Droplet Emission Research Articles

Optical diagnostics of vacuum arcs in the process of DC interruption

Abstract Vacuum circuit-breakers (VCBs) are commonly used in emerging mechanical DC circuit-breakers. The electrical performance of VCBs in DC interruption duties has been extensively investigated, but basic physics such as the excitation temperature of plasma species, electron/vapor densities and emission of droplets in such applications were rarely reported. In this paper, we focus on how the short high frequency (HF) current pulse affects erosion of electrodes as well as generation of flying droplets by using laser shadow photography. Spectroscopy was adopted to obtain spectra and calculate the excitation temperature of CuI. Shack–Hartmann wavefront sensors were applied to measure neutral and electron density, in tests where di/dt, breaking current and the direction of the injected HF current varied. Indeed, the in-phase injection of the HF current caused an intense arc, but the pulse was too short to make either the cathode or anode melt. Since the plasma is in a nonequilibrium state, the excitation temperature of CuI is relatively low. Electron densities in different tests all approach similar values right before current zero. Neutral densities show strong fluctuation and independence on instantaneous current.

Modelling the first droplet emission from an electrified liquid meniscus hanging at the nozzle tip

Electrohydrodynamic (EHD)-induced droplet emission is an efficient method for the production of micron- and submicron-sized droplets in technological applications. Existing studies propose several scaling laws to determine the size of the emitted droplet. However, they have usually focused on the tip streaming phenomena of a droplet when subjected to a uniform electric field. In most applications, a non-uniform distribution of the electric field is created owing to the nozzle-to-plate configuration. Here, we employ an arbitrary Lagrangian–Eulerian method to demonstrate the mechanism of the first droplet emission from an electrified liquid meniscus with a fixed volume hanging at the nozzle tip. The critical condition when tip streaming occurs is determined using our numerical results. A phase diagram in terms of the electric field and initial liquid volume is presented to obtain the commonly used jetting mode. The effects of the liquid volume, electric field strength and electrical conductivity of the liquid on the processes of jet formation and breakup are further investigated. We find a particularly non-monotonic dependence of the size of the emitted droplet on the electrical conductivity. These findings could be useful for generating microdroplets and improving injection frequency in EHD printing technology.

Preliminary study of the electrospray DPE peculiarities from the liquid surface in the presence of the CSWs

The Ultrasonic Electric Propulsion (UEP) system is a cutting-edge propulsion technology that is mostly used on platforms for small satellites (less than 10 kg). The characteristics of droplet partial emissions (DPEs) in the UEP system are investigated using a high-speed imaging technique (an ultra-high speed camera (NAC HX-6) and a long-distance microscope) in this work. The experiments demonstrate that there are a few partial emission modes, including left-side emission, double-side emission, and right-side emission, that are present in the droplet emission process of the UEP system. These modes are primarily caused by the partial formation of capillary standing waves (CSWs) on the emission surface of the ultrasonic nozzle. The emission rate for single- and double-sided emissions varies at different times, indicating that there are different CSWs engaged in droplet emission due to variations in the liquid film thickness and charge state of the liquid cones. Additionally, as the droplets emit continuously, a raised area on the emission surface appears, with several droplets emitting there as a result of charge accumulation. Additionally, photos of the CSWs with emitting droplets are obtained, which highlights the CSWs’ distinctive wave morphology.

Characteristics of droplets emission immediately around mouth during dental treatments

Control measures aiming for removing droplets generated during dental treatments have been attracted increasing attention after COVID-19 pandemic, but the emission characteristics of droplets around the source (namely, the mouth) have still not been fully understood. The present study investigated experimentally the emission characteristics of droplets around mouth during typical dental treatments. A manikin being treated by portable dental instruments simulated the dental treatments, while the laser light scattering technique and particle image velocimetry (PIV) technique were employed to perform the measurements. The peak mass fraction of droplets occurs around the droplet size from 20 μm to 100 μm, while the mass fraction of droplets with the diameter less than 200 μm accounts for over 80 %. By fitting with the Rosin-Rammler equation, the mean size of droplets (d‾) are 54.96 and 98.93, and the size distribution parameter (n) are 1.35 and 1.58, respectively, for the process of air-powder-polishing and ultrasonic scaling. The dominant emission direction of droplets is towards the dummy’ head and chest. The emitted droplets formed approximately a cone shape, and its angle varied from 25° to 83° for different conditions. The maximum velocity of droplets at the location near the dummy's mouth exceeds 1.0 m/s for most conditions. The injecting flow rate and treated tooth position are the major factors influencing the emission direction, angle, and velocity. It is expected that the results can further increase our understanding of emission characteristics of droplets and provide accurate initial conditions for CFD simulations.

Triboactive CrAlN+MoWS coatings deposited by pulsed arc PVD

Cathodic arc physical vapor deposition (arc PVD) is widely used due to its high deposition rate and degree of ionization. However, reducing droplet emission and surface roughness without compromising the deposition rate remains challenging. CrAlN+XS coatings (X = Mo, W) have shown potential for friction and wear reduction under dry-running conditions by forming solid lubricants MoS2 and WS2. Nevertheless, the evaporation of electrically low conductive materials like MoS2 or WS2 in arc PVD presents difficulties. This study investigates the use of pulsed arc PVD as a promising technology to address these challenges. CrAlN+XS coatings were developed using pulsed arc PVD, with a focus on analyzing their properties, including the incorporation of triboactive elements (S, Mo, W). Additionally, variations in coating architecture, substrate material, and process temperature were explored for CrAlN+MoS coatings. Subsequently, tribological investigations were conducted under dry-running conditions using a pin-on-disk tribometer. The results revealed that pulsed arc PVD effectively decreased droplet emission and thus surface roughness of the coatings. Moreover, the triboactive coatings exhibited significantly lower wear and friction compared to uncoated steel references. Raman spectroscopy was employed to examine the tribofilms on coated parts. This work represents the first-time deposition of CrAlN+XS coatings (X = Mo, W) using pulsed arc PVD. Notably, the CrAlN+MoS coating displayed the highest sulfur content of xS = 9 at.-%, as verified by electron probe microanalysis. The tribofilm analysis provided evidence that CrAlN+MoS coatings deposited by pulsed arc PVD can form solid lubricant MoS2 under tribological load, resulting in a reduced coefficient of friction. In conclusion, pulsed arc PVD shows promise for applications that require the evaporation of electrically low conductive target materials during coating deposition, while achieving a low surface roughness. The findings contribute valuable insights into the performance of CrAlN+XS coatings and their potential for enhancing tribological applications.

Thermofluid dynamics and droplets transport inside a large university classroom: Effects of occupancy rate and volumetric airflow

The airborne droplets released by humans while speaking or coughing are the main sources of human-to-human contagion for airborne transmissible diseases. Due to the recent SARS-Cov2 pandemic, indoor transmission in classrooms and other environments is an interesting topic investigated by the scientific community. In this work, the thermofluid dynamic conditions and the droplet transport was analysed in a large University Classroom (UC), as a function of the inlet airflow and the occupancy rate. These parameters assume a relevant role in the design stage. However, their effects on droplet transport were not deeply investigated in the literature. Therefore, different conditions were analysed, such as: full occupancy, which represents the design condition where no social distancing is enforced; half occupancy, which is representative of social distancing; and empty UC, to be considered as a term of comparison. The main novelty of this work is related to the analysis of the combined effect of thermal plume and airflow in a large UC. The numerical model for the thermofluid dynamics simulation was validated in a previous work and the droplet emission model was retrieved from the literature. High volumetric airflow is not always beneficial for the reduction of the airborne transmission risk. An important aspect is related to the combined effect of the inlet airflow and the thermal plume, which cannot be ignored if the occupancy rate assumes a significant value. Moreover, the evacuation rate and the age of the droplets are relevant indicators to assess the cleaning time.

Bubble bursting in a weakly viscoelastic liquid

We study the bursting of bubbles in weakly viscoelastic liquids. The dissolved macromolecules form a monolayer at the water–air interface, influencing the bubble dynamics during the cavity collapse. For an optimum polymer concentration, the interfacial effects dampen short-wavelength waves, which intensifies the focusing of energy powering the jet ejection. This results in a significant increase (decrease) in the first-emitted droplet velocity (radius). The jet formation produces strain rates leading to a significant increase in the extensional viscosity. This extensional thickening reduces (increases) the first-emitted droplet velocity (radius). Bulk viscoelasticity produces a large difference between the velocity of the jet front at the tank surface level and the velocity of the first-emitted droplet. This droplet coalescence with others that are subsequently emitted, even for small polymer concentrations. Overall, viscoelasticity considerably hinders the ejection of small droplets, even for quasi-Newtonian liquids. The droplet emission is suppressed for smaller polymer concentrations when the bubble radius is decreased.

On modal decomposition as surrogate for charge-conservative EHD modelling of Taylor Cone jets

The dynamics of the interface of a fluid jet subjected to an external electrostatic field forming a Taylor cone jet is investigated using a numerical approach. A charge-conservative electrohydrodynamic model is used. In this model, the reduced form of Maxwell’s equations for an electrostatic field, a transport equation for electric charges and the Navier–Stokes equations for a laminar flow are solved. The connection between the electric field and the hydrodynamic flow is established by the Maxwell stress tensor, which ensures the description of the electric surface forces. A fluid volume model based on surface compression provides an efficient and robust modulation of the overall behaviour of the electric drive of immiscible flows. Test cases from the literature López-Herrera et al. (2011); Roghair et al. (2015) help us to validate the numerical model. The order of accuracy of the spatial and temporal discretization of the electric field equations is computed through a quasi one-dimensional test case. The primary decay of a Taylor cone beam is then simulated with uniform droplet emission. This simulation is validated with experimental results from the literature Taylor (1969). To analyse the dynamics of the flow, it is proposed to use decomposition methods such as Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) using snapshots of the flow field. The dynamic structures formed by the velocity field and the dynamics of the electric charge determine the fundamental dynamics of the droplet emission. An analysis of the sensitivity of the modes to the sampling frequency is performed. The influence of different inlet velocities on the decomposition is analysed in the spatial and frequency domain. A reduced-order model (ROM) is interpolated from the base POD, and we demonstrate that it can be used to successfully construct the velocity and electric charge along the field. We anticipate numerous applications of this new type of ROM, such as for accelerating electrohydrodynamic calculations as surrogate models in digital twins.

Angular Patterns of Nonlinear Emission in Dye Water Droplets Stimulated by a Femtosecond Laser Pulse for LiDAR Applications

Femtosecond laser-induced fluorescence (FLIF) and femtosecond laser-induced optical breakdown spectroscopy (FIBS) are important tools for remote diagnostics of atmospheric aerosols using LiDAR (Light Identification Detection and Ranging) technology. They are based on light emission excitation in disperse media via multiphoton nonlinear processes in aerosol particles induced by high-power optical pulses. To date, the main challenge restraining the large-scale application of FLIF and FIBS in atmospheric studies is the lack of a valued theory of the stimulated light emission in liquid microparticles with a sufficiently broad range of sizes. In this paper, we fill this gap and present a theoretical model of dye water droplet emission under high intensity laser exposure that adequately simulates the processes of multiphoton excited fluorescence and optical breakdown plasma emission in microparticles and gives quantitative estimates of the angular and power characteristics of nonlinear emission. The model is based on the numerical solution to the inhomogeneous Helmholtz equations for stimulating (primary) and nonlinear (secondary) waves provided by the random nature of molecule emission in particles. We show that droplet fluorescence stimulated by multiphoton absorption generally becomes more intense with increasing particle size. Moreover, far-field plasma emission from liquid particles demonstrates a larger angular diversity when changing the droplet radius in comparison with multiphoton excited fluorescence, which is mainly due to the excitation of the internal optical field resonances in spherical particles.

ADDI-Spraydrift: A comprehensive model of pesticide spray drift with an assessment in vineyards

Spray drift is a major contributor to pesticide losses in the atmosphere leading to nontargeted ecosystem exposure. An overview of currently used ground-application spray drift models highlighted gaps in the description of the processes occurring during and after spraying. The ADDI-Spraydrift model was developed to bridge some of these gaps and provide a comprehensive, yet concise description of spray drift processes. The model is based on a random walk approach that describes droplet emission, dispersion and evaporation, ground deposition and canopy interception, and accounts for atmospheric stability regimes, and in-canopy turbulence. It predicts airborne concentration, canopy interception and deposition to the ground downwind from the sprayer. The sensitivity analysis to wind speed, active matter content, leaf angle, canopy height, leaf area index, ejection angle and velocity demonstrated the consistency of the model behaviour. The model was calibrated and evaluated against the Ganzelmeier et al. (1995) sedimentary spray drift data in vineyards. The model satisfactorily predicted droplets deposition to the ground downwind from the field boundary with a mean deviation between modelled and measured deposition of 1.3%. A discrepancy was observed at 3 and 5 m downwind from the field boundary attributed to the sensitivity to spraying conditions for which several hypotheses are discussed. Bearing in mind the need to explicitly describe the emitted droplet size and velocity, the overall predictive performance of the model appeared to be sufficient for assessing and comparing application techniques efficiency, quantifying pesticide loss and bystanders or resident exposure and evaluating the efficiency of mitigation measures.

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.

Intensive Emission of Droplets during Melting of Metal Samples in a High-Frequency Inductor

Intensive Emission of Droplets during Melting of Metal Samples in a High-Frequency Inductor

Large eddy simulation of droplet transport and deposition in the human respiratory tract to evaluate inhalation risk.

As evidenced by the worldwide pandemic, respiratory infectious diseases and their airborne transmission must be studied to safeguard public health. This study focuses on the emission and transport of speech-generated droplets, which can pose risk of infection depending on the loudness of the speech, its duration and the initial angle of exhalation. We have numerically investigated the transport of these droplets into the human respiratory tract by way of a natural breathing cycle in order to predict the infection probability of three strains of SARS-CoV-2 on a person who is listening at a one-meter distance. Numerical methods were used to set the boundary conditions of the speaking and breathing models and large eddy simulation (LES) was used for the unsteady simulation of approximately 10 breathing cycles. Four different mouth angles when speaking were contrasted to evaluate real conditions of human communication and the possibility of infection. Breathed virions were counted using two different approaches: the breathing zone of influence and direction deposition on the tissue. Our results show that infection probability drastically changes based on the mouth angle and the breathing zone of influence overpredicts the inhalation risk in all cases. We conclude that to portray real conditions, the probability of infection should be based on direct tissue deposition results to avoid overprediction and that several mouth angles must be considered in future analyses.

Superradiant Droplet Emission from Parametrically Excited Cavities.

Superradiance occurs when a collection of atoms exhibits a cooperative, spontaneous emission of photons at a rate that exceeds that of its component parts. Here, we reveal a similar phenomenon in a hydrodynamic system consisting of a pair of vibrationally excited cavities, coupled through their common wave field, that spontaneously emit droplets via interfacial fracture. We show that the droplet emission rate of two coupled cavities is higher than the emission rate of two isolated cavities. Moreover, the amplified emission rate varies sinusoidally with distance between the cavities, as is characteristic of superradiance. We thus present a hydrodynamic phenomenon that captures several essential features of superradiance in optical systems.

Numerical modeling of a sneeze, a cough and a continuum speech inside a hospital lift

The global COVID-19 and its variants put us on notice of the importance of studying the spread of respiratory diseases. The most common means of propagation was the emission of droplets due to different respiration activities. This study modeled by computational fluid dynamics (CFD) techniques a high risk scenario like a hospital elevator. The cabin was provided with an extraction fan and a rack for air renewal. Inside, a sneeze, a cough and a continuum speech were simulated. Inside the lift, two occupants were introduced to observe the risk of propagation of emitted droplets and the impact in diseases spreading risk. The fan effectivity over the droplets ejection was analyzed, as well as environmental condition of a clinical setting. For this purpose the amount of droplets inside were counted during whole time of simulations. The effect of the fan was concluded as able to eject the 60% of small droplets, but also a high performance in spreading particles inside. Among the three cases, the riskiest scenario was the continuum speech due to the saturation of droplets in airborne.

A diagnostic for quantifying secondary species emission from electrospray devices.

Measuring the polydisperse beam of charged species emitted by an electrospray device requires accurate measurements of current. Secondary species emission (SSE) caused by high-velocity nanodroplet or molecular ion impacts on surfaces contributes to substantial uncertainty in current measurements. SSE consists of both positive and negative species; hence, mitigating measurement uncertainty requires different considerations other than plasma diagnostic techniques. The probe and analysis methods described herein distinguish between current contributions from positive SSE, negative SSE, and primary species. Separating each contribution provides positive and negative SSE yield measurements and corrected current measurements that reflect the true primary current. Sources of measurement uncertainty in probe design are discussed, along with appropriate mitigation methods. The probe and analysis techniques are demonstrated on an ionic liquid electrospray operating in a droplet emission mode to obtain an angular distribution of positive and negative SSE yields for an ionic liquid electrospray.

On the nature of droplet production in DC glows with a liquid anode: mechanisms and potential applications

The interactions between plasma and liquid solutions give rise to the formation of chemically reactive species useful for many applications, but the mass transport in the interfacial region is usually limited and not fully understood. In this work, we report on the observation and explanation of droplet ejection at the plasma–liquid interface of a one-atmosphere glow discharge with the liquid anode. The impact of droplets emission on plasma properties is also analyzed by spectroscopy. The process, which is an efficient mass and charge transport mechanism, apparently occurs during discharge operation and thus constitutes a feedback vehicle between the discharge and the liquid. Distinctive from the well-known Talyor cone droplets associated with liquid cathodes, the observed droplets originate from the bubbles due to electrolysis and solvated air which does not require strong electric field at liquid surface. Instead, the droplets are ejected by bubble cavity rupture at the plasma–liquid interface and their size, initial speed are strongly dependent on the gravity, inertia and capillarity. The droplets emerge near the plasma attachment and are subsequently vaporized, emitting intense UV and visible light, which originated from excited OH radicals and sodium derived from the liquid electrolyte. Spectroscopy analysis confirmed that the bursting droplets generally reduce the gas temperature while their effects on electron density depend on the composition of the liquid anode. Results also show that droplets from NaCl solution increase the plasma electron density due to the lower ionization potential of sodium. These findings reveal a new mechanism for discharge maintenance and mass transport as well as suggest a simple approach to dispersing plasma-activated liquid into the gas phase and thus enhancing plasma–liquid interaction.

The effect of relative air humidity on the evaporation timescales of a human sneeze.

The present paper investigates droplet and aerosol emission from the human respiratory function by numerical and experimental methods, which is analyzed at the worst-case scenario, a violent sneeze without a face covering. The research findings develop the understanding of airborne disease transmission relevant to COVID-19, its recent variants, and other airborne pathogens. A human sneeze is studied using a multiphase Computational Fluid Dynamics (CFD) model using detached eddy simulation coupled to the emission of droplets that break up, evaporate, and disperse. The model provides one of the first experimental benchmarks of CFD predictions of a human sneeze event. The experiments optically capture aerosols and droplets and are processed to provide spatiotemporal data to validate the CFD model. Under the context of large random uncertainty, the studies indicate the reasonable correlation of CFD prediction with experimental measurements using velocity profiles and exposure levels, indicating that the model captures the salient details relevant to pathogen dispersion. Second, the CFD model was extended to study the effect of relative humidity with respect to the Wells curve, providing additional insight into the complexities of evaporation and sedimentation characteristics in the context of turbulent and elevated humidity conditions associated with the sneeze. The CFD results indicated correlation with the Wells curve with additional insight into features, leading to non-conservative aspects associated with increased suspension time. These factors are found to be associated with the combination of evaporation and fluid-structure-induced suspension. This effect is studied for various ambient air humidity levels and peaks for lower humidity levels, indicating that the Wells curve may need a buffer in dry climates. Specifically, we find that the increased risk in dry climates may be up to 50% higher than would be predicted using the underlying assumptions in Wells’ model.

Evaluation of Respiratory Particle Emission during Otorhinolaryngological Procedures in the Context of the SARS-CoV-2 Pandemic.

Understanding the risk of infection by routine medical examination is important for the protection of the medical personnel. In this study we investigated respiratory particles emitted by patients during routine otolaryngologic procedures and assessed the risks for the performing physician. We developed two experimental setups to measure aerosol and droplet emission during rigid/flexible laryngoscopy, rhinoscopy, pharyngoscopy, otoscopy, sonography and patient interview for subjects with and without masks. A high-speed-camera setup was used to detect ballistic droplets (approx. > 100 µm) and an aerosol-particle-sizer was used to detect aerosol particles in the range of 0.3 µm to 10 µm. Aerosol particle counts were highly increased for coughing and slightly increased for heavy breathing in subjects without masks. The highest aerosol particle counts occurred during rigid laryngoscopy. During laryngoscopy and rhinoscopy, the examiner was exposed to increased particle emission due to close proximity to the patient’s face and provoked events such as coughing. However, even during sonography or otoscopy without a mask, aerosol particles were expelled close to the examiner. The physician’s exposure to respiratory particles can be reduced by deliberate choice of examination technique depending on medical indication and the use of appropriate equipment for the examiners and the patients (e.g., FFP2 masks for both).

Holographic slurry droplet monitor: Design and its application to 1000 MW coal-fired power unit

Wet flue gas desulfurization (WFGD) is a commonly utilized desulfurization technique. A major application of WFGD can be found in the coal-fired power plant, with up to 90% share of its market. Despite the fact that WFGD has been well developed and proven, its negative effects persist, e.g., entrainment pollution stemming from slurry droplet emission. In this study, we demonstrate in situ monitoring of flue gas slurry droplet emission in the coal-fired power unit using a self-made instrument based on digital holography, named the holographic slurry droplet monitor (HSDM), and gain insight into the key characteristics of slurry droplet emission through further statistics. HSDM exhibits superior accuracy in the respect of mass concentration, compared with conventionally accepted Mg2+ tracer method, and the maximum bias of the two is 5.61%. The application of HSDM in the real scenario of a 1000 MW coal-fired power unit is performed. After that, real-time slurry droplet measurement data is analyzed, revealing the key characteristics of slurry droplets, including size distribution and mass concentration, as well as factors associated with emission. Overall, high accuracy, real-time availability with a response time of 10 min, and multi-info acquisition in both size distribution and mass concentration make HSDM a remarkable alternative to the conventional method, enabling it to play a critical role in slurry droplet emission measurement.