Droplet Impact Dynamics Research Articles

Impact dynamics and deposition of perovskite droplets on PEDOT:PSS and TiO2 coated glass substrates

Perovskite solutions are widely used, to develop next generation photovoltaic solar cells and printed electronics. Droplet-based coating route, comprising inkjet printing, aerosol jet, and spray coating, is a viable deposition approach for high volume manufacturing of such devices. Perovskites are emerging ionic solutions deposited on unconventional substrates, where their droplet impact dynamics and deposition behavior is unexplored. In this work, we studied the impact dynamics of popular perovskite solution droplets, i.e. methylammonium lead halides (CH3NH3PbI3 and CH3NH3PbI3-xClx) on three surfaces, viz. glass coated with thin films of PEDOT:PSS, and compact and mesoporous TiO2, widely used in emerging optoelectronic devices. Droplets of two solutions, with initial diameter ∼2.2 mm, were impacted onto three substrates, with impinging velocities of 1.40, 1.72, and 1.98 m/s, generating 18 experimental conditions, in the range of 176 < We < 402, where We is the Weber number. Using top- and side-view high-speed imaging, we studied temporal evolution trend in kinematic and spreading stages, and maximum droplet spreading in the wetting stage. We also analyzed the prediction power of existing scaling laws, theoretical and semi-empirical models for maximum spreading diameter with respect to droplet initial diameter, βmax, against perovskite solution droplets. We found that for the common coated substrates used here, and also in photovoltaic devices, the relative roughness with respect to droplet size was small, and the coated substrates were quite wetting with small contact angles. Thus, the perovskite solution droplet spreading was found to be a function of the We number, according to an existing scaling law and correlation for common liquids, where the coefficient of the correlation was tuned for methylammonium perovskite solution droplets by curve fitting, yielding a modified empirical correlation in the form of βmax=0.8We0.25.

Impact dynamics of a polystyrene suspension droplet on nonwetting surfaces measured using a quartz crystal microresonator and a high-speed camera

We investigated the impact dynamics of droplets containing various sizes and concentrations of polystyrene microparticles (PSM) on superhydrophobic surfaces using a high-speed camera and a quartz crystal microresonator (QCM). Gold nanoflake structures were synthesized on QCM surfaces and treated with perfluorooctane ethylthiol molecules to obtain superhydrophobic characteristics. Upon the collision onto the superhydrophobic surface, the droplet spread, retracted, and bounced off the surface due to its nonwetting characteristics. Within the experimental range of PSM size and PSM concentration, the high-speed camera detected only negligible changes in the impact dynamics; however, abnormally large changes in the resonance frequency were observed with the QCM when the concentration or sizes of the PSM exceeded a certain threshold. The abnormal change in frequency was attributed to the formation of a shear-induced PSM structure.

Dynamics of droplets impacting hydrophilic surfaces decorated with a hydrophobic strip

Mixed-wettability surfaces have been extensively adopted to manipulate impact dynamics of droplets. In this work, a three-dimensional numerical model is developed to investigate the splitting behaviors of droplets when they impact a hydrophobic strip decorated on a hydrophilic background surface. The volume of fluid (VOF) method incorporated with mesh-refinement technique and dynamic contact angle model is employed to capture the shape evolution of droplets accurately. Dynamic behaviors of impact droplets are analyzed for various strip widths, strip hydrophobicities, and surface hydrophilicities at three typical Weber numbers. The results show that the droplet can be successfully split into two symmetric small droplets for all the three Weber numbers, so long as the strip width and wettability contrast between the strip and background surface are properly designed. The non-axisymmetric spreading and retraction of the film triggered by the strip lead to a shorter spreading and a faster retraction on the hydrophobic strip than the hydrophilic surface, which causes the formation of a liquid bridge on the strip and reduces the viscous dissipation on the strip. Thus, the non-axisymmetric spreading and retraction dynamics as well as the reduced viscous dissipation are responsible for the droplet splitting. A simplified theoretical model is developed to predict the splitting or no splitting behavior on the basis of energy conservation principle, which agrees with the present simulations well. Finally, the simulated results are assembled in phase diagrams to obtain the criterion for the droplet splitting from the critical strip width and the critical wettability contrast between the strip and background surface.

Dynamics of single droplet impact on cylindrically-curved superheated surfaces

Liquid droplet impact on superheated symmetric surfaces such as flat and spherical geometries has been extensively investigated in the past. However, the effect of a superheated asymmetric curvature on the collision dynamics remains unknown. In this study, experiments are performed to elucidate the thermo-hydrodynamic behavior of milli-metric water droplets impacting on superheated cylindrical surfaces having convex and concave profiles. These geometries offer an asymmetric zone for droplets to expand and retract. Mono-dispersed water droplets are impinged on convex and concave surfaces maintained in the temperature range of 125–290 °C. The impact Weber number is varied between 5 and 65 and droplet evolution is visualized using the high-speed imaging technique. It is observed that, droplet morphology and spreading characteristics are entirely different for convex and concave surfaces compared to a flat substrate due to momentum anisotropy. The maximum spread factor and residence time of the water droplet are significantly influenced by the impact Weber number while the surface superheat has negligible effect. Importantly, the asymmetric distribution of momentum facilitated 16% and 24% reduction in contact time for convex and concave surfaces respectively. The preferential extension of fluid in the azimuthal direction assisted by the gravity causes greater spreading of droplets on a convex surface. In contrary, these conditions enforced a reduction in contact time for droplets on a concave topography.

Temperature-regulated directional bounce of impacting droplets on gradient grooves

Droplet impact dynamics on gradient textures at room temperature have been intensively studied in the past decade. The wetting gradient surfaces are reported to direct the droplet motion. In this work, we show an undirectional rebound behavior of impacting droplets on gradient grooved surfaces at high temperature. The impinging droplet rebounds towards the denser microstructure region at contact boiling state, but towards to the sparser region when at Leidenfrost film boiling state. Our analysis indicates that the non-directional rebounding direction is attributed to the unbalanced Young's forces or vapor pressure difference between the droplet and the substrate. In contact boiling regime, the droplet rebounds towards the denser region due to the unbalanced Young's force. However, in the Leidenfrost film boiling region, adequate vapor generated underneath the droplet impedes the direct contact and larger vapor pressure in the denser region causes the droplet to rebound oppositely. This study is envisioned to promote the understanding of droplet dynamics at high temperature and provide promising applications in various systems where regulating droplet motion is needed.

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...

Dynamic behavior of droplets under interfacial jamming of nanoparticles

Interfacial particle jamming can lock a droplet in a non-spherical shape; however, the effects of jamming on the dynamic behaviors of droplets are not well known. In this paper, droplet impact dynamics are studied on a mechanically weak superhydrophobic surface whose outer nanoparticles were detached by an impinging droplet and jammed at the droplet surface during retraction. The number and coverage of jammed particles were increased by increasing the impinging velocity, and unusual parachute-like shapes appeared at the rebounding moments for polyethylene oxide (PEO) droplets. Also, oscillation prohibition, bounce damping, and final deformation of the PEO droplets were each enhanced with increased velocity. The critical impinging velocity for the final deformation was much smaller than that required to generate complete droplet coverage, indicating that considerable local jamming was sufficient for generating droplet deformation. Local jamming was difficult to achieve during impact when droplet viscosity was low, as in the case of pure water, because particles were well dispersed under high surface flowability.

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films.

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

Effect of Feather Elasticity of Kingfisher Wing on Droplet Impact Dynamics

We experimentally studied droplet impact dynamics onto wing feathers of kingfishers. Distilled water droplets with a fixed diameter of 2.06 mm were used as drop liquid and the initial impact velocities of droplets varied from 0.28 m·s−1 to 1.60 m·s−1. Two high-speed cameras were utilized to capture the impact process of water droplets onto the wing feather surface from both horizontal and vertical directions. Two states of the feathers (elastic and inelastic) were considered to study the influence of elasticity. At the entire impact velocity range we studied, regular rebound, bubble trapping and jetting, partial pinning and partial rebound of droplets on inelastic wing feather surface were observed as the initial impact velocity increased. However, only one dynamic behavior (regular rebound) was found on the elastic wing feather surface. The elasticity plays a more important role in the direction difference of droplet spreading than wing feather microstructure. The contact time of water droplets on the elastic wing feather surface was less than that on the inelastic surface within the range of Web numbers from 1.06 to 36 under test conditions.

Droplet impact on cross-scale cylindrical superhydrophobic surfaces

Reducing the contact time between impacting droplets and superhydrophobic surfaces has attracted much attention in recent years due to the importance of controlling heat and mass transfer. Previous researchers have proposed several methods, such as lifting the droplets before the retraction, accelerating the retraction process, or splashing the droplets. One example includes symmetry-breaking surfaces, which were used to accelerate the droplet retraction to realize the fast detachment. However, the dependence of the contact time on impact velocity and surface structure scale remains unclear. Here, we experimentally study the droplet impact dynamics on cross-scale cylindrical superhydrophobic surfaces. The reduction of the contact time is achieved on the surfaces with a ridge smaller or larger than the droplets, spanning different bouncing regimes. We describe the droplet behaviors and propose theoretical models from the view of retraction speed to explain the contact time variations. The maximum reduction is observed to occur when the ridge diameter is close to that of the droplets, which is also predicted by the models.

The dynamics of droplet impact on a heated porous surface

In this paper, droplet impact on a porous surface is experimentally investigated over a wide range of Weber numbers and surface temperatures. Regime transition criteria have been deduced to determine droplet post-impingement behaviour as a function of the Weber number and surface temperature for which a droplet impacting on a porous surface. Based on the energy balance, an analytical model with improved boundary layer description is proposed to predict maximum spreading of droplet following impact on porous surfaces when the effect of heat transfer is negligible. The results of the model indicate that the spreading process after droplet impact on porous surfaces is governed by the viscous dissipation and matric potential. The maximum-spread model predictions agreed well with experimental measurements reported in this paper and the literature over a large range of Weber numbers and different porous surfaces.

Computational Study of Single Droplet Deposition on Randomly Rough Surfaces: Surface Morphological Effect on Droplet Impact Dynamics

Tremendous efforts have been devoted to the modeling of droplet deposition on smooth and patterned surfaces with ordered structures. However, systematic work focusing on randomly rough surfaces, which are the most common bare substrate surfaces, can be hardly identified. A phase field modeling method together with a unique analysis approach has been developed in this work to characterize single droplet deposition on randomly rough surfaces. It is interesting to observe that the droplet shape evolves quite differently on a randomly rough surface, as compared to a patterned surface, even if two surfaces have the same roughness. Parametric studies have been carried out to explore how the morphological metrics of randomly rough surfaces, which include the Wenzel roughness parameter (Wr) and the root-mean-square roughness (Rr), can affect droplet impact dynamics. The quantitative relationships derived from this study can eventually shed light on droplet shape on-aim control during the deposition process, which...

3D lattice Boltzmann simulation for a saturated liquid droplet at low Ohnesorge numbers impact and breakup on a solid surface surrounded by a saturated vapor

In this paper, new simulation results of a saturated liquid droplet impact dynamics on hydrophilic (θ = 80°) and hydrophobic (θ = 170°) solid surfaces surrounded by a saturated vapor are obtained based on a 3D lattice Boltzmann method (LBM). A novel piecewise relaxation time is proposed to overcome numerical instability at high liquid/vapor density ratios and low liquid droplet viscosities. Simulations are carried out for low Ohnesorge (Oh) numbers in the range from 0.01 to 0.05, corresponding to a water droplet diameter in the range of 1 µm to 130 µm. The impinging droplet deformation process including breakup phenomena is illustrated on hydrophilic and hydrophobic surfaces at different Weber (We) numbers. Droplet spread factors obtained from simulations match well with existing experimental data and theoretical values, validating the accuracy of the present 3D LBM. It is found that a water droplet, after reaching its maximum spread factor, breaks up into a toroid shape with a vapor cavity or a liquid film left at its center at large We numbers on a hydrophilic and a hydrophobic surface, respectively. A map in terms of We number versus Oh number is obtained for predicting the breakup occurrence. The critical We number is monotonically increasing with the increase of the Oh number, meaning that the smaller diameter water droplet requires a larger We number for breakup. Droplet breakup is more likely to occur on a hydrophobic surface than on a hydrophilic surface at larger Oh numbers, while the behavior is opposite at lower Oh numbers.

Oblique impact of droplets on microstructured superhydrophobic surfaces

This paper studies the oblique impact of droplets on microstructured superhydrophobic surfaces with the lattice Boltzmann method. Three-dimensional simulations are performed based on the pseudopotential multiphase model, with the Carnahan-Starling equation of state and the multiple-relaxation-time collision operator adopted to overcome the numerical instability problems at high liquid/gas density ratio and low fluid viscosity. The droplet impact dynamics is simulated in a wide range of normal Weber numbers (0–30), Ohnesorge numbers (0.002–3), and impact angles (15°–90°), with particular interest in understanding different types of bouncing behavior and their effects on the contact time. The results show that, among the four possible types of bouncing identified in the simulation, including conventional retracting bouncing, incomplete-retracting bouncing, impaled retracting bouncing, and tumbling bouncing, incomplete-retracting bouncing and tumbling bouncing have less contact time and could be facilitated by increasing the impact obliqueness. The increase in the normal Weber number leads to a transition from conventional retracting bouncing, to incomplete-retracting bouncing, and then to impaled retracting bouncing. The contact time shows a non-monotonic trend of “decrease”-“increase”-“decrease” with enlarged Ohnesorge number, and the sheer drop in the contact time at high Ohnesorge numbers is due to the occurrence of tumbling bouncing.

Dynamics and heat transfer of a hollow droplet impact on a wetted solid surface

A numerical model based on the coupled level set and volume of fraction (CLSVOF) method is developed to investigate dynamics of a hollow droplet impacting on a wetted solid surface and the associated heat transfer process. Numerical results show different impact behaviors of the hollow droplet as compared to a continuous dense droplet. One of the distinctive flow features of the hollow droplet is a central counter jet, which is not present in continuous dense droplets. Analysis of pressure distribution indicates that the central counter jet is mainly due to the pressure gradient inside the hollow droplet during the impact and spreading. Simulation results also show that the impact velocity of the hollow droplet determines its spread factor, dimensionless spreading edge-jet height and central counter-jet height, all with positive relations. The complicated transient heat transfer between the impacting droplet and the surface was analyzed. The average heat flux increases with higher impact velocity. Overall heat transfer during hollow droplet impact is found to be lower than that during continuous dense droplet impact. These results provide better understanding of hollow droplet impingement and heat transfer on wetted surfaces.

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.

Supercooled water droplet impact on superhydrophobic surfaces with various roughness and temperature

This paper investigates the impact behaviors between supercooled droplets and various superhydrophobic surfaces. We analyze the thermodynamic reason of the liquid adhesion at low temperature and discuss the influence of the substrate roughness, temperature and wetting property on the impact dynamics of supercooled droplets both experimentally and theoretically. The liquid adhesion results from the ice nucleation rather than the enhanced viscous effect induced by the low temperature. The dramatic increase of the liquid–solid contact area induced by the high-speed collisions between droplets and textured surfaces leads to a larger nucleation rate and a lower critical adhesion temperature. The analytical argument of the liquid–solid contact area fraction at different impact speed throws light on the understanding of the controversy if hierarchical superhydrophobic surfaces are icephobic at low temperature. Valuable suggestions are given that smooth superhydrophobic surfaces with nanoscale roughness near the critical nucleus radius perform better water-repellency against high-speed supercooled droplets.

Preparation and droplet impact dynamics of superhydrophobic nano-aluminum films on metal substrates

A simple electrophoretic deposition method followed by stearic acid modification was developed to prepare superhydrophobic multiscale nano-aluminum films on a series of metal substrates such as nickel, copper, titanium and stainless steel. The surface morphology and chemical compositions were characterized by a field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscope, Fourier-transform infrared spectrophotometer and white light interferometer. The resultant nano-aluminum films exhibited excellent superhydrophobicity with a water contact angle of 168.5° and a water sliding angle of 2°. Furthermore, the obtained nano-Al films showed the desirable stability and durability after the related testing. In addition, the impact dynamic behaviors of water droplet on the superhydrophobic nano-Al films were described via the corresponding parameters, such as Weber number (We), spreading factor (β), contact time (tc), etc. Based on these characteristics, the superhydrophobic nano-Al films presented excellent water-repellent property. The EPD process is an efficient method to prepare superhydrophobic nano-Al films on common engineering metal materials, which can be applied to various industrial or manufacturing fields, including oil–water separation, anti-icing, self-cleaning, anti-corrosion, microelectronic fabrication and so forth.

Numerical study of water droplets impacting on cylindrical heat transfer pipes

ABSTRACTPoor performance in the condensers in power plants and chemical plants is due to the fact that condensed water is deposited on the heat transfer pipes. The dynamics of condensed water droplets forming on the surface of heat transfer pipes have a significant effect on the heat transfer efficiency of heat exchangers. In the present study, a numerical approach using a coupled level-set and volume of fluid (CLSVOF) method was adopted to investigate the impact of water droplets on cylindrical pipes. The numerical model was verified by an experiment, and both sets of results showed qualitative and quantitative agreements. The effects of the surface wettability, impact velocity and relative size of the droplet to the pipe on the droplet impact dynamics are systematically investigated. Moreover, the regularities of the contact area between the liquid and the pipe during the impacting process as well as the volume of residual liquid remaining on the pipe post-impact are also analyzed; these two parameters are the key factors which affect the heat transfer efficiency of heat transfer pipes, and they cannot be acquired very accurately using experiments.