Droplet Detachment Research Articles

Liquid film formation: prediction accuracy of different numerical approaches

In counteracting fouling phenomenon in gas turbines, which leads to system inefficiencies and performance degradation, water washing technique is very often adopted. Water droplets sprays are injected and, hitting the solid surfaces, remove the dirt deposition. Among the collateral undesirable phenomena related to water washing, blades erosion and liquid film formation are the most remarkable. Despite the former issue was extensively assessed by the authors in previous works, up to the authors’ knowledge the risk of liquid film formation due to water washing was scarcely investigated. Liquid film formation and spreading on a solid surface is a complex phenomenon involving a large number of physical events, such as: droplets impact on a solid surface, splashing phenomena, liquid film dragging under the effect of the carrier phase and droplets separation from the film in proximity of geometry discontinuities. In this paper, an extensively used experimental test case involving all these phenomena was used to test different numerical wall film models available in literature. The test case consists in the injection of a liquid jet in a high velocity crossflow. Some of the liquid jet mass impacts on the opposite solid surface generating a wall film which develops under the dragging effect of the crossflow. A Lagrangian approach was used to track the suspended droplets within the flow field by also considering the turbulent dispersion by means of a Random Walk model. Droplets-wall interaction is considered according to the Stanton-Rutland model, which provides the outcome of a collision (deposit, rebound or splashing), depending on the local impact conditions. If a droplet sticks on a solid boundary, a liquid film generates. Droplets atomization is also accounted for by using the Madabhushi model while Friederich separation model was selected to take into account the detachment of droplets from the film at the geometry edge. Three different numerical simulations have been performed based on different approaches used to solve the liquid film evolution, namely Eulerian one-way coupling, Eulerian two-way coupling and Lagrangian two-way coupling. Numerical results have been compared with the experimental ones from both a qualitative and a quantitative point of view. The wall film shape, its spatial distribution and the variation of the film thickness of the wall centreline have been compared between experimental and numerical simulations proving that the Lagrangian 2-way coupling approach better reproduces the liquid film dynamics observed in the experiments.

Deep-learning based analysis of metal-transfer images in GMAW process

Gas metal arc welding (GMAW) is a widely used metal-joining method in industrial manufacturing. In this method, the metal-transfer process plays an important role in determining welding quality. It is common industrial practice to optimize GMAW waveforms and process parameters using high-speed videography. The assessment of videos is currently done by subjective human interpretation, which is time consuming and prone to errors. Computer vision techniques before deep learning have not been able to overcome the confounding aspects of the images. This work uses deep learning segmentation models to isolate droplets in the video footage of the GMAW metal-transfer process. Segmentation masks are used to compute the geometric and kinematic properties of the droplet to illustrate the dynamic characterization process. The proposed deep learning model is a fully convolutional network (FCN) approach. Several architectures are considered and compared here. The main result shows that the FCN-based approach can reliably segment droplets within an image with the benefit of processing thousands of images within minutes. The image features isolated in this work allow for the calculation of valuable process variables such as droplet trajectory, velocity, acceleration, and detachment frequency, which agree with those found in the literature.

Hydrodynamics and mass transfer of liquid-liquid two-phase flow in circular milli-channels: Sizing-up effect

Background: Microfluidic is one of the most promising technologies for safety production in chemical industry while scale-up is a major challenge for its industrialization. Methods: In this work, hydrodynamics and mass transfer of liquid-liquid two-phase flow in different circular milli-channels were experimentally and numerically studied to investigate the sizing-up effect. Significant findings: Slug flow was observed in toluene-water system and the droplet length was measured. The results showed shorter droplets tended to present under higher flow rates, inlet mode one and smaller T-mixers. The buoyancy exerted on the toluene phase accelerated droplet detachment. For n-butanol-water system, a repetition of ‘parallel flow - annular flow’ was observed due to the prominent gravitational effect. The flow was asymmetric and the phenomenon was especially notable in the serpentine channel with a larger diameter. The detailed velocity field was demonstrated by numerical simulation. Mass transfer enhancement was observed in smaller channels and serpentine channels, resulting from shorter diffusion distance and secondary flow, respectively.

Parametric analysis of interfacial friction factor for liquid film dynamics sheared by turbulent gas flow

Water droplet impingement on a low-pressure steam turbine blade causing erosion has been recognized as a crucial issue. It is essential to elucidate a comprehensive droplet detachment mechanism, not only from the trailing edge but also from the liquid film surface. In the present paper, we investigate the influence of interfacial friction factor against liquid film dynamics on a wall sheared by a turbulent gas flow, including the liquid film thickness, liquid film velocity and entrained droplet detached from liquid surface for both pipe flow and plate flow conditions. We conduct the analyses by using a liquid film dynamics model, recently established, considering the three-dimensional destabilized waves and droplet entrainment from the liquid surface. As a result, the film thickness and velocity greatly depends on the interfacial friction factor. Interestingly, the rate of entrained droplet to initial liquid film has a minimum value when the interfacial friction factor equals to the inverse of the liquid film Reynolds number, while the remaining liquid film flow rate becomes maximum.

Numerical study of satellite droplet formation in dripping faucet

• Dynamic relation between the liquid/gas interface, pressure and velocity profile has been investigated. • Physics of capillary wave generation in liquid filament has been analyzed. • Satellite drop generation depends on the acceleration and distribution of the axial velocity at the cone-filament junction. • Satellite drop is less likely to be formed at high inlet velocity and low surface tension coefficient. Dynamics of primary and satellite drop formation has wide engineering applications. Drop formation from a vertical capillary tube in air goes through various stages that include necking, bifurcation, recoil, wave generation and secondary necking and bifurcation. In this study, the problem has been studied numerically where the free surface of the pendant drop is tracked by a coupled level set and volume-of-fluid method with the surface tension force determined by the continuum surface force model. The evolution of the complete droplet separation process has been simulated. The numerical results have been compared with published experimental data. The mechanism for the recoiling of the liquid filament after droplet detachment and the subsequent formation of a satellite droplet has been studied and dynamic correlation among liquid/gas interface topology, velocity and pressure field has been investigated. Bell shaped distribution of axial velocity profile and fast acceleration at the cone-filament junction has been found to lead to satellite drop generation. Parametric studies on the effects of inlet velocity, surface tension coefficient and viscosity have been performed. Satellite droplet is less likely to be formed at high inlet velocity and low surface tension coefficient. Viscosity, on the other hand, has been found to play an important role on the shape of liquid filament and satellite droplet. These findings can help better understanding of drop-on-demand liquid dispensing, inkjet printing, spray combustion and many more engineering applications.

Coalescence-induced jumping of droplets from superhydrophobic surfaces—The effect of contact-angle hysteresis

Droplets coalesce and jump from superhydrophobic surfaces, a result that stems from the dominance of capillary and inertial forces and the presence of high contact angles. This phenomenon has been a subject of intensive numerical research mostly for cases when the degree of hydrophobicity is described by a single contact-angle value (a static contact angle). The introduction of various degrees of contact-angle hysteresis complicates the numerical modeling of the jumping process due to the sensitivity of the results to the effective value of the contact angle. We have developed and validated a comprehensive volume-of-fluid–immersed boundary numerical framework that accounts for the effect of hysteresis by focusing on the representation of actual (i.e., effective) values of contact angles. By comparing the behavior of jumping droplets on superhydrophobic surfaces with several degrees of hysteresis (up to 15°), we quantified the influence of hysteresis on the jumping process and identified various stages of the merged droplet's detachment and re-attachment to the surface. The latter phenomena were observed in all our simulations with droplets of different initial radii. In all the cases with hysteresis, the merged droplet eventually jumps, but we point out the decrease in the jumping velocity as compared to cases with only a static contact angle imposed. Finally, by using the Kistler dynamic contact-angle model, we demonstrate and quantify the importance of accurately capturing the dynamic receding contact angle when droplets jump from superhydrophobic surfaces with various degrees of hysteresis.

Role of surfactant-induced Marangoni stresses in retracting liquid sheets

In this work, we study the effect of insoluble surfactants on the three-dimensional rim-driven retraction dynamics of thin water sheets in air. We employ an interface-tracking/level-set method to ensure the full coupling between the surfactant-induced Marangoni stresses, interfacial diffusion and inertia. Our findings are contrasted with the (Newtonian) dynamics of a liquid sheet edge, finding that the surfactant concentration can delay, or effectively prevent, the breakup of the rim. Our simulations use the fastest growing Rayleigh–Plateau instability to drive droplet detachment from the fluid sheet (rim). The results of this work unravel the significant role of Marangoni stresses in the retracting sheet dynamics at large elasticity numbers. We study the sensitivity of the dynamics to the elasticity number and the rigidification of the interface.

Metal Transfer Behavior of Metal-Cored Arc Welding in Pure Argon Shielding Gas

The metal transfer behavior of gas metal arc welding in a pure argon shielding gas was evaluated through experiments using a standard solid wire and a metal-cored wire. The investigation was conducted using observation techniques based on recording images by a high-speed camera equipped with laser assistance and bandpass filters in a range of welding currents. It was observed that the metal transfer mode became a streaming transfer mode when the welding current increased in the solid wire. Meanwhile, in the metal-cored wire, the droplet transfer frequency increased, and the droplet diameter decreased without changing the metal transfer mode in the globular transfer mode. We surmised that the streaming transfer in the solid wire would be caused by the spread of argon plasma at the wire tip, which decreases the effect of the electromagnetic force on droplet detachment. Conversely, due to the presence of flux inside the metal-cored wire, the argon plasma could not spread and was attached close to the iron vapor plasma at the overhead of the droplet. Hence, the electromagnetic force acting on the side of the unmelted flux was ineffective at promoting droplet detachment, preventing the transition to a streaming transfer mode. Furthermore, weld bead formation in the metal-cored wire was better than that in a conventional solid wire.

Study on droplet motion behavior on a rough gas diffusion layer surface of a PEM fuel cell

Understanding the dynamic characteristics of the droplets on the gas diffusion layer (GDL) of the proton exchange membrane fuel cell (PEMFC) is important to improve fuel cell performance. This paper studies the dynamic characteristics of water droplet detachment from a simplified rough GDL surface by volume of fluid (VOF) method. The rough GDL surface was restructured by designing grooves with cubic pillars on the GDL surface. Results indicate that the gas velocity, space distance, and wettability of the cubic pillars remarkably affect the water droplet detachment. On a high hydrophilic, water droplet exhibits a shorter detachment time regardless of spatial distance and gas velocity. Whereas on a moderate hydrophilic GDL, a large space distance benefits the droplet detachment. However, in both situations referred to above, droplets tend to deviate and adhere to the corner between the channel wall and the GDL surface. On a high hydrophobic GDL, a large space distance and a high gas velocity favor droplet detachment, moreover, droplet shows Cassie mode and can be easily drained out. These findings can guide the optimal design of GDL for water management of high-performance fuel cells.

Kinetics of wettability alteration and droplet detachment from a solid surface by low-salinity: A lattice-Boltzmann method

The dynamics of droplet detachment from a surface is a fundamental topic studied in coating engineering, fluid mechanics, and subsurface engineering applications. This topic has direct relevance to wettability alteration using the modified ionic composition of water in contact with oil droplet, low salinity waterflooding (LSWF). Previous experimental studies of LSWF have shown a very long timescale in wettability alteration which cannot be explained using bulk diffusion coefficient. In the present study, we address both the time scale of detachment, as well as the impact of buoyancy and interfacial forces (referred to as Bond number) on droplet detachment by proposing an advanced GPU-enhanced lattice Boltzmann model. In this model, the immiscible two-phase flow has been coupled with wettability alteration due to the salinity dilution. After full validation of the model against the former experiments, a set of computational setups with distinct Bond numbers were designed to investigate the effect of interfacial tension and droplet size on the dynamics of droplet detachment. Results demonstrate that droplet detachment from a surface is not a unique function of Bond number and diffusion length scale is critical in detachment time. In summary, this study provides a fully validated model of LSWF and delineates the significant difference in impacts of interfacial tension and bubble size on detachment time, which can be used to predict capability for LSWF, to determine favorable conditions, as well as to observe and explain low-salinity-effect.

Impact of salinity on fluid/fluid and rock/fluid interactions in enhanced oil recovery by hybrid low salinity water and surfactant flooding from fractured porous media

Wettability alteration of the porous medium from oil-wet (OW) to neutral-wet or even water-wet (WW) is proposed as one of the main mechanisms behind the enhanced oil recovery (EOR) by the Low Salinity Water Flooding (LSWF). However, our recent studies for a specific crude-oil/brine/rock of the oil reservoir in the Middle East, shows that LSW (diluted sea water) is not that much effective, neither to change the wettability of the surface nor to significantly increase the oil recovery factor. This obstacle becomes more challenging when we deal with fractured formations, where the imbibition of injected fluid from fracture into matrix and its propagation within the matrix, is the main macroscopic mechanism of the EOR. Chemical EOR, in which the chemical additive is added to the injecting brine, is one of the hybrid techniques which can boost oil recovery in such cases. In this study, CTAB as a cationic surfactant was added to brine solutions with different ionic strengths (i.e., sea water (SW), 10 times diluted sea water (10dSW) and 2 times concentrated sea water (2cSW)). Contact angle, zeta potential of both fluid/fluid and fluid/rock surfaces, and IFT tests were conducted to shed insight on interactions in crude oil/brine and rock/brine interfaces and the role of ionic strength. Finally, dynamic micromodel tests were performed in oil-wet fractured porous media to observe the impact of the added surfactant along with different ionic strength on displacement mechanisms and oil recovery.Experimental results show that neither electrical double layer expansion nor multiple ion exchange mechanisms cannot justify the observed detachment of oil droplets from the oil-wet substrates in the presence of the CTAB. Surfactant are absorbed on the oil/brine interface by the embedment of their hydrophobic tails in oil. Both surfactant molecules and the non-polar oil components (that are attracted to hydrophobic tail of surfactant molecules), are attracted to the solution via the hydrophilic parts of the surfactants. The effectiveness of this mechanism is dependent on both the concentration of the surfactant and concentration of the ions in brine.In the case that the aforementioned mechanism is adequately dominant and effective, the small droplets of oil will be detached from continuous oil bulk, resulting in oil to be broken to smaller droplets which enhances the detachment of the oil bulk from oil-wet substrates. This fluid/fluid interaction mechanism also promotes the imbibition of the aqueous phase into the matrix and also its propagation within the matrix.

Two-phase Flow Dynamics at the Interface Between GDL and Gas Distributor Channel Using a Pore-Network Model

For improved operating conditions of a polymer electrolyte membrane (PEM) fuel cell, a sophisticated water management is crucial. Therefore, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. Pore-scale modeling of diffusion layers and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the cathode layers, a particular challenge is the combination and interaction of the multi-phase flow in the porous material of the gas diffusion layer (GDL) with the free flow in the gas distributor channels. The formation, growth and detachment of water droplets on the hydrophobic, porous surface of the GDL have a major influence on the mass, momentum and energy exchange between the layers. A dynamic pore-network model is used to describe the flow through the porous GDL on the pore-scale. To capture the droplet occurrence and its influence on the flow, this dynamic two-phase pore-network model is extended to capture droplet formation and growth at the surface of the GDL as well as droplet detachment due to the gas flow in the gas distributor channels. In this article, the developed model is applied to single- and multi-tube systems to investigate the general drop behavior. These rather simple test-cases are compared to experimental and numerical data available in the literature. Finally, the model is applied to a GDL unit cell to analyze the interaction between two-phase flow through the GDL and drop formation at the interface between GDL and gas distributor channel.

High-Efficient Fog Harvest from a Synergistic Effect of Coupling Hierarchical Structures.

Fog harvesting is an important method to solve the water shortage in arid and semi-arid areas by collecting water from air. Improving fog harvesting efficiency is still a big challenge to be overcome. Herein, under the inspiration of natural creatures, a novel harvesting structure that couples a hierarchical microchannel (HMC) needle with the Janus membrane by taking a conical pore as their junction is proposed. Such an HMC-conical pore-Janus membrane system can improve the harvesting efficiency by regulation of liquid behavior in the whole fog harvesting process involving droplet capture from air, high speed transport on the microchannel, and droplet detachment from Janus. The synergistic effects of the hierarchical channel-conical pore-Janus structure are exploited in terms of capture, transport, and detachment capabilities, and their underlying mechanism to enhance fog harvesting efficiency is built. Compared with the traditional harvesting structure, the proposed hierarchical channel-conical-Janus coupling mode was demonstrated to improve fog harvesting efficiency by 90%. Such a coupled system has potential applications in efficient fog harvesting systems, microfluidic devices, and liquid manipulation.

Ultrascalable Surface Structuring Strategy of Metal Additively Manufactured Materials for Enhanced Condensation.

Metal additive manufacturing (AM) enables unparalleled design freedom for the development of optimized devices in a plethora of applications. The requirement for the use of nonconventional aluminum alloys such as AlSi10Mg has made the rational micro/nanostructuring of metal AM challenging. Here, the techniques are developed and the fundamental mechanisms governing the micro/nanostructuring of AlSi10Mg, the most common metal AM material, are investigated. A surface structuring technique is rationally devised to form previously unexplored two‐tier nanoscale architectures that enable remarkably low adhesion, excellent resilience to condensation flooding, and enhanced liquid–vapor phase transition. Using condensation as a demonstration framework, it is shown that the two‐tier nanostructures achieve 6× higher heat transfer coefficient when compared to the best filmwise condensation. The study demonstrates that AM‐enabled nanostructuring is optimal for confining droplets while reducing adhesion to facilitate droplet detachment. Extensive benchmarking with past reported data shows that the demonstrated heat transfer enhancement has not been achieved previously under high supersaturation conditions using conventional aluminum, further motivating the need for AM nanostructures. Finally, it has been demonstrated that the synergistic combination of wide AM design freedom and optimal AM nanostructuring method can provide an ultracompact condenser having excellent thermal performance and power density.

Investigation of the detachment of oil droplets driven by water shear field on solid surfaces with different wettabilities

The movement and detachment of tiny oil droplets in water on a substrate is the key technology to strengthen oil–water separation by using corrugated plate oil–water separation technology. Here, the detachment behaviors of oil droplets adhered to substrates with different wettabilities in a laminar flow field are studied experimentally and theoretically. The morphologic changes of mother and daughter oil droplets are obtained when the adhered oil droplets are continuously separated. The shedding mechanism of oil droplets is analyzed in detail, and the critical condition for the effective detachment of oil droplets is obtained, which can be used to guide the design of the surface wettability and water flow velocity in the actual production of oil–water separation.

Exploring the Role of Initial Droplet Position in Coalescence-Induced Droplet Jumping: Lattice Boltzmann Simulations

Coalescence-induced droplet jumping on superhydrophobic surfaces with different initial positions was numerically simulated using the 2D multi-relaxation-time (MRT) Lattice Boltzmann method (LBM). Simulation results show that for coalesced droplets with radii close to the structure length scale, the change of initial droplet positions leads to a significant deviation of jumping velocity and direction. By finely tuning the initial droplet positions on a flat-pillared surface, perpendicular jumping, oblique jumping, and non-jumping are successively observed on the same structured surface. Droplet morphologies and vector diagrams at different moments are considered. It is revealed that the asymmetric droplet detachment from the structured surface leads to the directional transport of liquid mass in the droplet and further results in the oblique jumping of the coalesced droplet. In order to eliminate the influence of initial droplet position on droplet jumping probability, a surface with pointed micropillars is designed. It is demonstrated that compared to flat-topped micropillars, a surface with pointed micropillars can suppress the initial droplet position effects and enhance droplet jumping probability. Furthermore, the effect of droplet/structure scale on droplet jumping is investigated. The influence of initial positions on coalescence-induced droplet jumping from the refined surface can be ignored when the droplet scale is larger than three times the structure scale. This study illustrates the role of initial droplet position in coalescence-induced droplet jumping and provides guidelines for the rational design of structured surfaces with enhanced droplet self-shedding for energy and heat transfer applications.

Optical investigation on the interaction between a fuel-spray and an oil wetted wall with the focus on secondary droplets

Since the technology of direct injected engines has become more and more prevalent, combustion anomalies such as pre-ignition events have been occurring with increasing frequency. The interaction between the fuel-spray and the oil wetted cylinder liner is considered to be a partial cause for these phenomena. The incoming fuel jet can lead to a detachment of secondary droplets, which may affect pre-ignition events due to their changed flammability properties. To characterize these secondary droplets, spatially and temporally highly resolved optical investigations on the fuel-oil-film-interaction are carried out in this work. Using laser induced fluorescence and two high-speed cameras both fluids are optically separated. Thereby the secondary droplets can be detected and characterized in size, velocity vector, and composition.

Vibration-induced detachment of droplets on superhydrophobic surfaces

Vibration is a robust and efficient method for droplet removal from superhydrophobic surfaces. In the present work, we experimentally investigated the dynamics of droplets on vibrating superhydrophobic surfaces, and established a mass-spring-damper theoretical model, to clarify the underlying physics of vibration-induced droplet detachment. Different droplet oscillation patterns were experimentally observed, namely, the droplet motion is in the same/opposite direction with the substrate when the vibration frequency is less/higher than the resonance frequency, respectively, and the transition occurs at the resonant frequency. The motion of the droplet is found to be composed of a transient response by the droplet free oscillation and a steady-state response by vibrating substrates. The critical condition for droplet detachment was identified to be the droplet energy (sum of the surface energy and kinetic energy) exceeds the surface adhesion energy. At the resonance frequency, the vibrational energy of the substrate is progressively accumulated in the droplet with the highest energy maintenance efficiency, and a small amplitude of vibration could therefore yield droplet detachment. The separation time shortens with the vibration amplitude and minimizes at the resonance frequency at a given amplitude. Our findings are helpful for the utilization of vibrating superhydrophobic surfaces for droplet removal.

Spectacular Behavior of a Viscoelastic Droplet Impinging on a Superhydrophobic Mesh.

Spray formation using the droplet impact on superhydrophobic mesh surfaces is particularly important because of its application in different industries. The present study revealed that adding a trivial amount of the poly(ethylene oxide) (PEO) polymer to a water droplet can considerably change the impact phenomena on the superhydrophobic mesh surfaces and suppress the spray formation. Droplet rebound is observed only in a narrow range of impact velocities of PEO aqueous droplets when the tiny filaments still connect the surface and droplet. Rebound suppression and deposition of the PEO aqueous droplet is attributed to the higher interaction between the polymer chains and the superhydrophobic mesh surface. After a critical impact velocity and We number which is independent of the PEO concentration, the liquid penetrates the mesh pores. The penetrated liquid formed the ligaments that grow until they reach the maximum length and surprisingly retract back to the mesh surface and the mother droplet. The ligaments destabilized at low PEO concentrations (c = 0.5 and 1 g/L) and a mesh opening size of H = 357 μm to the crest swell droplets when the droplet size is reduced by increasing the impact velocity. The ligament fragmentation and droplet detachment are observed only at high impact velocities when c = 0.5 and 1 g/L and H = 357 μm. The result shows that the PEO additive does not significantly affect the maximum spreading diameter. An empirical model to calculate the maximum spreading factor is developed.

Dynamics of nonmagnetic inclusions in a microchannel with a magnetic fluid in an inhomogeneous magnetic field

The article presents the results of studying the dynamics of non-magnetic bubbles and droplets in a magnetic fluid placed in an inhomogeneous magnetic field. Using a digital high-speed system for video recording, the mechanism of the detachment of a non-magnetic droplet from the major part of the magnetic fluid has been considered in detail. It is experimentally demonstrated that the size of formed non-magnetic inclusions in such a magnetic-fluid system does not depend on the flow rate and hydrostatic pressure. The possibility to control the size of non-magnetic liquid and gas inclusions by changing the parameters of magnetic fluid and magnetic field can be used for microdosers or gas meters in microfluidic systems.