Dynamics Of Water Droplets Research Articles

Droplet Dynamics On a Body Surface In a Gas Flow

The results of previous studies of the water droplet dynamics on a horizontal surface blown by air are briefly reviewed. The experiments were carried out on rectangular wing model profile as an example. The inclined plane method was used to measure the limiting hysteresis of the contact angle with respect to the surface properties. The values of the flow velocity at which the drop begins to move are found. The dependence of the drop velocity on its characteristic size and air velocity is obtained. A theoretical description of the drop dynamics is given, and a semiempirical expression for its velocity is proposed. The developed experimental-theoretical algorithm can be used for a wider range of control parameters (surface tension coefficient of the fluid, contact angle, gas flow velocity, droplet size).

Modeling and analysis of water droplet dynamics in the dead-ended anode gas channel for proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cells usually operate in dead-ended anode mode due to a comparatively simple system. Nevertheless, flooding in the anode channels in dead-ended mode is severe than that in flow-through configuration, which causes cell performance degradation and durability decrease. In this study, water droplet dynamics in the anode channel is investigated numerically using volume of fluid method to study water accumulation and drainage in the PEM fuel cell with the dead-ended anode. Simulations are divided into a dead-ended stage and a purge stage to study the two-phase flow behaviors. Impact of water accumulating volume is taken into consideration and, cases of different wettability of the gas diffusion layer surface and the bipolar plate surface are compared. The numerical results reveal that water droplets emerge from the water inlets and then accumulate and coalesce in the dead-ended stage. Most water droplets are drained out of the gas channel along the channel corners in the purge stage. It is found that larger water accumulating volume results in higher eliminating rate. The total purge time is mainly affected by the wettability of the bipolar plate surface.

Post-collision hydrodynamics of droplets on cylindrical bodies of variant convexity and wettability

Droplet impact, dynamics, wetting, and spreading behavior on surfaces impose rich and interesting physics, in addition to extensive understanding of processed employing droplets and sprays. The physics and mechanisms become more interesting and insightful when the geometry and wettability of the surface provide additional constraints to the fluid dynamics. Post-impingement morphology and dynamics of water droplets on convex cylindrical surfaces, having diameters similar to that of the droplet, have been explored experimentally. Droplet impact and post-impact feature studies have been conducted on hydrophilic and superhydrophobic (SH) cylindrical surfaces. Effects of the impact Weber number (We) and target-to-drop diameter ratio on the wetting and spreading hydrodynamics have been studied and discussed. The post-impact hydrodynamics have been quantified employing dedicated non-dimensional variables, such as the wetting fraction, the spreading factor, and the non-dimensional film thickness at the north pole of the target. The observations reveal that the wetting fraction and spread factor increase with an increase in the impact We and a decrease in the target-to-drop diameter ratio. An opposite trend is noted for the non-dimensional film thickness at the target’s north pole. It is also deduced that the spread factor is independent of the target wettability, whereas the wetting fraction is remarkably low for SH targets. The lamella dynamics post spreading has also been observed to be a strong function of the wettability, the impact We, and the diameter ratio, and the same has been explained based on wetting and inertial principles. An analytical expression for temporal evolution of film thickness at the north pole of the cylindrical target is derived from first principles. The article also proposes a theoretical model based on energy conservation for predicting the maximum wetting fraction for variant cylindrical targets in terms of the governing We and Capillary number. It is observed that the experimental measurements are in good agreement with the theoretical predictions.

Water droplet dynamics on a heated nanowire surface

When a liquid droplet is deposited onto a heated surface, evaporation occurs. If the temperature of the surface is sufficiently high, bubbles are released from activated nucleation sites, making the heat transfer more efficient. However, if the temperature of the surface is further increased above the Leidenfrost point, a vapour cushion will form underneath the droplet, deteriorating the heat transfer between the surface and the droplet. In this work, we show that patterned Si nanowires can allow shifting the Leidenfrost temperature while maintaining a minimum droplet evaporation lifetime. In particular, it is observed that the Leidenfrost point is reached when the phase-change time scale compared to the wicking time scale becomes dominant. In this situation, the energy of the lift-off process is not sufficient for allowing the droplet to reach a sufficient height from where the droplet can penetrate in the porous surface.

Comparison of single droplet impact behaviors on the burning shallow/deep pool

Abstract Dynamics of single water droplet impinging on burning heptane surface were investigated experimentally in shallow/deep pool. The pool regimes were newly defined and a translation region was found in burning pools. At low Weber numbers, the crater-jet only occurs for shallow pools while the crater-secondary jet does for deep pools. The bubble was observed at high Weber numbers for both pools. Compared to Weber number, liquid depth has a significant effect on the number of jet (jet and secondary jet). Some characteristic parameters, such as maximum crater depth, width and jet height were analyzed as a function of impacted dimensionless depth (1.9–11.1) and Weber numbers (246–638). Maximum crater depth and width are mainly determined by liquid depth in shallow pools, while they significantly depend on Weber number and impact behavior in deep pools. Maximum jet height is highly related to liquid depth and temperature. Additionally, the damping effect of fire plume on droplet impact velocity was explored and theoretical correlation between droplet impact velocity and impact height was proposed.

Water droplet dynamics in a dead-end anode proton exchange membrane fuel cell

A numerical model was developed in ANSYS Fluent to investigate the dynamic behavior of a liquid water droplet emerging from the gas diffusion layer into the anode flow channel of a dead-end anode proton exchange membrane fuel cell. The volume of fluid method was used with a porous model to account for the multiphase flow and porous media transport through the gas diffusion layer. To mimic the purging effect on water droplet dynamics, the anode flow channel outlet was closed and opened periodically. The closing duration was varied from 15 to 20 ms, while the purging duration was set constant at 3.5 ms, over a total flow time of 50 ms. The results show that the closing duration can be extended to over 20 ms at a low current density. However, it may result in pressure build-ups. For the conditions studied, the current density of 1.0 A/cm2, operating with a closing duration of 20 ms resulted in the best performance with the least hydrogen wastage (7.12%) and the most water evacuated (69.72%).

Impact Dynamics of Aqueous Polymer Droplets on Superhydrophobic Surfaces

Controlling the impact of water droplets on nonwetting surfaces is difficult due to their superior liquid repellency. Herein, we experimentally demonstrate that the impact dynamics of water droplets on superhydrophobic surfaces can be effectively altered by adding a very small amount of high molecular weight polymer. The presence of polymer chains in water enhances droplet–surface interactions and liquid viscoelasticity, which couple with the hydrodynamic effect and thus affects the dynamic behaviors of impinging droplets. Whereas various impact phenomena with altered transitional boundaries in the phase diagram have been observed for droplets of aqueous polymer solutions at low concentrations, only deposition was identified for aqueous polymer droplets with high concentrations. Nevertheless, the polymer addditive does not influence droplet spreading but siginificantly slows down droplet retraction and increases the contact time of rebounding droplets on the superhydrophobic surface. Moreover, the induced...

Multi-phase smoothed particle hydrodynamics modeling of supercooled large droplet dynamics for in-flight icing conditions

Understanding the dynamics of a single large water droplet is needed for accurate simulations of the in-flight icing phenomenon. Obtaining information on the ratio of ejected to deposited water and the post-impact droplet distribution should improve the numerical modeling of the bulk of impinging droplets. In this study, a weakly compressible multi-phase Smoothed Particle Hydrodynamics (SPH) method with shifting algorithm and surface tension model is presented to simulate the single droplet dynamics. The validity of the approach has been proved by modeling the classical problems of Rayleigh–Taylor instability, dam break, and droplet formation by comparing against other numerical and experimental data in the literature. Finally, droplet impingement on a liquid film and dry solid surface has been simulated and compared against the experimental data. The effect of impact angle and film thickness on the crown formation is studied to demonstrate the importance of modeling SLD impingement for in-flight icing conditions.

Effect of Latent Heat Released by Freezing Droplets during Frost Wave Propagation.

Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing droplets locally heats the substrate, leading to evaporation of nearby droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times ( tf), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation.

Investigation of time dependent water droplet dynamics on porous fuel cell material via synchrotron based X-ray imaging technique

Proper water management in proton exchange membrane fuel cells (PEMFCs) remains a challenge, hindering the commercialization of this technology. Liquid water emerges in the form of droplets from micrometer-scaled pathways of porous gas diffusion layers (GDLs) into millimetre-scaled gas flow channels. Afterward, droplets grow to certain critical sizes before being removed by the convective gas flow. The opaque nature of fuel cell components, including GDL materials, poses significant challenges to understand droplet dynamics and develop more efficient water management techniques. In this work, an advanced synchrotron X-ray radiographic imaging technique available at the Canadian Light Source (CLS) was employed to visualize time dependent droplet behaviour in a serpentine PEMFC flow channel for the first time. High spatial and temporal resolution coupled with high energy photons of an X-ray beam provided high resolution images of growing droplets. From the captured images, the whole growth cycle of liquid droplets was obtained under various gas flow rates. A cyclic pattern of droplet dynamics including emergence, growth, and detachment was visualized from high resolution images. From the X-ray images, the time-evolution of dynamic contact angles, height, and chord of a growing droplet was quantitatively analyzed. At higher gas velocities, the critical detachment diameter of droplets became smaller and contact angle hysteresis was found be lower, while larger contact angle hysteresis and bigger droplets were observed at lower gas velocities. The critical droplet size at which the detachment of droplets occurred was compared to the predicted results from an analytical approach and good agreement was found.

Durable and regenerable superhydrophobic coatings for brass surfaces with excellent self-cleaning and anti-fogging properties prepared by immersion technique

In this work, superhydrophobic coatings on brass surfaces were prepared by two-step (chemical etching with a mixture of hydrochloric and nitric acids, followed by treatment with lauric acid) and one-step (treatment with lauric acid). The surface morphology shows the presence of rough microstructures on treated brass surfaces. Superhydrophobicity with high water static contact (>173°)and low sliding angles (<4°) is achieved by both processes. Besides, the dynamics of water droplets on the coated samples also were studied. Additionally, the durability of the coatings also was examined by performing thermal, chemical, and mechanical stability tests. Along with, these coatings are also found to be excellent regeneration ability as verified experimentally. Further, coatings exhibit self-cleaning and anti-fogging properties.

Influence of atmospheric pressure dielectric barrier discharge on wettability and drying of poly(ether-ether-ketone) foils

Wettability and water droplet drying dynamics on poly(ether-ether-ketone) (PEEK) foils treated by atmospheric pressure air dielectric barrier discharge (DBD) has been investigated. It has been found that plasma treatment causes significant increase of PEEK wettability that is predominantly connected with alterations of its chemical composition (oxidation) induced by DBD plasma. The hydrophilization of PEEK surface was not temporally stable and substantial increase of water contact angle up to 67° was observed with increasing storage time, which is consistent with loosening of polar groups as confirmed by means of XPS. Characteristic restoration time of the contact angle was 6.7 days. Furthermore, a large alteration of the dynamics of water droplets drying on PEEK after the plasma treatment was also observed: whereas for untreated PEEK three drying phases were clearly distinguishable, the phase of constant contact angle disappeared in the case of PEEK exposed to the atmospheric pressure air plasma. In spite of substantial decrease of PEEK wettability with storage time the constant angle phase didn't appear within 51 days after the plasma treatment. As a result of this plasma treated and aged PEEK exhibits much higher water contact hysteresis as compared to untreated PEEK. This effect may be explained by the formation of randomly distributed nanostructures on PEEK surface exposed to DBD plasma that increase the spatial heterogeneity of the PEEK surface and enhance droplet pinning during its drying.

Water Droplet Dynamics on a Hydrophobic Surface in Relation to the Self-Cleaning of Environmental Dust

The dynamic motion of a water droplet on an inclined hydrophobic surface is analyzed with and without environmental dust particles on the surface. Solution crystallization of a polycarbonate surface is carried out to generate a hydrophobic surface with hierarchical texture composed of micro/nanosize spheroids and fibrils. Functionalized nanosize silica particles are deposited on the textured surface to reduce contact angle hysteresis. Environmental dust particles are collected and characterized using analytical tools prior to the experiments. The droplet motion on the hydrophobic surface is assessed using high-speed camera data, and then, the motion characteristics are compared with the corresponding analytical results. The influence of dust particles on the water droplet motion and the amount of dust particles picked up from the hydrophobic surface by the moving droplet is evaluated experimentally. A 40 μL droplet was observed to roll on the hydrophobic surface with and without dust particles, and the droplet slip velocity was lower than the rotational velocity. The rolling droplet removes almost all dust particles from the surface, and the mechanism for the removal of dust particles from the surface was determined to be water cloaking of the dust particles.

Developing a non-optical platform for impact dynamics analysis on nanostructured superhydrophobic surfaces using a quartz crystal microbalance

Quantitative analysis of water droplet behavior under dynamic conditions is one of the critical challenges for applications of wettability-controlled surfaces. Currently, various optical analysis techniques have been employed to analyze impact dynamics. Despite the convenience of direct observation of water droplets, most of these techniques have limited applicability to microscopic and quantitative investigations. In an effort to overcome these limitations, here, we suggest a complementary analysis platform using a quartz crystal microbalance (QCM) to study impact dynamics. A high-speed camera and QCM were applied together to study the behavior of water droplets that impact wettability-controlled surfaces with various We numbers (Weber number). For these experiments, ZnO nanowire surfaces were prepared and chemically modified by alkyl-thiol molecules with various carbon chain lengths (C0–C12) to control the surface energy. For nanowire surfaces with high surface energies (C0–C6) and for the lowest surface energy sample (C18), both methods exhibited highly consistent impact dynamics, showing stable wetting and dewetting properties, respectively. In addition to these apparent behaviors, QCM was further able to provide detailed microscopic information regarding the penetration and deformation of water droplets in a quantitative way based on acoustic sensing. More interestingly, QCM was able to determine the metastable water repellency of a C12-modified surface with a high We number, which could not be detected by the high-speed camera. These results suggest the significant potential of QCM as a new platform to analyze the impact dynamics of water droplets via quantitative, microscopic investigations.

Collision dynamics of a single water droplet impinging on a high-temperature pool of oil

The article presents the dynamic process of a single water droplet impinging on a hot oil surface with various temperatures ranging from 205 to 260 $$^\circ \hbox {C}$$ . Distilled water is used to produce water droplets with different diameters. The impact behavior is recorded by using a high-speed digital camera with the speed of 2000 fps. The result shows that two typical phenomena, including crater–jet–bubble and vapor explosion, can be observed. The vapor explosion occurs when the oil temperature is higher than 210 $$^\circ \hbox {C}$$ . The oil temperature, the droplet size, and the Weber number are found to have significant influence on the vapor explosion time. The higher the oil pool temperature is, the earlier the vapor explosion occurs. Vapor explosion time increases with the droplet size, while decreases as the droplet Weber number increases. Moreover, the maximum heat absorption for a single water droplet immersing into the hot oil is calculated considering the changes of the droplet size. Both dimensionless maximum crater depth and maximum jet height increase with the pool temperature due to the surface tension, viscous force and decreasing density of the hot oil.

Investigation of droplet behaviors for spray cooling using level set method

The droplet dynamics and heat removal is one of the popular research streams in spray cooling applications. The increase in the water droplet temperature as a result of its impact on the hot solid surface would determine the efficiency of the heat removal that is crucial in some sensitive applications such as spray cooling in depressurization systems in commercial nuclear power plants. Computer modelling of the underlying mechanism of the liquid droplet interaction with the hot solid surface would be necessary. The accuracy and the reliability of these models are important in simulating the multiphysics phenomena such as droplet dynamics and heat removal. In present work, the level set method coupled with heat transfer was used to simulate the water droplet impact on an isothermal solid surface. The changes in the temperature of water droplet were found to be highly dependent upon its size and the impingement speed. Topological variations in the droplet shape as a result of its impact onto the solid wall would cause abrupt changes in the temperature of the water droplet. In addition, the droplet detachment, coalescence and flattening were found to strongly influence the temperature of the droplet which would evidently affect the heat removal efficiency in the spray cooling systems employed in the commercial nuclear power plants. Furthermore, we demonstrated that the level set method had the ability to produce more accurate estimations of the water droplet dynamics when compared to the Volume-Of-Fluid (VOF) method in simulating the droplet behavior.

Droplet Bouncing and Breakup during Impact on a Microgrooved Surface.

We experimentally investigate the impact dynamics of a microliter water droplet on a hydrophobic microgrooved surface. The surface is fabricated using photolithography, and high-speed visualization is employed to record the time-varying droplet shapes in the transverse and longitudinal directions. The effect of the pitch of the grooved surface and Weber number on the droplet dynamics and impact outcome are studied. At low pitch and Weber number, the maximum droplet spreading is found to be greater in the longitudinal direction than the transverse direction to the grooves. The preferential spreading inversely scales with the pitch at a given Weber number. In this case, the outcome is no bouncing (NB); however, this changes at larger pitch or Weber number. Under these conditions, the following outcomes are obtained as a function of the pitch and Weber number: droplet completely bounces off the surface (CB), bouncing occurs with droplet breakup (BDB), or no bouncing because of a Cassie to Wenzel wetting transition (NBW). In BDB and NBW, the liquid partially or completely penetrates the grooves beneath the droplet as a result of the wetting transition. The former results in droplet breakup alongside bouncing, while the latter suppresses the bouncing. These outcomes are demarcated on the Weber number-dimensionless pitch plane, and the proposed regime map suggests the existence of a critical Weber number or pitch for the transition from one regime to the other. CB and BDB are quantified by plotting the coefficient of restitution of the bouncing droplet and the volume of the daughter droplet left on the surface, respectively. The critical Weber number needed for the transition from CB to BDB is estimated using an existing mathematical model and is compared with the measurements. The comparison is good and provides insights into the mechanism of liquid penetration into the grooves. The present results on microgrooved surfaces are compared with published results on micropillared surfaces in order to assess the water-repelling properties of the two surfaces.

Large-Area Fabrication of Droplet Pancake Bouncing Surface and Control of Bouncing State.

Superhydrophobic pillar arrays, which can generate the droplet pancake bouncing phenomenon with reduced liquid-solid contact time, have huge application prospects in anti-icing of aircraft wings from freezing rain. However, the previously reported pillar arrays, suitable for obtaining pancake bouncing, have a diameter ≤100 μm and height-diameter ratio >10, which are difficult to fabricate over a large area. Here, we have systematically studied the influence of the dimension of the superhydrophobic pillar arrays on the bouncing dynamics of water droplets. We show that the typical pancake bouncing with 57.8% reduction in contact time with the surface was observed on the superhydrophobic pillar arrays with 1.05 mm diameter, 0.8 mm height, and 0.25 mm space. Such pillar arrays with millimeter diameter and <1 height-diameter ratio can be easily fabricated over large areas. Further, a simple replication-spraying method was developed for the large-area fabrication of the superhydrophobic pillar arrays to induce pancake bouncing. No sacrificial layer was needed to reduce the adhesion in the replication processes. Since the bouncing dynamics were rather sensitive to the space between the pillars, a method to control the contact time, bouncing shape, horizontal bouncing direction, and reversible switch between pancake bouncing and conventional bouncing was realized by adjusting the inclination angle of the shape memory polymer pillars.

Tunable structures of compound droplets formed by collision of immiscible microdroplets

We study the dynamics of a small water droplet colliding with an immiscible large sessile oil droplet in the air. Such a three-phase system forms compound microdroplets with tunable structures, depending on whether the small droplet can penetrate into the large one. A structure of penetrative-engulfing is distinguished from structures which are determined by balancing of the three interfacial tensions among the three phases, i.e., partial-engulfing and complete-engulfing. We develop a three-phase volume-of-fluid method to investigate the collision dynamics numerically, considering the evolution of the triple-line among the three interfaces. Regime maps of the structures for different spreading parameters and heights of the oil droplet are obtained regarding the impact velocity and the viscosity ratio of oil and water. We find that the oil droplet is impenetrable when the viscosity ratio is higher than a critical value. For lower ratio, the structure varies non-monotonically with the impact velocity to cause two transitions. We propose a simple model for the lower transition by incorporating droplet deformation, viscous resistance, and spreading condition. The upper transition boundary is influenced by the spreading of the oil droplet, resulting in an increase in the required penetration length to prevent penetrative-engulfing. Understandings from this work may provide valuable guidelines for generating compound microdroplets with desired structures.

A robust superhydrophobic surface and origins of its self-cleaning properties

A hierarchical surface was fabricated by electrodeposition of copper coating and chemical oxidation to form copper oxide, and the surface energy was lowered by chemical modification. The optimum parameters including seven days of chemical modification, 0.12mol/L of (NH4)2S2O8, 2.5mol/L of KOH and 60°C of oxidation temperature were used to fabricate the superhydrophobic surface with a water contact angle up to around 160° and a sliding angle about 3° on a steel substrate. Silver mirror effect and simple calculation showed that the wetting state between a water droplet and the hierarchical superhydrophobic surface was the Cassie state. This superhydrophobic surface had excellent self-cleaning properties for two different sizes (∼50μm and 150μm) of fly-ash cenospheres, and we gave the reason for its self-cleaning properties by the force involved at the interface. We also investigated the dynamics of water droplets impinging onto the superhydrophobic surface with different impact velocities, ranging from 0.31m/s to 1.71m/s, and found that all the water droplets could rebound from the superhydrophobic surface, with no trace of adhesion. In addition, a variety of tests were performed to assess the robustness of the superhydrophobic surfaces.