Liquid Bridge Research Articles

Documenting weld pool behavior differences in variable-gap laser self-melting and wire-filling welding of titanium alloys

In large-scale welding processes involving complex geometries, variations in gap dimensions and the transition of filler wire form distinctive molten pool flow behaviors, significantly affecting weld formation and quality. This paper investigates the differences in the laser self-melting and wire-filling welding pool behavior of titanium alloy with variable gaps through both simulation and experimentation. An innovative three-dimensional transient heat-flow coupling model is developed to simulate laser welding with variable gap structures, incorporating the dynamics of the welding wire-filling process. The different laser welding modes and the filling process under the variable gap structure were numerically simulated. The results indicate that the maximum flow rate of the laser self-melting pool remains stable in proximity to the keyhole, reaching a maximum of approximately 2.491 m/s, with molten metal entering through the area surrounding the keyhole. As the gap size gradually increases to 0.2 mm during the laser self-fusion welding stage, keyhole instability becomes more pronounced. In transitioning from laser self-fusion to wire-filling welding, the welding wire behavior can be divided into three stages. The molten metal at the end of the welding wire is transferred to the liquid melt pool through a liquid metal bridge. The flow rate of this bridge initially rises before gradually decreasing, reaching a maximum flow rate of 1.606 m/s. Notably, the welding wire lags approximately 0.6 mm behind the laser's focal point, with the narrowest width of the liquid bridge measuring about 0.89 mm. Based on the simulation results and considering the melting transition time of the welding wire, the initiation time for the welding wire has been further adjusted to the first 10 ms when the gap threshold reaches 0.2 mm. Experimental verification confirms the successful laser welding of a 2 mm variable gap structure thin plate. This study elucidates the melt pool flow, keyhole fluctuations, and metal transition behaviors during the laser welding process, contributing to a deeper understanding of the complex heat and mass transfer dynamics inherent in variable gap structure laser self-melting and wire-filling welding. Ultimately, these insights aim to enhance the application of laser welding in adaptive welding for large structural components.

Prediction of liquid bridge rupture between two plates combining artificial neural network with grey wolf optimization algorithm

Prediction of liquid bridge rupture between two plates combining artificial neural network with grey wolf optimization algorithm

Evaporating capillary bridges of pure and binary liquids

We present a numerical and experimental study on the evaporation of microliter capillary bridges of both pure and binary liquids. Specifically, we focused on capillary bridges of a binary liquid composed of water and isopropanol confined between poly-dimethylsiloxane coated surfaces. We developed a finite-element method-based numerical model to solve Laplace equations for vapor diffusion of the two species present in the capillary bridge, considering quasi-steady and diffusion-limited evaporation. We applied a modified version of Raoult's law, incorporating activity coefficients for binary liquids. The Galerkin finite element method was employed in axisymmetric cylindrical coordinates. The numerical model was validated against in-house experiments of side visualization on an evaporating capillary bridge. We quantified the effect of confinement from the plates on slowing down the diffusion of liquid vapor. The volume evolution of the binary liquid capillary bridge was found to be nonlinear, strongly influenced by the initial concentration of isopropanol in the capillary bridge. This nonlinearity is attributed to the faster diffusion of isopropanol vapor compared to water vapor. We examined the effects of height, substrate radius, contact angle, and composition on the evaporation characteristics. We proposed a computationally efficient reduced-order model for determining evaporation kinetics, which yields predictions very close to those of the numerical model.

Movement behavior and morphological change of droplets on vertically crossed fibers

Droplet capture and coalescence are critical processes for enhancing emulsion separation efficiency in oily wastewater treatment. In this paper, four kinds of motion behaviors and secondary droplet formation behaviors of oil droplets on vertically crossed fibers in a water environment were investigated using high-speed camera technology. The research shows that during the droplet capture process, droplets on large-diameter horizontal fiber exhibit greater lateral spreading, and smaller vertical elongation. Increasing the diameter of horizontal fiber significantly shortens droplet capture time. The capture time on lipophilic fiber intersection of 190–420 μm is 14 ms less than that of 190–190 μm. During the droplet coalescence process, the liquid bridge on large-diameter horizontal fiber is wider, and the droplet contraction ratio is smaller. The droplet contraction ratio on fiber intersection of 190–420 μm is 0.3 smaller than that of 190–190 μm. Increasing the diameter of horizontal fiber can promote rapid fusion between droplets, but lipophobic fibers hinder the degree of fusion and deformation. Furthermore, the diameter and wettability of horizontal fibers have significant effects on the volume of secondary droplets. The volume of secondary droplets increases with the diameter of lipophilic horizontal fiber in the capture process. The volume of secondary droplets on lipophilic horizontal fiber is approximately three times larger than that on lipophobic horizontal fiber in the coalescence process. These findings contribute to understanding the details of coalescence separation and optimizing the design of fiber fillers.

Flow behavior of polypropylene reactor powder in horizontal stirred bed reactors characterized by X-ray imaging

Horizontal stirred bed reactors (HSBRs) are widely used in the commercial production of polypropylene (PP). Despite their commercial significance, a comprehensive understanding of the flow behavior in HSBRs remains elusive, primarily due to the lack of detailed experimental data. This study investigates the influence of operating parameters on the particle flow behavior of two types of PP reactor powder in a laboratory-scale HSBR using X-ray imaging. Our results indicate that the overall flow behavior and phase holdup in the HSBR are dominated by agitation. Moreover, gas injection through the inlet points at the bottom of the HSBR results in spouting behavior, which can lead to reduced gas-solid contacting and, in extreme cases, complete bypass. Finally, the presence of liquid (in this study, isopropyl alcohol) adversely affects the flow behavior of the PP reactor powder due to liquid bridging at the contact points of particles. Powders that comprise particles with relatively small sizes and dense surface morphology are particularly prone to reduced flow behavior when exposed to liquid.

Low-temperature treatment optimization for diesel-contaminated kaolin: Mutual impacts of generated pyrolytic carbon and particle agglomeration

Efficiency improvement in low-temperature treatment for diesel-contaminated sites is urgent because changes in soil properties and the generation of new substance during the remediation process can influence the duration and energy utilization. This paper focuses on low-temperature treatment optimization based on the mutual impacts of pyrolytic carbon and kaolin aggregation. Results reveal that the peak mass loss rate occurred between 100 and 150 °C, with minimal loss beyond 200 °C. Samples thermally treated at 150–200 °C exhibited darker colors, indicating pyrolytic carbon formation, corroborated by x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR), and three-dimensional fluorescence spectrum (3D-EEM) analyses. Additionally, diesel contamination influenced the fractal dimension of aggregates by influencing adhesion forces (<10000 mg/kg) and forming liquid bridges (≥10000 mg/kg) in untreated kaolin, resulting in an initial increase and subsequent fall in fractal dimension with increasing concentration. Decline rates of pollutant gas concentration were closely correlated with fractal dimension changes under thermal conditions due to pollutant volatilization and pyrolytic carbon formation. Based on the consistency between fractal dimension and decline rate, two critical remediation concentrations (C0) and temperatures (T0) indices were identified to optimize the low-temperature remediation.

Gravity-driven flow of liquid bridges between vertical fibres

Liquid bridges are formed when a flowing liquid interacts with multiple parallel fibres, as relevant to heat and mass transfer applications that utilize flow down fibre arrays. We perform a comprehensive experimental study of flowing liquid bridges between two vertical fibres whose spacing is controlled dynamically in our experimental apparatus. The bridge patterns exhibit a regular periodic spacing typical of absolute instability for low flow rates, but become spatially inhomogeneous above a critical flow rate where the base flow is convectively unstable. The shapes of individual bridges and their associated dynamics are measured, as they depend upon the liquid properties, and fibre geometry/spacing. The bridge length scales similarly to static bridges between parallel fibres. The bridge dynamics exhibits a dependence on viscosity and scale with the impedance. A simple energy balance is used to derive a scaling relationship for the bridge velocity that captures the general trend of our experimental data. Finally, we demonstrate that these scalings similarly apply when the fibres are dynamically separated or brought together.

Equilibrium States of a Liquid Bridge between Flexible Sheets.

We study the equilibrium states of a drop between flexible sheets clamped on both ends. Revisiting first the case of parallel sheets, we find multiple equilibria, which we classify in a parameter space. In solution branching diagrams, we identify hysteresis cycles and folds indicating abrupt transitions, yet not necessarily leading to channel collapse. Between nonparallel sheets, a drop can stay in equilibrium away from the ends even when the liquid is not totally wetting, which is impossible between nonparallel rigid plates. We also show that nonparallel sheets can delay or prevent collapse in comparison to straight channels, suggesting thus a mechanism that protects slender structures inside microdevices from damage by surface tension.

Microstructure and Mechanical Properties of a Weld Seam from Magnetron High-Current CO2 Welding

External magnetic field (EMF)-assisted high-current CO2 welding is beneficial for improving the large spatter and poor performance of the welding heat-affected zone for mild steels under high-current welding specifications. In this paper, the droplet transfer behaviors were determined using a high-speed camera on a self-developed magnetically controlled CO2 welding system. Based on these welding specifications, a three-dimensional, transient, multi-energy field coupling welding system model to investigate the mechanism of the droplet and molten pool in EMF-assisted welding was developed. The microstructure and mechanical properties of the welded joint were systematically studied. The results show that the Lorentz force applied by the EMF to twist the droplet decreases the accumulated energy in the short-circuited liquid bridge and changes the liquid metal flow condition, both of which reduce the spatter by 7% but increase the welded joint hardness by 10% and tensile strength by 8%.

Effect of surface roughness on the liquid bridge between two rigid spheres

We aimed to investigate the impact of surface roughness on liquid bridges between spherical particles. Sandblasting was used to control the particle size and produce glass beads and plates with different surface roughness. First, by measuring the advancing and receding contact angles of droplets on different rough surfaces, we analyzed the effects of surface roughness on wettability and hysteresis. Next, we used a custom-made liquid-bridge stretching device to measure the capillary forces of the liquid bridges between spherical particles with different surface roughness values. A charge-coupled device camera acquisition system was set up to capture the morphological changes of the liquid bridge during stretching, and Image View software was used to extract the morphological parameters of the liquid bridge. Theoretically, we reasonably simplified and solved the differential equations for the liquid bridge morphology and used the Young–Laplace equation to calculate the theoretical capillary force of the liquid bridge, providing an in-depth analysis of the influence of surface roughness on the capillary force. Finally, we studied the impact of surface roughness on the volume ratio of the liquid bridge during static stretching and the residual liquid remaining after the liquid bridge breaks. Experimental results indicated that, as the surface roughness increased, the hydrophobicity and wettability hysteresis of the solid surface also increased. The increased hydrophobicity of the surface reduces the solid-liquid contact area of the liquid bridges between the particles, making it easier to form “columnar” or “convex” liquid bridges. Additionally, the enhanced wettability hysteresis causes the solid-liquid contact boundary to lag during the stretching of the liquid bridge, resulting in a decrease in the solid-liquid contact angle. These factors directly alter the geometric shape of liquid bridges during static stretching, thereby affecting capillary forces. Meanwhile, the increase in surface roughness weakens the effect of gravity on the morphology of the liquid bridge, resulting in less liquid mass remaining on the lower sphere after the liquid bridge breaks as the surface becomes rougher.

A reinvestigation on combined dry and wet adhesive contact considering surface tension

The present study theoretically explores combined dry and wet adhesive contact between a rigid sphere and elastic semi-half substrate, in which dry contact is encircled by liquid bridge. We consider threefold effects of liquid bridge on contact behavior, namely Laplace pressure induced by the curved surface of liquid meniscus, surface tension at the triple-phase junction and alternation of adhesion energy between solid surfaces ascribed to liquid immersion. A clear novelty in this study is the investigation on the effect of surface tension at the vapor-liquid-solid junction on the adhesive contact response, in contrast to previous studies. The model solution predicts that the contact behavior and adhesive strength are strongly dependent on surface wettability (manifested by contact angle), liquid volume and the contact system's rapidity in achieving thermodynamic equilibrium. It is found that the transition of the pull-off force is evidently different from Maugis-Dugdale model in terms of a couple of interesting characteristics. Moreover, it is unveiled that the jump instabilities and hysteresis of force-separation curves are highly affected by surface wettability and liquid volume. These theoretical results can not only shed lights on the mechanism of liquid-mediated adhesion employed by animals and plants, but also provide us inspiration for development of biomimetic adhesive devices.

Weld Pool Flow Characteristics in Double-Wire Arc Welding of Aluminum Alloys: Research by Numerical Simulations

Double-wire arc welding involves simultaneously feeding two wires into a molten pool, improving the efficiency and flexibility of traditional welding techniques. However, the interactions between the two wires and the molten pools are complex, which increases the difficulties in process and composition control. This work focuses on the weld pool flow characteristics in double-wire TIG arc welding. A CFD model incorporating a liquid bridge transfer model was developed to simulate the fluid flow phenomenon. Results show that the bead-forming appearances and flow characteristics of double-wire arc welding show no significant differences from single-wire arc welding. Welding current and welding speed have significant effects on the weld bead dimensions, while only welding current has effects on the flow characteristics. Wire feed XOZ angles show no significant influences on weld bead forming appearances and molten pool flow characteristics. Wire feed XOY angles influence the symmetry of the weld bead and the fluid flow. In 5B71/7055 heterogeneous double-wire arc welding, achieving a uniform distribution of alloy elements is difficult due to the complex convection patterns within the molten pool.

Analytical modeling of capillary force hysteresis in liquid bridge between two spheres of unequal sizes

Analytical modeling of capillary force hysteresis in liquid bridge between two spheres of unequal sizes

Study on Coalescence Characteristics and Mechanisms of Salt-laden Droplets Under Electric-magnetic Coupling Field

Abstract Electric-magnetic coupling dehydration technology is a novel method that can significantly improve the efficiency of water-oil separation in water-in-oil (W/O) emulsions. However, the electric-magnetic coupling mechanisms and its influence on droplet coalescence have not been understood from a microscopic perspective, which has become a bottleneck restricting the progress of this technology. In this study, the high-speed microscopic experiment and molecular dynamics (MD) simulation were combined to study the kinetic properties of droplets coalescence under an electric-magnetic coupling field, and the potential mechanisms of electric-magnetic coupling field strengthening droplet coalescence were articulated from the perspective of salt ions migration and energy evolution. The results show that the experiment and simulation have excellent qualitative consistency. As water molecules polarise and migrate, the contact interface between adjacent droplets forms a cone tip during the approach process. The contact point of the droplet forms hydrogen bonds due to the mutual attraction of water molecules, which are then transformed into a continuously expanding liquid bridge. The salt ions in the droplet undergo migration deflection by the electric-magnetic coupling effect, which may affect the surface charge distribution of the droplets. During the process of droplet coalescence, strong electrostatic attraction occurs between molecules, leading to a significant increase in coulomb potential and dominating the overall potential energy variation. However, excessively small molecular spacing induces exchange repulsion, thereby reducing the van der Waals potential. The study results are of guiding significance for sounding droplet dynamics theory and guiding the research and development of electric-magnetic dehydration equipment.

Dynamic analysis of adsorbate behavior in nanopores: Liquid bridge formation, cavitation, and contact angle evaluation via molecular dynamics simulations

To explore the internal transport mechanisms and evolutionary processes within nanopore, molecular dynamic simulations were employed to explore the formation of liquid bridges and the evolution of menisci. The adsorption–desorption isotherms were obtained, and the dynamic behavior of the adsorbate was analyzed. Results revealed two distinct processes governing the behavior of the adsorbate within nanopore. At low pressure ratios, a molecular adsorption layer initially grows continuously on the pore surface. As the pressure ratio increases, an additional mechanism emerges, characterized by the formation of a liquid bridge near the nanopore mouth. This leads to pore filling as the liquid bridge expands and retreats towards the pore’s closed end. During desorption, cavitation occurs, which can be characterized as an activation process. Once initiated, cavitation intensifies, progressively increasing the number of cavities. The convergence of these cavities undermines the stability of the liquid bridge, ultimately resulting in its collapse and the rapid evaporation of the adsorbate molecules. Additionally, we further calculated the contact angle at the vapor–liquid interface using the Kelvin equation. The results indicated a deviation of less than 5 % compared to our simulations, suggesting that the Kelvin equation is applicable for materials with pore sizes of at least 3.6 nm.

Droplet transition behavior and compositional homogenization of TiAl alloys fabricated via dual-wire electron beam-directed energy deposition

The dual-wire electron beam-directed energy deposition (EB-DED) process presents considerable advantages for fabricating titanium aluminide (TiAl) alloys via in situ reactions between Ti and Al. However, variations in the thermophysical properties of pure Ti, pure Al, and TiAl alloys pose challenges in achieving consistent droplet transition behavior and uniform chemical composition. This study explored the effects of wire feeding modes on the forming quality of TiAl alloys and droplet transition behaviors and discussed compositional homogenization and elemental evaporation in TiAl alloy components. Both single-side and double-side feeding modes were utilized, and the electron beam directly melted the Ti wire in the double-side mode, thereby achieving superior surface morphology. Random disturbances and suboptimal wire feeding angles led to the wires deviating from the electron beam center and then being inserted into or passing through the molten pool, thereby degrading the forming quality. By positioning the Al wire tip beneath the Ti wire tip, a common droplet emerged as the Ti droplets melted the Al wire. The droplet transition behavior was regulated by the distance between the wire tips and component surfaces, achieving a liquid bridge transition at distances ranging from 0.75 to 5.82 mm. Although the droplet composition was generally uniform owing to elemental evaporation, the fluctuations among the droplets exceeded those observed in the as-deposited components. The extensive molten pool and keyhole effect enhanced compositional homogenization within the TiAl alloy components. During the deposition process, the evaporation rate of Al surpassed that of Ti due to its higher saturation vapor pressure, consequently yielding a lower actual Al content. The loss rate of Al initially decreased and then increased as the calculated Al content increased, which was influenced by the Al content in the molten pool and temperature variations. This research advances the fundamental understanding of TiAl alloy fabrication via in situ reactions and contributes to the development of the EB-DED process.

Three-dimensional mesoscopic investigation of directional coalescence of two droplets impacting on a wall with wettability difference

In this work, a three-dimensional (3D) nonorthogonal pseudopotential lattice Boltzmann method (LBM) was proposed to investigate the coalescence dynamics of two droplets impacting on a wall with wettability difference. The influences of the wettability difference, Weber number, offset distance on the low-wettability side on the coalescence dynamics and the contact-line evolution processes were systematically examined. Both symmetric and asymmetric distributions of the droplet-coalescence behaviors were considered. Our findings reveal that the wettability difference has a significant influence on the asymmetric-retracting and wetting-equilibrium stages, identifying three modes: pin-slip, slip and no-rebound, and slip and rebound. The rebound time is dominated by the high-wettability wall. At a larger Weber number, droplets exhibit a large retracting velocity, which results in increased pumping velocity and earlier rebound time. In addition, a dramatic retraction of the three-phase contact line (TPCL) on the low-wettability wall is observed, leading to the detachment of the liquid bridge from the low-wettability wall, and the formation of a cavity. With increasing offset distance on the low-wettability wall, three different evolution modes are found: coalescence-rebound, coalescence-separation, and non-coalescence. A power function relationship is reported between the Weber number We and the offset distance L* both on the high-wettability wall and low-wettability wall for three modes of coalescence behavior with We∼L*α. The value of the exponent α ranges from 4.6 to 7.4. This study showcases the effectiveness of the 3D nonorthogonal pseudopotential LBM in predicting the complex interface phenomena and characteristics of the multiphase flow structures under investigation.

Improved Liver Intravital Microscopic Imaging Using a Film-Assisted Stabilization Method.

Intravital microscopy (IVM) is a valuable method for biomedical characterization of dynamic processes, which has been applied to many fields such as neuroscience, oncology, and immunology. During IVM, vibration suppression is a major challenge due to the inevitable respiration and heartbeat from live animals. In this study, taking liver IVM as an example, we have unraveled the vibration inhibition effect of liquid bridges by studying the friction characteristics of a moist surface on the mouse liver. We confirmed the presence of liquid bridges on the liver through fluorescence imaging, which can provide microscale and nondestructive liquid connections between adjacent surfaces. Liquid bridges were constructed to sufficiently stabilize the liver after abdominal dissection by covering it with a polymer film, taking advantage of the high adhesion properties of liquid bridges. We further prototyped a microscope-integrated vibration-damping device with adjustable film tension to simplify the sample preparation procedure, which remarkably decreased the liver vibration. In practical application scenarios, we observed the process of liposome phagocytosis by liver Kupffer cells with significantly improved image and video quality. Collectively, our method not only provided a feasible solution to vibration suppression in the field of IVM, but also has the potential to be applied to vibration damping of precision instruments or other fields that require nondestructive ″soft″ vibration damping.

Measurement and prediction of void fraction and pressure drop during air drying process of wet particle packed bed

This study aimed at development of a pressure drop prediction method at a drying process by flowing air through a wet particle packed bed saturated with distilled water. At first, the pressure drop measurement during the drying process of the wet particle packed bead of glass sphere was carried out, and a method for predicting the pressure difference of the bed was proposed by investigating the effect of liquid saturation on the bed void fraction. It was clarified that the effect could be predicted by a simple model by considering liquid bridges formed between particles. Next, the liquid saturation when the measured pressure difference suddenly decreased was investigated as the residual saturation. The value was like that of past research. Finally, considering the effect of liquid saturation on the bed void fraction and the influence of the residual saturation on the relative permeability of liquid phase, more than 90 % of all pressure drop data could be predicted with an error of less than ±15 %. Furthermore, in the range where the gas-phase velocity was large, it was important to predict the pressure difference considering the change of the bed void fraction and the residual saturation.

The simplification and analytical verification of the static liquid bridge force model for particle adhesion in inferior seedling substrates

This study examines the adhesion mechanism in varying water contents of inferior seedling substrate blocks, focusing on the adhesion between particles‒particles and particles‒wall of the seedling tray. A simplified static liquid bridge force model based on the Young–Laplace equation is proposed. The correlation between the liquid bridge force and volume is derived and validated against existing experimental. The results indicate that the static liquid bridge force between particles decreases as the distance increases. A critical distance region is identified, where below this distance, a smaller liquid bridge volume results in a greater static liquid bridge force, and beyond this, the trend reverses, except when the solid–liquid contact angle is 50°. Additionally, the solid‒liquid contact angle has a minimal influence on the liquid bridge force between the particles and the inner sidewall of the seedling tray. This study also analyses the change in critical rupture distance during the transition of liquid bridges from pendulum to ring rope type, and this change increases with the liquid bridge volume.