Droplet Impact Research Articles

Insight into Role of Arc Torch Angle on Wire Arc Additive Manufacturing Characteristics of ZL205A Aluminum Alloy.

The arc torch angle greatly affected the deposition characteristics in the wire arc additive manufacturing (WAAM) process, and the relation between the droplet transition behavior and macrostructure morphology was unclear. This work researched the effect of torch angle on the formation accuracy, droplet transition behavior and the mechanical properties in the WAAM process on a ZL205A aluminum alloy. The results suggested that at the obtuse torch angle, part of the energy input was used to heat the existing molten pool, which was optimized for the longer solidification period of the molten pool. Therefore, the greater layer penetration depth at 100° resulted in the improved layer-by-layer combination ability. The obtuse torch angle was associated with the better formation accuracy on the sidewall surface due to the smaller impact on the molten pool, which was influenced by both the arc pressure and droplet impact force. The eliminated pores were optimized for the mechanical properties of depositions at a torch angle of 100°; thus, the tensile strength and elongation attained maximum values of 258.6 MPa and 17.1%, respectively. These aspects made WAAM an attractive mode for manufacturing large structural components on ZL205A aluminum alloy.

Hydrovoltaic–piezoelectric hybridized device for microdroplet-pressure dual energy harvesting and piezo sensing

Previous droplet-based nanogenerators have made significant advancements in mechanical energy harvesting from liquid–solid contact electrification. However, these generators still face certain issues, such as neglecting substrate deformation energy, inability to store energy, and surface deterioration due to continuous impact of droplets, thus narrowing the scope of their potential applications. Here, a novel hydrovoltaic-piezoelectric hybridized device (HPeD) is reported that integrates hydrocapacitor, aluminum–air (Al–air) battery, and piezoelectric nanogenerator technologies through a green design strategy, which can effectively address the proposed problems. Impinged by a 40-μL water droplet, the HPeD yields a potential window of 1.54 V within approximately 1 min, further boosting to 1.86 V under external vertical pressure. The generated energy was greatly stored for up to 5.7 days. This all-solid-state device effectively inhibits electrolyte leakage during idle periods and controls reversible reactions to minimize corrosion, thereby extending its lifespan. Furthermore, the HPeD is successfully used as a piezoelectric nanogenerator/sensor that can operate continuously to detect external mechanical stimulus. In piezo sensor mode, the device demonstrated good sensitivity of 0.098 kPa−1 in the low-pressure range (0–8.2 kPa), a remarkable response/relaxation time of 0.3 s, and durability lasting over 8000 cycles. This study paves the way for the advancement of next-generation flexible hybrid devices capable of multifunctionality in both energy harvesting and sensing.

Water Impact Dynamics and Mechanism Analysis of Polypropylene and Polypropylene/poly(ethylene-co-octene) Replica Surfaces with Nanowires under Low Temperature Conditions.

In this work, polypropylene (PP) and PP/poly(ethylene-co-octene) (PP/POE) blend replica surfaces with densely arranged nanowires are fabricated using a molding process. Morphology analysis shows that the nanowires on these replica surfaces have sharp tips and high aspect ratios. The droplet impact behaviors on the surfaces of the PP/POE blend replicas and PP replicas are investigated before and after freezing at -20 °C for 4 h. The height of the nanowires on the PP/POE blend replica surfaces is higher than that on the PP surface before and after freezing. The static wettability and droplet impact tests on the surfaces of PP/POE blend replicas and PP replica show that adding POE into the PP matrix can significantly increase the toughness of the nanowires on the PP/POE blend replica surfaces during the droplet impact at low temperatures. These investigations are favorable to broaden the practical applications of superhydrophobic surfaces in some severe environments.

Dynamic behaviors of shear-thinning droplet impacting on a spherical particle surface

Non-Newtonian droplet impact is common in nature and industry. The viscosity of non-Newtonian liquids changes with shear stress, leading to different impact behaviors compared to Newtonian liquids. This study prepared shear-thinning solutions by adding HPMC polymer into deionized water and investigated its impact dynamics on hydrophobic spherical surface. The evolution patterns of droplet spreading factor, film thickness, and dynamic contact angle are investigated under different impact conditions. The results show that addition of polymers increases the energy dissipation during impact and suppresses droplet rebound from spherical surface compared to Newtonian droplet with similar viscosity. The droplet-impacting process is distinguished into three phases: (I) initial deformation, (II) inertia-dominated, and (III) viscosity-dominated phases. The temporal evolution of film thickness in Phases I and II is fitted by simple equations. In addition, an empirical formula that integrates Weber and Reynold numbers is proposed to predict the maximum spreading factor of HPMC droplets.

Experimental study of the collision behavior between moving and sessile droplets on curved surfaces

In this study, we used pure water droplets to investigate the collision interactions of two droplets on curved surfaces. We captured the collision dynamics at different droplet impact velocities with the use of a high-speed camera. We observed three main modes during droplet collisions: deformation, coalescence, and rebound. We also plotted a collision regime map for different surfaces using the Weber number We and presented the critical We value for coalescence and rebound. With the increases of curvature radius, the critical We variates around 6.7 to 12.3 and presenting the tendency of increases first and then decreases. Furthermore, we quantitatively analyzed the transformation laws of droplet rebound height and compression value. Our main findings describe how the surface curvature radius influences droplet interactions, thus affecting the contact time, compression value and droplet deformation factor. The outcomes of droplet collisions on curved surfaces contribute to the development of technologies related to droplet manipulation and their applications in chemical and industrial processes.

Lagrangian Split-Step Method for Viscoelastic Flows.

This research addresses and resolves current challenges in meshless Lagrangian methods for simulating viscoelastic materials. A split-step scheme, or pressure Poisson reformulation of the Navier-Stokes equations, is introduced for incompressible viscoelastic flows in a Lagrangian context. The Lagrangian differencing dynamics (LDD) method, which is a thoroughly validated Lagrangian method for Newtonian and non-Newtonian incompressible flows, is extended to solve the introduced split-step scheme to simulate viscoelastic flows based on the Oldroyd-B constitutive model. To validate and evaluate the new method's capabilities, the following benchmarks were used: lid-driven cavity flow, droplet impact response, 4:1 planar sudden contraction, and die swelling. These findings highlight the LDD method's effectiveness in accurately simulating viscoelastic flows and capturing large deformations and memory effects. Even though the extra stress was directly modeled without any regularization approach, the method produced stable simulations for high Weissenberg numbers. The stability and performance of the the Lagrangian numerics for complex temporal evolution of material properties and stress responses encourage its use for industrial problems dealing with polymers.

Steerable droplet precise bouncing on a superhydrophobic surface with superhydrophilic stripes

The precise rebound of a droplet upon hitting a solid surface has garnered significant attention in recent years due to its critical applications in self-cleaning, printing industries, and the design of heat exchanger surfaces, among others. This study introduces an innovative approach that combines femtosecond laser processing with a high-temperature stearic acid modification to create surfaces that feature superhydrophilic (SHL) stripes on a superhydrophobic substrate. By controlling the offset distance between the droplet's impact point and the SHL stripe, we achieved a directional and precise rebound of the droplets. Our findings indicate that the lateral displacement of the droplet increases with the offset distance and always tilts toward the direction of the SHL stripe. This study also incorporates numerical simulations to validate the findings, shedding light on the energy conversion mechanisms at the liquid–solid interface during the impact, particularly during the retraction phase. This discovery is significant for more accurately predicting the specific landing spots of rebounding droplets.

Numerical Study on the Impact Pressure of Droplets on Wind Turbine Blades Using a Whirling Arm Rain Erosion Tester

The leading-edge erosion of a wind turbine blade was tested using a whirling arm rain erosion tester, whose rotation rate is considerably higher than that of a full-scale wind turbine owing to the scale effect. In this study, we assessed the impact pressure of droplets on a wet surface of wind turbine blades using numerical simulation of liquid droplet impact by solving the Navier–Stokes equations combined with the volume-of-fluid method. This was conducted in combination with an estimation of liquid film thickness on the rotating blade using an approximate solution of Navier–Stokes equations considering the centrifugal and Coriolis forces. Our study revealed that the impact pressure on the rain erosion tester exceeded that on the wind turbine blade, attributed to the thinner liquid film on the rain erosion tester than on the wind turbine blade caused by the influence of centrifugal and Coriolis forces. This indicates the importance of correcting the influence of liquid-film thickness in estimating the impact velocity of droplets on the wind turbine blade. Furthermore, we demonstrated the correction procedure when estimating the impact velocity of droplets on the wind turbine blade.

High-Speed Stress Field Measurement In A Soft Substrate During Droplet Impact

In this study, we utilized high-speed photoelastic tomography to quantify dynamic stress fields within a soft substrate during droplet impact. This manuscript details the measurement technique, which employs a high-speed polarization camera and the principles of photoelastic tomography. Our method successfully enabled the quantitative measurement of dynamic stress fields in a soft substrate during droplet impact. Additionally, we conducted an analysis of the impact force derived through the spatial integration of the measured stress field. The discussion explored the interplay between the maximum impact force, droplet viscosity, and substrate elasticity. Our findings indicate that the maximum impact force 𝐹max can be expressed as a function with a self-similar variable of 𝑅𝑒 / 𝐶𝑎^2 , where 𝑅𝑒 is Reynolds number representing the droplet inertial force and droplet viscous force and 𝐶𝑎 is Cauchy number representing the ratio of the droplet inertial force and substrate elastic force. This combination, 𝑅𝑒 / 𝐶𝑎^2 , represents the relationship between the relaxation time of droplet and substrate deformation ( 𝜂/ 𝐸), the contact time of the droplet (𝑅 / 𝑉), and the ratio of substrate elastic force to droplet inertial force. Consequently, the maximum impact force 𝑅𝑒 / 𝐶𝑎^2 is determined by the balance between the relaxation and contact times. We believe that our developed method will significantly enhance the understanding of droplet impact phenomena. Furthermore, it holds potential for broader applications in various engineering processes, such as analyzing stress distribution in materials caused by liquid jet impact and studying cavitation bubbles in viscoelastic materials. This method's ability to provide detailed quantitative measurements of dynamic stress fields offers a valuable tool for future research and technological advancements in related fields.

Understanding Vortex Ring Instability During Droplet Impact Onto Thin Liquid Films Using High Speed PIV

This study investigates vortex ring instability during droplet impact on thin liquid films using high-speed Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF). Experiments were conducted using two different setups, capturing the droplet impact from both bottom and side views. The study examines how different dimensionless parameters (Reynolds number, Weber number and dimentionless thickness) influence vortex ring behavior in thin liquid films. Vortices were analyzed by identifying the centers using Gamma_2 scalar and calculating the vortex circulation through a two superposed Lamb-Oseen vortex fitting. The effect of film thickness on vortex dynamics reveals that thinner films lead to greater instability due to higher energy at the moment of wall collision, facilitating the formation of additional vortex structures. Experimental phase mapping highlights the critical threshold of Reynolds number and dimensionless thickness for vortex ring instability. Moreover, the number of fingers formed due to vortex ring instabilities increases with higher Reynolds numbers and thinner films, providing a clear relationship between film thickness, impact energy, and the complexity of resulting fluid structures.

Contact time of impacting nanodroplets on cylinder surfaces

Engineering surfaces that can accelerate droplet detachment are of great importance to a wide range of practical applications, including dropwise condensation, anti-icing, and self-cleaning. Considering the pragmatic effect of reducing contact time (τc), the cylindrical surface is one of the most popular passive approaches for achieving this purpose. Owing to the absence of a suitable way for the experimental observation, we use molecular dynamics (MD) simulations to obtain an in-depth understanding of droplet impingement on the curved surface at the molecular level together with the contact time variation. Accordingly, the regimes of τc upon curved surfaces are well recognized by summarizing the τc variation, which are stable reducing τc regime and hybrid τc regime. We propose a law of τc ∝ α-0.226 to predict the τc variation in the stable reducing τc regime. And for the hybrid τc regime, the τc variation is specifically defined as three parts, including the delayed τc, limited τc, and transitional τc regions. Moreover, we investigate the effect of the dimensionless length of curved surfaces on the τc variation in detail. This work paves the way to design a more effective curved surface to promote the nanodroplet rebound.

Study on dynamic characteristics of anionic surfactants containing ethoxy groups wetting bituminous coal

Coal dust restricts coal mine safety and green production. Wet dust removal is widely used in coal mining, processing and transportation. High-efficiency dust suppressants help control coal dust and improve the working environment. The wetting behaviors of dust suppression droplets on coal affect the dust reduction efficiency. In order to select high-quality and efficient water-based materials, four ethoxy-containing surfactants with similar structures, fatty alcohol ether sulfate (AES), lauryl alcohol sulfosuccinate disodium (MES), fatty acid methyl ester ethoxylates (FMEE) and fatty acid methyl esters ethoxylate sodium sulfonate (FMES), are selected to prepare solutions with different concentrations. Through the droplet impact wetting experimental system, the dimensionless spreading coefficient of droplets is compared. The results show that the spreading behavior of the four surfactant-containing droplets is better than that of distilled water droplets, and the maximum dimensionless spreading coefficient follows the rule of AES > MES > FMES > FMEE >distilled water. The maximum dimensionless spreading coefficient of the droplet first increases sharply with the increase of the concentration, and then increases slowly when the concentration reaches the critical micelle concentration (CMC). The adsorption configuration of water-surfactant-coal system is simulated by molecular dynamics (MD) simulation. The relative concentration distribution of each component and the mean square displacement (MSD) of water molecules are analyzed. The interaction energy of the system is calculated. The simulation results verified the effectiveness of the impact wetting experiment results.

An analytical model for ice accretion on the engine strut surface

To predict flight icing more widely and practically, an ice accretion numerical framework that incorporates both the water droplet splash and the ice crystal sticking is developed. By proposing a deformation hypothesis, we deduce the modified energy conservation expression and the force balance relation for water droplet impingement. Subsequently, a new threshold determination and the probabilities for the droplet splash and ice crystal sticking are obtained, which are applicative across a wide range of Weber number after the validation. Through the interface tracking for a single droplet with the volume of fluid method, the droplet impingement dynamics are further explored, and the results of interaction with the wall serve the boundary treatments of droplet impingement in the discrete phase model. Additionally, the probability statistics method is employed to determine the parameters of the secondary droplets. Through the dynamic mesh technique, the retentive water droplets and the collected ice crystals are transformed into the accumulated ice in real time to update the ice accretion on the strut surface. Results demonstrate that the diameter, velocity, and content of droplets or crystals play significant roles in the impingement and the icing phenomena. Based on our numerical model, the predictions show that the ice accretion on the engine strut is influenced by flight parameters and environmental conditions, providing crucial guidance for the icing protection processes.

Successive rebounds of obliquely impinging water droplets on nanostructured superhydrophobic surfaces

The impact and rebound of water droplets on superhydrophobic surfaces frequently happen in nature and also in a number of industrial processes, which has thus stimulated strenuous efforts to explore the underlying hydrodynamics. Despite that massive achievements have been made over the past decades, existing works are mostly focusing on the short-time bouncing dynamics after a single impact; however, the long-term, successive droplet rebounds, which are practically more important, only received very limited attention. In this work, we perform an experimental investigation on the impact of water droplets on inclined nanostructured superhydrophobic surfaces at low Weber numbers, where massive complete rebounds arise. It was found that an obliquely impinging droplet would undergo many impacts on the superhydrophobic surface, accompanying with sliding on the surface, jumping in air, and complex shape evolutions. Based on the kinematic analyses, we demonstrate that the droplet motion on the surface can be decomposed into a perpendicular impact, which is dominated by the capillary and inertial forces, and a translational motion under the drive of gravity. By contrast, the jumping motion after droplet rebound is solely governed by the gravitational force, yet relevant droplet characteristics are affected by the energy loss during the impact on superhydrophobic surface, which sets the maximum height that the droplet rebounds to. In addition, three distinct shape evolution modes–namely, oscillation, rotation and their combination–were identified on jumping droplets, and the direction of a rotational droplet can be altered via the following impingement on the superhydrophobic surface.

A comparison of models for predicting the maximum spreading factor in droplet impingement

The maximum spreading factor during droplet impact on a dry surface is a pivotal parameter of a range of applications, including inkjet printing, anti-icing, and micro-droplet transportation. It is determined by a combination of the inertial force, viscous force, surface tension, and fluid–solid interaction. There are currently a series of qualitative and quantitative prediction models for the maximum spreading factor rooted in both momentum and energy conservation. However, the performance of these models on consistent experimental samples remains ambiguous. In this work, a comprehensive set of 785 experimental samples spanning the last four decades is compiled. These samples encompass Weber numbers ranging from 0.038 to 2447.7 and Reynolds numbers from 9 to 34 339. A prediction model is introduced that employs a neural network, which achieves an average relative error of less than 16.6% with a standard error of 0.018 08 when applied to the test set. Following this, a fair comparison is presented of the accuracy, generality, and stability of different prediction models. Although the neural network model provides superior accuracy and generality, its stability is weaker than that of Scheller's We-Re-dependent formula, chiefly due to the absence of physical constraints. Subsequently, a physics-informed prediction model is introduced by considering a physical loss term. This model demonstrates comprehensive enhancements compared to the original neural network, and the average relative and standard errors for this model are reduce to 13.6% and 0.010 59, respectively. This novel model should allow for the rapid and precise prediction of the maximum spreading factor across a broad range of parameters for various applications.

Characterisation of hydrophobic surfaces by droplet impact

Characterisation of hydrophobic surfaces by droplet impact

Droplet impact and rebound dynamics on superhydrophobic surfaces

The impact and rebound dynamics of droplets on superhydrophobic surfaces were investigated through numerical analysis employing the phase field method. The influences of contact angle, impact velocity, surface tension, and dynamic viscosity on the fields of pressure and velocity as well as the spreading factor and central height were described comprehensively. The results indicate that there are a series of stages of impingement, spreading, transition, retraction, and rebound in order throughout the life cycle of a droplet. The droplet exhibits distinct pressure and velocity profiles upon impingement stage, with the maximum pressure at the lower center and higher velocities at the upper periphery, spreading around. Velocities are predominantly upward, peaking at the bottom of the droplet during the rebound stage. A larger contact angle, viscosity, surface tension, and lower impact velocity contribute to a reduced maximum spreading factor. Deposition is more likely to occur when the impact velocity, surface tension is lower, and the viscosity is larger. Droplets tend to rebound when the contact angle, impact velocity, and surface tension are larger. Thresholds for impact velocity, surface tension, and viscosity were delineated for droplet rebound. Furthermore, a correlation for predicting the maximum spreading factor of droplets on superhydrophobic surfaces was proposed.

Droplet impact outcomes: Effect of wettability and Weber number

In this study, we experimentally explored the impact outcomes of droplets under a wide range of Weber numbers (0.2 ≤ We ≤ 200) and contact angles (91° ≤ θ ≤ 162°). Five impact outcomes were identified: deposition, rebound, partial rebound, receding breakup, and prompt splash. Compared to the literature, we gathered more comprehensive data on the impact outcomes at various contact angles, which were then organized into a complete phase diagram. Furthermore, we corroborated the accuracy of these outcomes through comparisons with other studies. We conducted a comprehensive analysis of the associated phenomena and underlying mechanisms of these outcomes. By introducing the concept of surface hysteresis energy, we also proposed identification criteria for partial rebound. This innovative approach provides an important reference for further understanding of droplet impact behavior and provides guidance for future research in this field.

Bounds on the spreading radius in droplet impact: the inviscid case

We consider the classical problem of droplet impact and droplet spread on a smooth surface in the case of an ideal inviscid fluid. We revisit the rim–lamella model of Roisman et al. (Roisman et al . 2002 Proc. R. Soc. Lond. A 458 , 1411–1430 ( doi:10.1098/rspa.2001.0923 )). This model comprises a system of ordinary differential equations (ODEs); we present a rigorous theoretical analysis of these ODEs, and derive upper and lower bounds for the maximum spreading radius. Both bounds possess a W e 1 / 2 scaling behaviour, and by a sandwich result, the spreading radius itself also possesses this scaling. We demonstrate rigorously that the rim–lamella model is self-consistent: once a rim forms, its height will invariably exceed that of the lamella. We introduce a rational procedure to obtain initial conditions for the rim–lamella model. Our approach to solving the rim–lamella model gives predictions for the maximum droplet spread that are in close agreement with existing experimental studies and direct numerical simulations.

3D Modelling of Layer-by-Layer Heat and Mass Transfer in Wire Arc Additive Manufacturing

A comprehensive three-dimensional transient computational fluid dynamics (CFD) model was developed to analyze the thermophysical phenomena in the arc wire additive manufacturing (WAAM) process. The model includes droplet impact, gravity, heat and mass transfer, molten metal flow, and solid-liquid phase changes. By integrating the mass, energy, momentum, and volume of fluid (VOF) equations, a layer-by-layer additive process was successfully simulated. The accuracy of the model was validated by comparing the simulation results of single-pass single-layer weld beads at three welding positions with experimental data. The established model was utilized to quantitatively investigate the effects of three crucial welding process parameters–welding direction, droplet transfer frequency, and initial temperature–on the multi-layer cladding process. These findings suggest that the use of opposite welding directions in two adjacent layers can result in a well-distributed overall morphology of the weld bead. Moreover, the high initial droplet temperature enhanced the inclination of the weld bead morphology while decreasing the height of each layer. However, the high droplet transfer frequency caused both the height and width of the weld to increase. This research contributes to explaining the formation mechanism of the multi-layer cladding process and offers insights for improving weld bead morphology. This provides valuable theoretical guidance for the process optimization and control of arc additive manufacturing technology.