Water Droplet Impact Research Articles

Harvesting energy from high‐frequency impinging water droplets by a droplet‐based electricity generator

AbstractHarvesting energy from water, in the form of raindrops, river, and ocean waves, is of considerable importance and has potential applications in self‐powered electronic devices and large‐scale energy needs. Recently, the droplet‐based electricity generator has shown an increase by several orders of magnitude in electrical output, overcoming the drawback of traditional droplet‐based device limited by interfacial effects. Despite this exciting result, the output performance of this novel droplet‐based electricity generator is limited by relatively low frequency of impinging droplets owing to the formation of a continuous liquid film at high impact frequency, which might hinder its practical applications. To overcome this challenge, here, we report the design of a superhydrophobic surface based droplet electricity generator, referred to as SHS‐DEG, which can timely shed water droplets from the surface without the formation of liquid film at high impact frequency, and thereby generating enhanced average electrical output. Moreover, our SHS‐DEG exhibits many distinctive advantages over conventional design including robustness, long‐term durability, and power generation stability even in harsh environments. We envision that the ability to harvest electrical energy from water droplets at high impact frequency has promising applications in various energy‐harvesting systems.image

Breakup and collision of water droplet for mass injection pre-compression cooling

The breaking and bouncing coefficients of water droplets in water injection pre-compression cooling are tested in this paper. The test results show that there appears bimodal structure on the droplet size distribution of water droplets due to the combination and secondary crushing of water droplets under the action of high-speed airflow; water droplets are mainly broken by shear under most measuring conditions; when the spray pressure is higher than a certain pressure value (7 MPa), the interference between the nozzles is significantly reduced; the impingement of water droplets against blades would increase the average diameter of particles. By test verification, the optimal rebound coefficient and elastic recovery coefficient are obtained. The experimental results can improve the computational accuracy of the two-phase flow of CFD calculation and make it much closer to the real test or situation.

Numerical investigation of droplet impact on a solid superhydrophobic surface

The impact of microscale water droplets onto a solid superhydrophobic surface is numerically investigated. The multiphase problems are modeled by the three-dimensional incompressible Navier–Stokes equations and the liquid–gas interface is captured by the level set method. The numerical model is verified with our experimental impact results via the comparison of spreading factor ξ, which is defined as the ratio of the wetted surface area to droplet initial diameter. The simulation results suggest that when the droplet impacts with constant impact velocity and diameter, the maximum spreading parameter increases with the ambient temperature. As Weber and Reynolds numbers increase, the impact turns into doughnut-breakup regime; the droplet breaks up into a toroidal shape and a cavity is formed at the center. The results indicate that the diameter of the central cavity grows linearly related to the non-dimensional time. Finally, a new droplet impact spread/splash model that is governed by Weber and Reynolds numbers is proposed for superhydrophobic surface based on our numerical and experimental results.

A novel piezoelectric structure for harvesting energy from water droplet: Theoretical and experimental studies

Toward applications for supplying low-energy consumed devices like remote sensors, a novel structure of piezoelectric substrate is proposed by combining polyvinylidene fluoride beams with elastic film, which shows higher performance than typical cantilever and fixed-fixed structures in collecting energy from water droplet impact. To evaluate the performance, the electromechanical behaviors of substrate are studied by conducting droplet impact tests and also with the aid of the developed theoretical model. Results show that there exists an optimal design of parameters which can lead to higher peak voltage and energy collected than single cantilever beam. The predicted results also show that based on similarity principle, the beam surface can be greatly enlarged to collect more energy from the same impact momentum of water droplet so as to satisfy different scales of energy requirement. By making use of the great elasticity of film under fixed-fixed configuration, the tension in the film can be adjusted to adapt with various occasions of utilization that when the film is successively tightened, the peak voltage is gradually reduced while the vibration frequency is increased. Moreover, it is not sensitive to the impact location of water droplet as for the cantilever beam, which can indeed provide larger collecting area of water in practical utilizations. Overall, improvements have been made by the proposed structure in energy conversion efficiency, the scale of energy recovery and flexibility in applications. This work also lays a foundation for further practical utilizations of the proposed structure in a wide range of engineering fields such as the rain energy harvesting technology.

Self-cleaning and bouncing behaviour of ion irradiation produced nanostructured superhydrophobic PTFE surfaces

The bouncing dynamics of impacting water droplet on low energy ion beam produced superhydrophobic PTFE surfaces was investigated for the self-cleaning application. Ion beam with 300 eV and 800 eV energies was used to produce nanostructures on PTFE surfaces having the bouncing behaviour. The bouncing dynamics of water droplet was studied using dynamics parameters like spreading factor (β), contact time (tc), time of flight (ta) and number of bounces. The maximum spreading factor for the impact velocity of 0.5 m s−1 and 1.0 m s−1 was found to be 1.4 and 2.0, respectively for both 300 eV and 800 eV irradiated surfaces. The contact time of impacted droplet on 800 eV irradiated surface with 0.5 m s−1 velocity was found to be much lower (tc = 13.00 ms) than 300 eV ion irradiated surface (tc = 19.67 ms). Post impact bouncing number as high as 14 was found for 800 eV irradiated surface. Depending on ion energy and irradiation time the surface was converted from complete wetting to complete bouncing at low impact velocity of 0.5 m s−1 and from complete bouncing to partial bouncing at 1.0 m s−1. The self-cleaning property on 800 eV irradiated surface was successfully demonstrated using carbon powder.

High Transmittance Superhydrophobic Coatings with Durable Self-Cleaning Properties

One of the most important factors determining a significant reduction in optical devices’ efficiency is the accumulation of soiling substances such as dust, which, especially in solar power plants, implies higher costs and materials ageing. The use of superhydrophobic (SH) coatings, water contact angle (CA) greater than 150°, represents a suitable solution to improve the self-cleaning action while at the same time providing high transmittance for energy conversion. A mixed organic–inorganic SH coating with surface roughness below 100 nm was prepared by an easily scalable spray method and employed, allowing us to modulate the covered area and transparency. The coating has been also investigated while simulating pollution agents like acid rain, harsh environments, and the impact of continuous water droplets and dust particles with different physicochemical properties. The spray coating method allows us to obtain a modulated SH and self-cleaning surface showing CA > 170°, high transmittance in UV-Vis range and the ability to completely restore its initial properties in terms of wettability and transmittance after durability and soiling tests.

Bouncing behavior of a water droplet on a super-hydrophobic surface near freezing temperatures

Super-hydrophobic surfaces (SHSs) have demonstrated outstanding capability to mitigate the problem of ice nucleation in various engineering applications. Hence, experiments were conducted to understand the bouncing behavior of a water droplet on SHS under the condition of large temperature difference between the SHS and the impinging water droplet. The SHS was fabricated by spray-coating of a suspension of acetylene black and trimethylsiloxysilicate on a copper substrate. The ice-phobic nature of SHS was evaluated for a wide range of SHS temperature maintained around the freezing point of water (283 K–248 K). In addition, the effects of increasing the droplet temperature and droplet size on the droplet rebounding behavior were also analyzed. To evaluate the dynamics of water droplet during impact, the contact time and rebounding height were measured and employed as performance parameters. The results revealed that either by lowering the SHS temperature (from 283 K to 248 K) or by increasing the droplet temperature (from 278 K to 293 K) or droplet diameter (from DS = 2.07 mm to DL = 2.99 mm), an increase in the droplet-SHS contact time or a decrease in rebounding height were observed. Above findings were elucidated by the fact that when an impinging water droplet of relatively higher temperature came into contact with an SHS maintained at much lower temperature, evaporation took place from the droplet and at low temperature of SHS, the empty space inside the micro/nano surface textures of SHS was filled due to the condensation of evaporated water vapor. Thus, water bridges were formed within the micro/nano surface cavities and the wetting transition from the Cassie-Baxter state to the Wenzel state took place and the super-hydrophobicity of the surface could no longer be preserved. To overcome the adhesion between the impinging droplet and water bridges within micro/nano surface cavities, a larger amount of kinetic energy was spent during the droplet rebound. Consequently, for an impinging droplet, a noticeable decrease in the residual energy was observed.

Axial spreading of droplet impact on ridged superhydrophobic surfaces

HypothesisDue to the complex hydrodynamics of droplet impact on ridged superhydrophobic surfaces, quantitative droplet spreading characteristics are unrevealed, limiting the practical applications of ridged superhydrophobic surfaces. During droplet impacting, the size ratio (the ratio of the ridge diameter to the droplet diameter) is an important factor that affects droplet spreading dynamics. ExperimentsWe fabricated ridged superhydrophobic surfaces with size ratios ranging from zero to one, and conduct water droplet impact experiments on these surfaces at varied Weber numbers. Aided by the numerical simulations and theoretical analysis, we illustrate the droplet spreading dynamics and reveal the law on the maximum axial spreading coefficient. FindsThe results show that the droplet spreading and retraction dynamics on ridged superhydrophobic surfaces are significantly asymmetric in the axial and spanwise directions. Focusing on the maximum axial spreading coefficient, we find it decreases first and then increases with increasing size ratios, indicating the existence of the critical size ratio. The maximum axial spreading coefficient can be reduced by 25-40% at the critical size ratio compared with that on flat surfaces. To predict the maximum axial spreading coefficient, two theoretical models are proposed respectively for size ratios smaller and larger than the critical size ratio.

A novel wrinkle-gradient strain sensor with anti-water interference and high sensing performance

• The strain sensor was fabricated by a simple one-step pre-stretching method. • The strain sensor exhibited good super-hydrophobicity. • The strain sensor can automatically remove the impacting droplets. • The sensor exhibits a broad strain range, high sensitivity, and stability. The application of strain sensors in extreme environments (such as moisture or rain droplets) is limited by the influence of water molecules on their sensing performance. Endowing strain sensors with superhydrophobicity is a promising strategy to solve this problem. However, it remains a challenge for superhydrophobic strain sensors to completely avoid the effects of water droplets. Herein, we developed a superhydrophobic strain sensor with gradient structure for the first time by a simple one-step pre-stretching method. The strain sensor exhibits high sensitivity (maximum gauge factor of 1199.10), a broad strain range (up to 400%), robust stability (3000 cycles), a low detection limit (0.1% strain), and fast response (88 ms), which can be used to monitor full-range human body motions. Furthermore, the gradient wrinkle structure with low surface energy makes the strain sensor have gradient superhydrophobic performance, so that the impacting water droplets can automatically deviate and bounce off the surface by the Young's force, avoiding the interference of water droplets on the sensor's electrical signal. Even when the strain is up to 400%, the impacting droplet can still offset and rebound, which makes the superhydrophobic strain sensor have stable sensing performance in extreme environments.

Water droplet impingement erosion performance of WC-based coating sprayed by HVAF and HVOF

In this work, a comparative study between two coating processes is explored to combat water droplet impingement erosion (WDIE) phenomenon. High velocity air fuel (HVAF) and high velocity oxygen fuel (HVOF) processes are used to spray WC-10Co-4Cr powder on Ti-6Al-4V. The WDIE tests were performed at 250, 300 and 350 m/s impact velocities. At 350 m/s, both HVAF and HVOF coating showed similar behaviour. This was attributed to the high induced stress which is significantly higher than the fracture strength of a WC-10Co coating. At 250 m/s and 300 m/s, gradual damage occurs due to accumulated impact and jetting, where HVAF outperformed HVOF coating. HVOF lower performance is attributable to the formation of unwanted brittle W2C phase.

Impacting Water Droplets Can Alleviate Dust from Slanted Hydrophobic Surfaces

Water droplet impacting on a slanted dusty hydrophobic surface is examined in relation to dust mitigation from surfaces. Impacting droplet characteristics including droplet spreading/retraction rates, slipping length, and rebound heights are analyzed via high-speed recording and a tracker program. The environmental dust characteristics in terms of size, shape, elemental composition, and surface free energy are evaluated by adopting the analytical methods. The findings reveal that the dynamic characteristics of the impacting droplet on the slanted hydrophobic surface are significantly influenced by the dust particles. The maximum droplet spreading over the dusty surface becomes smaller than that of the nondusty surface. The presence of the dust particles on the slanted hydrophobic surface increases energy dissipation, and the water droplet slipping length over the surface becomes less than that corresponding to the nondusty surface. Impacting droplet fluid infuses over the dust particle surface, which enables mitigation of dust from the surface to the droplet fluid. A dust-mitigated area on the slanted surface is larger than that corresponding to the horizontal surface; in which case, the area ratio becomes almost six-fold, which slightly reduces with increasing Weber number. The optical transmittance of the dust-mitigated surface by the impacting droplet remains high.

Dynamic behaviours and drying processes of water droplets impacting on superhydrophilic surfaces

ABSTRACT The dynamic water contact performances and drying processes of two commercially available superhydrophilic coatings, QS200, used for glass antifogging, and QS3100, used for the self-cleaning of solar photovoltage panels, both with static contact angles near zero, have been evaluated. We investigated the drying behaviours of both statically and dynamically deposited water droplets on these two coatings by measuring droplet diameters and mass changes, and found that their dynamic water droplet behaviours were substantially different, although their static contact angles were essentially the same. The dynamic behaviour of water droplet impact on the QS200 coating was basically independent on the water droplet height (impact speed), while that on the QS3100 coating changed with droplet height. The drying times of droplets on the QS200 coating, both statically and dynamically deposited, were longer than on the QS3100 coating, although the initial droplet area on the QS200 coated surface was larger.

3D Lattice Boltzmann simulations for water droplet's impact and transition from central-pointy icing pattern to central-concave icing pattern on supercooled surfaces. Part II: Rough surfaces

In this paper (Part II of a paper series), a 3D pseudo-potential lattice Boltzmann method in combination with a solid-liquid phase change model with volume expansion taken into consideration (the same model used in Part I) is applied to study a water droplet's impact and its subsequent freezing patterns on rough supercooled substrates, consisting of an array of micro-pillars in a saturated vapor environment. Freezing patterns of the droplet in three different modes on rough surfaces depending on the contact angle are investigated. In the Hemi-wicking Cassie-Baxter mode where the water droplet fills into grooves between micro-pillars completely, heat conduction rate increases because of the increasing contact area of the water droplet and the micro-pillars, and the contact angle becomes smaller which results in the central-concave icing pattern. When the droplets are in the Wenzel mode and the Cassie-Baxter mode on the rough surface, saturated vapor exists between the droplet and the micro-pillars, which brings additional thermal resistance between the supercooled substrate to droplets. This results in lower heat conduction rate which delays ice nucleation time and allows the droplet to complete its recoiling motion that leads to the formation of the central pointy icing pattern. Because static contact angles of rough substrates in these two modes are higher than those of the corresponding smooth substrates, droplets’ recoiling motions on rough substrates are stronger which lead to the formation of central-pointy icing patterns. Regime maps, showing effects of contact angle and Stefan number on formation of central-pointy icing or central-concave icing patterns of a water droplet (with D0 = 100 and Pr = 13.5) after its impact on rough substrates at supercooled temperatures at We= 320 and Re = 164.9 are presented.

Dynamic characteristics of droplet impingement on carbon nanotube array surfaces with varying wettabilities

This work experimentally investigated the water droplet impingement behaviors on carbon nanotube (CNT) array surfaces characterized by different wettabilities from hydrophilicity to superhydrophobicity. By varying the impact velocity, six typical dynamic behaviors of droplet impact were identified and a related droplet regime map was developed under different wettabilities. To further investigate the evolution behavior of droplet impact, the dynamic contact angle (DCA) and spreading factor were measured and analyzed. The damped oscillations of DCA were observed for wettable and mildly hydrophobic CNT array surfaces, while the approximate impact inertia independence was verified for high hydrophobicity or even superhydrophobicity. The contact line velocity in the vicinity of triple phase interface was found to be highly related to the DCA variation at the initial stage of spreading. The discrepancy of DCA on different wettable surfaces reduced significantly with the increase of Weber number. The reduction in post-impact CAs and resultant wettability enhancement after droplet impingement was mainly attributed to self-assembly structures caused by the capillary action during the spreading phase.

3D Lattice Boltzmann simulations for water droplet's impact and transition from central-pointy icing pattern to central-concave icing pattern on supercooled surfaces. Part I: Smooth surfaces

A water droplet's impact and its subsequent spreading, recoiling and freezing on a smooth substrate at a supercooled temperature is studied numerically using a 3D pseudo-potential lattice Boltzmann method, in combination with a solid-liquid phase-change model with volume expansion of water at 0°C taken into consideration. Simulated results show that a water droplet after its impact on a smooth surface at a supercooled temperature can form either a central-pointy icing pattern with a single ice peak, or a central-concave icingpattern in a donut shape, depending on the contact angle and the supercooled degree of the wall. Itisshown that the recoiling motion of the droplet after its impact on the supercooled substrate plays the dominant role in the formation of the central-pointy icing or central-concave icing pattern. The central-pointyicingpattern is formed on a substrate having a large contact angle where the recoiling motion of the droplet is strong while the central-concave icing pattern is formed on a substrate having a high degree of supercooled temperature (i.e., a large Stefan number) where the recoiling motion is terminated prematurely because of early nucleation of freezing on the supercooled substrate. The volume expansion during liquid to ice phase-change process at 0°C does affect the movement of the liquid phase although its effect on the formation of icing patterns is small. Effects of movement of freezing fronts on the shape of the icing patterns are illustrated. At fixed values of We= 320 and Re = 164.9, a map showing effects of contact angle and Stefan number (bottom wall temperature) on the formation of central-pointy icing or central-concave icingpatterns after a water droplet's impact (with D0= 100 and Pr = 13.5) on smooth supercooled surfaces is presented.

Highly transparent and robust superhydrophobic coatings fabricated via a facile sol-gel process

A simple sol-gel method was utilized to fabricate SiO2-based coatings on glass substrates. After decorating the coatings with fluorosilane, the coatings became superhydrophobic, with a maximum water contact angle of 154°. After a systematic investigation of the coating processing parameters, the transparency and hydrophobicity of the coatings were well balanced. A highly transparent superhydrophobic coating was fabricated. The coated sample has an optical transmittance that is only <1% lower than the bare substrate. The coating also demonstrates high durability as evaluated by the water droplet impact test. After the test, the water contact angle of the coating was reduced by 2°. Moreover, the coating is oleophobic as well, with an oleic acid contact angle of 130°.

Direct observation of the impact of water droplets on oil replenishment in EHD lubricated contacts

Water is one of the most significant causes of lubrication failure. There is little research on the direct observation of the impact of water on lubrication properties. In this study, the influence of water on oil replenishment under different elastohydrodynamic (EHD) lubricating conditions is evaluated using optical interferometry and infrared microscopy, and a dimensionless criterion when water influences the film thickness is proposed. Evidence shows that the scour displacing effect and emulsification of water/oil are the main reasons for the reduction in film thickness. Once a water droplet enters an oil reservoir around the critical contact zone, it hardly moves away. This aggravates starvation, reduces the center film thickness of the contact, and leads to lubrication failure of the mechanical components.

Fabrication of durable corrosion-resistant polyurethane/SiO2 nanoparticle composite coating on aluminium

In this work, SiO2 nanoparticle–embedded polyurethane superhydrophobic coating on aluminium substrate was fabricated using spin coating technique. By embedding SiO2 nanoparticles into the polyurethane matrix, superhydrophobicity is successfully achieved with water static contact angle of 156 ± 3° and tilt angle of 6 ± 1°. Coating exhibits excellent self-cleaning and antifouling properties. Wetting stability of coating was further evaluated by performing water jet, bending, sand falling, tape peeling, floating, annealing, and chemical stability tests. It is found that coating is mechanically, thermally, and chemically stable. Water droplet impact test shows the bouncing, pinning, and splashing behaviour of water droplets at different impact velocities. The electrochemical experiment has demonstrated that the corrosion potential of aluminium after coating increases and the corrosion current density decreases, revealing the corrosion resistance property of coating. We envision that the self-cleaning, antifouling, and anticorrosive superhydrophobic coating of the polyurethane/SiO2 nanoparticle composite can effectively prevent the corrosion of aluminium and the aforesaid coating has great industrial applications.

The Splashing of Melt upon the Impact of Water Droplets and Jets

Small-scale experimental studies of melt splashing upon the impact of water are presented here, with a focus on the fluid dynamics and thermal aspects of these interactions. Gravity-accelerated droplets and forced short-duration water jets interacted with liquid Rose’s alloy superheated to 100–200 degrees above its melting point. A repeatability study was performed for better-control of the gravity fall of a droplet. The amount of perturbation on the melt surface was obtained from a video recording, and indicated the existence of three principal stages of interaction. The cases using the forced water jet demonstrated the occurrence of a cumulative jet of melt following the collapse of the cavity caused by the water impact. Also, it was shown that numerous small-diameter melt droplets were scattered by the primary impulse, and small-scale micro-eruptions were observed, which generated small but fast melt droplets at the later stages of interaction.

Hydrophobic N-halamine based POSS block copolymer porous films with antibacterial and resistance of bacterial adsorption performances

Functional films with antimicrobial efficacies and resistance of bacterial adhesion have great potential applications. In this paper, polyhedral oligomeric silsesquioxanes-(poly(trifluoroethyl methacrylate))8 (POSS-(PTFEMA)8) block copolymer and olefinic N-halamine monomer precursor containing quaternary ammonium groups (HQS) were used to prepare antibacterial porous films. The HQS compound was chlorinated (HQS-Cl) with tert-butylhypochlorite to achieve antibacterial functionality, and the stability and regenerability of HQS-Cl were examined. The porous film was prepared from POSS-(PTFEMA)8 by the breath figure approach and the stability of the porous films was examined. The surface morphology, stability and mechanical property of the porous films were studied. Moreover, the adding and spraying methods were carried out with HQS-Cl on the pincushion porous films to obtain hydrophobic antibacterial films. The static and dynamic hydrophobic properties were investigated by water contact angle and water droplet impact behavior. In addition, the bacterial adsorption of S. aureus and E. coli O157:H7 on the flat, porous and pincushion structure films were studied. The antibacterial testing results showed that the HQS-Cl modified pincushion films had great antibacterial activity and could completely inactivate 2.38 × 106 CFU/sample of S.aureus and 3.07 × 106 CFU/sample of E. coli O157:H7 within 30 min of contact time.