Droplet Number Density Research Articles

Fabricating Tunable Superhydrophobic Surfaces Enabled by Surface‐Initiated Emulsion Polymerization in Water

AbstractFabricating controllable superhydrophobic surfaces remains challenging in various fields ranging from chemical industries to biomedical engineering. Conventional methods commonly require volatile organic solvents and the assistance of special surface deposition and modification equipment, which are detrimental to environment and limit their applications in micro‐devices. Herein, an equipment‐free method is reported to directly transform fluorinated monomer micro‐droplets into hydrophobic polymer particles on flat substrate surfaces in water, simultaneously depositing hydrophobic coatings with tunable surface structures. The as‐prepared surfaces show superior superhydrophobicity and great stability in extreme conditions (e.g., varying acidity, basicity, and heating conditions), and excellent anti‐fouling property. Meanwhile, surface hydrophobicity can be manipulated by adjusting emulsion droplet number density and reaction time. Hence, superhydrophobic surfaces with tunable hydrophobicity gradients have been successfully fabricated in one pot. This study provides an equipment‐free method to facilely fabricate controllable superhydrophobic surfaces, with great potential in the development of smart superhydrophobic materials in various engineering and industrial applications.

Aerosol-cloud-precipitation interaction during some convective events over southwestern Iran using the WRF model

Four convective precipitation events in southwestern Iran are examined in this study using the aerosol-aware bulk microphysical scheme implemented in the Weather Research and Forecasting (WRF) model. Two simulations were conducted for each of the events, which included the control and polluted simulations. In the control simulation, the concentration of aerosols in the current climate is considered, while the concentration of aerosols was increased by a factor of 5 at all grid points in the polluted simulation. The aim of this research is to evaluate the effects of aerosols on cloud microphysics and precipitation. During the convective events, southerly to southwesterly warm and dry winds dominated, causing a substantial transport of aerosols and humidity. Relatively high values of planetary boundary-layer height (PBLH) are simulated in the polluted simulations, which indicate a higher convective activity and more efficient transport of aerosols and moisture in the vertical. Also, the convective available potential energy (CAPE) increases in polluted simulations, implying that an increase in the concentration of aerosols is associated with more favorable thermodynamic conditions for convection. In all simulations, the reflection of shortwave radiation by clouds increases, indicating that the first indirect effect of aerosols plays a key role in the energy budget of the Earth. On the other hand, an increase in the concentration of aerosols only slightly influences longwave radiation. Altitudes of the lifting condensation level (LCL) and the level of free convection (LFC) increase in all polluted compared to control simulations. This indicates that the air parcel needs to rise to a higher altitude to get saturated. The impact of an increase in the concentration of aerosols on cloud development is substantial in a simulation that contains a higher convergence of vertical moisture flux and stronger winds over the region. The convergence of the vertical moisture flux in the three simulations indicates that more water vapor is available in these simulations to be condensed on aerosols, which increases the cloud water content. Thus, the number density of cloud droplets is higher in the polluted compared to control simulations. The altitude of the maximum mass density of cloud droplets is nearly the same in the simulation (between 3 and 6 km). Due to higher specific humidity in these altitudes, higher water vapor can be condensed on condensation nuclei. An increase in ice and snow, which indicates a higher lifting of droplets to the freezing level, is seen in the simulations with the negative convergence of vertical moisture flux. This indicates that these regions may help the large-scale collection of moisture and its lifting. In simulations with a divergence of moisture in the northern and the whole domain, the cloud water content decreases. In simulations with a higher moisture difference, there is higher precipitation in the polluted compared to control simulations because, in the humid atmosphere, there is enough water vapor to be condensed on aerosols, which leads to the formation of larger cloud droplets. Thus, the collision of cloud droplets is more efficient, and precipitation increases. In addition, due to a lower cloud base, there is less chance for the evaporation and melting of precipitation.

Label-free third harmonic generation imaging and quantification of lipid droplets in live filamentous fungi

We report the utilization of Third-Harmonic Generation microscopy for label-free live cell imaging of lipid droplets in the hypha of filamentous fungus Phycomyces blakesleeanus. THG microscopy images showed bright spherical features dispersed throughout the hypha cytoplasm in control conditions and a transient increase in the number of bright features after complete nitrogen starvation. Colocalization analysis of THG and lipid-counterstained images disclosed that the cytoplasmic particles were lipid droplets. Particle Size Analysis and Image Correlation Spectroscopy were used to quantify the number density and size of lipid droplets. The two analysis methods both revealed an increase from 16 × 10−3 to 23 × 10−3 lipid droplets/µm2 after nitrogen starvation and a decrease in the average size of the droplets (range: 0.5–0.8 µm diameter). In conclusion, THG imaging, followed by PSA and ICS, can be reliably used for filamentous fungi for the in vivo quantification of lipid droplets without the need for labeling and/or fixation. In addition, it has been demonstrated that ICS is suitable for THG microscopy.

Characterization of the in-focus droplets in shadowgraphy systems via deep learning-based image processing method

It is important to accurately identify and measure in-focus droplets from shadowgraph droplet images that typically contain a large number of defocused droplets for the research of multiphase flow. However, conventional in-focus droplet identification methods are time-consuming and laborious due to the noise and background illumination in experimental data. In this paper, a deep learning-based method called focus-droplet generative adversarial network (FocGAN) is developed to automatically detect and characterize the focused droplets in shadow images. A generative adversarial network framework is adopted by our model to output binarized images containing only in-focus droplets, and inception blocks are used in the generator to enhance the extraction of multi-scale features. To emulate the real shadow images, an algorithm based on the Gauss blur method is developed to generate paired datasets to train the networks. The detailed architecture and performance of the model were investigated and evaluated by both the synthetic data and spray experimental data. The results show that the present learning-based method is far superior to the traditional adaptive threshold method in terms of effective extraction rate and accuracy. The comprehensive performance of FocGAN, including detection accuracy and robustness to noise, is higher than that of the model based on a convolutional neural network. Moreover, the identification results of spray images with different droplet number densities clearly exhibit the feasibility of FocGAN in real experiments. This work indicates that the proposed learning-based approach is promising to be widely applied as an efficient and universal tool for processing particle shadowgraph images.

Spatial evolution of multi-scale droplet clusters in an evaporating spray

Evaporative sprays are encountered in a wide range of engineering applications. Since clustering of droplets in sprays leads to strong inhomogeneity in the spatial distribution of droplet concentration that impacts mass, momentum, and energy exchange between the spray and the surrounding flow, a detailed investigation of droplet clustering in evaporating sprays is important. In the current research work, we experimentally investigate the spatial evolution of droplet cluster characteristics in an evaporating acetone spray injected from an air-assist atomizer. The droplet size and velocity are measured using Interferometric Laser Imaging for Droplet Sizing technique. In detail, characterization of the droplet clusters is achieved by the application of Voronoi analysis to particle image velocimetry images of the spray droplets. This approach not only identifies the droplet clusters but also provides area, length scale, and local droplet number density within the clusters. The identified droplet clusters are multi-scale and could be classified into either large- or small-scale clusters, which scale with spray half-width and Kolmogorov length scale, respectively. Experiments are also conducted in water spray under the same operating conditions. Despite the similarity in the droplet clustering process between the two sprays at small scales of air turbulence, some distinct trends are observed for the large-scale clusters in the acetone spray. This is attributed to the higher evaporation rate of acetone droplets, which promotes preferential accumulation of droplets.

Droplet cluster evolution and collective gasification of droplet groups in a fuel spray: A comparative study under non-reacting and reacting conditions

In this paper, the goal is to understand the influence of spray combustion on the evolution of droplet clusters and collective gasification processes of droplet groups in a fuel spray. Experiments were conducted in an acetone spray from a pressure swirl injector under both non-reacting and reacting conditions for the same fuel flow rate through the injector. The PIV technique was employed to capture Mie-scattering images of the spray and to measure droplet velocity, while droplet sizing was achieved by application of the ILIDS technique. Voronoi analysis of the PIV images facilitated identification of droplet clusters. Accordingly, the cluster length scale and local droplet number density within clusters could be obtained. In addition, the Group evaporation/combustion number (G) for individual clusters was also evaluated. Either in the presence or absence of spray combustion, wide range of cluster size exists in the spray which highlights multi-scale clustering of droplets. Accordingly, G varies in a broad range that indicates multi-mode evaporation/combustion of the droplet clusters. Distinct behavior of droplet clusters is identified depending on the cluster size relative to the mean cluster area. While the evolution of small clusters is not sensitive to the presence of the spray flame, the length scale of the large-scale clusters and gasification process of such groups of droplets are modified due to spray combustion compared to the non-reacting condition.

Experimental Investigation on the Spray Behaviour of Bluff Body Air-Assisted Atomizer Designs

This study investigates the gas dynamic effects and atomization behavior of a novel sonic bluff body-assisted two-fluid atomizer with three different geometric configurations based on airflow orifice diameters (d) of 2.0 mm, 3.0 mm, and 4.0 mm. Along with a 280 µm annular liquid sheet, atomizers that employed a central bluff body (cone) with 6.0 mm cone distance (Lc) are compared based on the range of different air and liquid (water) flow rates. The spray-bluff body-impacted secondary atomization was characterized through volume-normalized droplet size distribution (DSD) and cumulative droplet distribution, excentricity plots, Sauter mean diameter (SMD), and relative span factor (Δ). Droplet number density decreases with the increase in radial location, with lesser droplet density for the 3.0 mm atomizer. DSD and cumulative droplet distribution become less uniform with the increase in the radial locations with wider distribution for larger diameter atomizers (4.0 mm). Droplet excentricity follows an inverse relationship with the droplet diameter such that high diameter droplets have low excentricity (%) and vice versa. SMD and relative span factor (RSF) showed opposite trends when plotted (line plots) against the air-to-liquid ratio (ALR) with larger fluctuation in the SMD than the RSF (Δ) value. The spray pattern spread increases gradually with increasing liquid loading and with decreases in the ALR value for all atomizers.

Internal group combustion of droplet clouds and its transition to external group combustion under two-stage autoignition conditions

The present study is an extension of our prior study on the external group combustion (EGC) of droplet clouds under two-stage autoignition conditions [Zhou and Liu, Combust. Flame 234 (2021) 111689]. Effects of droplet heating and different n-alkane fuel candidates have been further considered. By comparing numerical results from n-heptane and n-dodecane, it was found that the fuel volatility has little effect on the combustion modes of droplet clouds. Among all the computed cases, internal group combustion (IGC) occurs for relatively low value of group ignition number Gig (= 4πn2/3d2, n and d are the droplet number density and diameter, respectively). When Gig further increases, the combustion mode can switch to EGC. IGC modes can be sustained by cool or hot flame. IGC of cool flame exhibits a partial burning structure, while IGC of hot flame is firstly premixed, followed by non-premixed structure. These flame structures were analyzed in details. A regime diagram based on Gig and ambient temperature Ta was developed for all the IGC modes. It was found that for high Ta (1500 K), Gig can be used to predict ICG modes, but for low Ta (900 K) and intermediate Ta (1200 K), additional parameters might be needed for the prediction, especially for the cases when Gig is around 0.03. The inapplicability is due to the strong interaction between droplet behavior (droplet heating and vaporization) and chemical kinetics (low and high temperature reaction) that were not inherent to description of Gig. To clarify the interaction, time scale analysis of these processes was performed for better complementation of the regime diagram characterized by Gig and Ta for various IGC modes.

Planar Rainbow Refractometry For Size, Refractive Index And Position Measurement Of Droplets In A Plane

Rainbow refractometry can simultaneously measure the temperature and geometric parameters of droplets by analyzing the recorded primary rainbow scattering signal of the droplet. The technique therefore has high potential in measurement applications involving reacting/non-reacting sprays. This study develops the planar rainbow refractometer technique, expanding the measurement dimension from a single "line" measurement to a "planar" measurement. The position of the droplet in the plane is determined by the contour of the rainbow signal, whereas the size and refractive index of the droplet are derived from the extracted rainbow image using the calibrated scattering angle. A simple and compact rainbow optical measurement system is constructed based on horizontal slit modulation, enabling the recording of spatially discrete rainbow signals of droplets in a plane. A novel scattering angle calibration method using a precisely traversed monodisperse droplet stream with a priori parameters is proposed and the algorithm for obtaining the calibration coefficients is evaluated by simulation. Spray verification experiments were carried out. The signal processing consists of image recognition, particle localization, and the rejection of non-spherical droplets. The possibility of estimating droplet number density is also discussed.

Response of spray number density and evaporation rate to velocity oscillations

A theoretical investigation of the effect of gas velocity oscillations on droplet number density and evaporation rate is presented. Oscillations in gas velocity cause a number density wave, i.e. an inhomogeneous, unsteady variation of droplet concentration. The number density wave, as it propagates downstream at the mean flow speed, causes modulation of the local evaporation rate, creating a vapour wave with corresponding oscillations in equivalence ratio. The present work devises an analytical formulation of these processes. Firstly, the response of a population of droplets to oscillations in the gas velocity is modelled in terms of a number density wave. Secondly, the formulation is extended to incorporate droplet evaporation, such that an analytical expression for the evaporation rate modulation is obtained. Subsequently, the droplet 1D convection-diffusion transport equation with the calculated evaporation source term is solved using an appropriate Green’s function to determine the resulting equivalence ratio perturbations. The dynamic response of equivalence ratio fluctuations to velocity oscillations is finally characterized in terms of a frequency-dependent transfer function. The aforementioned analytical approach relies on a number of simplifying approximations, nevertheless it was validated with good agreement against 1D Euler-Lagrange CFD simulations.

Resolved and subgrid-scale crossing trajectory effects in Eulerian large eddy simulations of size-dependent droplet transport

We study the dispersion characteristics of slightly buoyant droplets in a turbulent jet using large eddy simulations (LES). The droplet number density fields are represented using an Eulerian approach, with the dispersed phase modelled using the Fast-Eulerian method (Ferry & Balachandar, Intl J. Multiphase Flow, vol. 27, issue 7, pp. 1199–1226, 2001) that includes the droplet rise velocity. Radial concentration profiles and turbulent concentration fluxes for droplets of different sizes are analysed to quantify the ‘trajectory crossing effect’, when relative motions between particles and turbulent eddies tend to reduce turbulent diffusion. For finer LES grid resolutions, the model captures the differential, size-based dispersion characteristics of the droplets with the transverse dispersion of the larger droplet sizes suppressed, since trajectory crossing effects are explicitly resolved in LES. We examine a similarity solution model for the size-dependent radial concentration profiles based on a modified Schmidt number derived from the theory of turbulent diffusion of particles in the atmosphere proposed by Csanady (J. Atmos. Sci., vol. 20, pp. 201–208, 1963). The results are validated with the high resolution LES data and show good agreement. Then the size-dependent Schmidt number model is reformulated as a model for unresolved subgrid-scale trajectory crossing effects and used to calculate the subgrid concentration flux in a coarse LES of a turbulent jet, with slightly buoyant droplets injected at the centreline in the self-similar region of the jet. The results are compared to a simulation with higher grid resolution and a coarse simulation with a constant Schmidt number subgrid-scale model. We find that the subgrid model enhances the prediction accuracy of the concentration profiles and turbulent concentration flux for the coarse LES.

Large scale unsteadiness during self-pulsation regime in a swirl coaxial injector and its influence on the downstream spray statistics

The focus of the present paper is to understand the large scale unsteadiness in the atomization of swirl coaxial jets and correlate it with the downstream spray statistics. A gas-centered swirl coaxial atomizer, in which a gaseous jet fragments an annular swirling liquid sheet, displays pulsating flow at certain momentum flux ratios (MFRs). These conditions were chosen to study the unsteady dynamics in the spray formation. Various zones of the spray such as near orifice region, primary atomization zone and far field spray were diagnosed using high-speed shadowgraphy technique. Proper orthogonal decomposition was employed on the time-resolved spray images to understand various unsteady modes. Three modes observed prominently were identified as large scale unsteady structures viz. axisymmetric pulsating mode, explosive mode and swirling mode. The pulsating mode was found to be more dominant in the pulsation regime, whereas the other modes gained significance at higher MFRs when the pulsation regime ceases to exist. Fourier analysis of temporal coefficients pertaining to pulsating mode showed a definitive frequency. The analysis of liquid shedding rate was found to be in synchronization with the pulsating mode which shows the upstream influence on periodic atomization. Spatio-temporal measurements of droplet sizes were carried out far from the atomizer. The temporal variation in the droplet number density and Sauter Mean Diameter depicted unsteady behavior; however, there is no preferred frequency in the power spectrum. This clearly indicates that the far field spray loses its memory of upstream periodic atomization in the pulsation regime of swirl coaxial atomizer.

Condensation heat transfer on phase change slippery liquid-infused porous surfaces

Phase change slippery liquid-infused porous surfaces (PC-SLIPSs) are presented with emphasis on surface wetting characteristics and droplet dynamics influencing the water vapor condensation and the corresponding heat transfer. The functionalized nano-porous copper plate is infused with paraffin wax-xylene solution via dip coating method to prepare PC-SLIPSs., where the droplet dynamics are found to be affected by low adhesion (sliding angle α of 45±5°), high adhesion (α of 60±5°) and slippery (α of 3 ± 1°) states enabled by solid (at 48 °C), mush (at 58 °C) and liquid (at 66 °C) phases, respectively. Besides the wettability and droplet adhesion characterization, the condensation heat transfer is particularly explored on PC-SLIPSs in custom-built vacuum-assisted condensation rig. Two major dropwise condensation mechanisms are unveiled depending on the phase of the infused phase change material, such as coalescence-induced droplet shedding coupled with droplet sweeping in the solid and mush phases, while discrete-droplet shedding and sweeping in the absence of additional coalescence are reported in the liquid phase. Approximately, 136.8% higher heat transfer coefficients for PC-SLIPS in the liquid phase are reported when compared with the pristine copper surface at low sub-cooling temperatures. In the liquid phase of PC-SLIPSs, theoretical heat transfer modeling on dropwise condensation has been further carried out to elucidate the heat transfer mechanism via a single droplet coupled with the droplet number density. The operational durability of PC-SLIPSs has been found to last for 8 ± 1 h as confirmed during rigorous condensation experiments. In summary, different wetting features, various droplet shedding mechanisms, promising dropwise condensation modes, high heat transfer coefficient in the liquid phase, and effective time-dependent durability are the salient findings associated with PC-SLIPSs, rendering them competitive alternatives. The most important implication is that the dropwise condensation mode can also underperform if the droplet dynamics is inefficient.

Two-phase nozzles performances CFD modeling for low-grade heat to power generation: Mass transfer models assessment and a novel transitional formulation

The use of two-phase nozzles for low-grade heat valorization by electricity production increases the energy recovery rate using Trilateral Flash or Wet to Dry cycles. A model benchmark for nozzle flow rate and efficiency estimation was conducted on experimental data from the literature. These data come from a series of tests made on geothermal energy production water two-phase nozzles. A new transitional bubble-to-droplet interfacial area density formulation (TA-BD model) is presented by the paper. It is compared to three models from literature. Two nozzles operating with different inlet and outlet conditions were modeled. The calibration flexibility and the robustness of the models are discussed in association with physical analysis. The paper shows how the models using single or redundant adaptation parameters fail to provide good results simultaneously on flow rate and efficiency. It appeared that the TA-BD model is the more flexible and robust. The Homogeneous Relaxation Model (HRM) model gives also good results. Furthermore, TA-BD model gives the lowest average discrepancies. Especially at the best efficiency point of the first test case, TA-BD model shows<1% discrepancy where the HRM model has 18% discrepancy in efficiency. The TA-BD model appeared to be easier to calibrate than the HRM model. Finally, regarding the proposed TA-BD model, the sensitivity to the geometry and operating conditions shows that the interfacial area density formulation could be completed to include the effect of nozzle's section profile, the effect of the inlet temperature on bubbles number density, and the effect of outlet pressure on the droplets number density.

Droplet Formation and Growth Mechanisms in Reaction-Induced Spontaneous Emulsification of 3-(Trimethoxysilyl) Propyl Methacrylate.

Spontaneous emulsification of 3-(trimethoxysilyl) propyl methacrylate (TPM) can produce complex and active colloids, nanoparticles, or monodisperse Pickering emulsions. Despite the applicability of TPM in particle synthesis, the nucleation and growth mechanisms of TPM emulsions are still poorly understood. We investigate droplet formation and growth of TPM in aqueous solutions under quiescent conditions. Our results show that in the absence of stirring the mechanisms of diffusion and stranding likely drive the spontaneous emulsification of TPM through the formation of co-soluble species during hydrolysis. In addition, turbidity and dynamic light scattering experiments show that the pH modulates the growth mechanism. At pH 10.1, the droplets grow via Ostwald ripening, while at pH 11.5, the droplets grow via monomer addition. Adding surfactants [Tween, sodium dodecyl sulfate (SDS), or cetyltrimethylammonium bromide] leads to <100 nm droplets that are kinetically stable. The growth of Tween droplets occurs through addition of TPM species while the number density of droplets is kept constant. In addition, in the presence of the ionic surfactant SDS, electrostatic repulsion between the solubilized TPM species and SDS leads to a significant increase in the number density of droplets as well as additional nucleation events. Finally, imaging of the solubilization of TPM in capillaries shows that in the absence of a surfactant, TPM hydrolysis is likely the rate-limiting step for emulsification, whereas the presence of silica particles in the aqueous phase likely acts as a catalyst of TPM hydrolysis. Our experiments highlight the importance of diffusion and solubilization of TPM species in the aqueous phase in the nucleation and growth of droplets.

Physical characteristics of splash and spray clouds produced by heavy vehicles (trucks and lorries) driven on wet asphalt

Heavy vehicles rolling on wet roads produce splash and spray clouds. These aerosols reduce the visibility of other drivers, contribute to a small, but quantifiable proportion of road traffic accidents and affect the operational capabilities of autonomous vehicles travelling near them. Even though knowing the physical properties of these aerosols is essential for testing and validating sensors for environment perception and recognition of autonomous vehicles, there is little information about them.In this work the physical characteristics of spray clouds produced by heavy vehicles rolling on wet asphalt were measured by optical methods. Time resolved droplet size, mass concentration, number density, light extinction and contrast attenuation parallel and perpendicular to the travelling direction of the vehicle were measured. Vehicle velocity, vehicle configuration and water depth were varied during the tests.Results show that the average droplet diameter ranges between 100 and 400 μm with maximum diameters of almost 4 mm. Mass concentration gamuts between 0,2 and 0,7 kg/m3 with peaks surpassing 1 kg/m3 while number density spans between 20 and 40 cm−3 and occasionally exceeds 100 cm−3. Light extinction can reach levels as high as 0,2 m−1 and contrast, evaluated from images, can reach values under 0,1.

Experimental investigation on fluid structure and full-cone spray characteristics of a jet-swirl atomizer using shadowgraph technique

Full-cone spray is quite important in spray cooling and catalytic combustion applications; however, it is not extensively studied. Besides, the liquid spray is relatively a non-uniform structure especially along longitudinal axis which includes different sizes and distribution of droplets. The few published experimental studies are limited to calculate some of the spray characteristics on a certain plane located downstream of the nozzle exit. Therefore, the spray parameters representing fluid structure, droplets mean diameter, and their distribution in different cross sections from nozzle exit are considered in this study. Accordingly, a jet-swirl atomizer with pressure-swirl full-cone spray is investigated where all important full-cone spray characteristics are considered at different planes from nozzle exit. The spray images are obtained with a shadowgraph technique and are analyzed to obtain the Sauter mean diameter (SMD), D10, and droplet size distribution along with the spray structure, spray cone angle, and discharge coefficient. The experimental results are verified based on the pre-published numerical studies on the same atomizer. The experimental and numerical results show good agreement. Moreover, the results show that the SMD is increased by moving away from center of spray to its edges, and the droplets number density is increased in central regions. The increased droplets number density leads to the greater external forces which create smaller droplets. In contrast, larger particles exist in peripheral parts due to the less droplets concentration. Furthermore, and far away from the exit nozzle, the SMD values are decreased due to the increased aerodynamic forces and oscillations. The droplets dispersion including spray density in radial and axial directions is also observed using spray density images.

Mid-infrared spectroscopy of hemispherical water droplets

As an important component of atmospheric aerosols, water is profoundly related with aerosol hygroscopicity and provides a medium for atmospheric heterogeneous reactions. The quantitative analysis of water content in aerosol droplets is instrumental to understanding atmospheric chemistry, as well as to addressing the related environmental issues, such as air pollution and climate change. Fourier transform infrared (FTIR) spectroscopy has been widely adopted to quantify the amount of water content in atmospheric aerosols, which is based on the absorbance of OH functional group in proportion to water content. However, in the OH stretching vibration band around 3400 cm−1, spectral distortions may occur, making a quantitative analysis impossible. In addressing this issue, here we put forth a model to simulate the FTIR absorption of hemispherical water droplets, along with a quantitative description of the spectral distortion. Our model prediction was benchmarked with the microscopic-FTIR experiments conducted on sodium sulfate droplets, and good agreements between theoretical and experimental results were found. We observed that the absorbance spanning across the mid-wavenumber infrared region increases with water absorption coefficients; while such an increasing trend was not seen in the 3400 cm−1 band. We speculate that the spectral saturated absorption is related to the absorption coefficient of water and the ratio of the projected area of droplets to the aperture area. In addition, the effects of droplet size and number density on the absorption spectra were investigated. The waveband range of the saturated absorption broadens with an increase in droplet radius. On the other hand, as the number density of water droplets increases, the absorption at 3400 cm−1 is enhanced, and the characteristic peak of condensed water becomes increasingly sharper, asymptotic to the typical infrared spectra of water collected by the pressing method.

How coronavirus survives for hours in aerosols.

COVID (CoronaVirus Disease)-19, caused by severe acute respiratory syndrome-CoronaVirus-2 (SARS-CoV-2) virus, predominantly transmits via airborne route, as highlighted by recent studies. Furthermore, recently published titer measurements of SARS-CoV-2 in aerosols have disclosed that the coronavirus can survive for hours. A consolidated knowledge on the physical mechanism and governing rules behind the significantly long survival of coronavirus in aerosols is lacking, which is the subject of the present investigation. We model the evaporation of aerosolized droplets of diameter m. The conventional diffusion-limited evaporation is not valid to model the evaporation of small size (μm–nm) droplets since it predicts drying time on the order of milliseconds. Also, the sedimentation timescale of desiccated droplets is on the order of days and overpredicts the virus survival time; hence, it does not corroborate with the above-mentioned titer-decay timescale. We attribute the virus survival timescale to the fact that the drying of small (m–nm) droplets is governed, in principle, by the excess internal pressure within the droplet, which stems from the disjoining pressure due to the cohesive intermolecular interaction between the liquid molecules and the Laplace-pressure. The model predictions for the temporal reduction in the aerosolized droplet number density agree well with the temporal decay of virus titer. The findings, therefore, provide insight on the survival of coronavirus in aerosols, which is particularly important to mitigate the spread of COVID-19 from indoors.

Investigation of theoretical scaling laws using large eddy simulations for airborne spreading of viral contagion from sneezing and coughing.

Using a set of large eddy point-particle simulations, we explore the fluid dynamics of an ejected puff resulting from a cough/sneeze. The ejection contains over 61 000 potentially virus-laden droplets at an injection Reynolds number of about 46 000, comparable to an actual cough/sneeze. We observe that global puff properties, such as centroid, puff volume, momentum, and buoyancy vary little across realizations. Other properties, such as maximum extent, shape, and edge velocity of the puff, may exhibit substantial variation. In many realizations, a portion of the puff splits off and advances along a random direction, while keeping airborne droplet nuclei afloat. This peeled-off portion provides a mechanism for virus-laden droplets to travel over large distances in a short amount of time. We also observe that the vast majority of droplets remain suspended within the puff after all liquid has evaporated. The main objectives of the study are to (i) evaluate assumptions of Balachandar's et al. theory [Int. J. Multiphase Flow 132, 103439 (2020)], which include buoyancy effects, shape of the puff, and droplet evaporation rate, (ii) obtain values of closure parameters, which include location and time of the virtual origin, and puff entrainment and drag coefficients, and (iii) evaluate the accuracy of the theory in predicting the shape, size, and location of the puff, as well as droplet number density long after ejection. The theory adequately predicts global puff properties including size, velocity, and distance traveled, the largest size of droplets that exit the puff due to settling, and the droplet size distribution within the puff long after ejection.