Droplet Impact Research Articles

Reconsideration on the maximum deformation of droplets impacting on solid surfaces

AbstractDroplet impact on solid surfaces is widely involved in diverse applications such as spray cooling, self‐cleaning, and hydrovoltaic technology. Maximum solid‒liquid contact area yielded by droplet spreading is one key parameter determining energy conversion between droplets and surfaces. However, for the maximum deformation of impact droplets, the contact length and droplet width are usually mixed indiscriminately, resulting in unignored prediction errors in the maximum contact area. Herein, we investigate and highlight the difference between the maximum contact length and maximum droplet width. The maximum droplet width is never smaller than the maximum contact length, and the difference appears once the contact angle exceeds 90° (which becomes more significant on superhydrophobic surfaces), regardless of impact velocities, liquid viscosities, and system scales (from macroscale to nanoscale). A theoretical model analyzing the structure of the spreading rim is proposed to demonstrate and quantitatively predict the above difference, agreeing well with experimental results. Based on molecular dynamics simulations, the theoretical analysis is further extended to the scenario of nanodroplets impacting on solid surfaces. Reconsideration on the maximum deformation of impact droplets underscores the often‐overlooked yet significant difference between maximum values of contact length and droplet width, which is crucial for applications involving droplet‒interface interactions.

High‐Performance Droplet‐Based Triboelectric Nanogenerators: A Comparison of Device Configuration and Operating Parameters

AbstractDroplet‐based electricity generators (DEGs) harness liquid‐solid electrification to convert water droplets impacts into electrical energy. This study systematically examines how droplet height, droplet volume, flow rate, and substrate tilt angle influence DEG performance using polytetrafluoroethylene (PTFE) as a triboelectric layer and deionized water. Three electrode designs (double, top, bottom) are evaluated, revealing that the double‐electrode configuration delivers the highest output. This enhanced performance arises from synergistic droplet motion, electrical double‐layer formation, and charge discharge, as validated by an equivalent circuit model. By varying droplet heights from 1–20 cm, volumes of 7.7–50 µL, flow rates of 50–300 drops/min, and tilt angles of 0–90°, an optimized setup yields −70 V and 22 mA, translating to a power density of 0.28 µW cm−2. High‐speed imaging correlates these outputs with droplet impact dynamics and the resulting charge transfer. Additionally, the optimized DEG can power small electronic devices, charge capacitors, and monitor artificial acid rain in real‐time, displaying distinct electrical signals compared to typical rainwater. These findings underscore the potential of DEGs as renewable energy harvesters and smart environmental sensors, paving the way for advanced on‐demand power generation in diverse settings.

Experimental Investigation of Dispersant on Dynamics of Impact of Al2O3 Nanofluid Droplet

Spray cooling, of which the essence is droplet impacting, is an efficient thermal management technique for dense electronic components in unmanned aerial vehicles (UAVs). Nanofluids are pointed as promising cooling dispersions. Since the nanofluids are unstable, a dispersant could be added to the fluid. However, the added dispersant may influence the droplet, thereby impacting behaviors. In this work, the effects of dispersant on the nanofluid droplet-impacting dynamics are studied experimentally. The base fluid is deionized water (DI water), and Al2O3 is the selected nanoparticle. Sodium dodecyl sulfate (SDS) is used as the dispersant. Five different concentrations of nanofluids are configured using a two-step method. Droplet impacting behaviors are observed by high-speed imaging techniques. The other effects, i.e., the nanofluid particle volume fraction and the Weber number on droplet impact dynamics, are also systematically investigated. The results illustrate that the surface tension of the Al2O3 nanofluid increases with increased nanofluid concentrations. The surface tension of Al2O3 nanofluid with SDS is lower than that of DI water. And the increase in droplet impact velocity increases the spreading morphology. Nanofluid droplets exhibit spreading and equilibrium process when SDS is added. Furthermore, as the concentration of the nanofluid increases, the spreading process is inhibited. Whereas without SDS, the droplets undergo spreading, receding, and equilibrium processes. Moreover, there is no appreciable change in the impacting process with concentration increase. The empirical models of maximum spreading factor should be established without SDS and with SDS, respectively. This study can provide theoretical basis and specific guidance for experimental characterization of UAVs’ electronic devices based on the mechanism of nanofluid droplet impact on the wall.

Surface wettability effects on droplet dynamics and heat transfer on heated stainless-steel foils

Abstract We report the dynamics and heat transfer characteristics of water droplets impacting on a thin stainless-steel foil maintained at different temperatures. The hydrophobic characteristics are imparted to the surface through polysiloxane coating and water droplets impact the uncoated and coated heated surfaces at different velocities. High-speed videography is utilized to capture the dynamics of the droplet upon impact, while the temperature field of the substrate, during the phenomenon, is simultaneously recorded using high-speed infrared thermography. Heat transfer to the droplet over different surfaces is determined through energy balance on the foil using the captured thermographs. The results reveal that the spreading phase and its duration are strong functions of droplet impact velocity, irrespective of the surface wettability, whereas surface wettability primarily affects the receding phase. The coated hydrophobic surfaces exhibited lower resistance to motion at the three-phase contact line, resulting in reduced spread ratios during the receding phase. It is noted that majority of heat transfer occurred during the initial spreading and receding phases, driven primarily by forced convection. The maximum heat fluxes were observed along the three-phase contact line, particularly at the onset of the receding phase. The coated surface demonstrated lower overall heat transfer rates compared to non-coated surfaces, with the difference increasing at higher surface temperatures. Additionally, an increase in surface temperature to 75°C enhanced the hydrophobicity of the coated surface, leading to prolonged receding phases and extended time to reach the sessile state.

Investigation of the Droplet Impact Upon An Inclined Curved Non-Piezoelectric Substrate Propagating Lamb Waves

Investigation of the Droplet Impact Upon An Inclined Curved Non-Piezoelectric Substrate Propagating Lamb Waves

Flexible Mushroom-Like Cross-Scale Surface with Extreme Pressure Resistance for Telecommunication Lines Anti-Icing/Deicing.

Ice accretion caused by freezing rain or snowstorms is a common phenomenon in cold climates that seriously threatens the safety and reliability of telecommunication lines and other overhead networks. Various anti-icing strategies have been demonstrated through surface engineering to delay ice formation. However, existing anti-icing surfaces still encounter several challenges; for example, surfaces are prone to ice-pinning formation due to the impact of supercooled droplets, which leads to a loss of anti-icing effectiveness. In this study, a mushroom-like cross-scale surface (MCS) with extreme pressure resistance and superior anti-ice-pinning property was reported. Specifically, the designed MCS, featuring multiscale microfeatures, re-entrant structure, heterogeneous sidewalls, and nanoscale particles, exhibits excellent anti-icing properties. Ice formation was determined to occur through a process involving liquid penetration, condensation, icing, and frost filling. By establishing an anti-ice -pinning model and a bubble column model, the relationship between structural characteristics and anti-icing performance was clarified. The MCS demonstrates excellent static liquid repellency (contact angle >167°) and robust dynamic impact resistance (water impact with Weber number ≥300). Furthermore, it exhibits an ultralow ice adhesion strength of 0.46 kPa. Notably, the ice adhesion strength remains below 5 kPa even after 15 deicing cycles. The anti-ice-pinning mechanism and robust icephobicity induced by the micromorphologies of MCS provide valuable insights for effective anti-icing prospects in telecommunication line surfaces and other areas in the field of information and communication technology.

The droplet dynamics of asymmetrical impingement on moving ridged surface.

The depth of research into the mechanism of droplet impacting structured surfaces dictates the efficacy of their applications. The impact stress generated when a droplet impacts a surface is a pivotal factor influencing the efficiency of surface applications, ultimately determining the extent of surface wear. Despite the systematic examination of impact force, there remains a scarcity of research on impact stress and its mitigation strategies. Consequently, the objective of this study is to delve into the mechanisms that reduce maximum impact stress following the introduction of aerodynamic boundary layer and structural design. Based on experimental investigations, we examined the dynamic behavior of droplet impacting moving ridged surface with varying offset ratios across different tangential and normal Weber numbers. This study emphasizes the impact behavior phase diagram, droplet spreading length, and other relevant parameters. Furthermore, we systematically analyzed the evolution of the flow field and surface stress during the impact process benefit from a numerical simulation of a two-phase laminar flow model. This study shows that the impact of droplets on moving ridged surfaces significantly influences stress reduction, with a focus on asymmetric droplet impacts. The research establishes a phase diagram for droplet impact behaviors across varying tangential Weber number (Weτ), normal Weber number (Wen) and offset ratio (χ), and some unified models about the maximum spreading diameter (Dmax) of the droplet are obtained based on the above variables. Besides, a model based on Prandtl's boundary layer theory emphasizes and proves the positive effect of the introduction of air layer on the reduction of maximum impact stress. Notably, the offset ratio significantly influences stress distribution, the maximum impact stress shows a non-monotonic change from increasing first to decreasing as χ increases considering the rotation of droplet. This research contributes valuable insights into surface design strategy for enhancing the resistance to droplet impact wear. These findings have broad implications for industrial applications where managing droplet impingement is crucial.

Simulation and experimental investigation on kinetic and thermodynamic characteristics of liquid nitrogen droplets impacting superheated wall

Liquid nitrogen droplets impacting superheated wall is an essential phenomenon in cryogenic phase-change spray cooling. In this study, the Volume of Fluid (VOF) method was used to develop a numerical model to investigate the kinetic and thermodynamic characteristics of liquid nitrogen droplets impacting superheated wall. The experiment for the impact of liquid nitrogen droplets was conducted to validate the simulation. The findings indicate that liquid nitrogen droplets impacting superheated wall exhibit three boiling regimes: contact boiling, transition boiling, and film boiling. During contact boiling, as We increases, droplets undergo three modes sequentially: spreading, forming fingering-like structures, and fragmentation. During transition boiling and film boiling, as We increases, droplets exhibit spreading, splashing, and fragmentation. Increasing the wall temperature leads to the formation of vapor pockets and vapor film, which results in deteriorated heat transfer at the solid–liquid contact area, and meanwhile reducing the extent of droplet spreading. Increasing We promotes droplet spreading, reducing vapor pockets and vapor film thickness, increasing wetting area, and postpones the onset of heat transfer deterioration. Increasing the wall temperature and We both lead to a higher decreasing rate of liquid volume of droplet which indicates an intensified vaporization rate. The larger the contact angle of the wall, the less the droplet spreads, and the lower the heat transfer between the droplets and the wall.

Dual-interfacial alloying mechanism in Ti-steel laminated metal composite fabricated by wire-arc directed energy deposition using a Cu-Ni interlayer

Ti-steel laminated metal composites have been widely used in shipbuilding and the chemical industry due to the superior corrosion resistance of Ti and the favorable mechanical properties of stainless steel. In this paper, Ti-steel laminated metal composites without Ti-Fe IMCs have been fabricated by directed energy deposition-arc method using a Cu-Ni transition interlayer. Based on the analysis of microstructure analysis, the cross-sectional microstructure can be divided into 304SS / Cu-Ni interface containing (Fe, Cr) and (Cu) solid solution, Cu-Ni deposition layer containing (Fe, Cr) and (Cu) solid solution, and TA2/Cu-Ni interface containing Ti2FeCu and Ti-Cu IMCs. Besides, the Cu-Ni deposition layer contains a small content of recrystallized grains with {100}<100> texture and a large content of deformed grains with{100}<110> texture due to the impact of arc and droplets during continuous depositions. Furthermore, the peak hardness is decreased to 518 Hv and the mean value of compressive shear strength is enhanced to 195.27MPa, which can be attributed to the elimination of interfacial Ti-Fe IMCs, defect-free microstructure, and residual stress relief. Based on polarization curves and electrochemical impedance spectroscopy, the corrosion resistance reduction of the cross-section is due to the formation of a mass of primary battery systems. Furthermore, based on the thermodynamics calculation of Fe-Cu-Ni, Ti-Fe-Cu, and Ti-Ni-Cu ternary systems, the alloying mechanism is that Ti-based IMCs with lower Gibbs free energy will be formed instead of brittle Ti-Fe IMCs, resulting in the elimination of Ti-Fe IMCs and the formation of less brittle phases at both 304SS / Cu-Ni interface and TA2 / Cu-Ni interface.

Investigation of Single Saltwater Droplet Impact Solidification and Prediction Method for Macroscopic Ice Accretion on a Flat Surface

Investigation of Single Saltwater Droplet Impact Solidification and Prediction Method for Macroscopic Ice Accretion on a Flat Surface

A REVIEW ON THERMO-FLUIDIC STUDY OF DROPLET IMPACT IN SPRAY COOLING

Spray cooling exhibits outstanding cooling performances compared to other liquid cooling techniques, which offers robust thermal management for numerous applications facing high heat flux challenges. In spray cooling, coolant droplets generated from a spray nozzle continuously impinge onto a hot surface at high flow rates. The interaction between the droplets and the surface - whether they land on a pre-existing liquid film or directly on the heated area - depends on the fluid saturation temperature and the surface temperature. Understanding the dynamics and heat transfer during droplet impact is crucial for advancing spray cooling research. The present work summarizes the recent advancements in the study of droplet impact dynamics and heat transfer in spray cooling from two aspects. The first aspect is about the statistical analyses of droplet behaviors and liquid film conditions in spray cooling, examining their influence on cooling efficiency. The second one is regarding the droplet-surface interactions in spray cooling, ranging from single droplet to spray by increasing the complexity of droplet condition and surface condition. It includes the single droplet impacting a dry heated surface, multiple droplets impacting a dry heated surface, and droplets impacting the heated flowing film.

Viscous dissipation upon water droplet impact on an oil layer atop water pool

Viscous dissipation upon water droplet impact on an oil layer atop water pool

Studying a droplet impaction on a vibrating porous medium

This study investigates the influence of vibration on droplet dynamics when a droplet impacts a three-dimensional (3D) structured porous medium, focusing on intrinsic properties such as porosity and wettability, along with the effect of vibration phase angle. Using an Allen-Cahn equation-based lattice Boltzmann method (A-C LBM), the research analyzes multiphase flow dynamics. The results compare droplet spreading and penetration on vibrating versus non-vibrating porous media. Without vibration, droplets get trapped within the pores, reaching equilibrium due to adhesive, viscous, and capillary forces. However, sufficient vibrational forces can overcome surface adhesion, allowing droplets to pass through the porous medium. This phenomenon is influenced by the droplet's contact angle and initial impact inertia. The study finds that hydrophobic surfaces, characterised by higher contact angles, significantly reduce liquid infiltration into the medium under both vibrational and non-vibrational conditions. This reduction is due to dominant surface tension and repellent forces, which, in combination with vibrational forces, minimise droplet spreading and penetration, even in highly porous media. Conversely, on hydrophilic surfaces, adhesive and capillary forces enhance the droplet's inertia, causing sudden penetration into the porous medium. Further, the study reveals that the phase angle of vibration critically affects the droplet's behaviour. With sinusoidal vibration, lower phase angles cause the porous medium to move in the opposite direction to the droplet, enhancing the synergy between vibration and inertia forces, leading to rapid infiltration. Higher phase angles, aligning the medium's movement with the droplet's impact path, result in reduced infiltration. Overall, this research provides crucial insights into how porosity, wettability, and vibration phase angle collectively influence droplet dynamics on structured porous media, offering valuable implications for applications involving fluid transport and filtration in porous structures.

Droplet impact on a flexible disk

In this paper, we simulate the process of two-dimensional axisymmetric fluid–structure coupling of a droplet impacting on a flexible disk. The effects of dimensionless disk stiffness (K = 0.1–1000), Weber number (We = 1–500) and contact angle (θ = 130° and 60°) on the dynamics of the droplet impacting on the flexible disk are analysed. The results indicate that there are five typical impact modes for a hydrophobic surface (θ = 130°) and four typical impact modes for a hydrophilic surface (θ = 60°) within the range of considered parameters. The analysis of spreading factor reveals that a part of the energy is transferred to the substrate, which is manifested as a weakening of the droplet spreading, when the substrate deforms downwards due to the droplet impact; the squeezing of the droplet causes a tendency to flow from the centre of the droplet to the edge, which is manifested as an enhancement of the droplet spreading, when the substrate recovers from the downward deformation. The effect of the substrate flexibility on the maximum spreading factor depends on the competition of the two mechanisms above. Based on this, a modified scaling law of βmax has been proposed by introducing the effective Weber number (Wem). The analysis of impact force demonstrates that the peak of the impact force is related to the deflection of the flexible substrate which is different from that of a rigid wall; and three typical processes of impact force variation have been summarised. In addition, unlike the rigid substrate scenario, there is an energy interaction between the droplet and the flexible substrate after impact occurs, which is classified as three typical energy transformation processes.

Nanosized caltrops enable selective capture and directional maneuvering of water droplets

AbstractSurface design by tailoring topographical features and interface function groups to modulate dynamic or kinetic behaviors of liquid droplets, has been an increasing hotspot due to its broad spectrum of applications in biochemical diagnosis, microfabrication, and energy conversion systems. Here we report an engineered surface decorated by packed nanosized caltrops resulting from two perfectly articulated oxidation processes, where self-assembled nanoislands generated in the 1st plasma oxidation serve as protective masks in the 2nd chemical oxidation. As caltrops per design can effectively block lateral motion, the present surface can anchor contact lines of advancing water films when being hydrophilic and selectively capture impinging droplets when being hydrophobic. Furthermore, biphilic patterns can be readily obtained by integrating nanocaltrops with other surface asperities, engendering directional droplet maneuvering and designated droplet arraying. This work provides guidelines in designing nanostructures that achieve on-demand manipulation of droplets and flow patterns for multifunctional applications.

Prediction of hydrodynamic behavior of impinging Glycerin hollow droplet on a concave surface using jet counter and dynamic contact angle

PurposeThis study aims to study hollow droplet collisions for their hydrodynamic behavior and jet properties.Design/methodology/approachThe volume of fluid (VOF) method was used to simulate a hollow impact using OpenFoam software (VOF).FindingsThe height of the edge-jet decreased as the air diameter (d) and length of the concave surface (L) increased. Height is specific for case 1 at t = 4 ms and its value is 3 mm. The minimum height is 0.585 mm in case 5. Also, the length of the edge-jet changed with time and decreased with the increasing length of concave and air diameter. The maximum length observed in case 1 was 9.23 mm, and the minimum appeared in case 5, in which the length was 0.68 mm.Originality/valueThe impact of a hollow droplet on a solid concave surface was numerically analyzed in this paper at various lengths of surface and shell thicknesses.

Erratum: “Physics of droplet impact on various substrates and its current advancements in interfacial science: A review” [J. Appl. Phys. 133, 030701 (2023)

Erratum: “Physics of droplet impact on various substrates and its current advancements in interfacial science: A review” [J. Appl. Phys. 133, 030701 (2023)

Mechanism and strategy of self-assembly of quaternary ammonium surfactant molecules to regulate pesticide droplet impact and wetting of hydrophobic surfaces.

Surfactants regulate the interaction between pesticide droplets and the surfaces of plants on which they are sprayed. The influence of the key structural functional groups of surfactants on the interaction between pesticide droplets and hydrophobic pear leaves has not been explored. The behavior of Imidacloprid (Imid) droplets regulated by cationic quaternary ammonium surfactants with different structures on hydrophobic pear leaves and their bouncing dynamics were studied. The properties of pesticide droplets regulated by rosin-based bicationic quaternary ammonium salt and ethylene (dodecyl polyoxyethylene/tetradecyl polyoxyethylene) chloride/ammonium bromide were well matched with those of pear leaves with a waxy layer. This structure was closely related to the double-chain structure corresponding to that of double N-head groups in quaternary ammonium surfactants. Quaternary ammonium surfactants regulate the wetting of droplets by forming semi-micellar structures near the three-phase contact line, which drives the droplets to wet and spread on the leaf. The quaternary ammonium surfactant containing the double N-head structure enabled strong wetting and adhesion of pesticide droplets on the hydrophobic surface. The key structural functional groups of different quaternary ammonium surfactants directionally modified the impact kinetics of Imid droplets on the leaf surfaces and their changing trend. The double N-head structure played a key role in the molecular structure of quaternary ammonium surfactants, and the hyperbranched ethylene oxide (EO)chain played a small role in the molecular structure. These results clearly indicate how the structure of key functional groups of quaternary ammonium surfactants regulated the interface adhesion of pesticide droplets on the leaf surfaces and explain the microscopic mechanism of their interaction. © 2024 Society of Chemical Industry.

Long-Term Dynamics of Water Droplet Impact on Rotating Hydrophilic Disk

Long-Term Dynamics of Water Droplet Impact on Rotating Hydrophilic Disk

On formation and breakup of jets during droplet impact on oscillating substrates

Droplet impact on substrates is the cornerstone of several processes relevant to many industrial applications. Imposing substrate oscillation modifies the impact dynamics and can, therefore, be used to control the ensuing heat, mass, and energy transfer between the substrate and the impacting droplet. Previous research has shown that substrate oscillation strongly influences the spreading behavior of the droplet. In this study, we extend this understanding to examine how substrate oscillations can further modulate the retraction dynamics of the droplet, consequently affecting its long-term behavior, with a particular focus on induced jetting and subsequent breakup. We systematically examine the breakup of jets formed by the recoiling droplet through experimental investigations across a range of oscillation frequencies and amplitudes. Our findings reveal two distinct jet breakup modes: early and late, each governed by different time scales. Subsequently, we present a mechanistic description of the jetting process. Furthermore, we derive a simple scaling analysis based on energy balance to identify the critical condition required for jet breakup. Finally, we compare the experimental data with the scaling analyses to show its efficacy.