High-speed Droplet Research Articles

Movement Characteristics of Droplet Deposition in Flat Spray Nozzle for Agricultural UAVs

At present, research on aerial spraying operations with UAVs mainly focuses on the deposition outcomes of droplets, with insufficient depth in the exploration of the movement process of droplet deposition. The movement characteristics of droplet deposition as the most fundamental factors affecting the effectiveness of pesticide application by UAVs are of great significance for improving droplet deposition. This study takes flat spray nozzles as the research object, uses the Particle Image Velocimetry (PIV) technique to obtain movement data of water droplet deposition under the influence of rotor flow fields, and investigates the variation characteristics of droplet deposition speed under different influencing factors. The results show that the deposition speed and the distribution area of high-speed (>12 m/s) particles increase with the increase of rotor speed, spraying pressure, and nozzle size. When the rotor speed increases from 0 r/min to 1800 r/min, the average increase in maximum droplet deposition speed for nozzle models LU120-02, LU120-03 and LU120-04 is 33.26%, 19.02%, and 7.62%, respectively. The rotor flow field significantly increases the number of high-speed droplets, making the dispersed droplet velocity distribution more concentrated. When the rotor speed is 0, 1000, 1500, and 1800 r/min, the average decay rates of droplet deposition speed are 36.72%, 20.00%, 15.47%, and 13.21%, respectively, indicating that the rotor flow field helps to reduce the decrease in droplet deposition speed, enabling droplets to deposit on the target area at a higher speed, reducing drift risk and evaporation loss. This study’s results are beneficial for revealing the mechanism of droplet deposition movement in aerial spraying by plant protection UAVs, improving the understanding of droplet movement, and providing data support and guidance for precise spraying operations.

Pressure and wall shear stress from high-speed droplet impact

We report on experimental and numerical results of the impact of millimeter sized droplet with high-speed projectile. The impact velocity Vimp ranges between 70m/s and 245m/s and is sufficiently high that the compressibility of the liquid becomes important. High-speed images reveal non-axisymmetric lamella spreading, hydrodynamic and secondary cavitation and the ejection of a thin jet from the distal side of the droplet. The spreading velocity is supersonic for the impact velocities tested. Secondary cavitation at the distal side of the droplet is found for Vimp≳ 120m/s. The experiments are compared and analyzed further with a CFD model based on a compressible volume of fluid (VoF) method. Excellent agreement of the early droplet and lamella dynamics is obtained with the experimental data. Additionally, we provide quantitative data for the pressure loading and the shear stress on the projectile using spatio-temporal maps.

Study on the microscopic characteristics of pentanol/highly active fuel spray based on high-speed droplet tracking velocimetry technology

Pentanol blending with highly reactive fuels for internal combustion engines holds considerable promise. The atomization of sprays closely influences the efficiency and combustion characteristics of the system. Based on Particle Tracking Velocimetry and high-speed shadow imaging technology, we propose a method termed High-speed Droplet Tracking Velocimetry for analyzing spray particle size. Prior to conducting our study, we validated the reliability of High-speed Droplet Tracking Velocimetry through comparisons with the Malvern and Phase Doppler methods. This study utilized long working distance micro high-speed shadow imaging technology in a visualized constant volume combustion chamber to investigate the spray micro-characteristics of pentanol blended with highly reactive fuels, including Fatty Acid Methyl Ester, Hydrogenated Catalytic Biodiesel, and Diesel. The results indicate that under all operating conditions, the droplets of P20H80 (the volume fraction of 20 % n-pentanol mixed with 80 % Hydrogenated Catalytic Biodiesel) are smaller and more uniformly distributed, demonstrating the minimum Sauter Mean Diameter, characteristic diameter, and width of droplet size distribution. The droplet velocities of P20H80 are the lowest, followed by P20Di80 (the volume fraction of 20 % n-pentanol mixed with 80 % Diesel), and P20B80 (the volume fraction of 20 % n-pentanol mixed with 80 % Fatty Acid Methyl Ester) exhibits the highest velocities. These velocity differences among the three fuels generally remain within 10 %-20 % across various injection pressures. P20Di80 shows the highest Reynolds and Weber numbers, followed by P20H80, with P20B80 having the smallest values due to its intermediate physical properties. The gaps between each fuel type range from 20 % to 30 %.

Study on the influence of collision conditions on the surface morphology of compound droplets

In this study, a three-dimensional compound droplet collision numerical model is established by using volume of fluid. The morphological evolution of compound hollow droplets affected by high-speed solid droplet was studied in detail. Parameterized analysis is conducted on the velocity VS, center distance ϕ, and diameter ζ of high-speed small droplets. Through the analysis of the compound droplets flow field, it is found that the broken mode of compound droplets is caused by the increase in Pn (dimensionless pressure) and θ (velocity angle). The results show that the surface Pn of compound droplets is positively correlated with the velocity VS of high-speed small droplets, while there is a more complex relationship with the dimensionless center distance ϕ and dimensionless diameter ζ. When the values of ϕ and ζ are appropriate, Pn can reach its maximum value. The broken mode of compound droplets can be divided into three categories: shear deformation, shear crushing, and violent crushing.

Droplet manipulation of smart ferrofluid on covalently grafted slippery surface

Remote manipulation of droplets plays an important role in fields such as energy, bioanalysis and microfluidics. Despite efforts to develop many surfaces that respond to external stimuli for droplet manipulation, not all surfaces are responsive in practical applications, making droplet manipulation difficult to achieve. Manipulating droplets on unresponsive surfaces presents a formidable challenge. Our study unveils a covalently grafted slippery surface (CGSS) that is capable of proficiently manipulating ferrofluid droplets in the presence of a magnetic field. By adjusting the strength of the magnetic field applied, a horizontal CGSS can achieve efficient, long-range, and high-speed droplet transportation. On the tilted CGSS, droplets can be transported and aggregated against gravity. Magnetic field guidance allows for droplets to be programmatically guided along a desired trajectory. This research offers feasible strategies for developing new methods to manipulate droplets, which are anticipated to have significant applications in fields like microfluidics, drug delivery, and intelligent robotics.

Simulation and analytical modeling of high-speed droplet impact onto a surface

The fluid dynamics of liquid droplet impact on surfaces hold significant relevance to various industrial applications. However, high impact velocities introduce compressible effects, leading to material erosion. A gap in understanding and modeling these effects has motivated this study. We simulated droplet impacts on solid surfaces and proposed a new analytical model for impact pressure and droplet turning line, targeting at predictions for enhanced cavitation. The highly compressed liquid behind the droplet expands sideways, causing lateral jetting. As the droplet encounters a shock wave, it reflects as a rarefaction wave, leading to low-pressure zones within the droplet. These zones converge at the droplet's center, causing cavitation, which, upon collapse, induces another shock wave, contributing to erosion. Using the well-established model for the low-velocity impact shows a significant discrepancy. Hence, an analytical model for the turning line radius is introduced, incorporating the lateral jetting's characteristic length scale. Comparing our model with existing ones, our new model exhibits superior predictive accuracy.

Hybrid Smoothed-Particle Hydrodynamics/Finite Element Method Simulation of Water Droplet Erosion on Ductile Metallic Targets

Erosion of metallic surfaces due to the permanent impact of high-speed water droplets is a significant concern in diverse industrial applications like turbine blades, among others. In the initial stage of water droplet erosion, there is an incubation regime with negligible mass loss whose duration is strongly dependent on water droplet sizes and velocities, the initial state of the surface, and the material properties of the target. The prediction of the incubation period duration is one of the main topics of research in the field. In this work, the interaction of the water droplets with a metallic surface is simulated using a hybrid Smoothed-Particle Hydrodynamics/Finite Element Method modeling scheme. The effect of multiple random impacts on representative target areas for certain ranges of impact angles and velocities was studied using a combination of simple material and damage models for the target surface of Ti-6Al-4V titanium alloy. The simulation is able to reproduce the main dependencies of the incubation regime and the first stages of water droplet erosion on the impact angle and velocity as reported in the literature. This framework can be considered a foundation for more advanced models with the goal of a better understanding of the physical mechanisms behind the incubation regime in order to devise strategies to extend it in real applications.

Compressible multicomponent flow simulations and data-driven modeling of high-speed liquid droplet impingement

Accurate prediction of the force exerted on a droplet during high-speed liquid droplet impingement (LDI) is challenging yet important for the safety management of nuclear/fossil power plants. This paper describes compressible multicomponent flow simulations and data-driven modeling of high-speed LDI. The compressible multicomponent flow simulations are conducted for various impact Mach numbers ranging from Ma = 0.045 to 0.077. Whereas the peak force associated with the water hammer shock exhibits dependency on the impact Mach number, a self-similar structure for the pressure field within the small region about the impact plane is observed during high-speed LDI. An important observation is that the nondimensional mass-averaged droplet velocity does not vary with the impact Mach number. Finally, a data-driven LPP approach is presented to obtain fast and highly realistic simulations of high-speed LDI.

Influence of aerodynamic pressure on dust removal by supersonic siphon atomization

At present, there are many problems in coal mine operation, such as formation of large amounts of dust, complex dust migration behavior, severe respiratory harm, poor on-site performance of spray dust removal technology under windy conditions, and low collection efficiency of respirable dust. To solve the above problems, the finite element method of computational fluid dynamics (CFD) was used. The Spalart-Allmaras turbulence model of COMSOL software and the tracking module for atomized particles broken by droplets were used. They were combined with the Fraunhofer diffraction principle to study the distributions of the velocity of the airflow field, droplet size, droplet velocity and wind resistance for a supersonic water siphonic atomization device. The influence of the above factors on dust removal at different aerodynamic pressures was studied experimentally. The relationships between spray characteristics, flow field velocity distribution, dust removal effect and spray characteristics were revealed. The research showed that with increasing pressure, the supersonic component accounted for an increased proportion of the flow field inside the nozzle, average velocity of the flow field increased, atomization efficiency increased, droplet particle size decreased, and droplet velocity increased. The particle size of the droplets first increased, then decreased and then increased with increasing spraying distance. Additionally, the range of high-speed fine droplet size fog increased with increasing pressure. At the same transverse wind speed, with increasing pressure, the deflection point of the fog field moved backward, and the wind resistance of the spray was enhanced. With the increase in aerodynamic pressure, the dust isolation and negative pressure effect of spray and the dust collection efficiency increased, and there was an optimal value for dust removal in a limited space. The reason was that when the pressure increased to a certain extent, the air flow reversed in the coil at the bottom of the limited space, which blew the dust that was not aggregated at the bottom away from the area where dust was falling. Thus, the dust was entrained in fog droplets but could not be removed from the air flow. The dust dispersion in the air flow after purification differed greatly at different pressures because the removal efficiency of dust in the 2.5–10 μm particle size range was mainly affected by the pressure. In a field study of a fully mechanized mining face, the fog curtain formed by the device covered the whole section under the condition of airflow disturbance with large transverse velocity, and the comprehensive respirable dust removal efficiency of the working area reached 92.3%. Thus, this method showed good wind resistance and dust removal performance. The study of this technology can provide a clean working environment and theoretical guidance for the safe production of coal in the future, especially for fully mechanized mining faces.

Spreading of high-speed tin droplets on copper substrate surfaces with ultrasonic vibration: Experiments and modeling

The paper studied the spreading of pure tin droplets on a copper substrate that was ultrasonically vibrated at a frequency of 28 kHz. High-speed photography was used to capture the complex spreading process. The results showed that the ultrasonic vibration improved the wetting of the molten tin droplets on the copper substrate when the substrate temperature was above the tin melting point. The effect of ultrasonication was diminished when the substrate temperature was below the tin melting point. The study revealed the differences in the spreading behavior of molten tin droplets on ultrasonically vibrated copper surfaces and explained the reasons for the differences from the perspective of the energy changes generated by sonication. The ultrasonic action first affects the kinetic energy, then the surface energy, and finally the adhesion energy of the molten droplets. This work provides a theoretical basis for ultrasonic-assisted molten-drop deposition techniques and can be used in technologies such as jet deposition, additive manufacturing, and microcircuit printing.

Splashing of tungsten-based anode during arc discharge

A unique mechanism of splashing from a tungsten-based anode was identified during arc discharge. Splashing occurred by breakoff of a liquid metal column, which elongates after a local concavity formed on the molten anode surface. Blue–violet luminescence, emitted by cerium ions originating from additives in the tungsten-based anode, was captured before the concavity formation. The surface temperature exceeded the boiling point of the additives at the time of splashing. The measured droplet speeds suggested that an electromagnetic force contributes the high-speed ejections. Energy dispersive spectrometry mapping also exhibited a remnant of the additives on the longitudinal cross-section of the anode after arc discharge. Based on these experimental facts, the mechanism of anode splashing in arc discharge was deduced as follows: bubble formation of additives at temperatures above their boiling point, bubble bursting at the surface, micro-plasma jet generation, liquid-column elongation and breakoff under an electromagnetic force, and consequent high-speed droplet ejection.

Elucidating the immunological landscape of acute myocardial infarction using an integrated single-cell enzyme and antibody quantification (iSEAQ) assay

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): National Research Foundation Background Characterizing enzyme secretion with surface antigen expression of circulating leukocytes provides deep immunological insights that may benefit diagnostics and therapeutics of acute myocardial infarction (AMI). Current methods are limited by laborious sample preparation, lengthy assay time and limited analytical throughput. Purpose We aimed to develop a point-of-care platform for real-time, granular immune profiling of AMI during index hospitalization. Methods We developed a system for a 40-min assay termed integrated single-cell enzyme and antigen quantification (iSEAQ), consisting of an integrated microfluidic device for a streamlined process to recover and isolate single leukocytes from whole blood, and a customized 5-channel high-speed droplet fluorescence cytometer for sensitive quantification of key protein activity resembling live leukocyte activity in-vivo (Figure 1). Using a panel of 2 secreted enzymes (granzyme B (GzB) and neutrophil elastase (NE)) and 3 surface antigens (cluster of differentiation (CD) 66b, CD3 and CD31), we tracked the longitudinal shift of immune functionality of 9 patients admitted for acute ST-segment elevation myocardial infarction (STEMI) patients (median age 52 years, 100% males) at 3 timepoints during the index hospitalization: 1) on admission prior to coronary revascularization, 2) within 24 hours of revascularization, and 3) pre-discharge. The 5-channel fluorescence data were imported using a python script and pre-processed, before piping into Numpy, Pandas, Scipy and UMAP library for data preparation and classification. Seaborn and Matplotlib were used for all data visualization and plots. Data was analyzed using student two-tailed t-tests, with a significance level of <5%. Results Leveraging on its ability to dissect enzyme secretion by phenotype class, iSEAQ unraveled distinct contributions of CD31, NE and GzB from peripheral granulocytes, T lymphocytes and other cells (Figure 2). The majority immune response is expressed in CD66b+ granulocytes showing increased NE+ secretion and CD31 surface antigen expression denoted in Class 0 in Figure 2. Compared to samples from healthy donors and STEMI patients on admission, the NE+ and CD31+ expressions were significantly lower in pre-discharge samples of STEMI patients (p<0.05). iSEAQ also distinguished anterior from inferior STEMI, as evident from a significantly elevated NE+CD31-secretion for CD66b+ granulocytes in anterior STEMI (365±70 vs 83±35 fg, p < 0.001), showing preliminary promise in the ability of iSEAQ to risk stratify. Conclusion iSEAQ provided a comprehensive granular immune atlas of the longitudinal immune responses to STEMI during hospital admission. With its open design for assay panel using commercial biosensors, iSEAQ is a potentially powerful tool for taking functionality snapshots of immune system in many diseases.

High speed water droplet impact erosive behavior on dry and wet pulsed waterjet treated surfaces

During water droplet impact onto a dry or wet rough solid surface, several phenomena affect the surface erosion process, such as splashing, crown formation, and small droplet emission to name a few. These phenomena have been extensively studied for various simple target surface geometries. However, droplet impact studies on complex irregular and asymmetric target surface topographies resulting from a waterjet treatment have never been conducted. Furthermore, very limited reports are found on the role of target surface topography and water droplet deformation development on the resulting target stress state. In the present study, high speed droplet impingements on surfaces exhibiting coarse topographical features associated with ultrasonic pulsed waterjet treatment are modeled to understand the underlying mechanisms causing erosion. Impacts on surfaces with various roughness values and water film thicknesses are modeled using a three-dimensional coupled Eulerian–Lagrangian approach. A detailed comparative analysis of the model with experimental ultrasonic pulsed waterjet erosion features and material loss is provided. It was found that the synclastic curvature of the modeled coarse surface features increases the shock wave's strength as many compression wavelets are simultaneously emitted at each water droplet contact location with the surface, resulting in concentrated high-pressure zones. The ultrasonic pulsed waterjet treated surface features and water film thickness also greatly influence the onset of water droplet splashing, subsequent finger, secondary droplet characteristics, and crown stability. According to the numerical results, strong splashing patterns and droplet breakup are generated and create high stress zones capable of accelerating surface erosion, explaining the enhanced performance of ultrasonic pulsed waterjet process.

Coacervate-Enhanced Deposition of Sprayed Pesticide on Hydrophobic/Superhydrophobic Abaxial Leaf Surfaces.

Deposition of high-speed droplets on inverted surfaces is important to many fundamental scientific principles and technological applications. For example, in pesticide spraying to target pests and diseases emerging on abaxial side of leaves, the downward rebound and gravity of the droplets make the deposition exceedingly difficult on hydrophobic/superhydrophobic leaf underside, causing serious pesticide waste and environmental pollution. Here, a series of bile salt/cationic surfactant coacervates are developed to attain efficient deposition on the inverted surfaces of diverse hydrophobic/superhydrophobic characteristics. The coacervates have abundant nanoscale hydrophilic/hydrophobic domains and intrinsic network-like microstructures, which endow them with efficient encapsulation of various solutes and strong adhesion to surface micro/nanostructures. Thus, the coacervates with low viscosity achieve high-efficient deposition on superhydrophobic abaxial-side of tomato leaves and inverted artificial surfaces with a water contact angle from 170° to 124°, much better than that of commercial agricultural adjuvants. Intriguingly, the compactness of network-like structures dominantly controls adhesion force and deposition efficiency, and the most crowded one leads to the most efficient deposition. The tunable coacervates can help comprehensively understand the complex dynamic deposition, and provide innovative carriers for depositing sprayed pesticides on abaxial and adaxial sides of leaves, thereby potentially reducing pesticide use and promoting sustainable agriculture.

Optimization of CMT Characteristic Parameters for Swing Arc Additive Manufacturing of AZ91 Magnesium Alloy Based on Process Stability Analysis.

The droplet transfer behavior and stability of the swing arc additive manufacturing process of AZ91 magnesium alloy based on the cold metal transfer (CMT) technique were studied by analyzing the electrical waveforms and high-speed droplet images as well as the forces on the droplet, and the Vilarinho regularity index for short-circuit transfer (IVSC) based on variation coefficients was used to characterize the stability of the swing arc deposition process. The effect of the CMT characteristic parameters on the process stability was investigated; then, the optimization of the CMT characteristic parameters was realized based on the process stability analysis. The results show that the arc shape changed during the swing arc deposition process; thus, a horizontal component of the arc force was generated, which significantly affected the stability of the droplet transition. The burn phase current I_sc_wait presented a linear function relation with IVSC, while the other three characteristic parameters, i.e., boost phase current I_boost, boost phase duration t_I_boost and short-circuiting current I_sc2, all had a quadratic correlation with IVSC. A relation model of the CMT characteristic parameters and IVSC was established based on the rotatable 3D central composite design; then, the optimization of the CMT characteristic parameters was realized using a multiple-response desirability function approach.

Controlling High-Speed Droplet Splashing and Superspreading Behavior on Anisotropic Superhydrophobic Leaf Surfaces by Ecofriendly Pseudogemini Surfactants.

Efficient deposition of high-speed droplets on superhydrophobic leaf surfaces remains an important challenge. Especially for the anisotropic wired superhydrophobic leaf surface, the splashing phenomenon is more serious, which leads to the low effective utilization of pesticides by biological targets. The lost pesticides have caused serious ecological environment pollution. Therefore, it is urgent to develop a green and sustainable strategy in a cost-effective to achieve efficient deposition of high-speed droplets on anisotropic superhydrophobic leaf surfaces at low dosage. A kind of green pseudogemini surfactants is constructed based on fatty acids and hexamethylenediamine by electrostatic interaction to control the splashing and spreading of high-speed droplets on superhydrophobic surfaces. The formed surfactant can not only achieve completely inhibition of the bouncing of droplets, but also promote the rapid spreading on superhydrophobic leaf surfaces at a very low usage. The efficient deposition and superspreading phenomenon are attributed to the rapid migration and adsorption of the surfactant from the dynamic spherical micelles at the newly formed solid-liquid interface, the network-like aggregated spherical micelles, and the Marangoni effect caused by surface tension gradient. Moreover, the surfactant shows excellent synergistic effect with herbicides to control weeds by inhibiting droplets splashing. This work provides a more simple, effective and sustainable approach to utilize aggregated spherical micelles rather than conventional vesicles or wormlike micelles to improve the droplet deposition on superhydrophobic leaf surfaces and reduce the impact of surfactants and pesticides on the ecological environment. This article is protected by copyright. All rights reserved.

Deposition and Spread of Aqueous Pesticide Droplets on Hydrophobic/Superhydrophobic Surfaces by Fast Aggregation of Surfactants.

Deposition and spread of aqueous droplets on hydrophobic/superhydrophobic surfaces are of great significance in many practical applications, such as spraying, coating, and printing, and particularly in improving pesticide utilization efficiency because the intrinsic hydrophobicity/superhydrophobicity of most plant leaves results in serious loss of water-based pesticides during spraying. It has been found that proper surfactants can promote the droplet spread on such surfaces. However, most reports involved the effects of surfactants on the spread of the gently released droplets over hydrophobic or highly hydrophobic substrates, while the situation on superhydrophobic substrates has rarely been explored. Moreover, high-speed impact makes it extremely difficult to deposit and spread the aqueous droplets on superhydrophobic surfaces; thus, the deposition and spread have just been achieved by surfactants in recent years. Here, we give an overview concerning the influence factors on the deposition and spreading performance of gently released and high-speed impacted droplets on hydrophobic/superhydrophobic substrates and emphasize the effects of fast aggregation of surfactants at the interface and in solution. We also outline perspectives on the future development of surfactant-assisted deposition and spreading after high-speed impact.

Liquid–Solid Impact Mechanism, Liquid Impingement Erosion, and Erosion-Resistant Surface Engineering: A Review

Liquid impingement erosion has been known as mechanical degradation, where the original material is removed progressively from a solid surface due to continued exposure to impacts by high-speed liquid droplets. This is a major issue in many industries, including aerospace and aviation and power generation, particularly gas and steam turbines, nuclear power plants, and wind energy. Tremendous numerical and experimental studies have been performed so far to understand the physical phenomena involved in this process and to improve the erosion resistance of different surfaces. In this review paper, first, the liquid–solid impact in a wide range of relative velocities is reviewed fundamentally. Then, the liquid impingement erosion of metals, including damage regimes and damage accumulation mechanisms, as well as the role of solid properties on erosion performance are explained. Finally, promising water droplet erosion-resistant materials and surface treatments are discussed. This review paper is intended to summarize the present knowledge of the different mechanisms involved in the liquid impingement erosion process.

Effect of turbulence modulation caused by thread structure on coaxial air-blast atomization

This study aims at investigating the influence of turbulence modulation caused by a thread structure on coaxial air-blast atomization by means of high-speed flow visualizations and droplet particle size techniques. The medium in the central channel of an atomizer is water while the annular channel is airflow. The results show that the thread structure added to the inner surface of an annular channel plays an important role in atomization effect. The generated liquid ligaments on the jet present more structures, which grow shorter and breakup faster than that without thread. To compare the difference in jet breakup length and droplet diameter caused by the thread structure, we establish a new breakup length model and then use the ratio of structure factors to describe the change in the droplet diameter. The results in this experiment confirm the improvement of atomization performance brought by optimization design of the thread structure.

Surface durability during high-speed spray impingement onto heat-exchanger materials and modified surfaces

Surface durability was investigated for materials used in electronics and heat exchangers during water spray impingement at ambient conditions. Smooth, rough, and modified copper and aluminium samples were tested with surface hardness, roughness and contact angle ranging from 52–100 HV, 0.53–79.28 μm and 48-153°, respectively. Although material loss and hardness alterations were insignificant after the high-speed droplet collisions, changes in surface morphology were observed. Both smooth and rough surfaces were resistant to 36-hour sprays with droplet velocities from 11–22 m s−1. However, sprays did cause changes in surface roughness and static contact angle, and the water repellent coatings were damaged, losing their low wettability.