper reached the junction from the stem channel, it selected one of the two routes equivalently. Three or five camphor papers which were placed on a stem channel exhibited either alternate or random route selection depending on the characteristic length between the leading and following papers, Lc. That is, the alternate route selection of the camphor papers for the two-branched water channels was observed at Lc ≤ 25 mm. By contrast, the alternate route selection was broken at Lc > 25 mm. The physicochemical meaning of the threshold value, Lth ∼ 26 mm, between the alternate and random route selections was discussed based on the experimental results. In addition, the distribution length of camphor molecules developed from the leading camphor paper and the change in the spatial gradient of surface tension around the junction supports the value of Lth. These results suggest that autonomous phenomena using inanimate self-propelled objects are important to understand collective motion in living organisms.

Numerical simulation of slag movement from Marangoni flow for GMAW with computational fluid dynamics

This study analyzed the behavior of molten slag, which is an oxide generated during spray mode gas metal arc welding, with computational fluid dynamics. The molten slag, composed mainly of silicate (SiO2), is located on the molten pool surface. It is generally assumed that the molten slag is generated around the arc plasma boundary. Therefore, in the numerical simulation in this study the slag is modeled as a spherical particle, which has a specific density and size. To compare the Marangoni flow effect, this study investigated two different cases where the surface tension gradients were different (positive and negative). In both the numerical simulation and experimental results it was found that negative surface tension gradient formed trapped slag on the bead edge while the positive surface tension gradient formed trapped slag on the center of the top surface.

Buoyancy-driven convection of droplets on hot nonwetting surfaces.

The global linear stability of a water drop on hot nonwetting surfaces is studied. The droplet is assumed to have a static shape and the surface tension gradient is neglected. First, the nonlinear steady Boussinesq equation is solved to obtain the axisymmetric toroidal base flow. Then, the linear stability analysis is conducted for different contact angles β=110^{∘} (hydrophobic) and β=160^{∘} (superhydrophobic) which correspond to the experimental study of Dash etal. [Phys. Rev. E 90, 062407 (2014)PLEEE81539-375510.1103/PhysRevE.90.062407]. The droplet with β=110^{∘} is stable while the one with β=160^{∘} is unstable to the azimuthal wave number m=1 mode. This suggests that the experimental observation for a droplet with β=110^{∘} corresponds to the steady toroidal base flow, while for β=160^{∘}, the m=1 instability promotes the rigid body rotation motion. A marginal stability analysis for different β shows that a 3-μL water droplet is unstable to the m=1 mode when the contact angle β is larger than 130^{∘}. A marginal stability analysis for different volumes is also conducted for the two contact angles β=110^{∘} and 160^{∘}. The droplet with β=110^{∘} becomes unstable when the volume is larger than 3.5μL while the one with β=160^{∘} is always unstable to m=1 mode for the considered volume range (2-5μL). In contrast to classical buoyancy driven (Rayleigh-Bénard) problems whose instability is controlled independently by the geometrical aspect ratio and the Rayleigh number, in this problem, these parameters are all linked together with the volume and contact angles.

Marangoni Force Assisted Spreading and Printing of Nanometer‐Thick Polymer Films for Ubiquitous Optoelectronic Devices

AbstractPrintable organic electronics are attractive owing to its high throughput and easy fabrication. Traditional coating methods (such as inkjet printing) are liquid‐solid contact‐based that deposit liquid solutions onto solid surfaces. Film quality is limited by surface energy of target surfaces and surface tension of the solvents. Furthermore, these printing methods are challenging to directly fabricate films on non‐flat or curved surfaces owing to the fluidity of liquid solution drops. In this work, a water transfer printing method that is solid‐solid contact‐based and assisted by Van der Waals force is reported. Nanometer‐thick films can be deposited on various surfaces: both high‐ and low‐surface energy, both flat and non‐flat surfaces. Water plays two roles: 1) as the transfer medium substrate and providing weak adhesion force between water and the transferred film; 2) high surface tension of water providing room to create surface tension gradient and producing Marangoni force to unfold the crumpled nanometer‐thick films. Electronic and optoelectronics on wrinkled skin and curved surfaces are demonstrated with this water transfer printing method.

Unified Model for Laser Doping of Silicon from Precursors.

Laser doping of silicon with the help of precursors is well established in photovoltaics. Upon illumination with the constant or pulsed laser beam, the silicon melts and doping atoms from the doping precursor diffuse into the melted silicon. With the proper laser parameters, after resolidification, the silicon is doped without any lattice defects. Depending on laser energy and on the kind of precursor, the precursor either melts or evaporates during the laser process. For high enough laser energies, even parts of the silicon’s surface evaporate. Here, we present a unified model and simulation program, which considers all these cases. We exemplify our model with experiments and simulations of laser doping from a boron oxide precursor layer. In contrast to previous models, we are able to predict not only the width and depth of the patterns on the deformed silicon surface but also the doping profiles over a wide range of laser energies. In addition, we also show that the diffusion of the boron atoms in the molten Si is boosted by a thermally induced convection in the silicon melt: the Gaussian intensity distribution of the laser beam increases the temperature-gradient-induced surface tension gradient, causing the molten Si to circulate by Marangoni convection. Laser pulse energy densities above H > 2.8 J/cm2 lead not only to evaporation of the precursor, but also to a partial evaporation of the molten silicon. Without considering the evaporation of Si, it is not possible to correctly predict the doping profiles for high laser energies. About 50% of the evaporated materials recondense and resolidify on the wafer surface. The recondensed material from each laser pulse forms a dopant source for the subsequent laser pulses.

Wetting and Drying of Aqueous Droplets Containing Nonionic Surfactants CnEm.

This paper presents a systematic study of the wetting and drying of aqueous pico-liter droplets containing nonionic surfactants polyoxyethylene alkyl ethers (CnEm; n = 10, 12, 14, m = 6 or 8) in comparison with the anionic surfactant sodium dodecyl sulfate (SDS). The spreading and drying of droplets on hydrophilic substrates were studied by tracking the three-phase contact line (TCL) and by interferometry. CnEm droplets undergo phase separation during drying: a water-rich droplet retracts and leaves behind a thin film that is postulated to be a surfactant mesophase. This thin film either retracts or breaks up into small droplets on a longer time scale. The receding contact angle of the water-rich droplet on the thin film in the late stage of drying of CnEm droplets is independent of hydrophobicity of substrates, supporting the inference that a mesophase is present on the surface. Both CnEm and SDS solutions inhibit spreading on hydrophilic surfaces, which is attributed to Marangoni contraction as a result of a surface tension gradient across the gas-liquid interface. More pronounced suppression of spreading is observed in the case of CnEm solutions, possibly due to the phase transition of surfactant solution in the vicinity of the initial TCL leading to a viscous phase at the TCL that pins the droplet. Tracer particle measurements reveal that mild Marangoni flows exist for droplets with surfactant concentrations well above the critical micelle concentration (CMC). Origins of the surfactant gradients that result in Marangoni flows are discussed.

Thermal effect on the film of a solution with a volatile component

Interfacial convection is a widespread phenomenon occurring in various fields of technology, including chemical technology. Marangoni convection is of greatest interest in the case of thin liquid films. Phase transitions significantly affect the convective flow by changing the surface tension coefficient. In this work, we study the behavior of a thin film of a binary homogeneous solution when it is heated. A change in the temperature of the free surface together with the escape of the volatile component leads to two opposite effects in the direction of the surface tension gradient. The necessity of taking into account the motion of the volatile component over the solution surface is substantiated, analytical estimates and solutions are obtained.

Influence of Sulphur Content on Structuring Dynamics during Nanosecond Pulsed Direct Laser Interference Patterning.

The formation of melt and its spread in materials is the focus of many high temperature processes, for example, in laser welding and cutting. Surface active elements alter the surface tension gradient and therefore influence melt penetration depth and pool width. This study describes the application of direct laser interference patterning (DLIP) for structuring steel surfaces with diverse contents of the surface active element sulphur, which affects the melt convection pattern and the pool shape during the process. The laser fluence used is varied to analyse the different topographic features that can be produced depending on the absorbed laser intensity and the sulphur concentration. The results show that single peak geometries can be produced on substrates with sulphur contents lower than 300 ppm, while structures with split peaks form on higher sulphur content steels. The peak formation is explained using related conceptions of thermocapillary convection in weld pools. Numerical simulations based on a smoothed particle hydrodynamics (SPH) model are employed to further investigate the influence of the sulphur content in steel on the melt pool convection during nanosecond single-pulsed DLIP.

Study on bifurcation to chaos of surface tension gradient driven flow

As the primary heat and mass transfer mechanism in space through natural convection, surface tension gradient-driven convection is a complex nonlinear process concerning strong coupling between fluid flow and heat transfer. It is also a multiple parameter coupling process that exhibits complex spatial-temporal characteristics. Therefore, the mechanism of the surface tension gradient-driven convection becomes a hotspot in microgravity fluid physics. It also has many important applications, such as in space fluid and energy management. In this paper, recent experimental and numerical results on the transition of surface tension gradient-driven convection are reviewed, especially the nonlinear analysis on the flow bifurcations to chaos. There are several numerical methods to obtain the corresponding bifurcation diagrams. One is to integrate the model forward in time starting from different parameters and initial values, and others are to calculate the asymptotic flow states and bifurcation points directly. The direct numerical simulation method and time series analysis are widely used, but searching for bifurcation points from a large number of data is burdensome. Bifurcation points can be computed directly with the numerical bifurcation method, but such calculations are more difficult to implement than the direct numerical method.

Convective and Absolute Instabilities Induced by Viscous Dissipation in the Thermocapillary Convection With Through-Flow

Abstract The mixed convection in a thin liquid film flow over a horizontal plate is investigated under finite Prandtl numbers. The gas–liquid interface is considered free, nondeformable and subject to surface tension gradients and convection, while gravity is assumed negligible. Therefore, thermocapillary instead of buoyancy effects appears due to the unstable temperature stratification induced by the internal heating generated by viscous dissipation. A linear and modal stability analysis of this model is then performed to identify its convective/absolute nature. This is achieved by solving the resulting differential eigenvalue problem with a shooting method. Longitudinal rolls are the most unstable at the onset of instability for most parametric conditions. Otherwise, transverse rolls are the first to become convectively unstable. Finally, longitudinal rolls are absolutely stable. A transition to absolute instability occurs through transverse rolls, but only within a limited region in parametric space.

Influence of process parameters on heat transfer of molten pool for selective laser melting

The stainless steel 316L metal powder is layered on the substrate and then directly heated by a laser beam on the surface of the powder, forming a molten pool. A three-dimensional numerical model is established to investigate the heat transfer in the molten pool. The influence of the thermal evaporation, Marangoni effect, laser beam radius, and surface tension gradient on the molten pool are discussed based on the dimensions, temperature distribution, and Marangoni number. In this study, the energy density is introduced as the laser condition in the simulation settings. The results indicate that the thermal evaporation effect obviously affects the dimensions of the molten pool. The fluid flow caused by the Marangoni effect results in the molten pool forming into a deep and narrow shape. The dimensions, including the depth and width of the molten pool, are expanded as laser beam radius increases. The large surface tension gradient leads to a high Marangoni number and aspect ratio of the molten pool. The shape and dimension of the molten pool obtained from the experimental micrograph are close to the predictions with a surface tension gradient of 0.45 × 10−3N/m-K.

Surfactant Driven Marangoni Spreading in the Presence of Predeposited Insoluble Surfactant Monolayers.

When an insoluble surfactant is deposited on the surface of a thin fluid film, stresses induced by surface tension gradients drive Marangoni spreading across the subphase surface. The presence of a predeposited layer of an insoluble surfactant alters that spreading. In this study, the fluid film was aqueous, the predeposited insoluble surfactant was dipalmitoylphosphatidylcholine (DPPC), and the deposited insoluble surfactant was oleic acid. An optical density-based method was used to measure subphase surface distortion, called the Marangoni ridge, associated with propagation of the spreading front. The movement of the Marangoni ridge was correlated with movement of surface tracer particles that indicated both the boundary between the two surfactant layers and the surface fluid velocities. As the deposited oleic acid monolayer spread, it compressed the predeposited DPPC monolayer. During spreading, the surface tension gradient extended into the predeposited monolayer, which was compressed nonuniformly, from the deposited monolayer. The spreading was so rapid that the compressed predeposited surfactant could not have been in quasi-equilibrium states during the spreading. As the initial concentrations of the predeposited surfactant were increased, the shape of the Marangoni ridge deformed. When the initial concentration of the predeposited surfactant reached about 70 A2/molecule, there was no longer a Marangoni ridge but rather a broadly distributed excess of fluid above the initial fluid height. The nonuniform compression of the annulus of the predeposited monolayer also caused tangential motion ahead of both the Marangoni ridge and the boundary between the two monolayers. Spreading ceased when the two monolayers reached the same final surface tension. The final area per molecule of the DPPC monolayer matched that expected from the equilibrium DPPC isotherm at the same final surface tension. Thus, at the end of spreading, there was a simple surface tension balance between the two distinct monolayers.

Subgrain microstructures and tensile properties of 316L stainless steel manufactured by selective laser melting

316L stainless steel samples were manufactured by selective laser melting (SLM). The microstructure of SLM-made 316L stainless steel and the room temperature tensile properties both perpendicular and along the building direction were studied and characterized. The static temperature field during the molten pool formation was simulated by finite element simulation. It indicates that the nonlinear asymmetrical inclined temperature gradient in SLM process produces a large surface tension gradient. The melt forms a Marangoni flow with different convection modes under the action of surface tension as well as a micro-molten pool morphology with subgrain structures such as strip, hexagonal and elongated cellular structures. In addition, there are also epitaxially grown columnar grains. The growth of columnar crystals is not affected by the boundary of the molten pool. Subgrain structures and low-angle grain boundaries make the tensile strength and the elongation of SLM-made 316L sample higher as compared to those of the cast and wrought samples. The room temperature tensile strength of the sample perpendicular to the building direction is higher than that of the sample along the building direction, while the elongation is lower than that of the sample along the building direction.

Marangoni fireworks: Atomization dynamics of binary droplets on an oil pool

Surface tension gradient due to concentration and temperature differences induces Marangoni forces. The Marangoni effect has been extensively studied to understand its fundamental underlying physics and its industrial applications. This paper describes the spreading and atomization dynamics of an aqueous 2-propanol (IPA) solution on sunflower oil. The spreading and self-atomization of droplets by the Marangoni effect with the evaporation of volatile components are herein observed. With 40 wt. % IPA solution droplets on sunflower oil, firework-like behavior was demonstrated with the Marangoni effect. To better understand the interplay between the evaporation and spreading/atomization characteristics, the temperature field on the oil pool was visualized and quantified using an infrared camera. The Marangoni flow driven by the temperature gradient near the spreading front was estimated and compared with the experimental spreading velocity. The experimental spreading velocity of the liquid film was found to roughly agree with the model prediction. By the atomization of the spreading IPA solution, thousands of atomized daughter droplets were counted, and the size distribution was determined. Additionally, fingering instability at the interface of the IPA solution and sunflower oil was quantitatively discussed, and the resulting wavelength on its interface was compared with the capillary model. We hope that our demonstration stimulates further studies that will yield deeper insights into the spreading and atomization dynamics of volatile binary droplets on a liquid pool.