Droplet Emission Research Articles

Experiments and modeling of droplet emission from tungsten under transient heat loads

Tungsten in the form of macrobrush is foreseen as one of the candidate materials for the ITER divertor. Melting of tungsten, the melt motion and melt splashing are expected to be the main mechanisms of damage determining the lifetime of plasma-facing components. Experimental investigations of droplet emission from the W melt layer for ELM-like heat loads were carried out at the plasma gun facility QSPA-T. Distribution functions of droplet velocity and droplet sizes for different heat loads were determined.In the paper, the main physical mechanism (the Kelvin–Helmholtz instability) of the melt splashing under the heat loads being applied at QSPA-T and those anticipated after the ITER transients is analyzed. Analytical distribution functions for droplet sizes and droplet velocities are defined. Numerical simulations demonstrated a reasonable agreement with the experimental data on the droplet sizes and droplet velocities and allowed projections of the melt splashing at ITER conditions.

Experimental measurements and large eddy simulation of expiratory droplet dispersion in a mechanically ventilated enclosure with thermal effects

Understanding of droplet transport in indoor environments with thermal effects is very important to comprehend the airborne pathogen infection through expiratory droplets. In this work, a well-resolved Large Eddy Simulation (LES) was performed to compute the concentration profiles of monodisperse aerosols in non-isothermal low-Reynolds turbulent flow taking place in an enclosed environment. Good care was taken to ensure that the main dynamical features of the continuous phase were captured by the present LES. The particle phase was studied in both Lagrangian and Eulerian frameworks. Steady temperature and velocity were measured prior to droplet emission. Evolution of aerosol concentration was measured by a particle counter. Results of the present LES were to compare reasonably well with the experimental findings for both phases.

Tailoring the hydraulic impedance of out-of-plane micromachined electrospray sources with integrated electrodes

Hydraulic impedance is a critical parameter for the operation of electrospray emitters, and for preventing flooding when spraying from arrays of emitters. Controlling flow rate by tuning the flow impedance allows accessing different operating modes, such as droplet, ionic, or pulsating. We report on a method to tailor the hydraulic impedance of micromachined capillary out-of-plane emitters with integrated extractor electrodes by filling them with silica microspheres. Spraying the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), we demonstrate the ability to tune from droplet emission to pure ion emission depending on microbead diameter, obtaining stable emission from single emitters and from arrays of 19 emitters.

Experimental and theoretical investigation of droplet emission from tungsten melt layer

Tungsten in form of macrobrush structure is foreseen as one of candidate materials for the ITER divertor and the dome. Melting of tungsten and the following melt motion and melt splashing are expected to be the main mechanisms of damage which determine the lifetime of plasma facing components. New experimental investigations of droplet emission from the W melt layer for the Edge Localised Mode (ELM)-like heat loads have been carried out at the plasma gun facility quasistationary plasma accelerators (QSPA-T). In these experiments the threshold for droplet emission and the distributions of velocity on emission angles and amplitude of the ejected droplets were determined. In the paper the main physical mechanism (the Kelvin–Helmholtz instability) of the melt splashing under the heat loads being applied at QSPA-T and those anticipated after the ITER transients is analyzed. These numerical simulations demonstrated a reasonable agreement with the experimental data on the droplet sizes and droplet velocities and allowed the projections upon the W melt splashing at ITER conditions.

Quality control of inkjet technology for DNA microarray fabrication

A robust manufacturing process is essential to make high-quality DNA microarrays, especially for use in diagnostic tests. We investigated different failure modes of the inkjet printing process used to manufacture low-density microarrays. A single nozzle inkjet spotter was provided with two optical imaging systems, monitoring in real time the flight path of every droplet. If a droplet emission failure is detected, the printing process is automatically stopped. We analyzed over 1.3 million droplets. This information was used to investigate the performance of the inkjet system and to obtain detailed insight into the frequency and causes of jetting failures. Of all the substrates investigated, 96.2% were produced without any system or jetting failures. In 1.6% of the substrates, droplet emission failed and was correctly identified. Appropriate measures could then be taken to get the process back on track. In 2.2%, the imaging systems failed while droplet emission occurred correctly. In 0.1% of the substrates, droplet emission failure that was not timely detected occurred. Thus, the overall yield of the microarray manufacturing process was 99.9%, which is highly acceptable for prototyping.

A Simple Model for the Disintegration of Highly Charged Solvent Droplets during Electrospray Ionization

This work uses a minimalist model for deciphering the opposing effects of Coulomb repulsion and surface tension on the stability of electrosprayed droplets. Guided by previous observations, it is assumed that progeny droplets are ejected from the tip of liquid filaments that are formed as protrusions of an initially spherical parent. Nonspherical shapes are approximated as assemblies of multiple closely spaced beads. This strategy greatly facilitates the calculation of electrostatic and surface energies. For a droplet at the Rayleigh limit the model predicts that growth of a very thin filament is a spontaneous process with a negligible activation barrier. In contrast, significant barriers are encountered for the formation of larger diameter filaments. These different barrier heights favor highly asymmetric droplet fission because the dimensions of the filament determine those of the ejected droplet(s). Substantial charge accumulation occurs at the filament termini. This allows each progeny droplet to carry a significant fraction of charge, despite its very small volume. In the absence of a long connecting filament, relieving electrostatic stress through progeny droplet emission would be ineffective. The model predicts the prevalence of fission events leading to the formation of several progeny droplets, instead of just a single one. Ejection bursts are followed by collapse back to a spherical shape. The resulting charge depleted system is incapable of producing additional progeny droplets until solvent evaporation returns it to the Rayleigh limit. Despite the very simple nature of the model used here, all of these predictions agree with experimental data.

High- and low-frequency noise in Cs and in liquid metal ion sources

Fundamental Physics space missions set rigid thrust noise limits for liquid metal ion thrusters used as actuators on drag-free platforms aboard the spacecraft. We have measured current-, voltage- and thrust noise of Cs and In LMIS, foreseen as prime candidates in these missions. In the high-frequency range, quasiperiodic oscillations around ∼10 5 Hz can be observed for both types of emitters with frequency depending on emission current. In the low-frequency range (1–10 −3 Hz), which is particularly important for drag-free control, different types of noise events are observed, which in some instances show definite signs of deterministic chaos (period doubling, self-similarity). High-frequency current oscillations are generally ascribed to electro-hydrodynamic oscillations of the TAYLOR cone and the jet at its apex, with concomitant emission of charged nanodroplets. Comparison of theory and experiment shows unsatisfactory agreement in predicted vs. measured current oscillation frequencies and large disagreement in droplet emission frequencies. No theory is presently available for describing low-frequency noise events. In terms of a linearized Mair theory it is, however, shown that these noise events can be efficiently described by spontaneous variations in electrical emitter impedance. In spite of this impedance noise, the mission requirements for thrust noise (<0.1 μN/Hz 1/2) can be met by a thrust-stabilized In emitter.

Supercritical droplet dynamics and emission in low speed cross-flows

Droplet dynamics and emission of a supercritical droplet in crossing gas stream are numerically investigated. Effects of ambient pressure and velocity of nitrogen gas on the dynamics of the supercritical oxygen droplet are parametrically examined. Unsteady conservative axisymmetric Navier-Stokes equations. in curvilinear coordinates are preconditioned and solved by dual-time stepping method. A unified property evaluation scheme based on a fundamental equation of state and extended corresponding-state principle is established to deal with thermodynamic non-idealities and transport anomalies.

Oriented Fiber Filter Media

Coalescing filters are widely used throughout industry and improved performance will reduce droplet emissions and operating costs. Experimental observations show orientation of micro fibers in filter media effect the permeability and the separation efficiency of the filter media. In this work two methods are used to align the fibers to alter the filter structure. The results show that axially aligned fiber media improve quality factor on the order of 20% and cutting media on an angle from a thick layered media can improve performance by about 40%. The results also show the improved performance is not monotonically correlated to the average fiber angle of the medium.

Comparative study on EUV and debris emission from CO2 and Nd: YAG laser-produced tin plasmas

The emission characteristics of debris from laser-produced tin plasma were investigated for an extreme ultraviolet lithography (EUV) light source. The ions and droplets emitted from tin plasma produced by a CO2 laser or an Nd: YAG laser were detected with Faraday cups and quartz crystal micro-balance (QCM) detectors, respectively. A higher ion kinetic energy and a lower droplet emission were observed in the case of CO2 laser in compared with Nd: YAG laser for the same laser energy (50 mJ). The reason comes from the interaction of the laser pulse with the plasma. CO2 laser is absorbed in superficial region of the plasma due to the long wavelength, in contrast, Nd: YAG laser penetrates deeply into the plasma. Therefore, the absorbed energy density of CO2 laser will be much larger than that of Nd: YAG laser for the same laser energy. This causes the higher ion energy and complete vaporizing the target material.

A comparison of bulk precipitation chemistry during normal and typhoon events on subtropical Okinawa Island (Japan)

Atmospheric acidic deposition and typhoons are prevalent environmental problems on subtropical Okinawa Island. This study was conducted to examine the chemical composition of bulk precipitation at Gesashi estuary (P1 and P2 sites) for one year, and compare the results with precipitation chemistry during four typhoon events on the Island. The acidic precipitation (pH < 5.70) was only observed during normal precipitation events with an average pH value of 5.42 at P1, while the highest mean pH of 6.11 was revealed during typhoons. The sum of the acidic minus basic concentrations (i.e. [non-sea salt (nss)–SO 4 2− + NO 3 −] − [nss–Ca 2+ +NH 4 +]) revealed a high linear correlation with pH ( P < 0.001), indicating that the pH was largely controlled by nss–SO 4 2−, NO 3 −, nss–Ca 2+ and NH 4 +. The average values of nss–SO 4 2−, NO 3 − and NH 4 + concentrations were 27.8, 14.5 and 11.5 μeqL − 1 , respectively in the Gesashi samples. They decreased by ∼ 2 fold during typhoons, contrary to nss–Ca 2+ which showed increasing trend. The Na + and Cl − ions were the dominant ions, which showed high linear correlations ( P < 0.001), which suggest that they were largely from a marine source. The average concentrations of sea salt ions increased by ∼ 4 fold during typhoons compared to Gesashi samples. A positive linear relation between electric conductivity and daily average wind speed ( P < 0.005) for the four typhoon samples may indicate high emission of seawater droplets to the atmosphere owing to typhoon winds. Dissolved Al and silica showed ∼ double average concentrations during typhoons than Gesashi samples. A significant positive linear correlation ( P < 0.001) between them suggests similar origin, probably from wind-blown soil dust. The acidic deposition during normal precipitation events may enhance solubility and transport of Al from the Okinawan acidic red soil to surface waters while typhoons tend to mitigate the acidic pollutants.

Study of expiratory droplet dispersion and transport using a new Eulerian modeling approach

Understanding of droplet nuclei dispersion and transport characteristics can provide more engineering strategies to control transmission of airborne diseases. Droplet dispersion in a room under the conventional well-mixed and displacement ventilation is simulated. Two droplet nuclei sizes, 0.01 and 10 μm, are selected as they represent very fine and coarse droplets. The flow field is modeled using k–ε RNG model. A new Eulerian drift-flux methodology is employed to model droplet phase. Under the conventional ventilation scheme, both fine and coarse droplets are homogeneously dispersed within approximately 50 s. Droplet nuclei exhibit distinctive dispersion behavior, particularly for low airflow microenvironment. After 270 s of droplet emission, gravitational settling influences the dispersion for 10 μm droplets, and concentration gradient can still be observed for displacement ventilation.

High-aspect-ratio silica nozzle fabrication for nano-emitter electrospray applications

With the trend in chemical and biological analysis for a reduction in sample size, the demand is for smaller and more finely controlled droplet emission from electrospray micronozzles. One of the key issues is to make reliable electrospray emitters with smaller diameters and high aspect ratio (length to diameter) to ensure a high value of electric field at the nozzle tip. Silica nozzles with aspect-ratio of over 20 were designed and fabricated in both single and array formats. An investigation of silicon deep etch by DRIE was performed and achieved a depth greater than 200 μm for 10 μm diameter channels. The surface wettability was also studied and the result showed that a thin gold film with low surface energy would offer the desired level of hydrophobic performance and be sufficiently robust for most micronozzle applications. The microfabricated single and linear-array nozzles were tested in the operation modes of on-line pumped and off-line isolated. The on-line tests for the single nozzles resulted in stable cone-jet electrospray with flow rates as low as 100 nl/min at voltage of a few kV. The off-line tests for the linear-array nozzles showed cone-jet pulsed electrospray with the production of picolitre droplets.

Jet Wiping in Hot-Dip Galvanization

This paper presents an analysis of the gas-jet wiping process in hot-dip galvanization. This technique consists of reducing the liquid film thickness on a moving substrate by applying gas slot jets. A theoretical development allows the computation of the film thickness evolution in the wiping zone. It is further simplified to an engineering model which predicts directly the final coating thickness, in good agreement with wiping experiments. The limit of applicability of jet wiping is due to the occurence of a violent film instability, called splashing, which takes the form of a liquid droplet emission just upstream the nozzle. An experimental investigation of this phenomenon is conducted on a water-model facility. Two nozzle designs are tested. The effect of process parameters such as the strip speed, the nozzle pressure, the standoff distance, and the tilt angle of the nozzle on splashing is emphasized. A dimensionless correlation is established to predict the operating conditions leading to splashing occurence. It is successfully confronted to observations made on galvanization lines.

A Micro-Fabricated Linear Array of Electrospray Emitters for Thruster Applications

This paper reports the design, fabrication, and experimental characterization of an internally fed linear array of electrospray emitters intended for space propulsion applications. The engine uses doped formamide as propellant and operates in the single-Taylor-cone droplet emission regime. The engine implements the concept of hydraulic and electrodynamic flow rate matching to achieve electrical control. The engine uses a set of meso-scaled silicon deflection springs to assemble the hydraulics to the electrodes, allowing to decouple the corresponding process flows. The micro-fabrication of the engine is described and novel technologies that were developed are reported. Experimental results that demonstrate cumulative uniform and steady operation are provided. Current-flowrate characteristics of the engine are in agreement with a reduced-order model. Experimental data demonstrating the low divergence of electrospray emitter arrays operated in the single Taylor Cone is in qualitative agreement with a reduced-order mode that assumes the absence of a thermalized tail in the plume

Investigation on Erosion of Cu/W Contacts in High-Voltage Circuit Breakers

The prediction and understanding of the erosion process of the Cu/W arcing contacts in high-voltage circuit breakers is important for the improvement of the breaker lifetime. The change of the contact geometry due to erosion is a main factor, which influences the lifetime. It is known that the ablation of contact material depends at least on parameters like the arcing current-amplitude, arcing time, total charge, and contact material properties. Based on experimental results, a method for an improved prediction of the erosion was derived. The experiments have been carried out in a SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> self-blast circuit breaker test device with commercial high-voltage circuit breaker contacts at a current amplitude up to 45kA RMS and arcing time up to 17ms. From the power balance at the electrodes, the material erosion due to evaporation is calculated. The resulting erosion rates are in good agreement to the experimental results and explain that the specific erosion scales with current, polarity, and the arcing time. This indicates that erosion in this range of arcing stress occurs mainly due to evaporation of contact material and less to emission of droplets

Courtship behaviour of Drosophila melanogaster revisited

Nearly all studies of Drosophila melanogaster courtship have focused exclusively on male behaviour. Female precopulatory behaviour is often relegated to ‘accept’ or ‘reject’ mating, and how female behaviour interacts with that of males remains largely unknown. Using a video-computing approach, we measured 10 behavioural states and 22 elementary behaviours that occur during the precopulatory phase of Canton-S (Cs) flies. Male and female behaviours were studied under a variety of social conditions. This allowed us to identify which one(s) is relevant to courtship or to general activity. Our analysis showed that the courtship repertoire was not constant over time, as female and male behaviours varied during courtship, and that virgin Cs females showed several behavioural sequences whose frequency, latency and total duration could predict their mating success. The most striking index of females' willingness to copulate was the way they extruded the ovipositor: a partial extrusion stimulated male courtship, whereas total extrusion inhibited it. Furthermore, females that mated preened their abdomen more frequently than did females that did not mate. We also observed that male sexual activity was strongly increased when the female emitted a tiny volatile droplet at the tip of the ovipositor. We hypothesized that partial ovipositor extrusion, droplet emission and abdominal preening are related behaviours that result in the active dispersion of compounds spread on the female's abdomen. Such a detailed picture of the precopulatory behaviours should allow better characterization of the sensory stimuli exchanged during courtship in Drosophila.

NANOPARTICLES PRODUCTION FROM GLYCERIN VIA ELECTROSPRAY AND SIZE MEASUREMENT TECHNIQUES

Glycerin is a potential feedstock for hydrogen production because one mole of glycerin can produce up to fourmoles of hydrogen. In addition to glycerin, several potential biorenewable feedstocks, such as triglycerides and theirderivatives, could be used for hydrogen production. The major challenge in using such feedstocks in a hydrogen-producingreformer is atomizing these viscous feedstocks for the gas phase reaction to occur. Moreover, due to the difficulty ofatomization, the reforming reaction becomes mass transfer limited. This article presents a technique to produce nanoscaledroplets from a highly viscous liquid (glycerin). In addition, several measurement techniques to quantify nanoscale dropletsare discussed. The principle behind the nanospray formation is applying a sufficiently high electric field to the surface of theliquid, which causes emission of fine charged droplets. The mean and median sizes of the nanoparticles generated were36.46 nm and 25.77 nm, respectively, with a geometric standard deviation of 1.7.

Influence of Taylor cone size on droplet generation in an indium liquid metal ion source

Depending on the emission current, a liquid metal ion source (LMIS) produces microdroplets in addition to ions. It is demonstrated that this ratio is directly influenced by the geometry of the Taylor cone. Using analytical models and experiments, we can show that the droplet production rate is directly proportional to the Taylor cone base radius for low impedance emitters. This relationship is of particular importance to optimize emitters for either low or high droplet emission at various emission currents.

THE EFFECT OF NITROGEN GAS FLOW RATE ON THE PROPERTIES OF TiN-COATED HIGH-SPEED STEEL (HSS) USING CATHODIC ARC EVAPORATION PHYSICAL VAPOR DEPOSITION (PVD) TECHNIQUE

Cathodic arc evaporation (CAE) is a widely-used technique for generating highly ionized plasma from which hard and wear resistant physical vapor deposition (PVD) coatings can be deposited. A major drawback of this technique is the emission of micrometer-sized droplets of cathode material from the arc spot, which are commonly referred to as "macroparticles." In present study, titanium nitride ( TiN ) coatings on high-speed steel (HSS) coupons were produced with a cathodic arc evaporation technique. We studied and discussed the effect of various nitrogen gas flow rates on microstructural and mechanical properties of TiN -coated HSS coupons. The coating properties investigated in this work included the surface morphology, thickness of deposited coating, adhesion between the coating and substrate, coating composition, coating crystallography, hardness and surface characterization using a field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray (EDX), X-ray diffraction (XRD) with glazing incidence angle (GIA) technique, scratch tester, hardness testing machine, surface roughness tester, and atomic force microscope (AFM). An increase in the nitrogen gas flow rate showed decrease in the formation of macro-droplets in CAE PVD technique. During XRD-GIA studies, it was observed that by increasing the nitrogen gas flow rate, the main peak [1,1,1] shifted toward the lower angular position. Surface roughness decreased with an increase in nitrogen gas flow rate but was higher than the uncoated polished sample. Microhardness of TiN -coated HSS coupons showed more than two times increase in hardness than the uncoated one. Scratch tester results showed good adhesion between the coating material and substrate. Considerable improvement in the properties of TiN -deposited thin films was achieved by the strict control of all operational steps.