Droplet Impact Research Articles

Morphology and heat transfer of a train of microdroplets impinging on the heated surface for spray cooling

Owing to the superior cooling performance of the droplet impingement process, the spray cooling is widely used in various industries including aerospace, e-motors, data centres and so on. In the present study, the morphological evolution and heat transfer characteristics of vertical and oblique impacts on the heated surface were investigated experimentally and numerically. A droplet generation system and a substrate heating system were designed to observe continuous vertical and oblique droplets impinging on the hot surface by a high-speed camera. The coupled Level Set and VOF (CLS-VOF) model and adaptive mesh refinement technique were used to establish a mathematical model for a train of micro-sized droplets impinging on the heated surface, which was validated by experimental data. The film that spreads on the surface exhibits asymmetry due to the horizontal velocity of the droplets. When continuous droplets impinge vertically on the heated wall, the evolution of the crown and the wall temperature appear periodic. The effects of impingement angles on morphological patterns and heat transfer of a train of micro-sized monodispersed droplets with the same Weber numbers were analyzed further. The results indicated that with a smaller angle of impingement, the crown is more likely to splash, and the crown asymmetry is less obvious.

A study on the impact of liquid metal droplets onto metal and elastomer substrates

A study on the impact of liquid metal droplets onto metal and elastomer substrates

Evolution of liquid film in a crossflow tunnel: liquid film thickness measurement and effect of droplet impingement on film breakup

Abstract Liquid jet in crossflow tunnel has widespread applications in various industrial devices,
with the measurements of liquid film on the bottom surface pivotal in exploring relevant mechanisms
such as heat transfer and film breakup. This work reports the measurements of liquid film on the
bottom surface of a crossflow tunnel using the brightness-based laser induced fluorescence (LIF)
method under different flow conditions at ambient pressure and temperature. Film breakup
phenomena are observed downstream within the tunnel. Employing the shadowgraph method, two
distinct patterns of film breakup associated with the droplet impingement positions on the film wave
are identified, i.e., bag breakup and membrane breakup. The film thickness is subsequently calculated,
and jet impingement and spray impingement of injected liquid on tunnel bottom surface are classified
based on the centerline film thickness. A critical jet-to-crossflow momentum flux ratio (q) is determined to be proportional to the square of tunnel height. The averaged film thickness across the entire cross-section downstream at a distance of 50 mm from the nozzle is found to increase with the logarithm of q. Besides, the film boundaries are also identified under different flow conditions, which can be well predicted by a quadratic fit with the fitting parameters also correlated to the logarithm of q.

Impact and spread dynamics of a viscoelastic droplet on an inclined hydrophilic surface

In this work, the impact of a three-dimensional viscoelastic droplet on an inclined hydrophilic surface is investigated by means of direct numerical simulations. The volume-of-fluid method is adopted to capture the interface, and the Oldroyd-B model is used to describe the rheological behavior of the viscoelastic droplet. The effects of the Weissenberg number (Wi) and the Weber number (We) on the impacting and spreading processes are studied, including the viscoelastic droplet shape, velocity, energy transformation, and stress distribution. Our results are in good agreement with the experimental data in the literature. In particular, the elastic force markedly influences droplet deformation at intermediate Wi values, although this trend diminishes at higher or lower Wi values. With increasing We, the impacting viscoelastic droplet reaches its maximum deformation more rapidly, while the nonmonotonic peak of kinetic energy indicates that the droplet elasticity plays significant role at moderate We. Additionally, the inclination of the surface has a pronounced effect on the droplet spreading process, and the elongated viscoelastic droplet at larger inclination angle is likely to experience a stronger oscillation. According to further analyses, We exerts a modest influence on the change rates of the droplet potential energy and spreading length in the flow direction. However, a larger inclination angle reduces stress concentration and accelerates the change rates. Due to the oscillation dynamics, Wi exhibits a non-monotonic effect on the spreading process and induces a monotonous increase in potential energy of viscoelastic droplets. The above analyses provide insights into the impact mechanism of droplets on an inclined hydrophilic wall and, therefore, will guide the applications in the future.

Lattice Boltzmann study of droplet dynamic behaviors impinging on textured surfaces under an external electric field

Textured surfaces contribute to enhancing the cooling effectiveness of electrostatic spray, while the droplet impacting dynamics on such substrates under the influence of electric field are crucial for cooling efficiency. This study utilized a multiphase lattice Boltzmann method combined with the leaky dielectric model to systematically examine the dynamics of droplet impingement on textured surfaces when exposed to electric field. The impact of Weber number, microstructural surface parameters, and electric field strength on droplet impact behavior was discussed in detail. Simulation outcomes reveal that, without the presence of an electric field, the impingement of droplets on textured surfaces results in three distinct deposition states: the Cassie state, partial penetration state, and Wenzel state, primarily contingent upon the surface solid fraction and the droplet impingement velocity. In the Cassie impact regime influenced by an applied electric field, the droplet spreading behaviors exhibit minimal sensitivity to the electric field, with surface tension and inertia primarily governing the spreading dynamics. Throughout the retraction stage, the droplet elongated the direction of the electric field as a result of electric field forces, and eventually, as the electric field strength grows, it bounces off the surface. In the Wenzel impact regime, as the strength of the electric field escalates, the droplet undergoes upward stretching and splits into satellite droplets during the retraction phase, attributed to the dynamic pressure and electrostatic pressure at the apex exceeding the capillary pressure and gravity. These findings could aid in advancing electrostatic spray cooling technology.

Publisher's Note: “Droplet impact and rebound dynamics on superhydrophobic surfaces” [Phys. Fluids 36, 072103 (2024)]

Publisher's Note: “Droplet impact and rebound dynamics on superhydrophobic surfaces” [Phys. Fluids <b>36</b>, 072103 (2024)]

Liquid metal droplets impacting superhydrophobic surface in a viscous (non-oxidizing) environment

The hypothesis of the present research is the existence of distinct spatial-temporal characteristics of non-oxidized liquid metal (LM) droplets impacting a solid surface. To provide a quantitative claim to this hypothesis, we created a test matrix based on the well-known impingement regime map bounded by two dimensionless quantities—Weber number (We) and Ohnesorge number (Oh). The range of these quantities is from 10−2 to 102 (We) and 10−3 to 101 (Oh), leading to Reynolds number (Re) (≡We1/2/Oh) to vary from 10−2 to 104. The class of LMs opted for are post-transition metals—eutectic gallium alloys—due to their several desired practical features, such as low melting point, non-toxicity, and low vapor pressure. The research is conducted using numerical experiments performed using C++ OpenFOAM libraries. To ensure the reliability of the code, we tested our work with numerous impingement behaviors of fluids available in the literature. A plethora of droplet behaviors are reported, such as deposition, rebound, bubble entrapment, and splash. Several features of droplet impingement were critically examined, such as temporal spreading factor, maximum spreading factor, and contact time of droplets on the surfaces. Moreover, the conventional scaling laws regarding the impingement behavior of droplets were tested, with new ones proposed where deemed necessary. Furthermore, a distinct route for the entrapment of droplet is observed, caused by the bulging of LM droplet during the recoiling stage. Emphasis is made to form delineations for these impingement characteristics using dimensionless groups (i.e., We, Oh, and Re).

Droplet Impact on Superhydrophobic Mesh Surfaces.

Reducing the contact time of droplet impacts on surfaces is crucial for various applications including corrosion prevention and anti-icing. This study aims to explore a novel strategy that greatly reduces contact time using a superhydrophobic mesh surface with multiple sets of mutually perpendicular ridges while minimizing the influence of the impacting location. The effects of the impact Weber numbers and ridge spacing on the characteristics of the impact dynamics and contact time are studied experimentally. The experimental results reveal that, for the droplet impact on mesh surfaces, ridges can segment the liquid film into independently multiple-retracting liquid subunits. The retracted subunits provide the upward driving force, which may promote the splashing or pancake bouncing of droplets. At this point, the contact time has a negligible sensitivity for the impacting position and is significantly reduced by up to 68%. Furthermore, the time, dynamic pressure, and energy criteria for triggering splashing and pancake bouncing are proposed theoretically. This work provides an understanding of the mechanism and the design guidelines for effectively reducing the contact time of the impacting droplet on superhydrophobic surfaces.

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.