Jet Droplet Research Articles

Modeling for Formation of Microdroplets Prepared by Pulsated Orifice Ejection Method

Abstract This study investigated the preparation of metallic microdroplets by pulsed orifice ejection method. Computational fluid dynamics simulations, combined with volume-of-fluid model, were employed to analyze the jet break-up and droplet formation behaviors of various metallic materials within a certain Weber number (We) range. An energy-based physical model under inlet boundary conditions was established to precisely control the generation of droplets. The results indicate that the break-up state of metal jets is significantly influenced by material properties within the range that We=12~24, which makes the materials exhibiting higher surface tension and density relatively favorable to the production of mono-sized microdroplets. Moreover, the break-up behavior of the jet is closely related to the ratio of volume force to surface tension (Bond number). And the proposed model effectively predicts the properties of droplets. This research provides theoretical guidance and technical support for the precise preparation of metallic microdroplets.

Atmospheric pressure plasma jet interacting with a droplet on dielectric surface

Abstract The chemical processes at plasma–liquid interface has become a crucial point for plasmas’ various applications. In this study, the interaction between atmospheric pressure plasma jet and different-scale droplets were investigated by both experiments and modeling. The interaction transited from ‘annular’ mode to ‘solid’ mode when plasma involved with different size of droplets. As the droplet size increased, the high-field region moved from the plasma jet head to the gap between plasma jet head and droplet vertex surface. Additionally, the time averaged surface fluxes of the main active species were analyzed. For the flux of singlet oxygen (1O2), both small and medium-scale droplets reached the maximum value in the central region of the droplets, while for large-scale droplet, the maximum value was observed in the edge region of the droplet. This was due to the fact that, compared to small and medium-scale droplet, the edges of large-scale droplet are closer to the He–Air mixed boundary layer, where more oxygen molecules were provided in the gas environment, leading to enhanced electron collision reactions with oxygen molecules. The cause for these behaviors were also analyzed and discussed. This work shed light on the interaction mechanism for plasma–liquid interactions, which provides significant guidance for plasma medical or water treatment applications.

Preparation of protective coatings for the leading edge of wind turbine blades and investigation of their water droplet erosion behavior

The damage caused by rain droplet erosion to the leading edge of wind turbine blades is extremely severe. To reduce this issue, in this study, hydroxyl-terminated polybutadiene (HTPB) and isophorone diisocyanate (IPDI) were used as the polyurethane (PU) polyol and curing agent, respectively, to prepare a PU coating with a high resistance to water droplet erosion (WDE) for the protection of the leading edge of wind turbine blades. The effect of n (–NCO):n (–OH) (R-value, n is the molar ratio) on the mechanical properties and WDE resistance of PU coatings, the relationship between the two performances, and the influence of the erosion conditions on the WDE behavior were investigated for the first time. The results show the existence of a correlation between the mechanical properties (hardness, impact, flexibility, and tensile strength) and WDE resistance of the coating. While, a better abrasion resistance is found not to result in a better WDE resistance. The PU coating with an R-value of 1.2 shows an optimal WDE resistance in both atomization and jet erosion experiments. In atomized droplet erosion (ADE) experiments, the change in erosion velocity accelerates the incubation period of coating erosion damage and increases the erosion rate. In jet droplet erosion (JDE) experiments, the damage to the coating is closely related to the erosion angle, reaching a maximum at an impact angle of 60°. Furthermore, in the ADE experiments, various erosion morphologies, such as pits, grooves, cracks, and stepped texture, are observed on the damaged coating surface. While, regarding the JDE experiments, holes, grooves, and cracks are observed. Such damage is caused by the combined effects of water hammer pressure, lateral jets, and local permeation.

Experimental Investigation on Atomization of JP-10 Slurry Jets Containing Boron Nanoparticles

The present work aims to experimentally investigate the atomization of JP-10 based slurry jets containing boron nanoparticles. A coaxial airblast injector is used to atomize the slurry fuel. Different injector operating conditions are realized by varying the air and the fuel flow rates through the atomizer. For each case, different particle-to-fuel mass loading ratios ϕ (equal to 0, 5, 10, and 20%) are investigated. Time-resolved imaging is performed near the nozzle exit and 30Dl downstream of the injector exit to visualize the primary liquid jet breakup process and the resulting spray droplets, respectively. The instantaneous jet breakup length, droplet size, and velocity are obtained by processing the raw images. The influence of particle loading on the mean and fluctuations of jet breakup length, droplet size, and droplet size/velocity correlation is investigated in detail. The results highlight degradation in atomization quality for higher particle loading ratio in the base fuel. A monotonic increase in the mean jet breakup length with particle loading ratio is identified. The Sauter mean diameter increases by about 100% when ϕ is increased from 0 to 20%. It is found that ϕ must be accounted for, in addition to conventional nondimensional numbers, in the experimental correlations for jet breakup length and droplet size for nanofuel slurry.

Double emulsion generation in shear-thinning fluids under electric field effects

Double emulsions are encapsulated microstructures with medical, food, and cosmetics applications. Precise control over their parameters, such as volume, thickness, and size distribution, is crucial. This study numerically investigates, for the first time, the generation of double emulsions in a shear-thinning outer fluid under the influence of an external DC electric field. A dual co-flowing microfluidic device is chosen as the flow domain, and the shear-thinning behavior of the outer fluid is approximated by the Carreau–Yasuda rheological model. The governing equations are discretized on structured numerical grids using the VOF-CSF model and the finite volume method. The combined effects of the shear-thinning behavior and electric forces on compound droplet formation parameters, including volume, eccentricity, maximum thickness, and jet length, are studied carefully. Results show that for highly shear-thinning fluids, the application of electric forces can ensure the generation of desired compound droplets, which is not feasible without electric field effects. However, excessive electric forces can disrupt the droplet generation process, especially for moderate to low shear-thinning fluids. These findings provide new insights into the role of external electric fields in double emulsion generation and highlight their potential to optimize the production process.

Experimental and numerical studies of the processes of spraying coal-water slurries with pneumatic nozzle

The article presents the results of spraying tests of highly viscous coal-water slurries with a pneumatic nozzle and numerical studies of the structure of the gas-drop jet. An increase in the viscosity of coal-water slurries was ensured by introducing a third component into its composition – industrial waste (pyrogenetic liquid). The moderate effect of the slurry viscosity on the average droplet size of coal-water slurries in the jet is due to the high efficiency of the aerodynamic characteristics of the pneumatic nozzle. Introduction of pyrogenetic liquid in the amount of 20 % by weight into the composition of coal-water slurry significantly increases its viscosity. The value of the surface tension of such a slurry is more than doubled. The increase in the average droplet size was no more than 25 % when pyrogenetic liquid content in the composition of coal-water slurry was over 10 % by weight. The average droplet size of coal-water slurries varied in the range from 180 to 210 microns in the study area of gas droplet jet. The results of experimental and numerical studies have shown good convergence. High air velocities at short distances from the nozzle ensure high crushing efficiency of the jet of coal-water slurry. The breakdown of the boundary layer of the liquid from the periphery of the droplets of coal-water slurries and its subsequent destruction was observed at short distances from the nozzle. The values of the Weber number were 100<We<350. At considerable distances from the nozzle (more than 0.5 m), droplets of coal-water slurries were subjected to vibrational crushing (We<50).

Dynamic Cleavage-Remodeling of Covalent Organic Networks into Multidimensional Superstructures.

Superstructures with complex hierarchical spatial configurations exhibit broader structural depth than single hierarchical structures and the associated broader application prospects. However, current preparation methods are greatly constrained by cumbersome steps and harsh conditions. Here, for the first time, a concise and efficient thermally responsive dynamic synthesis strategy for the preparation of multidimensional complex superstructures within soluble covalent organic networks (SCONs) with tunable morphology from 0D hollow supraparticles to 2D films is presented. Mechanism study reveals the thermally responsive dynamic "cleavage-remodeling" characteristics of SCONs, synthesized based on the unique bilayer structure of (2.2)paracyclophane, and the temperature control facilitates the process from reversible solubility to reorganization and construction of superstructures. Specifically, during the process, the oil-water-emulsion two-phase interface can be generated through droplet jetting, leading to the preparation of 0D hollow supraparticles and other bowl-like complex superstructures with high yield. Additionally, by modulating the volatility and solubility of exogenous solvents, defect-free 2D films are prepared relying on an air-liquid interface. Expanded experiments further confirm the generalizability and scalability of the proposed dynamic "cleavage-remodeling" strategy. Research on the enrichment mechanism of guest iodine highlights the superior kinetic mass transfer performance of superstructural products compared to single-hierarchical materials.

Longitudinal transducer based membrane resonant ejector for high-viscosity and high-frequency micro-droplet jetting

Aiming at the requirement for high-precision on-demand droplet jetting of high-viscosity industrial inks, a longitudinal-transducer-based membrane resonant droplet ejector (LT-MRDE) is designed and fabricated in this work. The jetting and observing system of LT-MRDE is set up to quantitatively explore the variations of droplet volume and velocity with driving voltage. Moreover, the three-dimensional (3D) multi-relaxation-time (MRT) lattice Boltzmann (LB) model coupled immersed boundary (IB) is employed to elucidate the effects of driving voltage and liquid viscosity on droplet characteristics. Experimental results demonstrate that the LT-MRDE could stably eject the silicone oil with a viscosity of 100 mPa·s from ∼50 μm vibrating orifice. Furthermore, both the volume and velocity of droplet exhibit a proportionality to voltage and an inverse proportionality to viscosity. The optimal droplet volume and velocity of 100 mPa·s silicon oil are 67.27 pL and 9.07 m/s with the driving parameters of 75 Vpp/14.754 kHz. With high jetting frequency and controllable droplet volume, LT-MRDE provides a new alternative for on-demand precision jetting of high-viscosity industrial inks.

Experimental study on the spray characteristics of an internal-mixing gas-liquid injector

To determine the spray characteristics of internal-mixing gas-liquid injector under various operating conditions, the experimental facilities for spraying are established. By means of high-speed camera, laser particle size analyzer and PIVlab, the effects of gas and liquid flow conditions on the spray characteristics, such as position of jet impingement, spray angle, breakup length, velocity distribution and droplet size, have been investigated in detail. Experimental results demonstrated that there are two types of atomization for this injector, namely impact atomization and aerodynamic atomization. The impingement position of the liquid jets is outside of the injector exit. Alternatively, no impingement occurs between the jets. The spray angle increases with the increase of liquid flow rate, and decreases with the increase of gas flow rate. The breakup length decreases with the increase of the liquid or gas flow rate. The higher the liquid flow rate, the greater the velocity distributed along the axial and radial direction. However, the effect of gas flow rate on the velocity along the axial and radial directions acts differently at different spatial position. An increase in the liquid flow rate causes the droplet diameter to increase first and then decrease. The droplet size will decrease when the gas-liquid momentum ratio increases.

Data-driven surrogate modelling of multistage Taylor cone–jet dynamics

The Taylor cone jet is an electrohydrodynamic flow typically induced by applying an external electric field to a liquid within a capillary, commonly utilized in colloidal thrusters. This flow generation involves a complex multiphase and multiphysics process, with stability contingent upon specific operational parameters. The operational window is intrinsically linked to flow rate and applied electric voltage magnitude. High voltages can induce atomization instabilities, resulting in the production of an electrospray. Our study presents initially a numerical investigation into the atomization process of a Taylor cone jet using computational fluid dynamics. Implemented within OpenFOAM, our numerical model utilizes a volume-of-fluid approach coupled with Maxwell's equations to incorporate electric body forces into the incompressible Navier–Stokes equations. We employ the leaky-dielectric model, subjecting the interface between phases to hydrodynamic surface tension and electric stress (Maxwell stress). With this model, we studied the droplet breakup of a heptane liquid jet, for a range of operation of 1.53–7.0 nL s−1 and 2.4–4.5 kV of extraction. First, the developed high-fidelity numerical solution is studied for the jet breakup and acceleration of the droplets. Second, we integrate a machine learning model capable of extending the parametric windows of operation. Additionally, we explore the influence of extractor and acceleration plates on colloidal propulsion systems. This work offers a numerical exploration of the Taylor cone–jet transition and droplet acceleration using novel, numerically accurate approaches. Subsequently, we integrate machine learning models, specifically an artificial neural network and a one-dimensional convolutional neural network, to predict the jet's performance under conditions not previously evaluated by computationally heavy numerical models. Notably, we demonstrate that the convolutional neural network outperforms the artificial neural network for this type of application data, achieving a 2% droplet size prediction accuracy.

Effect of impinging behavior annular jet shielding gas and printing substrate on metal droplet stable ejection

Using coaxial shielding gas for low-oxygen protection in metal droplet-based 3D printing helps to promote flexible production and lightweight manufacturing. However, the presence of the printing substrate causes the shielding gas to exhibit complex annular impinging jet characteristics, making the stability of droplet ejection unpredictable. In the present work, the mechanisms of airflow pattern evolution on droplet formation and metal jet deflection were first revealed by incorporating shielding gas simulations, hydrodynamic modeling, and droplet ejection experiments. An innovative airflow disturbance suppression strategy for metal droplet ejection was proposed, which can remarkably reduce the shielding gas disturbance on droplet printing. Results show that the change in deposition distance leads to a transition between two typical airflow patterns, thus affecting the droplet ejection behavior. When the deposition distance exceeds 2.5 mm, metal jets would be stretched even to a secondary break under airflow pattern 1, accelerating droplets. For the deposition distance below 2.5 mm, metal jet shortening and droplet deceleration would occur under airflow pattern 2, deflecting jet trajectory. The negative airflow effect on droplet ejection could be avoided by controlling the deposition distance to the transition region of two airflow patterns. Furthermore, a ball grid array (BGA) chip ball-mounting and two thin-wall tube printing were realized based on metal droplet ejection in annular impinging jet shielding gas. This work provides theoretical and technical guidance for the stable ejection and accurate printing of metal droplets in an opening low-oxygen environment.

The Enhancement of Oil Delivery and Bearing Performance via a Guiding-Structured Nozzle under Oil–Air Lubrication

The oil–air lubrication method is specifically employed for high or ultra-high-speed spindle rolling bearings. Under high-speed conditions, the air curtain formed inside the bearing cavity obstructs oil delivery, thereby limiting further increases in spindle rotation speed. To enhance oil delivery capability, a guiding-structured nozzle has been developed to concentrate the jet flow and improve penetration through the air curtain. Tests were conducted on an oil–air lubricated bearing test bench to investigate the impact of nozzle structures and oil types on torque and temperature rise. The results demonstrate that compared to conventional nozzles, the guiding-structured nozzle requires smaller optimal amounts of oil supply, indicating its superior ability to deliver oil. Further examination of oil jet patterns and droplet distributions confirms that the guiding-structured nozzle provides a more concentrated jet flow with uniform distribution and smaller droplet sizes in diameter. These characteristics contribute to highly efficient oil delivery. Additionally, synthetic oils reduce droplet size, torque, and temperature rise in mixed lubrication regimes due to their formation of an anti-friction absorption layer on rubbing surfaces.

Droplet–jet collision following the monodispersedly dripping of coaxial binary droplets above a pool surface

In the present study, the collision between a falling droplet and a rising Worthington jet was experimentally studied. The event is followed by the monodispersedly dripping of coaxial binary droplets into a quiescent pool of glycerol solution. Different concentrations of the solution are considered. Unique droplet–jet collision characteristics are observed when the dripping flow rate is manipulated to release binary droplets. When the first droplet impacts the pool, a significant disturbance is imposed onto the pool, forming a deep crater followed by a Worthington jet. The second droplet is timed to collide with the rising jet to create a unique mushroom-shaped droplet–jet collision. Two jet pinch-off modes (tip pinch-off and no pinch-off) and four distinct collision regimes (partial rebounding, end-pinching, elongated, and clotted central jet collision) are recognized. Liquid viscosity and jetting mode significantly influence the collision dynamics and splattering characteristics. To achieve partial rebounding collision at low Weber number, a high-impact coefficient incorporating characteristic dimensions of the droplets and the Worthington jet is required, whereas a low-impact coefficient is required at high Weber number to attain clotted jet collision. The overall end-pinching phenomenon occurs due to the interaction between liquid flow toward the jet tip and the retraction of the tip, which causes the jet neck diameter to decrease on a capillary timescale. As the impact parameter decreases, the Worthington jet is inhibited, and the mushroom-shaped collision splash spreading is suppressed.

Singular jets during the impingement of compound drops upon lyophilic surfaces

An important phenomenon produced during the impingement of drops upon solid surfaces is the formation of singular jet, which is often followed by the pinch-off of satellite droplets. Great efforts have been made to investigate the jetting dynamics of low-viscosity single-phase drops impact upon sufficiently lyophobic surfaces. However, whether such singular jets can be produced during the impact of compound drops and how the liquid properties and surface wettabilities affect the dynamics have remained largely unexplored. Herein, we perform comparative and systematic experiments on the impact dynamics of single-phase water and silicon oil drops, as well as water-in-oil compound drops on lyophilic substrates. We show that singular jets only occur during the impact of compound drops. The critical values in terms of the Weber number depend on both the viscosity of the silicon oil and the volume ratio of the two liquids composing the compound drops. We also show that the singular jets break up and throw out satellite droplets only when they are considerably fast and thin. Power-law correlations between the jet velocities and the jet radii, between the jet neck radius and time, and between the maximum jet height and the jetting time are obtained. A linear correlation between the radii of the jet droplets and those of the singular jets is also found and analyzed.

Water–air transfer rates of microplastic particles through bubble bursting as a function of particle size

Microplastic (MP) particles can be ejected into the air by jet drops when gas bubbles burst at water surfaces. For a qualitative and quantitative understanding of this transport mechanism from the hydrosphere to the atmosphere, we studied the transfer of MP due to bubble bursting at the air–water interface in laboratory experiments. Gas bubbles were produced with filtered air that was pushed through a stainless-steel frit at two different volume flow rates in a glass flask filled with polystyrene (PS) particles of six different diameters (0.35 µm, 0.5 µm, 0.75 µm, 1 µm, 1.5 µm, 2 µm) suspended in deionized water. Airborne PS particle concentrations were measured by an optical particle counter. Additionally, size and volume of the bursting bubbles and the resulting jet droplets were analyzed with a camera. Depending on the volume flow rates, bubble bursting rates from 688 s−1 to 1176 s−1 and mean diameters of the bursting bubbles from 0.76 mm to 0.81 mm were observed. The mean diameters of the top jet drops were estimated to be between 0.10 mm and 0.11 mm. The measured number of jet droplets ranged from 2092 s−1 to 2391 s−1. For particle diameters from 0.35 µm – 2.0 µm, the airborne MP particle concentrations ranged from 4.2 l−1 to 348 l−1. We determined size-dependent transfer factors for the water–air transfer and found a maximum for 1 µm particles. For MP particles up to 1 µm diameter, the particle concentration in the jet droplets was enhanced compared to the bulk water concentration, indicating an enrichment of MP particles at the water–air-interface of bubbles.

Numerical investigation of nozzle length effects on Rayleigh jet breakup behaviors and droplet formation properties at different modulation amplitudes in liquid droplet radiator

Uniform droplet stream generation is required for a liquid droplet radiator to achieve the efficient heat dissipation of space nuclear power systems. In this study, Rayleigh jets breaking into continuous droplet streams at different modulation amplitudes are numerically simulated to investigate the nozzle length effects on jet breakup behaviors and droplet formation properties. First, simulated results confirm that the growth of secondary instability wave on jet free-surface causes the jet breakup pattern transition from downstream pinching to upstream pinching. As the modulation amplitude is from 0.04 up to 0.16, the nozzle length range where the upstream pinching occurs in the jet breakup is broadened from null to 1–5. The essential reason for the increase of jet breakup length is the reduction of fundamental amplitude and the increase of full velocity relaxation length. Then, a comprehensive prediction model for the velocity-modulated jet breakup length is developed in agreement with the experiments, and the average relative error is below 7.38 %. Finally, two types of satellite droplet formation regimes including rear-merging and forward-merging satellite droplets are identified by the oscillation mode of the jet tip. It is found that the primary droplet generation frequency is equal to the external disturbance frequency and independent of nozzle length and modulation amplitude. Increasing the nozzle length and modulation amplitude can both increase the satellite droplet diameter and primary droplet spacing, while the primary droplet diameter is almost constant at around 1.85.

Ultrafast Droplets Trajectories Modulation of Electrohydrodynamic Jet Printing for Pattern Refinement Using Electrostatic Deflection

AbstractElectrohydrodynamic jet (E‐Jet) printing is a highly promising non‐contact additive manufacturing technology. However, the fast jets generated by E‐Jet may cause droplet accumulation on small curvature features due to the slow motion control achieved by the mechanical stage, leading to a degradation of printed patterns in localized areas. Here, a method is proposed for ultrafast modulation of jet trajectories to achieve finely patterned E‐Jet printing by introducing jet‐deflecting electrodes around the jet, which can continuously transform the jet trajectory with lateral acceleration up to 104 ms−2. This extraordinary acceleration is two orders of magnitude higher than that can be achieved by using mechanical stage motion. Through a combination of numerical calculations and experiments, the effects of electric field parameters on the feature size and kinematic properties of jet droplets are investigated. In addition, deposition errors of &lt;5 µm are fabricated by selecting appropriate working parameters and using UV‐curable adhesives as printing ink. Subsequently, complex micropatterns with feature curvatures &lt;4 µm are successfully printed, demonstrating the consistency and reliability of this electrostatic deflection technique in achieving a uniform droplet deposition pitch. This innovative approach holds significant promise in the field of high‐quality microscale additive manufacturing.

Free surface tension modelling using particle-grid hybrid method without considering gas particles

In particle method, the number of particles is an important factor affecting the computational efficiency. For the free-surface flow, gas particles are usually necessary for surface tension calculation, but they have little impact to the liquid flow with a large density. If the free surface tension can be modeled without considering gas particles, it is of great significance to improve the computational efficiency. The existing potential-based surface tension model and the stress-based surface tension model don't need gas particles, but they have the problems of instability or are difficult to implement. Therefore, in this study, another free surface tension calculation method by coupling particles and grids is proposed. In the method, background mesh is applied to the entire computational domain, where the boundary of grid is extended outward by 3.1 l0 on the basis of particle boundary for curvature calculation. The level-set function is adopted to mark different phases on background mesh, whose value is set according to the normal distance between the grid and the nearest free surface particle. After obtaining the curvature and the level-set function gradient on background mesh, these variables are interpolated to corresponding particles to calculate the surface tension. Through the comparative study of area-weighted and distance-weighted interpolation schemes, it is found that the accuracy of distance-weighted interpolation scheme is higher than that of area-weighted interpolation scheme. Finally, the performance of the particle-grid hybrid method is verified by simulating four benchmark cases, including droplet oscillations, droplet spreading, capillary jet breakup and droplet impinging onto plate.

Atomization of composite liquid fuels in experimental setup with variated gas temperature and pressure

The efficiency of slurry fuel combustion can be improved by developing a predictive mechanism to estimate fuel atomization characteristics. However, no data are available on slurry fuel atomization behavior under near-real conditions. This paper presents the experimental research findings on the combined and separate effect of gas temperature and pressure on slurry fuel atomization characteristics. The experiments involved composite liquid fuels based on water and filter cake (typical coal processing waste) in variable concentrations. These conditions are the same as in advanced fuel slurry preparation units involving pre-combustors, swirlers, and other elements. In this research we have recorded such spray characteristics as droplet sizes and velocities, volume fraction of droplets with given sizes, jet angle, as well as the angle its deviation from the original trajectory. As has been observed, an increase in the ambient gas temperature leads to a 30% increase in the jet velocity and a 40–60% increase in the volume fraction of small droplets, whereas a pressure increase, on the contrary, reduces these parameters by 11–32% and 100%, respectively). Combined effects of chamber gas pressure and temperature on atomization characteristics have been identified: the variation of atomization characteristics did not exceed 18%. Mathematical expressions have been obtained to predict the atomization characteristics of composite liquid fuels in power-generating units with the known gas temperature and pressure. Maps have been plotted using a set of dimensionless parameters that can help control droplet size, velocity, jet angles, and angles of its deviation from its original path.

Comparison of the jet breakup and droplet formation between non-Newtonian and Newtonian fluids

Comparison of the jet breakup and droplet formation between non-Newtonian and Newtonian fluids