Surface Tension Pressure Research Articles

A study on the flow behavior and bubble evolution of circular oscillating laser welding of SUS301L-HT stainless steel

The flow characteristics, bubble formation and inhibition mechanisms of circular oscillating laser beam welding (OLBW) at distinct stages were elucidated in present work. The formation processes of bubbles mainly included the following manners. First, one or more of vortex P2, the recoil pressure flow P3, the migration flows FMF and BMF impinged the keyhole wall. The necking generated, gradually destabilized, and finally collapsed. Second, the self-instability of needle-like keyhole bottom (NKB) with large curvature induced the bubble formation. Third, complex melt flow broke the abnormal deformation of the keyhole or the bubble wall to form a new bubble. The bubble suppression mechanisms were summarized to three categories. First, the recoil pressure on the inner wall of bubble was at a low level, and the volume of bubble continually decreased until it collapsed. Second, high recoil pressure penetrated the metal liquid bridge between keyhole and bubble. Third, when recoil pressure was greater than the resultant force between surface tension and melt dynamic pressure, the bubble constantly fluctuated and gradually merged with the moving keyhole. As the frequency increased from 10 Hz to 200 Hz, porosity showed an exponential decrease from 7.16% to 0.35% in circular oscillation mode. A unified porosity prediction curve based on the Reynolds number was built with a coefficient of determination of 0.989. When fluid changed from laminar to turbulent flow, the critical Reynolds number was set as 1000, the predicted porosity was 0.912%, and the critical frequencies in circular and infinite oscillation modes were 143 Hz or 115 Hz, respectively, which were consistent with experiment results.

Identification of the seeding mechanism in the spinodal instability of dewetting liquids

HypothesisSpinodal dewetting is one of the basic processes inducing a spontaneous withdrawal of a liquid from a substrate surface. In the accepted theory, thickness fluctuations generated by thermally activated capillary waves are amplified by the competing actions of surface tension and disjoining pressure. Ubiquitous sub-nanometric substrate roughness also produces thickness fluctuations and may play a role analogous but even more efficient in seeding the process. ModellingAnalytic calculations valid at the early linear stage of the process and simulations extending the study to its whole non-linear development have been performed to compare features and the relative relevance of the two seeding mechanisms. FindingsCalculations and simulations have shown that substrate roughness can replace capillary waves in seeding spinodal dewetting. A typically larger amplitude and a steady nature compared to the transitory one of capillary waves allow us to conclude that, contrary to the common view, substrate roughness is the prevailing seed of the spinodal instability. The consequence of our statement is that spinodal dewetting loses most of its stochastic nature and becomes, in principle, a process that can be tuned by engineering substrate roughness.

Thin Molten Pool Behaviors in Blue Laser Microjoining

Thin copper materials are difficult to weld using infrared lasers (IR) due to their low energy absorption. A blue laser with a wavelength of 450 nm strongly increases energy absorption and successfully realized the microjoining of a copper material with a thickness of 0.1 mm. The dynamic behaviors of a thin molten pool were investigated by a numerical simulation using a new blue laser heat source model. At both the top and bottom surfaces of the thin molten pool, the liquid metal flowed outward under the driving of Marangoni stress. The stability of the thin molten pool was studied, showing that Marangoni stress promoted the melt-through of the thin molten pool, while the viscous resistant stress, buoyant force, and surface tension pressure had minor influences on it. The Peclet and Marangoni numbers were adopted to predict the stability of the thin molten pool.

Numerical modelling of in-situ alloying of Al and Cu using the laser powder bed fusion process: A study on the effect of energy density and remelting on deposited track homogeneity

In-situ alloying using laser powder bed fusion (LPBF) allows for formation of new alloys and complicated components with spatially controlled characteristics. A major issue in in-situ alloying is the macro-segregation of alloying elements, that results in chemical inhomogeneity. The precise control of composition is one of the most important steps in any effective in-situ alloying process. In this study, a mesoscale computational fluid dynamics (CFD) model for simulating single-tracks is developed to advance the in-depth understanding of in-situ alloying of dissimilar metals. Surface tension and recoil pressure driven flows at the free surface, rapid phase change, laser reflections, and other physical phenomena affecting the LPBF process have all been examined. Species conservation is used to trace the mixing of different metals. Simulations are performed by melting a layer of Al powder on a Cu substrate, and the impact of recoil pressure and Marangoni convection driven flows on Al-Cu mixing was addressed. The homogeneity of the simulated track is quantified by defining a mixing index. The effect of different laser energy densities and remelting on the homogeneity of the deposited track is investigated. Numerically, it was observed that the high thermal conductivity of both metals results in a sharp and short melt pool, and rapid solidification of such a melt pool generates a heterogeneous concentration dispersion. At high energy density, a low mixing index value is observed, indicating improved homogeneity in the deposited track. Furthermore, remelted tracks have a low mixing index value, indicating that remelting is an effective strategy for improving homogeneity. This study provides fundamental insights on the mixing of dissimilar metals and aims to support future research activities on the design and development of new alloys using LPBF process.

Micro-hole drilling of 2.5D C/SiC composite with picosecond laser: Numerical modeling and experimental validation on hole shape evolution

Micro-holes of 2.5D C/SiC composites produced by picosecond laser are widely used in aerospace, military, and automotive industries. An accurate numerical model is crucial to illustrate the formation mechanism of hole shape and ablation characteristics. This paper proposes a two-dimensional transient numerical model based on solid-liquid-gas coupling in micro-hole drilling of 2.5D C/SiC composites with a picosecond laser. In the stage of solid-liquid phase transition, original mass, energy, and momentum conservation equations are optimized by considering surface tension, gravity, and hydrostatic pressure. In the stage of liquid-gas phase change, the mass transfer during the material evaporation is considered and used as a source term to optimize the level set equation. The effectiveness of the proposed model is validated by picosecond laser micro-hole drilling experiments on 2.5D C/SiC composites. Compared with the existing research, the results show that the proposed can accurately depict hole shape evolution in picosecond laser drilling of 2.5D C/SiC composites, not only including entrance, exit diameter, and hole taper, but also the hole profile and its dynamic variation. In addition, the model effectively simulates the thermal effect phenomena, such as thermal damage, pressure concentration, and recast layer. And the thermal damage is found to be the main cause of the hole quality variation, which changes from severe to smooth along the hole entrance to exit. The research provides important support for high-quality micro-hole machining of 2.5D C/SiC composites with the picosecond laser.

Effect of amino acid additives in ammonia solution on SO2 absorption and ammonia escape using bubbling reactor for membrane contactor applications

Controlling the SO2 emissions is crucial because they result in several environmental problems and affect human health. Several techniques have been proposed for controlling SO2 emissions. Limestone or lime-based absorbents are widely used for SO2 removal. However, their low solubility limits their applications. Ammonia solution has attracted attention owing to its higher SO2 removal efficiency. However, owing to its high volatility, it has the propensity to escape, which can cause secondary environmental pollution. Therefore, we investigated the effect of six amino acid additives to improve the SO2 absorption performance and inhibit the ammonia escape in aqueous ammonia solution. The surface tension and theoretical breakthrough pressure of the amino acid-containing ammonia solutions was investigated to evaluate their applicability to a membrane contactor process. L-histidine (His) exhibited the best performance. Therefore, the effect of His concentration, inlet SO2 concentration, and absorption temperature on SO2 absorption performance was explored. The 13C nuclear magnetic resonance (NMR) and 1H NMR analyses were used to study the SO2 absorption mechanism. This study provides a strategy for preparing an eco-friendly and highly efficient SO2 absorbent that can overcome the disadvantage of current ammonia solutions for the removal of low-concentration SO2 from industrial gas emissions for the membrane contactor process application.

Droplet Spreading and Adhesion on Spherical Surfaces

Adhesion of a liquid droplet to a solid surface is a result of solid surface interactions with surrounding fluids, affected by its wettability and morphology. Unfortunately, the direct measurements of adhesion forces are rarely reported in the scientific literature, especially for solids with curvatures. In this study, by using a high-sensitivity microelectronic mechanical balance which vertically deposits and then pulls liquid droplets, the spreading and adhesion forces for water and ethylene glycol droplets on spherical surfaces of polyethylene terephthalate (PET) with radii of curvature from 2 to 8 mm were recorded. Results show that the surface curvature does not affect the advancing and most-stable contact angles but affects the extent of spreading and maximum adhesion forces. The solid surface curvature affects both surface tension and Laplace pressure forces at the spreading point, whereas it mainly affects the Laplace pressure force at the maximum adhesion point.

Contact line motion on heated patterned surfaces

We develop a lubrication-type model of contact line motion on a surface patterned by an array of square patches of decreased wettability. The model accounts for evaporation of the liquid due to surface heating, as well as effects of surface tension and disjoining pressure. Stick-slip motion and strong interface deformations are found from the simulations. The height of the capillary ridge becomes spatially nonuniform when the stable contact line travels over the region of decreased wettability. Coupling between the surface pattern and intrinsic contact line instability is investigated. For the weak evaporation limit, the model predictions are compared to previous studies of contact line dynamics under isothermal conditions.

Surface Wettability Is a Key Feature in the Mechano-Bactericidal Activity of Nanopillars.

Nanopillar-textured surfaces are of growing interest because of their ability to kill bacteria through physical damage without relying on antimicrobial chemicals. Although research on antibacterial nanopillars has progressed significantly in recent years, the effect of nanopillar hydrophobicity on bactericidal activity remains elusive. In this study, we investigated the mechano-bactericidal efficacy of etched silicon nanopillars against Pseudomonas aeruginosa at nanopillar hydrophobicities from superhydrophilic to superhydrophobic. Assessing cell viability and bacterial morphology in immersed wet conditions, we observed negligible bactericidal activity; however, air/liquid interface displacement during water evaporation established a bactericidal effect that strongly depends on substrate hydrophobicity. Specifically, bactericidal activity was highest on superhydrophilic surfaces but abated with increasing hydrophobicity, diminishing at substrate contact angles larger than 90°. Calculation of the surface tension and Laplace pressure forces during water evaporation for each substrate subsequently highlighted that the total capillary force, as an external driving force responsible for bacterial deformation, is significantly weaker on hydrophobic substrates. These findings suggest that superhydrophilic nanopillared surfaces are a superior choice for mechano-bactericidal activity, whereas superhydrophobic surfaces, although not bactericidal, may have antibiofouling properties through their self-cleaning effect. These findings provide new insights into the design and application of nanopillared surfaces as functional antibacterial materials.

Dynamic evolution of keyhole during multi-pulse drilling with a millisecond laser on 304 stainless steel

The efficiency of millisecond laser drilling is high due to the molten material ejection, and is applied in many industries like aerospace, automobile and electronics. However, there may be some defects such as hole blockage, low machining accuracy and low repeatability, especially for the process of multi-pulse drilling. The melt flow remaining inside keyhole due to inefficient ejection, is responsible for the formation of those defects. Hence, it is necessary to investigate the dynamic behavior of melt flow during multi-pulse drilling. In this paper, the dynamic evolution of keyhole is investigated combining in-situ observation and numerical simulation methods. According to the ejection efficiency, the multi-pulse drilling process can be divided into three periods, namely rapid drilling period, linear drilling period, and moderate drilling period. The melt flow behaves differently in each period. In rapid drilling period, a conical keyhole is formed in the end. The melt flow behavior is affected dominantly by the recoil pressure when the laser is on and by the surface tension when the laser is off. In linear drilling period, the blocked keyhole occurs occasionally. The melt flow behavior is affected by gravity and recoil pressure when the laser is on and by the surface tension and recoil pressure when the laser is off. In moderate drilling period, the keyhole profile is wavy. The melt flow behavior is affected by surface tension and recoil pressure dominantly during the whole period. The melt flow transition and its influence factors are investigated in this paper, which is helpful for understanding the physical processes during the multi-pulse laser irradiation process.

Study on the molten pool behavior and porosity formation mechanism in dual-beam laser welding of aluminum alloy

Based on the mechanism of dynamic coupling between molten pool and keyhole, a three-dimensional (3D) transient model for the dual-beam laser welding of aluminum alloy is established by considering the surface tension, Marangoni force, and recoil pressure. The morphology of the molten pool, porosity formation process, and the heat transfer mechanism during the process of laser welding under different parameters are analyzed. A double rotating 3D Gaussian heat source is used to represent the laser beam, and the volume of fluid method is used to track the gas-liquid free surface, and the gas-liquid interface force is transformed by the continuous surface force model. The results show that in the process of dual-beam welding, the interaction between the two keyholes enhances the fluid flow perpendicular to the spot line, and the shape of weld pool surface changes from elliptical to circular. Furthermore, welding speed and spot spacing have a significant effect on the shape of the molten pool. Furthermore, it is observed that dual-beam welding can improve the stability of the keyhole and reduce the maximum oscillation amplitude and the number of keyhole breakups. At a specific spot spacing, a unique process of separation and fusion is discovered in addition to the common stages of growth, maintenance, breakup, and shrinkage of the keyhole. The simulation results are in good agreement with the existing experimental results. Overall, this paper provides useful insights into the dynamics of the molten pool in dual-beam welding and reveals the molten pool behavior and porosity formation mechanism.

The mechanism of surface-seal splash during water entry

When a solid body impacts quiescent water at a certain velocity, the resultant “surface-seal splash” deforms and closes above the free surface. This study investigates the dynamics of the surface-seal splash by using a high-speed camera and particle image velocimetry (PIV) measurements. Based on PIV, an analytical model is developed to understand the mechanics of the surface-seal splash. The results indicate that the theoretical splash shapes are consistent with the experimental results. In addition, the effects of gravity, drag, surface tension, and aerodynamic pressure on the dynamics of surface-seal splash are also assessed, and the results indicate that the aerodynamic pressure plays a dominant role in the formation of the surface-seal splash.

Modelling of keyhole dynamics and melt pool flow in laser powder bed fusion process

The keyhole dynamics in the laser powder bed fusion (LPBF) process and its relationship with driving forces of surface tension, Marangoni force and recoil pressure have not been well addressed and quantified. In this work, through the modelling of melt pool dynamics, the keyhole life cycle including formation and drilling, fluctuation, and disappearance is captured. The results show that pores are sourced from the powder bed voids, the ejected keyhole protrusion, and the liquid eroded gas bubbles at the end of the track. An anticlockwise vortex is generated at the rear of melt pool, and the molten liquid travels in a clockwise path ahead of the melt pool. The variation of the powder layer thickness breaks the force equilibrium on the keyhole rim and leads to the depth fluctuation. The compressed rear keyhole is dominated by the surface tension, and the expanded front rim is controlled by the recoil pressure.

Linear instability analysis of a viscoelastic jet in a co-flowing gas stream

The instability characteristic of a viscoelastic jet in a co-flowing gas stream is studied comprehensively. The important role of the non-uniform basic velocity in the instability analysis of viscoelastic jets is clarified, which first induces an unrelaxed elastic tension, and then produces a coupling term between the elastic tension and perturbation velocity. The elastic tension promotes the instability of the jet, while the coupling term exhibits a stabilizing effect, which is essentially related to the nonlinear constitutive relation of viscoelastic fluids and the non-uniform basic velocity. The competition between these two factors leads to the non-monotonic effect of fluid elasticity on the disturbance growth rate, which can be divided into two different regimes characterized by the Weissenberg number with values smaller or larger than unity. In different regimes, the structure of the eigenspectrum is also significantly different. Furthermore, three instability mechanisms are identified using the energy budget analysis, corresponding to the predominance of the surface tension, elastic tension and shear and pressure of the external gas, respectively. By analysing the variations of the growth rate and phase speed of the disturbances, the general features of viscoelastic jet instability are obtained. Finally, the transitions of instability modes in parameter spaces are investigated theoretically and the transition boundaries among them are provided. This study provides guidance for understanding the underlying mechanism of instability of a viscoelastic jet surrounded by a co-flowing gas stream and the transition criterion of different instability modes.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation.

Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 μL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.

Long Time Behavior of a Quasilinear Hyperbolic System Modelling Elastic Membranes

The paper studies the long time behavior of a simplified model of an elastic membrane driven by surface tension and inner air pressure. The system is a degenerate quasilinear hyperbolic one that involves the mean curvature, and also includes a damping term that models the dissipative nature of genuine physical systems. With the presence of damping, a small perturbation of the sphere converges exponentially in time to the sphere, and without the damping the evolution that is \(\varepsilon \)-close to the sphere has a life span longer than \(\varepsilon ^{-1/6}\). Both results are proved using a new Nash–Moser–Hörmander type theorem proved by Baldi and Haus.

PBF-LB/M multiphysics process simulation from powder to mechanical properties

The laser powder bed fusion process is modelled holistically. The Discrete Element Method is used for powder spreading simulations yielding realistic powder beds. Smoothed Particle Hydrodynamics simulations are then used to study the thermo-viscous flow in the melt pool considering laser energy absorption, radiation, heat transfer, melting and re-solidification, surface tension, Marangoni currents and vaporization pressure. Material properties are obtained from thermodynamic CALPHAD simulations. The temperature field of the melt pool is coupled to a Cellular Automaton, which calculates the growth of dendritic grains and, thus, provides a prediction for the microstructure formed during solidification. This microstructure serves then as input for Crystal Plasticity Finite Element Analyses to qualitatively describe texture dependent mechanical properties.

Influence of residual pressure on the melting of a powder bed induced by a laser beam

The laser beam melting (LBM) process is a powder-based additive manufacturing able to produce parts layer-by-layer in a hermetic chamber. Selection of the appropriate process parameters and fabrication conditions plays a fundamental role in the final properties. Investigating the interaction between the laser and the powder bed helps to understand the melt-pool behavior. In fact, even if additive manufacturing processes offer appreciable advantages, some processing parameters have to be studied to improve the properties of the products. The conventional LBM process is carried out in a chamber filled with an inert gas, at approximately atmospheric pressure. During the melting of a metal powder, gas can be caught in the liquid and may cause pores to form. This work investigates the effect of vacuum pressure in the chamber (up to 10−2 mbar) on avoiding oxidation and improving the quality of the welding of stainless steel 316L. However, by modifying the residual pressure of the chamber, physical phenomena such as surface tension, Marangoni convection, and recoil pressure are amplified, creating more instabilities in the melt pool. This paper presents an initial analysis of the behavior of the powder under combinations of different residual pressures and laser speeds/powers. High-speed image acquisition brings to light the evolution of the spatters in vacuum compared to atmospheric pressure. Particle recovery coupled with Scanning Electron Microscopy (SEM) analyses shows the organization and morphologies of the ejections. Then, sectional views of single scan tracks are compared and related to the spattering. A considerable evolution in the results of the melting is found as one passes from atmospheric pressure to residual pressure. Hence, a powder pre-sintering solution is investigated in order to remedy the instabilities in vacuum and improve the process.

Archimedes’ principle with surface tension effects in undergraduate fluid mechanics

The general physics of how objects float is only partly covered in most undergraduate fluid mechanics courses. Although Archimedes’ principle is a standard topic in fluid statics, the role of surface tension in floating is rarely discussed in detail. For example, very few undergraduate textbooks, if any, mention that the total buoyancy force on a floating object includes the weight of the fluid displaced by the meniscus. This leaves engineering students without an understanding of a wide range of phenomena that occur at a low Bond number (the ratio of buoyancy to interfacial tension forces), such as how heavier-than-water objects can float at a gas-liquid interface. This article makes a case for teaching a more unified version of Archimedes’ principle, which combines the effects of surface tension and hydrostatic pressure in calculating the total buoyancy on floating objects. Sample problems at the undergraduate level and two classroom demonstrations are described that reinforce the basic science concepts.

New mathematical modeling of temperature-based properties of ionic liquids mixture: Comparison between semi-empirical equation and equation of state

Ionic liquids are a significant type of organic salts with attractive and superior physicochemical features for employing in a number of chemical processes. Ionic liquids include one large and asymmetric cationic or anionic functional group. The properties of cationic functional group are almost similar and it is main motivation of this study. In the present investigation, we extended various semi-empirical correlations to predict some temperature-based properties of ionic liquids mixture (IL-mixture). These properties are density, viscosity, surface tension and bubble point pressure. In the case of IL-mixture density, modified Rackett equation and Taylor equation with three scenarios, modified Gardas and Coutinho equation were considered and the obtained results were compared with Peng-Robinson equation of state (EoS), Patel-Teja EoS and perturbed-chain statistical associating fluid theory EoS. In the case of IL-mixture viscosity, two types of activation energy concept models were used and also, Lee-Kesler method and modified Brock and Bird were applied to predict IL-mixture bubble point pressure and IL-mixture surface tension, respectively. A used dataset was collected from reliable literature including 2731 data points of bubble pressure (23 binary system), 4643 data points of density (102 binary system), 1011 data points of viscosity (32 binary system) and 391 data points of surface tension (16 binary system). These data relevant to 12 cation-family and 36 solvents. The all of the data was haphazardly separated in two subsets of train and test with a computer program. The obtained results indicated all used semi-empirical correlations gives the best match with corresponding experimental data and the values of statistical parameters demonstrated this hypothesis.