Water Droplet Impact Research Articles

Superhydrophobic Candle Soot Coating Directly Deposited on Aluminum Substrate with Enhanced Robustness

In this study, superhydrophobic surfaces were developed by using a simple and environmentally friendly technique. The nano-network of candle soot (CS) as the byproduct of incomplete combustion of paraffin candle was directly coated onto both smooth and micro-rough aluminum (Al) substrates for various time periods of deposition. The simple technique of mechanical sanding was used to impart micro-rough structures onto Al substrates using different sandpaper grit sizes. The scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques were used to characterize the morphology and chemistry of the prepared surfaces. Wetting analysis of the prepared surfaces was performed by measuring both water droplet contact angle (CA) and sliding angle (SA). The prepared coatings showed superhydrophobic properties with high CAs and low SAs for CS surfaces coated on roughened Al substrates. Moreover, the robustness of the prepared surfaces was tested by continuous impingement of water droplets onto their surfaces from various heights. Post-testing wetting analysis showed that the micro-nano surfaces of candle soot coated on micro roughened Al substrates demonstrated improved robustness. These surfaces could be useful for self-cleaning, anti-corrosion and anti-icing applications.

Superhydrophobic coating based on organic/inorganic double component adhesive and functionalized nanoparticles with good durability and anti-corrosion for protection of galvanized steel

The excellent wetting resistance properties of superhydrophobic materials mean that they have a wide range of potential applications. However, there are problems that need to be solved urgently regarding mechanical stability, corrosion resistance and complex synthesis. Herein, we prepared a robust and durable superhydrophobic coating using an inorganic aluminum phosphate (AP) adhesive. This contained organic polydimethylsiloxane (PDMS) and silica/halloysite composite nanotubes (HNTs) modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES). The as-prepared F-SiO2/HNTs-AP@PDMS coating shows good superhydrophobicity (water contact angle = 158°), mechanical strength and durability. Superhydrophobicity was maintained even after 150 complete sandpaper wear cycles and 200,000 water droplet impact measurements. In particular, electrochemical impedance spectroscopy (EIS) results showed that the Rct value of the superhydrophobic coating was significantly increased by five orders of magnitude. After exposure to a corrosive environment for 10 days, the Rct value is still far higher than that of uncoated galvanized steel, indicating that it provides excellent long-term protection for metals. We believe this work not only provides a reference for the large-scale preparation of superhydrophobic materials, but also provides a strategy for the long-term protection of galvanized steel.

A diffusion-enhancing icing theory for the freezing transition of supercooled large water droplet in impact

The freezing behavior of supercooled large droplet(SLD) in impact causes severe threats to civil aircraft. However, the mechanism by which the impact dynamics of SLD accelerates the icing still remains unclear. In this paper the impact and freezing of water droplets with different supercooling on inclined surfaces of different wettability were investigated experimentally and theoretically. Compared to one/two freezing morphologies and linear supercooling-freezing time relation reported previously, four freezing morphologies were observed in different supercoolings, and the transition of freezing morphologies resulted in the piecewise nonlinear supercooling-freezing time relation. By comparing the characteristic times of droplet dynamics and horizontal icing, it was found that the dynamics-icing interaction could lead to higher heat conduction area in higher supercooling. Theoretical analysis shows that the unsteady heat conduction enhances the freezing rate in a short time initially. The coupled effect of large contact area and initial heat flux greatly enhances the diffusion of latent heat in freezing of droplet with high supercooling. Based on this machanism, a diffusion-enhancing icing theory is proposed. Compared with previous icing theories, this theory can give much better predictions about the freezing rate of impinging supercooled water droplets, especially in conditions of high supercooling and hydrophobic surface.

Modelling the particle trajectory and melting behaviour of non-spherical ice crystal particles

Existing ice crystal icing codes commonly neglect non-spherical particle rotation behaviour. This leads to uncertainties in modelling ice crystal icing as ice crystals are typically non-spherical. This paper develops a two-dimensional framework to model the particle trajectory and melting behaviour of rotating non-spherical ice crystal particles. Non-spherical particle translational and rotational motions are resolved using a framework of unit complex numbers. The effect of rotation on particle heat and mass transfer is implemented using a sub-model dependent on both rotational and translational particle Reynolds numbers. A test case of flow through a swan neck duct is presented. The model is validated through comparison to the discrete phase model in ANSYS Fluent and experimental water droplet impingement test data. Comparison of the developed framework with other simplified particle tracking methods (without modelling the non-spherical particle rotation) is first conducted. These results show large differences in the trajectory, velocity and melt ratio of individual particles and in overall particle impingement behaviours, especially for high aspect ratio particles. Particle rotation can indirectly affect single particle’s melting behaviour through its effect on particle trajectory and velocity. Both aspect ratio and porosity are seen to enhance particle melting behaviour and affect particle impingement behaviour. The numerical uncertainties in the DNS-based correlations of the particle forces and torques employed are also discussed, as well as the performance of the developed framework for the case of a flow past a test article.

Interfacial dynamics of viscous droplets impacting a superhydrophobic candle soot surface: Overview and comparison

The impact of droplets on textured or rough surfaces has garnered remarkable appreciation due to its multifarious applications such as self-cleaning, anti-icing, and anti-fouling, leading to a plethora of engineered superhydrophobic surfaces (SHPs) exhibiting different interfacial dynamics during impact. However, the prime limiting factors in using these surfaces abundantly arise from their long fabrication time and concurrent high cost. Here, we propose using carbon soot nanoparticle (CSNPs) coated fractal superhydrophobic surfaces prepared from flame deposition as an alternative to overcome the limitations. We establish our claim by exploring the dynamic wetting behavior of the soot-coated surface in terms of key droplet impact parameters such as rebounding, contact time, impalement transition, and splashing dynamics. A systematic investigation is undertaken by considering a vast range of viscosity and impact conditions. One of the significant observations is the absence of the partial rebound regime during the impact of water droplets on the CSNPs surface, unlike most of the existing superhydrophobic surfaces under similar impact conditions. Furthermore, the surface promotes droplet splashing for moderately viscous solutions at high impact velocities, also characterized by unified scaling laws based on different non-dimensional numbers. Finally, a regime map is proposed to elucidate the complete dynamic wetting characteristics of these CSNPs' surfaces for viscous fluids, which further reflects competitive and equal, if not superior, wetting behavior compared to a series of existing non-wetting surfaces. The results are expected to promote CSNPs based surfaces in applications such as self-cleaning, oil-water separation, and thermal management.

Numerical Investigation of Ice Shedding Model for Icing on Rotor Blade

Aircraft icing is the phenomenon that forms an ice layer on the solid surface by impingement of supercooled water droplets in the atmosphere. In icing on rotor blades, ice is shed from the blade surface by centrifugal force as the accumulated ice grows. Therefore, it’s necessary to establish an ice shedding model for icing simulations. In this study, the authors proposed an ice shedding model in which the condition for ice shedding is that the centrifugal force exceeds both the adhesion strength and the tensile strength. A numerical analysis was performed to validate the model, and the ice shedding conditions were investigated through comparison with the experimental results. The progress of icing on the rotor blade was predicted by UPACS. We simulated the change of centrifugal, adhesion, and tensile force over time in the icing process. The results showed that the present model is able to predict both the shedding timing and location with reasonable accuracy.

Numerical Analysis of the Effect of Ellipsoidal Shape on the Freezing Behavior of Impacting Water Droplets on Cold Surfaces

Numerical Analysis of the Effect of Ellipsoidal Shape on the Freezing Behavior of Impacting Water Droplets on Cold Surfaces

Spherical-micelle-driven deposition of high-speed impacting water droplets on superhydrophobic surfaces

This work utilizes modified small spherical micelles of surfactants to achieve complete deposition of high-speed impacting water droplets on superhydrophobic surfaces, without the assistance of high viscosity, low surface tension or large aggregates.

Alteration of freezing paradigms of an impact water droplet on different cold surfaces

The influence of surface properties on the dynamics and freezing of an impact water droplet is investigated under various impact velocities and surface temperatures. Three distinct icing paradigms are found, namely the widely observed Central-Pointy Icing paradigm (CPI), the annular paradigm with Exterior Freezing front First (EFF), and the annular paradigm with Interior Freezing front First (IFF). High impact velocities and low surface temperatures are beneficial to the IFF icing paradigm. Upon decreasing the impact velocity and/or increasing the surface temperature, the IFF paradigm transits to EFF and ultimately to CPI. The interior freezing front occurring prior to the exterior one is mainly attributed to the small difference of the thickness between the central and peripheral liquid, and the large temperature difference between the droplet and liquid-substrate interface. Raising the impact velocity and surface wettability enhances droplet spreading, resulting in a small thickness difference. The liquid-substrate interface temperature is collectively determined by surface temperature, density, thermal conductivity and specific heat capacity of the solid surface. A freezing paradigm map described by the dimensionless temperature and the maximum spreading diameter is proposed to evaluate the conditions under which one of the three distinct freezing paradigms appears. A large spreading area and a low liquid-substrate interface temperature are prerequisites for the emergence of IFF paradigm.

Investigations of water droplet impact and freezing on a cold substrate with the Lattice Boltzmann method

A multiphase Lattice Boltzmann model with phase change is presented for studying droplet impact and solidification on an airfoil. The proposed model combines a pseudo-potential multiphase model and a thermal single-component phase change model. These two models are verified separately. The pseudo-potential model with the Peng–Robinson equation of state is used to simulate large density ratios of multiphase flows. The thermal model is based on the total enthalpy and allows the phase change without using an iterative methodology. The coupling is made through the immersed moving boundary method that handles the solid–liquid interface. The generalization for curved surfaces is introduced through to a novel extrapolation method at boundaries. The effects of surface wettability, static contact angle and initial velocity of the droplet on the evolution of solid fraction and total freezing time are discussed and compared to other simulation and experimental works.

A simulation framework for rotorcraft ice accretion and shedding

In this work a novel three-dimensional simulation framework for modelling the safety critical problem of rotorcraft ice formation and shedding is presented. To enable an entirely three-dimensional framework, state-of-the-art numerical modelling techniques are adopted and the inter-dependency between the techniques is discussed. The numerical techniques introduced in this work include models to simulate the rotor flow-field, water droplet trajectories and impingement locations, phase change modelling during the ice accretion, and mesh deformation to account for the moving ice boundary. A set of benchmark test cases are initially used for preliminary validation of the icing framework. This allows for a comparison of the collection efficiency and ice shapes with high-quality experimental measurements. Icing wind tunnel tests conducted on the Spinning Rotor Blade (SRB-II) model rotor are used for an assessment of the numerical predictions specific to rotorcraft. Quantities used for comparisons include the ice thickness and shedding location. Numerical predictions are in good agreement with the measured data at all temperatures. Additionally, the outcome of influential parameters which directly impact rotor ice shapes are assessed. In particular, the model for the temperature profiles within the ice layer, and the centrifugally induced movement of the liquid film.

Durable and Superhydrophobic Aluminium Alloy with Microscale Hierarchical Structures and Anti-Drag Function Inspired by Diving Bell Spider

Coating materials with special surface wettability are widely applied in marine paint systems used in the naval industry to reduce the corrosion and viscous drag of seawater. However, traditional coatings are inefficient and limited, either by poor durability or insufficient anti-drag capacity. Here, inspired by the diving bell spider, a bionic superhydrophobic coating with multiscale hierarchical architecture was successfully prepared on the surface of aluminium alloy. It possesses excellent mechanical abrasion durability, chemical durability, and low adhesion. Remarkably, the water contact angles could remain over 150.9° after more than 15 abrasion cycles or strong acid/alkali conditions. In addition, the impacting water droplet lifted off the surface of bionic superhydrophobic aluminium alloy (BSAA) within 13 ms, illustrating an excellent low adhesion property. In fact, when the BSAA is immersed in water, it could absorb bubbles and form a gas membrane. The existence of the gas membrane could prevent water and anaerobic organisms from contacting and even corroding the BSAA. Meanwhile, the gas membrane acts as a lubricant and significantly deceases friction at the solid–liquid interface, reducing the drag for BSAA. The BSAA proposed in this work has broad application prospects, such as medical devices, microfluidic chips, gas separation and collection in water.

Water sprays formed by impinging millimeter-sized droplets on superhydrophobic meshes

Impinging a millimeter-sized liquid droplet on a leaky substrate—such as a porous mesh—can cause the formation of many small droplets from the ligament fragmentation. Although this phenomenon has been widely considered as a desirable strategy to produce liquid sprays of monodisperse droplets, the underlying mechanism has not yet been completely elucidated, and the spray needs detailed characterization. Herein, we experimentally investigate the atomization phenomena occurring in the recoiling and spreading stages of impinging water droplets on superhydrophobic meshes. We show that the spray formed during droplet recoiling is stimulated by the longitudinally symmetric air cavity collapse on the superhydrophobic mesh, and thus the size of the spray formation area on the mesh is almost identical to the size of the simultaneously generated upward jet. By contrast, the water spray produced during droplet spreading is expelled under the action of the inertia-induced hydrodynamic pressure, and the size of spray formation area on the mesh exhibits a power-law dependence on the Weber number; yet, the pore geometry restricts it to take a constant value when the Weber number is sufficiently high. By performing statistical analyses on the spray droplet sizes, we further demonstrate that the mean sizes of spray droplets are mainly set by the mesh pore sizes, but the complex atomization dynamics leads to a broad size distribution, which is beyond the expectation.

Experimental Study on Droplet Impact Phenomena on Burning Liquid Pool Surfaces

In this study, the phenomena of water droplet impact on burning methanol and n-heptane pool surfaces were experimentally investigated under various size and velocity conditions of impact droplet. In the n-heptane pool, the temperature increased to the deeper location of the pool and the droplet impact velocity was slower, as compared with those in the methanol pool. These results were caused by the higher heat release rate of the n-heptane pool. However, the impact droplet sizes on the burning methanol and n-heptane pool surfaces appeared to be similar. By visualizing the droplet impact phenomena, the impact pattern maps of burning methanol and n-heptane pool surfaces were constructed and compared with the previous impact pattern maps of their unburned pool surfaces. In the burning and unburned methanol and unburned n-heptane pools, patterns of single jet and splash with secondary jet were observed. On the contrary, in the burning n-heptane pool, patterns of single jet and canopy were observed.

Influence of alcohol additive and surface temperature on impact and spreading characteristics of a single water droplet

An experimental investigation is conducted to explore the effect of alcohol additive and surface temperature on the impact dynamics and spreading characteristics of a single water droplet on a heated copper surface. Non-azeotropic bi-component alcohol-water mixtures with 0–8 vol.% ethanol, 0–7 vol.% n-butanol and 0–2.5% n-pentanol are adopted as the working fluid covering wide ranges of surface temperature from 25 to 450 °C and Weber number from 5.8 to 71.9. The results of the visualization study on the instantaneous droplet behavior show that depending on the surface temperature and alcohol additive, the droplets exhibit noticeably different impact behaviors at the same impact height. A new phenomenon is found that some alcohol-water bi-component droplets with smaller Weber numbers show better spreading ability compared to water droplets with higher Weber numbers. This implies that the addition of alcohol additives can even counteract the negative influence of smaller impact energy. The variation of the maximum spreading factor is characterized by a contour map that is constructed over the tested ranges of Weber number and surface temperature for different droplets. The present paper also proposes a new piecewise correlation for the maximum spreading factor in different heat transfer regimes, which takes both the effects of alcohol additive and surface temperature into consideration.

Experimental Study of Unsteady Heat Transfer and Phase Changing Process Upon Impacting of Ice Crystals onto Heated Surfaces Pertinent to Aero-Engine Icing Phenomena

Ice accretion on exposed surfaces of aero-engine components has been widely recognized as a significant hazard to aviation safety in cold weathers. Icing process due to the impingement of the supercooled water droplets suspended in the cloud onto the cold surfaces of inlet components of aeroengines have been studied extensively for decades. Since ice particles were believed to simply bounce off from the exposed surfaces of aero-engine components, ice crystals in the clouds were initially considered not to pose a threat to aviation safety. Therefore, the ice accretion process due to the impacting of ice crystals onto the surfaces of hot engine componentes has not been studied until recently. It has been found recently that, tiny ice particles in the cloud may be partial/full melting upon impacting onto the hot surfaces of aero-engine components, such as heated Inlet Guide Vanes (IGV) and various probes. The partially/fully melted ice crystals were found to stick onto the hot surfaces and form thin water film, which would intercept more oncoming ice particles and lead to significant ice accretion over the surfaces of the hot engine components. The ice crystal induced ice accumulation on the critical aero-engine components has been found to cause significant engine performance loss and erroneous data being read from the probes. In the present study, a series of experimental investigations were conducted to elucidate the underlying physics of the dynamic ice accretion process pertinent to ice crystal icing phenomena. A novel ice crystal icing test rig with the capacity of generating controllable amount of ice crystals and flying speed up to 100 m/s was developed in a temperature-controllable environment chamber for the ice crystal icing studies. By using a high-speed imaging system, a digital particle image velocimetry(PIV), and an Infrared (IR) thermal imaging system, a comprehensive experimental campaign was performed to characterize the transient impacting process of ice crystals, dynamic ice accretion and unsteady heat transfer process associated with the impacting of ice crystals onto heated surfaces, in comparison to those due to the impingement of supercooled water droplets. By using an ultra-sensitive force sensor and a high-speed image system, a comparative study is conducted to examine the differences in the transient impinging dynamics of single water droplets, supercooled water droplets, and ice crystals onto solid surfaces with different wettability and stiffness. By upgrading the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) with additional ice crystal icing capability, a set of explorative studies are also conducted to examine the characteristics of the dynamic ice accretion processes over the heated surfaces of an aero-engine Inlet Guide Vane (IGV) model under both ice crystal icing and supercooled droplet icing conditions. The anti-/de-icing performance of a novel hybrid strategy by integrating icephobic coatings and minimized surface heating are also evaluated under both supercooled water droplet icing and ice crystal icing conditions. The new findings derived from the present studies are very helpful to gain further insights into the ice crystal icing phenomena for the development of more effective and robust anti-/de-icing strategies to ensure safer and more efficient aircraft/aero-engine operations in cold weathers.

Inside Front Cover

By fusion of superhydrophobic surface with transistor‐inspired architecture for high‐frequency water kinetic energy harvesting, we report the design of a superhydrophobic surface‐based droplet electricity generator, which allows for timely water droplet shedding at high impact frequency without the formation of unwanted liquid film and is capable of efficient electrical energy harvesting without compromising output performance. This work represents Harvesting energy from high‐frequency impinging water droplets by a droplet‐based electricity generator. image

Harvesting energy from light and water droplets by covering photovoltaic cells with transparent polymers

Energy harvesting of sunlight is often done using photovoltaic cells covered by a protective layer of polymer or glass. Currently, this layer does not have any other function than being transparent and protective, but its functionality could be improved and in fact contribute to electrical energy harvesting from the environment. This work reports new findings on the integration of silicon-based photovoltaic solar with a water droplet energy harvesting device based on contact electrification using readily available materials. The water droplet energy harvesting device utilizes hidden or transparent front electrodes in flat or curved geometries to increase the power output due to water droplets while at the same time minimizing the power loss from the photovoltaic cell. Three different designs are designed and tested, and the advantages and disadvantages are outlined. Particular emphasis is put on investigating the performance of the flat cell design that exhibited the largest electrical power output due to water droplet impact. The electrical energy harvesting efficiency of the commercial photovoltaic cell is about 4.4%, whereas for the water droplet energy harvesting device it is about 0.6%. The relative contributions of the two energy harvesting mechanisms are analyzed, and possible applications outlined.

Droplet impact dynamics on an aluminum spinning disk

Droplet impact on a spinning surface has been observed in different industries and plays an important role in the performance of industrial systems. In the current study, the dynamics of water droplet impact on a hydrophilic spinning disk is investigated. An experimental setup is designed in a way that droplet diameter, impact velocity, disk rotational speed, and location of impact are precisely controlled. While the droplet diameter is fixed in the present study, other mentioned parameters are changed and their effects on the droplet behavior are discussed. High-speed imaging is used to record the droplet dynamics under various operating conditions. It is demonstrated that after impact, droplet spreads on the surface due to a high adhesion between water and the hydrophilic substrate. It is indicated that the wetted area is a function of time, impact velocity, disk rotational speed, and centrifugal acceleration. Furthermore, depending on the mentioned parameters, different phenomena such as rivulet formation, fingering, and detachment of secondary droplet(s) are observed. In the angular direction, in general, the wetted length increases as time passes. However, in the radial direction, the droplet first spreads on the surface and reaches a maximum value, and then recedes until a plateau is attained. At this instant, a bulk of liquid, which is called wave in this study, moves radially outward from the inner boundary of the droplet toward its outer boundary due to the effect of centrifugal force. Once the wave reaches the outer boundary, depending on its size and momentum, fingers or rivulets are formed, and small droplet(s) may detach. The process is analyzed comprehensively, and different empirical correlations for wetted lengths in radial and angular directions, secondary droplet formation, number of fingers, the onset of fingering, and wave velocity are developed.

Effects of Upstream Component and Air Injection on Water Droplet Impingement Characteristics for Downstream Surfaces

Water droplet changes its movement direction and velocity when it bypasses an aircraft component with the surrounding airflow or gets blown by air injection from the inner part. When the deflected droplet impacts on the downstream surface, its impingement characteristics would be different from those without the frontal effects. In this article, a Lagrangian method was developed to include those upstream effects on the droplet collection efficiency. Validation cases were carried out for a cylinder and an MS (1)-0317 airfoil, whereas a multielement airfoil and an engine cone with a hot air film-heating anti-icing system were computed to investigate the effects of the upstream component and air injection on the impingement characteristics of downstream surfaces. It is found that the present collection efficiencies are in good agreement with the experimental data and the simulation results obtained by the Eulerian and traditional Lagrangian methods when not affected by those upstream factors. The droplet deflections and trajectory crossings are observed clearly under the influence of the upstream component, and the Lagrangian results of downstream surfaces differ from those of the Eulerian method. In addition, due to the air injection from the inner engine cone, the peak collection efficiency on the cone surface increases with the decrease of the droplet diameter and the value even exceeds one when the droplet is small. This work is helpful for the understanding of the droplet motion and the accuracy of aircraft icing simulation.