Droplet Flight Research Articles

Collision of water drops with a thin cylinder

The collision of water drops with a thin cylinder is studied. The droplet flight trajectory and the cylinder axis are mutually perpendicular. In the experiments, the drop diameter is 3 mm, and the diameter of horizontal stainless-steel cylinders is 0.4 and 0.8 mm. The drops are formed by a liquid slowly pumped through a vertical stainless-steel capillary with an outer diameter of 0.8 mm, from which droplets are periodically separated under the action of gravity. The droplet velocity before collision is defined by the distance between the capillary cut and the target (cylinder); in experiments, this distance is approximately 5, 10, and 20 mm. The drop velocities before the impact are estimated in the range of 0.2–0.5 m/s. The collision process is monitored by high-speed video recording methods with a frame rate of 240 and 960 Hz. The test liquids are water. Experiments and numerical simulation show that, depending on the drop impact height (droplets velocity) different scenarios of a drop collision with a thin cylinder are possible: a short-term recoil of a drop from an obstacle, a drop flowing around a cylindrical obstacle while maintaining the continuity of the drop, the breakup of a drop into two secondary drops, one of which can continue flight and the other one is captured by the cylinder, or both secondary droplets continue to fly, and the drop can be also captured by the cylinder, until the impact of the next drop(s) forces the accumulated drop to detach from the cylinder. Numerical modeling satisfactorily reproduces the phenomena observed in the experiment.

Investigating the in-flight droplets' atomization in suspension plasma-sprayed coating

A three-dimensional unsteady numerical model is employed to study the in-flight droplets/particles' atomization in the suspension plasma spraying (SPS) process. A user-defined function (UDF) written in C programming adds the electromagnetic fields to the fluid flow field. A two-way coupled Eulerian-Lagrangian technique simulates the sprayed droplets' interaction with the plasma flow. The developed model is applied to investigate the droplets' atomization behavior in the SPS process. Droplets' atomization is simulated by applying the Kelvin-Helmholtz Rayleigh-Taylor (KHRT) breakup model. This model considers the effects of both aerodynamic forces (Wave model) and Rayleigh-Taylor instabilities on the atomization process combing a liquid core region (Levich model). Previous works to estimate the KHRT breakup model's coefficients were performed on a liquid jet (e.g., fuel jet) into the air. Because the density ratio of suspension in the plasma is higher than that of the liquid jet into the air (suspension density is higher than a liquid droplet and plasma density is lower than air), the model's coefficients should be modified for the suspension atomization in the SPS process. For this reason, nine case studies are investigated to see the effect of KH and RT instabilities and the liquid core length on the droplets/particles' atomization. The results obtained from the suggested values show a good agreement compared to the existing data.

Simulation and experimental study on droplet breakup modes and redrawing of their phase diagram

Dimensionless numbers are often used to characterize the various modes of droplet breakup processes. However, the current methods of calculating these dimensionless numbers are not uniform—consequently, the calculation results are different, resulting in different phase diagrams of droplet separation. This paper first summarizes the methods of calculating the Weber number. The maximum transient velocity at the center of a nozzle is then used as the characteristic velocity for calculating the Weber number, and this formulation is used to solve certain scenarios in which the traditional Weber number cannot be applied, such as the strange phenomenon of the upward flight of separated droplets. A mathematical model is established to simulate the various separation forms of droplets, and the experimental study is also carried out. This upward flight of droplets is found to be the result of competition between the liquid inertial force, surface tension force, and suction effect of the nozzle mouth. The final velocity of the droplets depends on the existence of a stagnation surface and the corresponding sweep effect. Finally, the phase diagram of different droplet separation modes is drawn in the Ohnesorge–Weber number space.

(Invited) Low Temperature Plasma Electrochemistry with Microscopic Liquid Droplets in Flight

When microscopic-sized liquid droplets travel through a low temperature RF plasma [1] at atmospheric pressure a number of remarkable and unexpected effects have been observed. After a short flight time, ~0.1ms, there is evidence that chemical reactions induced by the plasma and gas flux proceed at a rate that is significantly faster that observed in plasma – bulk liquid studies and many orders of magnitude faster than in standard bulk chemistry.[2] We suspect this is due to the complex interplay between droplet charge, electric fields, both internal and external to the droplet, and high chemical fluxes arriving at the droplet surface.There exists a large potential to develop new plasma-liquid processes for medical, chemical, biological, environmental and materials applications, among others and we can highlight some unique features of the plasma – microdroplet system that may provide opportunities for exploitation, namely: (i) a controlled ambient environment, (ii) a large surface area to volume ratio, (iii) small volume, (iv) low droplet temperature, (v) in-flight chemical synthesis and encapsulation, and (iv) remote delivery. These features offer the possibility of delivering high fluxes of active chemical species and nanoparticles remotely and on demand for applications in, for example, plasma-medicine, agriculture and microreaction while keeping the plasma itself at a safe distance.We have measured reactive oxygen species (ROS) flux variation with distance, up to 150 mm beyond the plasma, along with its effect on bacterial cell viability, DNA and amino acids. We have investigated plasma interactions with single cells, each transported in its own droplet. We have used the individual droplets as chemical microreactors to produce nanoparticles in flight, at rates many orders of magnitude higher that via high energy radiolysis or chemical synthesis. These measurements form the basis for numerical simulation in the gas-plasma and liquid droplet phases. New measurement techniques, based on recently acquired facilities, are being investigated. These include mid-IR absorption studies of droplets and their environment in flight, using tunable supercontinuum and quantum cascade lasers, and freezing plasma-treated droplets in flight for in-situ transfer to XPS surface chemical analysis.Current theories of microparticle charging in a collisional plasma environment are very limited. While in-flight charge measurements represent a significant challenge, the relatively large size of the droplet (10 – 20 μm diameter) and the limited evaporation over the flight time, offer the prospect of using droplets as a spherical probe to develop enhanced collisional probe theories in the regime where the particle size is greater than Debye lengths or mean free paths. In-flight measurements indicate a minimum net charge of ~105 electrons, considerably higher than that obtained by other charging methods. Analytical – numerical and finite element simulations, in tandem with charge measurements, are being developed to better understand the droplet electrical environment and ultimately to link chemistry and charge in a consistent framework.

A ground-based work of droplet deposition manufacturing toward microgravity: Fine pileup of horizontally ejected metal droplets on vertical substrates

Uniform metal micro-droplets deposition manufacturing (MDDM) technology is assumed to be a very promising technique for advancing space manufacturing on account of without using large energy equipment or custom materials. However, the effects of gravity on manufacturing processes remain to be a work-in-progress before MDDM applied in outer space. To intuitively observe the gravity effects in terrestrial environment in the opening phase of the research, we turn the additive direction ninety degrees from upright direction. Therefore, the foremost work is to investigate the deposition process of horizontally ejected droplets on a vertical substrate. Here, freestanding pillar structure (the simplest building block) is planned as fabrication samples. As a prerequisite to printing a freestanding pillar structure, the stable deposition conditions for horizontally ejected droplets are experimentally investigated firstly. To ensure that the pillar behaves a straight profile, the deposition step distance for successive pileup droplets is mathematically modeled. A strategy for the fine pileup of the horizontally ejecting droplets is proposed and identified by experiments. The results suggest that the pileup accuracy mainly depends on the stand-off distance and initial velocity of the droplet. These parameters are normalized as a variable named time consumption for droplet flight, of which the threshold is 5.1 ms. The maximum effective pileup height of droplets decays with the function of pillar height increment and deposition step distance (Δ h/s -1). Finally, structures with high aspect ratio or pillar features are fabricated to verify the formability of the manufacturing process for droplets horizontal deposition. On this basis, toward the application in outer space, an aluminum alloy (aerospace light alloy) structure is built. This work provides meaningful guidance to control the deposition of horizontally ejected droplets and steadily build structures with good appearance on vertical substrates, and laids the foundation for ground research on microgravity-based droplet deposition manufacturing.

Face coverings and respiratory tract droplet dispersion.

Respiratory droplets are the primary transmission route for SARS-CoV-2, a principle which drives social distancing guidelines. Evidence suggests that virus transmission can be reduced by face coverings, but robust evidence for how mask usage might affect safe distancing parameters is lacking. Accordingly, we set out to quantify the effects of face coverings on respiratory tract droplet deposition. We tested an anatomically realistic manikin head which ejected fluorescent droplets of water and human volunteers, in speaking and coughing conditions without a face covering, or with a surgical mask or a single-layer cotton face covering. We quantified the number of droplets in flight using laser sheet illumination and UV-light for those that had landed at table height at up to 2 m. For human volunteers, expiratory droplets were caught on a microscope slide 5 cm from the mouth. Whether manikin or human, wearing a face covering decreased the number of projected droplets by less than 1000-fold. We estimated that a person standing 2 m from someone coughing without a mask is exposed to over 10 000 times more respiratory droplets than from someone standing 0.5 m away wearing a basic single-layer mask. Our results indicate that face coverings show consistent efficacy at blocking respiratory droplets and thus provide an opportunity to moderate social distancing policies. However, the methodologies we employed mostly detect larger (non-aerosol) sized droplets. If the aerosol transmission is later determined to be a significant driver of infection, then our findings may overestimate the effectiveness of face coverings.

Predictions of solute mixing in a weld pool and macrosegregation formation during dissimilar-filler welding of aluminum alloys: Modeling and experiments

The solute mixing phenomena of filler and base metal melts during dissimilar-filler metal-inert gas (MIG) welding are simulated by a multi-phase lattice Boltzmann (LB) model for predicting macrosegregation formation in the weld joint. The LB models incorporates the calculations of fluid field and solute transport, and the quantitative capalibity is validated through the problem of one-dimensional liquid diffusion. For the MIG welding of AA6063 sheets with an ER5183 filler wire, the simulation results reveal that the periodic impingement of heterogeneous filler metal droplets leads to symmetrical vortex flows accompanied by the unmixed zones formed in the center. These unmixed zones locate in the bulk weld pool and would develop into macrosegregation in the weld joint. In addition, the fluid flow near the fusion boundary remains almost stagnant, which gives rise to the appearance of the thereby continuous unmixed zone. The predicted distribution of unmixed zone in the weld pool agree well with the macrosegregation patterns in the weld joint observed in the experiments. For partially mimicking the effect of external magnetic field on droplet movement recorded by a high-speed camera, random droplet flight directions are arranged in the LB simulations. With the increase of droplet impact speed (Weber number) and the arrangement of random droplet flight direction, the simulated Mg concentration in the weld pool becomes more uniform, implying that the risk of macrosegregation formation is reduced. The LB simulations provides insight for better understanding the formation of weld defects during MIG welding with a dissimilar filler metal.

Understanding the formation mechanism of supersonic atmospheric plasma sprayed in-situ hypereutectic Al-25 wt%Si coating with nanostructured coupled eutectic: From powder, in-flight droplet, splat to coating

A comprehensive study on the formation mechanism of supersonic atmospheric plasma sprayed hypereutectic Al-25 wt%Si coating was performed by investigating microstructural evolution from powder, in-flight droplet, splat, to coating. Results show that feedstock Al-20 wt%Si powder exhibits sub-micron eutectic and micro-sized primary Si. Elements loss, surface oxidation, breakup and chemical diffusion occur when in-flight droplets are exposed to plasma jet. The splat exhibits an ideal structure of nano-sized coupled eutectic. The coating consists of featureless (~84.7 vol%) and ultrafine (~14.2 vol%) zones. Four Si morphologies coexist in the coating from scale of angstrom to sub-micron: Si atoms/clusters in supersaturated solid solution; round/rod-shape Si precipitations (~6.23 nm); nano-Si particulates (~34.37 nm) in featureless zone; and sub-micron Si particulates (~107.32 nm) in ultrafine zone. Further, physical/chemical changes mechanisms of in-flight droplets, rapid solidification mechanisms of splat and coating, formation mechanisms of featureless and ultrafine zones, precipitation and growth mechanisms of Si phase are extensively elucidated. Accordingly, the coating demonstrates ultra-high hardness (~259.8 HV0.1, better than previously reported values of hypereutectic Al-Si alloys with similar silicon content), and superior wear resistance. The unique microstructure with solid solution and grain refinement strengthening mechanisms, and in-situ fabricating higher Si-content Al-Si alloys contribute to the exceptional mechanical properties.

(Invited) Low Temperature Plasma Electrochemistry with Microscopic Liquid Droplets in Flight

When microscopic-sized liquid droplets travel through a low temperature RF plasma [1] at atmospheric pressure a number of remarkable and unexpected effects have been observed. After a short flight time, ~0.1ms, there is evidence that chemical reactions induced by the plasma and gas flux proceed at a rate that is significantly faster that observed in plasma – bulk liquid studies and many orders of magnitude faster than in standard bulk chemistry.[2] We suspect this is due to the complex interplay between droplet charge, electric fields, both internal and external to the droplet, and high chemical fluxes arriving at the droplet surface.There exists a large potential to develop new plasma-liquid processes for medical, chemical, biological, environmental and materials applications, among others and we can highlight some unique features of the plasma – microdroplet system that may provide opportunities for exploitation, namely: (i) a controlled ambient environment, (ii) a large surface area to volume ratio, (iii) small volume, (iv) low droplet temperature, (v) in-flight chemical synthesis and encapsulation, and (iv) remote delivery. These features offer the possibility of delivering high fluxes of active chemical species and nanoparticles remotely and on demand for applications in, for example, plasma-medicine, agriculture and microreaction while keeping the plasma itself at a safe distance. We have measured reactive oxygen species (ROS) flux variation with distance, up to 150 mm beyond the plasma, along with its effect on bacterial cell viability, DNA and amino acids.We have investigated plasma interactions with single cells, each transported in its own droplet. We have used the individual droplets as chemical microreactors to produce nanoparticles in flight, at rates many orders of magnitude higher that via high energy radiolysis or chemical synthesis. These measurements form the basis for numerical simulation in the gas-plasma and liquid droplet phases. New measurement techniques, based on recently acquired facilities, are being investigated. These include mid-IR absorption studies of droplets and their environment in flight, using tunable supercontinuum and quantum cascade lasers, and freezing plasma-treated droplets in flight for in-situ transfer to XPS surface chemical analysis. Current theories of microparticle charging in a collisional plasma environment are very limited. While in-flight charge measurements represent a significant challenge, the relatively large size of the droplet (10 – 20 μm diameter) and the limited evaporation over the flight time, offer the prospect of using droplets as a spherical probe to develop enhanced collisional probe theories in the regime where the particle size is greater than Debye lengths or mean free paths. In-flight measurements indicate a minimum net charge of 105 electrons, considerably higher than that obtained by other charging methods. Analytical – numerical and finite element simulations, in tandem with charge measurements, are being developed to better understand the droplet electrical environment and ultimately to link chemistry and charge in a consistent framework.

Hydrodynamics of formation of a microdispersed spray by the cup rotary atomizer

This article is devoted to the study of the movement of a single drop inside an air-liquid jet formed by a cup atomizer. The relevance of the research topic is proved. The problem, which has been identified, is about using modern fan sprayers for chemical protection of agricultural crops, in particular, orchards and vineyards. As for mechanical spraying, the process of movement of the droplet inside the torch, the trajectory of droplet flight, as well as the dependence of these indicators on the parameters of the rotary atomizers are currently very little studied. Therefore, this article considers the laws describing the movement of one droplet on the surface of the rotating working element and one droplet flight in atmospheric air after escape from the surface of the rotary atomizer’s cup. Research results are presented as the equations describing the dependence of the torch boundaries on the parameters of the rotary cup atomizer.

Impact and Adhesion of Surfactant-Amended Water Droplets on Leaf Surfaces Related to Roughness

Highlights The relationship between impacting droplet deposition and leaf surface roughness was examined. Complete deposition occurred on smooth leaves with roughness heights shorter than 1.5 micrometers. Droplet deposition on rough leaves was best achieved at low initial travel speeds and high surfactant concentrations. This research provides understanding of increasing droplet adhesion on plants for pesticide spray applications. Abstract. Understanding the effects of the microscopic-scale surface roughness of targeted leaves on spray droplet retention and spread can help develop optimal strategies to improve pesticide application efficiency. Improved spray deposition would minimize the amounts of chemicals to be applied and reduce off-target losses and production costs while protecting the environment. A 3D optical surface profiler was used to measure the arithmetic mean roughness height (Sa), providing a reliable metric for microscopic-scale leaf surface roughness. The relationship between leaf surface roughness and droplet deposition was examined by correlating droplet adhesion with the Sa of six leaf types for spray solutions at five surfactant concentrations (0.0% to 0.75%) and eight initial droplet horizontal travel speeds (0.5 to 4.5 m s-1). Leaf roughness and wettability, in terms of Sa and contact angle (θc), ranged from smooth and easy-to-wet to rough and difficult-to-wet (37° 1.6 μm), deposition decreased with increasing Sa and initial droplet horizontal flight velocity, while it increased as surfactant concentration increased. Thus, spray deposition on rough leaf surfaces could be greatly improved with slow initial droplet flight speeds and high surfactant concentrations. In contrast, greater deposition on smoother leaf surfaces could be achieved at any initial droplet flight velocity and with low concentrations of non-ionic surfactant, or possibly none.

An Image-Processing Method for Extracting Kinematic Characteristics of Droplets during Pulsed GMAW

Pulsed gas metal arc welding (GMAW) is widely applied in industrial manufacturing. The use of pulsed GMAW was found superior to the traditional direct-current (DC) welding method with respect to spatter, welding performance, and adaptability of all-position welding. These features are closely related to the special pulsed projected metal transfer process. In this paper, a monitoring system based on a high-speed camera and laser backlight is proposed. High-quality images with clear droplets and a translucent arc can be obtained at the same time. Furthermore, a novel image-processing algorithm is proposed in this paper, which was successfully applied to remove the interference of the arc. As a result, the edge and region of droplets were precisely extracted, which is not possible using only the threshold method. Based on the algorithm, centroid coordinates of undetached and detached droplets can be calculated, and more parameters of the kinematic characteristics of droplets can be derived, such as velocity, acceleration, external force, and momentum. The proposed monitoring system and image-processing algorithm give a simple and feasible way to investigate kinematic characteristics, which can provide a new method for possible applications in studying mathematic descriptions of droplet flight trajectory and developing a precise automatic welding system.

Defects in parts manufactured by selective laser melting caused by δ-ferrite in reused 316L steel powder feedstock

The presence of δ-ferrite in 316L stainless steel powder reused several times contributes to structural defect formation in selective laser melted parts built using the pin support structure. The virgin 316L stainless steel powder is fully austenitic. After several powder reuse cycles, reused powder has a finer particle size and about 6 vol. % of δ-ferrite. Phase change occurs due to the thermal cycles imposed on the particles near the melt pool, via spattering and further interaction of in-flight droplets with the laser beam. Phase transformation changes the magnetic behavior of the powder leading to particle clustering in the powder bed. The uniformity of the powder bed is affected causing defects such as porosity, delamination, warping and lack of fusion. These defects are more prone to occur at the beginning of the building process. The magnetic and non-magnetic fractions of the reused powder were separated from each other using magnetic separation. Powder characterization was performed using scanning electron microscopy, laser scattering particle size analysis, X-ray diffraction, and magnetization measurements. An explanation for the formation of such defects based on the magnetic behavior of δ-ferrite powder particles is proposed. The results suggest that magnetic separation should be used to remove magnetic particles after several reuse cycles.

Phase field simulation of dendrite growth in gas atomized binary Al–Ni droplets

Catalytic powders with fine microstructures can be produced from a rapidly solidified gas atomized Al–Ni alloy. The solidification microstructure of the droplets is closely linked with heat flow conditions. Thus, the heat transfer conditions between the gas and droplet are essential to microstructural evolution. In this study, a phase field model for simulating single and multiple dendrite growth in a binary Al–Ni alloy was constructed and the microstructural evolutions occurring during a gas atomization process were evaluated. Temporal variations in the heat transfer coefficient and the boundary heat flux were taken into account. The results revealed that the heat transfer coefficient of the atomized droplet in flight is correlated with the droplet size and the relative velocity between the droplet and the atomizing gas. During the simulation, the competition between boundary heat flux extraction and latent heat release from phase transition causes a recalescence process in thermal history, thereby affecting the gradient temperature distribution and, consequently, the dendrite morphology. Dendrite growth under the effects of the heat transfer coefficient is restrained continuously because of the decreasing amount of extraction. The computational results confirmed the fine homogeneous microstructure and low microsegregation levels of gas atomized powders.

Characterization of thermal inkjet droplets jitter

Due to the randomness in the nucleation process of vapor bubbles and the instability of ink drop breakup in the thermal inkjet drop ejector, the drop velocity varies slightly from drop to drop, such a variation is called the jitter of the droplets. Droplet jitter causes variations in drop volume, satellite drop formation, and drop placement error on the receiving substrate, which limit the high accuracy printing application for thermal inkjet. Traditionally, a multiple-exposure method is used to photograph the droplets in flight, which is unable to measure jitters of every drop due to the low sampling rate. In this study, a laser-based high-speed flying drop measurement system is used to study the jitter. We tentatively propose a simple model to explain the cause of droplet jitter, which is related to the latency phenomenon, the randomness in droplet formation and the difference in droplets velocity. The jitter is recorded under different driving conditions and the distribution of jitter is analyzed. As a conclusion, the conditions for operation to achieve lower jitter are determined. Under certain driving conditions, the jitter can be reduced by 90%.

Numerical analysis of in-flight freezing droplets: Application to novel particle engineering technology

The freezing of a stream of free-falling monodispersed droplets was simulated through the development of numerical models in this work. Prediction of the freezing time and temperature transition of a single droplet is beneficial for optimisation of novel continuous spray freeze drying (cSFD) processes. Estimations of the vertical free-falling distance of the droplets in a slip stream and predictions of the chances of droplet coalescence greatly enhance process understanding and can be leveraged to direct equipment design and process development. A design space of droplet diameters in the range from 100 μm to 400 μm and ambient temperatures from −120 °C to −40 °C was explored. The rate of supercooling within the design space was predicted to range from 48 to 830 °C s−1 depending on the ambient temperature and droplet size. A comparison of the vertical free-falling distances of solitary droplets and streams of droplets at different temperatures showed that the terminal velocity of a vertically falling stream of droplets is always in excess of the terminal velocity of a solitary droplet of the same size. A difference of 1.35 m was predicted for the free-falling distance of a 400 μm droplet compared to a stream of droplets at −42 °C. A comparison between flow rates for consecutively generated 100 μm droplets showed that droplet coalescence was predicted at 0.05 Lh−1, whilst at 0.02 Lh−1 a separation distance of 23 μm was maintained thus preventing coalescence.

Formation mechanisms of surfaces for osseointegration on titanium using pulsed laser spattering

Accelerated bone grow (osseointegration) can be achieved by modifying the surface of medical implants. For this purpose, pulsed lasers can be used to successfully texture such beneficial surfaces on titanium, e.g. a BioHelix™ structure. This surface typically includes ridges and droplets with a size range between 1 and 20 μm. This paper presents the results of an experimental program where a range of laser parameters was used to create different surface textures on titanium substrates, using pulsed laser spattering. The resultant surfaces are analysed by scanning electron microscope and X-ray Micro Computer Tomography. It is shown that optimisation of the laser parameters results in a robust process which produces a surface that has proven to be beneficial for osseointegration. The results are also deeper analysed, explaining how different types of surface are created by the laser-material interaction under different conditions. Further, droplet flight distances and the formation of the spongeous nano-scale surface that characterizes the surface structure depends on very fast cooling and is also evaluated.

Impact interaction of in-flight high-energy molten volcanic ash droplets with jet engines

The turbine technology incorporated in jet engines is inherently vulnerable to attack by environmental silicate debris. Amongst the various kinds of such debris, volcanic ash is a particular threat as its glass softens to a liquid at temperatures of 500–800 °C, far below jet engine operating temperatures of ∼1500 °C. As a result, ingested re-molten droplets impact and form splats on the protective thermal barrier coatings (TBCs). Investigation of the damage to jet engines ensuing from this process has, to date been restricted to forensic observations after critical encounters. Here, we employ a thermal spray technology to recreate the ‘in-situ’ generation of molten volcanic ash droplets and observe their morphological evolution and interaction with TBCs. The mechanism of splat formation is found to depend both on substrate topography and on in-flight droplet characteristics, whereby splat circularity increases with surface roughness and with the product of the Weber and Reynolds numbers. The experiments reveal that the molten ash droplet adhesion rate is dictated by droplet temperature and viscosity, ash concentration and substrate roughness. A new dimensionless number, S, is developed to quantify the molten ash droplet adhesion rate to both substrate topography and in-flight droplet characteristics. These findings provide a greatly improved basis for the quantification of the hazard potential of volcanic ash to jet engines and should be incorporated into protocols for operational aviation response during volcanic crises.

Flattening and solidification behavior of in-flight droplets in plasma spraying and micro/macro-bonding mechanisms

The microstructure and bonding strength of plasma sprayed coating are highly dependent on flattening and solidification behavior of in-flight droplets. In the present study, the flattening and solidification behavior of subsonic/supersonic droplets after impacting on a flat substrate was comparatively studied. Results show that a dimensionless Somerfeld number (K) of flattening droplets can be used as the splashing threshold. Due to the extremely rapid spreading and solidification of supersonic-droplet, some large-sized bubbles and grains that usually formed at the bottom of flattened subsonic-droplet were effectively restrained, leading to the improvement of splat/substrate micro-shear strength. The effect of grain size and growth feature on the macro-bonding property was further elaborated. It was found that the simultaneous increase of both velocity and temperature of in-flight droplets was an effective approach for the improvement of bonding strength of as-sprayed coating.

Comprehensive studies of air-brush spray deposition used in fabricating high-efficiency CH3NH3PbI3 perovskite solar cells: Combining theories with practices

Organic-inorganic perovskite solar cells (PSCs) are among the most attractive and efficient emerging thin film photovoltaic technologies. The perovskite films have been fabricated by lots of deposition methods, including one-step/two-step spin coating, thermal evaporation and vapor assisted processes. However, the laboratory-based fabrication methods are not well matched with large area manufacture for actual use. Spray coating deposition technique is being explored to prepare perovskite films to break the bottleneck plaguing large scale production. Herein, a comprehensive study is conducted in fabricating perovskite films by air-brush spraying method. The spraying process is classified into three stages, namely, 1) atomization process, 2) droplet flight and 3) film deposition. Each of them is discussed in detail from principles to parameters. Integrating the three aspects above, high quality perovskite films comparable to spin coated films are obtained. A best power conversion efficiency (PCE) of 16.68% is achieved for planar structure PSCs, which is prominent among the PSCs fabricated by spraying method. In addition, devices with spraying method show excellent optical and electrical properties. The principles we discuss here could be applied in the actual condition for mass production.