Drop Diameter Research Articles

Raindrop Size Distribution Characteristics of the Western Pacific Tropical Cyclones Measured in the Palau Islands

Due to the severe threat of tropical cyclones to human life, recent years have witnessed an increase in the investigations on raindrop size distributions of tropical cyclones to improve their quantitative precipitation estimation algorithms and modeling simulations. So far, the raindrop size distributions of tropical cyclones using disdrometer measurements have been conducted at coastal and inland stations, but such studies are still missing for oceanic locations. To the authors’ knowledge, the current study examines—for the first time—the raindrop size distributions of fourteen tropical cyclones observed (during 2003–2007) at an oceanic station, Aimeliik, located in the Palau islands in the Western Pacific. The raindrop size distributions of Western Pacific tropical cyclones measured in the Palau islands showed unlike characteristics between stratiform and convective clusters, with a larger mass-weighted mean diameter and smaller normalized intercept parameter in the convective type. The contribution of the drop diameters to the total number concentration showed a gradual decrease with the increase in drop diameter size. Raindrop size distributions of Western Pacific tropical cyclones measured in the Palau islands differed slightly from Taiwan and Japan. The helpfulness of empirical relations in raindrop size distribution in rainfall estimation algorithms of ground-based (Z–R, μ–Λ, Dm–R, and Nw–R) and remote-sensing (σm–Dm, μo–Dm, Dm–Zku, and Dm–Zka) radars are evaluated. Furthermore, the present study also related the rainfall kinetic energy of fourteen tropical cyclones with rainfall rate and mass-weighted mean diameter (KEtime–R, KEmm–R, and KEmm–Dm). The raindrop size distribution empirical relations appraised in this study offer a chance to: (1) enhance the rain retrieval algorithms of ground-based, remote sensing radars; and (2) improve rainfall kinetic energy estimations using disdrometers and GPM DPR in rainfall erosivity studies.

Air bubble entrapment during drop impact on solid and liquid surfaces

The phase-field modeling (PFM) of water drop impact onto a dielectric hydrophobic parafilm surface is performed to explore air entrapment and its influence on deposition and rebound phenomena. Local and global characteristics of the drop impact are taken into account by using the combined Cahn-Hilliard and Navier-Stokes equations. The modeling results of water drop impact are directly compared with our experimental measurements in terms of maximum spreading distance, and air bubble size. The simulation results reveal that air can be trapped under the liquid drop during the initial impact as well as during the retraction phase at the center of the drop due to the closure of the liquid layer above a cavity. It is found that the drop diameter and the impact velocity play significant roles in the air entrapment phenomena. The probability of air bubble formation is higher at lower impact velocity and for larger drop size. The model is also capable of simulating the case of drop impact onto a water surface, and the results are validated using prior literature data. In addition, the influence of the phase-field variables and the mesh adaptation scheme on the PFM is studied and discussed. Thus, our findings provide new qualitative and quantitative insights into the influence of air entrapment on drop deposition onto hydrophobic and liquid surfaces.

Transient surface temperatures upon the impact of a single droplet onto a heated surface in the film evaporation regime

Single droplet impingement on a heated surface is a fundamental problem of basic interest in understanding complex phenomena in spray cooling and other applications involving heat transfer to impinging droplets. The surface temperature experiences rapid temporal gradients upon droplet impact. These temperature transients are strongly dependent on the hydrodynamic and heat transfer regimes. Experimental results are presented for the transient surface temperature in the film evaporation heat transfer regime for water drop diameters of 2.1 and 3.5 mm at drop Reynolds numbers up to 34,000 and Weber number up to 5540 are reported for surface temperatures of 60 to 100 ∘C. High-speed video was used to capture the dynamics of the impact. Intrinsic thermocouples were manufactured to measure surface temperature with sufficient temporal resolution. It was found that the hydrodynamics of the impact were not significantly affected by the surface temperature. Heat removal was found to be more effective at lower impact velocities. The maximum temperature drop occurred at the maximum spreading diameter. An analytical model was developed to successfully predict the transient Nusslet number of the droplet motion on the surface for both free-falling and air-propelled droplets.

On the effect of concentration and wettability on polymer drops impact, dynamics and energy dissipation

The present study aims to understand the influence of small quantities of poly-ethylene oxide (PEO) diluted in pure distilled water with concentrations of 50, 100 and 200 parts per million (ppm) in the impact phenomenon of drops onto substrates of Parafilm M, aluminium and glass. Both the static aspects (i.e., maximum spreading area) and the dynamic aspect (i.e., recoiling velocities), and different parameters of contact angle oscillations of the droplet are analysed. Results show that the maximum spreading diameter, dissipation mechanisms, and oscillations period all increase with the release height. Unlike contact angles oscillations envelope which gets thinner, oscillations length decreases with the increase of release height. Polymer concentration influences the maximum spreading base diameter of drops on non-hydrophobic substrates, i.e., higher concentration slows down recoiling velocities, reduces oscillations length, enhances dissipation mechanisms, and influences oscillations period as well. In addition, the wettability of substrates influences the spreading phase, drop rebound, and can damp recoiling of the polymer drops. A hydrophilic substrate reduces the oscillations length, increases their period and lessens their amplitude decay. The dependence of oscillations on the concentration illustrates a clear different behaviour on Aluminium surfaces (hydrophilic) and on Parafilm M surfaces (hydrophobic). The initial amplitude and the damping rate were analysed and related to the concentration.

Influence of Reduced Anthropogenic Activities on Rain Microphysical Properties and Related Atmospheric Parameters Over an Urban Tropical Location

This letter reveals the prevailing scenario of raindrop size distribution (DSD) in terms of mass-weighted mean drop diameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> ) over a tropical metropolis, Kolkata (22.57°N, 88.37°E), India, in a contrasting aerosol environment that occurred during the COVID-19 pandemic in the absence of usual human activities. In the premonsoon months (March–May), the probability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> values exceeding 2 mm has increased in 2020, indicating the dominance of large raindrops, compared to the years 2017–2019. Increased number densities of larger drops have influenced the drop fall velocity spectrum as measured by a laser precipitation monitor in terms of the percentage occurrence of high-velocity small drops (superterminal) and low-velocity large drops (subterminal) for both convective and stratiform precipitations. As measured from a Ka-band microrain Doppler radar, the mean melting layer altitude during stratiform rain has decreased by ~800 m during the premonsoon of 2020 compared to 2017–2019. According to the ERA-5 reanalysis data, changing rain microphysical characteristics are related to decreasing zero-degree isotherm height and reduced wind shear.

Long-Term Impact of Continental and Maritime Airflow on Aerosol Environment and Rain Microstructure Near Land–Sea Boundary

The present study deals with the interplay of multiple atmospheric processes in changing the microphysical properties of precipitation during the pre-monsoon season (March–May) using the long-term experimental data of raindrop size distributions (DSDs) spanning over 15 years (2005–2019) obtained at an urban tropical location, Kolkata (22.57°N, 88.37°E), near the land–sea boundary. The changing pattern of air mass flows from the maritime and continental region, which contribute to the formation of precipitation processes, has been responsible for the varying characteristics of rain. Changes in raindrop sizes are related to aerosol properties, cloud features, temperature, and relative humidity that change mass-weighted mean drop diameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> ) differently in low and high rain rate regimes. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> has shown an increasing trend over time for low rain rates (< 15 mm/h), but a decreasing trend for high rain rate regimes (≥15 mm/h). An increase (decrease) in mean temperature (relative humidity) below the atmospheric boundary layer (< 1.6 km) has enhanced the evaporation of small raindrops and altered rain microphysical features. Based on satellite observations, it has been found that the increasing aerosol optical depth (AOD) has been accompanied by an increase in cloud effective radius (CER), resulting in the anti-Twomey effect, which is due to the dominance of maritime influence over continental activities. Because of the predominant maritime activities, sea salt aerosols have a greater presence, causing an increase in CER, which consequently prevents raindrops from becoming large enough before they fall, thereby reducing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> at high rainfall rates.

Rapid parallel generation of a fluorescently barcoded drop library from a microtiter plate using the plate-interfacing parallel encapsulation (PIPE) chip.

In drop-based microfluidics, an aqueous sample is partitioned into drops using individual pump sources that drive water and oil into a drop-making device. Parallelization of drop-making devices is necessary to achieve high-throughput screening of multiple experimental conditions, especially in time-sensitive studies. Here, we present the plate-interfacing parallel encapsulation (PIPE) chip, a microfluidic chip designed to generate 50 to 90 μm diameter drops of up to 96 different conditions in parallel by interfacing individual drop makers with a standard 384-well microtiter plate. The PIPE chip is used to generate two types of optically barcoded drop libraries consisting of two-color fluorescent particle combinations: a library of 24 microbead barcodes and a library of 192 quantum dot barcodes. Barcoded combinations in the drop libraries are rapidly measured within a microfluidic device using fluorescence detection and distinct barcoded populations in the fluorescence drop data are identified using DBSCAN data clustering. Signal analysis reveals that particle size defines the source of dominant noise present in the fluorescence intensity distributions of the barcoded drop populations, arising from Poisson loading for microbeads and shot noise for quantum dots. A barcoded population from a drop library is isolated using fluorescence-activated drop sorting, enabling downstream analysis of drop contents. The PIPE chip can improve multiplexed high-throughput assays by enabling simultaneous encapsulation of barcoded samples stored in a microtiter plate and reducing sample preparation time.

Condensation performance of superhydrophobic aluminium surface material used for cooled ceiling panels under highly humid indoor conditions

The application of radiant cooling systems is very limited in hot and humid areas due to condensation. Research on superhydrophobic surface (SHS) materials has shown the potential of restricting the size of condensate drops on these materials, which provides possibilities for preventing dripping and thereby alleviating condensation risks for cooled ceiling panels, but there are few studies on the anti-condensation performance of these materials under the scale and conditions of building applications. An experimental study of condensation on superhydrophobic materials under indoor conditions is presented in this article. Two material samples with a size of 2.5 cm, including a superhydrophobic aluminum sheet and a pure aluminium sheet, were affixed on a cooled ceiling panel to perform the experiment under the following condition: temperature is 25°C ± 0.5°C, relative humidity is 80% ± 5%, and air dew point is 21.4°C. The panel was cooled by chilled water of 6°C for eight hours. The measured temperature on sample surfaces was about 13.5°C during the experiment. After eight-hour condensation, the diameter of drops on the superhydrophobic aluminum sheet was less than 150 μm, while the max drop on the pure aluminum sheet was near 4 mm. The results suggested that the size of condensate drops on superhydrophobic surface materials can be largely restricted during a long-time indoor operation below the dew point, which shows their potential for constructing condensation-free radiant cooling panels.

Use of a 6-axis robot and ink piezo-jetting to print conductive paths on 3D objects. Printed circuit geometry, and conductivity predictive model

This paper presents an off-line programming approach for the automatic generation of trajectories and the printing program for a 6-axis robot in order to print accurate conductive paths on 3D objects using a silver microparticle solvent-based ink. The aim of this study was to develop a semi-predictive model in order to - Adapt the printing parameters of a 6-axis robot arm and a piezo jetting print head to the printing speed and substrate type. - Print 3D electronic circuits matching the targeted geometry and conductivity. For the jetting printing process, 2D patterns printed on substrates with different roughness and wettability were analysed as a function of the print head translation speed and jetting frequency which were used as base variables to develop correlations in order to predict line width, thickness and conductivity. The model was based on the assumption that the behaviour of a single drop impacting the printing substrate is close to that of a train of drops (i.e. lines). Thus, the diameter of individual drops on a specific substrate was used to account for support properties and jetting conditions. The study also proposes a methodology to tune the circuit morphology by adapting the jetting parameters as a function of the trajectory and the speed of the 6-axis robot. As a representative case study, a 3D circuit was printed on a disposable paper cup obtaining an excellent agreement between measured and predicted conductivity values.

The Role of Stochastic Time-Variations in Turbulent Stresses When Predicting Drop Breakup—A Review of Modelling Approaches

Many industrially relevant emulsification devices are of the high-energy type, where drop deformation and subsequent breakup, take place due to intense turbulent fluid–drop interactions. This includes high-pressure homogenizers as well as rotor-stator mixers (also known as high-shear mixers) of various designs. The stress acting on a drop in a turbulent flow field varies over time, occasionally reaching values far exceeding its time-averaged value, but only during limited stretches of time, after which it decreases down to low values again. This it is one factor separating turbulent from laminar emulsification. This contribution reviews attempts to take this intermittently time-varying stress into account in models predicting the characteristic drop diameter resulting from emulsification experiments, focusing on industrially applicable emulsification devices. Two main frameworks are discussed: the Kolmogorov–Hinze framework and the oscillatory resonance framework. Modelling suggestions are critically discussed and compared, with the intention to answer how critical it is to correctly capture this time-varying stress in emulsification modelling. The review is concluded by a list of suggestions for future investigations.

Detailed numerical simulations of primary atomization of airblasted liquid sheet

This paper investigates the primary atomization of airblasted liquid sheet using detailed numerical simulations. The atomization of liquid sheet under airblasting conditions involve complex mechanisms and a thorough understanding is necessary. A planar pre-filming airblast atomization configuration have been chosen to study the breakup of liquid sheet/film injected on a solid flat plate. We have investigated an operating point directly relevant for high altitude relight condition of the aircraft. This configuration has been chosen based on the experimental investigation of Gepperth et al. [S. Gepperth, A. Müller, R. Koch, H.-J. Bauer, Ligament and droplet characteristics in pre-filming airblast atomization, Proceedings of the ICLASS, 12th Triennial International Conference on Liquid Atomization and Spray Systems, September 2-6, Heidelberg, Germany, 2012] for the airblast atomization. The numerical simulations have been performed using in-house Navier–Stokes solver that uses consistent mass and momentum flux computation technique. The purpose of this work is to provide a comprehensive database and analyses of the airblast atomization of liquid sheet. This include studies on the effect of velocity profile on the atomization characteristics, occurrence of secondary atomization and drop coalescence, and extraction of near-field atomization characteristics. The qualitative analyses of the results from the simulations showed that there are two major atomization mechanisms of liquid film breakup — sheet/bag breakup and ligament breakup. The drop diameter and velocity distributions computed from the simulations was found to be of the same order of magnitude although under-predicting the experimental data. Based on the atomized drop data, both the secondary atomization and drop coalescence have been observed to occur in the simulations. The quantitative analyses of the near-field liquid ligaments results revealed the lengths of these ligaments are of the same order of magnitude as the experimental data while an under-prediction in the ligament velocity has been observed. Finally, an excellent agreement between simulations and experimental data has been found for the Sauter Mean Diameter (SMD) of the atomized droplets.

Experimental investigation on spray and ignition characteristics of plasma actuated bluff body flameholder

Stable combustion of aero-engines and ramjet combustors is greatly dependent on the quality of atomization. It is quite difficult to achieve wider ignition boundary and good combustion performance under low pressure and low temperature airflow, because the quality of fuel atomization is comparative bad and there is no enough surface area for fuel droplets to mix with surrounding air and evaporate. Internal mechanism of how plasma assisted ignition on spray performance in model aero-engine combustors and ramjet combustors remains not clear. In this study, a novel plasma-actuated bluff-body flameholder is proposed to help improve the quality of fuel atomization and thus enhance the ignition performance and stable combustion of original bluff-body flameholder. Spray and atomization properties of aviation kerosene (RP-3) were investigated under various operating conditions including inlet pressure, inlet temperature and gliding arc plasma actuation. A rotating arc discharge plasma column could be formed during the high-voltage tungsten electrode and the fins of bluff-body flameholder. Macroscopic as well as microscopic spray properties was analysed through simultaneous LIF/MIE technique along with particle/droplet image analysis (PDIA) system. Flame ignition limit at different inlet temperature was conducted under combustor inlet Mach number of 0.2. Results show that mean drop diameters were decreased with increase of inlet temperature and inlet pressure. Gliding arc plasma actuation help fuel injected from bluff-body flameholder breakup into smaller droplets and improve uniformity of fuel spray through heating and transport effect. With actuation of gliding arc plasma, Sauter mean diameter (SMD) was minished by 13%∼20% at inlet pressure of 30 kPa, while droplet uniformity index was increased by around 21.8%. Due to better fuel atomization quality together with more active components generated by gliding arc plasma, flame ignition limit of bluff-body flameholder was obviously extended by about 13%∼30%. The gliding arc plasma was verified to promote fuel droplets atomization and to achieve homogeneous distribution under low temperature and pressure, which is beneficial to wider flame ignition and stabilization boundary.

Collision of water drops with a thin cylinder

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

The dynamical behaviors of water drop impacting and bouncing on an inclined hydrophobic surface

Drop impacting on an inclined surface is currently a topical research area due to its broad industrial applications. However, a comprehensive investigation of drop bouncing outcomes is relatively lacking in the literature, especially at a high (> 60°) inclined angle. In this paper, the dynamical behaviors of water drop impacting and bouncing on an inclined hydrophobic surface are investigated with a numerical method. Specifically, we examine the effects of several key impacting factors including impacting velocity (Vi = 0.5–2.0 m/s), drop diameter (D0 = 1.5–2.5 mm), and inclined angle (β = 0°–80°) on impacting outcomes, sliding lengths of drop, drop leading and trailing edges (Ld, Llea, and Ltra), spreading diameters and maximum spreading diameters in both the tangential and lateral directions (Dtan, Dlat, Dtan_max, and Dlat_max). Four distinct outcomes take place on the variations of Vi, D0, and β. For the rebound and partial rebound outcomes, drop behaviors at β = 80° are quite different from β ≤ 60°, which is mainly caused by the liquid accumulation at drop head resulting from the drastic increase in tangential inertial force. For the same reason, the temporal variations of Dlat are more stable than Dtan, especially at a high β. Both Dtan_max and Dlat_max increase with Vi and D0 but reduce with β, owing to the increased normal inertial force. More significantly, scaling laws of the nondimensionalized maximum spreading diameters with the normal Weber number Dtan_max*∝WeNα1 and Dlat_max*∝WeNα2 are discovered where α1 and α2 decrease as β increases due to the smaller variations of Dtan_max* and Dlat_max* at larger β when impacting condition changes. These new findings provide a useful supplement to the currently insufficient understanding of the complicated phenomena of post-impacting drop behaviors on an inclined hydrophobic surface.

Quality estimation of the nozzle spray by measuring the brightness of the reflected light

This paper presents the results of the laboratory and numerical experiments performed to measure the sizes of transparent liquid droplets sprayed in air. The results of the laboratory experiments were mainly obtained using the Glare Point Technique (GPT) which gave information about the droplet size and the brightness of the light reflected by drops. The relationship between the brightness of the light reflected from the surface of droplets and their sizes was analyzed. Theoretically, the brightness of light scattered by a single spherical drop is proportional to the drop surface area and, accordingly, to the square of the drop diameter. It has been observed experimentally and verified numerically that the theoretical dependence obtained is relevant only for the brightest droplets because of nonuniform illumination. The results of the numerical experiments with a random sample of drops indicated the dependence of the total brightness of reflected light on the effective droplet size. It is shown that, for a fixed total volume, the total brightness of light reflected by drops is proportional to the droplet Sauter mean diameter.

Orographically Modified Ice-Phase Precipitation Processes During the Olympic Mountains Experiment (OLYMPEX)

AbstractOver mountainous terrain, windward enhancement of stratiform precipitation results from a combination of warm-rain and ice-phase processes. In this study, ice-phase precipitation processes are investigated within frontal systems during the Olympic Mountains Experiment (OLYMPEX). An enhanced layer of radar reflectivity (ZH) above the melting level bright band (i.e., a secondary ZH maximum) is observed over both the windward slopes of the Olympic Mountains and the upstream ocean, with a higher frequency of occurrence and higher ZH values over the windward slopes indicating an orographic enhancement of ice-phase precipitation processes. Aircraft-based in situ observations are evaluated for the 01-02 and 03 December 2015 orographically-enhanced precipitation events. Above the secondary ZH maximum, the hydrometeors are primarily horizontally oriented dendritic and branched crystals. Within the secondary ZH maximum, there are high concentrations of large (&gt; ~2 mm diameter) dendrites, plates, and aggregates thereof, with a significant degree of riming. In both events, aggregation and riming appear to be enhanced within a turbulent layer near sheared flow at the top of a low-level jet impinging on the terrain and forced to rise above the melting level. Based on windward ground sites at low-, mid-, and high-elevations, secondary ZH maxima periods during all of OLYMPEX are associated with increased rain rates and larger mass-weighted mean drop diameters compared to periods without a secondary ZH maximum. This result suggests that precipitation originating from secondary ZH maxima layers may contribute to enhanced windward precipitation accumulations through the formation of large, dense particles that accelerate fallout.

Exploration of modified cyclone separator for the enhanced recovery of inhalable spray dried cubosomal powder intended to be used for lung delivery

The study was conducted with the aim to fabricate a cyclone separator capable of providing cubosomal powder with enhanced recovery, uniform size, and high aerosolization efficiency. For that purpose, Classical cyclone design (CCD) and Texas cyclone design (TCD) process parameters (number of effective turns (Ne), inlet air velocity (Vi), pressure drop (ΔP), and cut point diameter (dpc)) were utilized to obtained five geometrical parameters (length of cyclone body (Lb), length of cyclone cone (Lc), height (H) and width (W) of inlet section and air exit diameter (De)) of modified cyclone (type XVII). The design analysis predicted the lowest dpc of 1.5 μm along with an enhanced amount of cubosomal powder collection. The Design of Experiment (DoE) and Signal to Noise (S/N) ratio were employed to evaluate the effect of formulation, process parameter, and cyclone design on device removal efficiency, aerosolization efficiency, fine particle fraction, and yield of cubosomal powder. The modified cyclone provides 1.8 fold enhanced cubosomal powder recovery, which has been associated with increased ΔP inside the cyclone separator. The optimized inhalable spray-dried cubosomal powder possess a sufficient geometrical diameter (dG) (3.9 ± 0.25 μm) and aerodynamic diameter (dA) (2.04 ± 0.15 μm) to deliver formulation to deeper lung tissues. Overall, the findings pointed towards the industrial benefits of developed cyclone design as it experimentally provides enhanced recovery of cubosomal powder in the size range of 1–5 μm with its desirable aerosolization properties.

Effect of cross-slot configuration in microfluidics on o/w emulsification at high throughput

The Liquid–liquid dispersion process is experimentally investigated to manufacture oil-in-water emulsion working at high flowrates in the cross-slot type of microfluidics. Two cross layouts, namely symmetric and asymmetric configurations, are compared via characterizing the droplet size and size distribution. Automated granulometry is implemented on the images taken by microscopy observations of the emulsion samples. High-speed shadow photography is carried out to discover the continuous- and dispersed-phase flow interactions in the vicinity of the collision region. The results show that the designed microsystems present great potential in terms of fabricating fine oil droplets distributed in the final emulsion. The arithmetic averaged diameter is less than 10 μm at all tested flow conditions and the minimum mean diameter reaches 3.9 μm at the highest energy consumption case. Because of the higher shear stress and more intensified interaction, the symmetric geometry of the cross-slot is beneficial to create fewer amounts of large droplets and dispersing the oil phase more uniformly at the same hydrodynamic conditions, especially in the low Reynolds flow case in this study. As the flowrate is enhanced, the disparity between them is diminished due to the instability inside the channel reaching a high level. The mean drop diameter for both systems is capable to be scaled with the emulsion velocity-based Weber number. The detrimental effect of the symmetric configuration is that the energy required to burst the dispersed streams is relatively a little higher than with asymmetric one.

Splashing generation by water jet impinging on a horizontal plate

The report of COVID-19 virus in municipal wastewater raises the question of whether viruses can become airborne during wastewater transport in sewer systems. The present work experimentally investigates a water jet impinging vertically onto a horizontal plate and the behaviours of the generated tiny droplets. Depending on whether the jet breaks into primary drops before the impingement, three regimes can be defined: non-splashing, jet-splashing and drop-splashing regimes. The splashing ratio, i.e., the portion of jet flow rate becoming splashing droplets, ranges from 1% to 70% in the drop-splashing regime, while it remains less than 2% in the jet-splashing regime. For the splashing droplets, their size and velocity distributions follow log-normal laws. Their diameters are mainly in the range from 0 to 0.3 of the impact jet or drop diameter with the median less than 0.1. Their velocities mostly range from 0 to 3.0 times of the impact velocity with the median around 1.0. The medians of both the dimensionless diameter and velocity of splashing droplets decrease with the impact Weber number. The ejection angles of splashing droplets obey a bell-shaped distribution with the maximum around 70° and the median ranging from 16° to 30°.

Size Effects on Droplet Displacing Process in Micropores by Multiscale Modeling

Transport mechanisms of small droplets on walls in micropores become significant for applications in energy, resource and biomedical engineering, however, a suitable numerical tool remains challenging. Macroscopic approach is ideal both in computing cost and simplicity but its applicability is doubted for nanoscale droplet, yet no clear evaluation on when exactly does it become invalid has been made. This work evaluates the applicability of macroscopic approach for the displacing process of droplet in a micropore and investigates relevant size effects, by comparing the simulation results of multiscale modeling and macroscopic method. Three types of size effects affecting the displacement results are identified: Laplace pressure, low interfacial density, and breakdown of macroscopic description. For the system studied, the Laplace pressure dominates for relatively big droplet, then low density region becomes significant for drop diameter smaller than 18 times molecule diameter, and finally macroscopic description gradually fails for drop diameter smaller than 13 times molecule diameter. We further investigate the influences of system scale and fluid type on these size effects and discuss the relative importance of each size effect under different conditions. Results indicate that traditional macroscopic approach may be invalid even when continuum assumption still holds due to other size effects, and corrections for those effects can be made to extend the applicability of macroscopic method.