Droplet Lifetime Research Articles

Evaporation characteristics of ethanol droplet in different gases under force convection condition

Ethanol is recognized as one of the finest alternative bio-fuels due to its natural characteristics. Fundamental studies on ethanol droplet evaporation process are mainly either in stagnant environment or using some semi-empirical co-relations. Here, a fully numerical model based on the first principle is solved to investigate the effects of various atmospheric gases (Ar, N2, O2 and CO2) on droplet evaporation phenomenon under force convective situation. Two-dimensional governing equations of species, momentum and energy transfer of spherical coordinate system are solved, and the simulation is validated quantitatively with the literature. Uniform convective strength (Re = 100) is maintained for all cases examined at T∞ = 500 K and P∞ = 0.1 MPa. From the simulation results, it is observed that the viscosity ratio (liquid to gas) has effect on droplet life time. The ethanol droplet life time increases in less viscous atmospheric gases. The ethanol droplet life time is shorter in Ar gas, but heat-up period, wet-bulb temperature, and the surface blowing effect are higher in Ar compared to other atmospheric gases. The heat-up periods of the ethanol droplet in Ar, CO2, N2 and O2 atmosphere occupy around 30%, 20%, 17% and 16.5% of the total life time of the droplet, respectively. It is also noticed that, the heat-up period increases with increase in the thermal conductivity ratio (liquid to gas) and vice versa. Furthermore, the flow pattern of both gas- and liquid-phase in terms of streamline and the internal temperature distributions of the droplet are visualized at various time instants.

Evaporation characteristics of sessile droplets on flat hydrophobic surfaces in non-boiling regime

The evaporation of the sessile droplets is a frequent and complicated interfacial movement phenomenon that is broadly used in engineering technologies such as spray cooling. The investigation of the droplet evaporation on hydrophobic surfaces is of great significance for passively enhancing heat and mass transfer. In this paper, the evaporative traits of sessile droplets on a hydrophobic surface in non-boiling regime at constant temperature were examined. The deionized droplets volume ranged from 0.5 μL to 4.0 μL, and the surface temperature ranged from 30 °C to 70 °C. The variation law of the parameters including contact angle, contact line and droplet lifetime during the droplet evaporation process was investigated. The results demonstrate that the droplet evaporation time is positively correlated with the initial volume and the surface hydrophobicity, but negatively correlated with the surface temperature during droplet evaporation on the hydrophobic surface. The change mode of contact angle is significantly affected by the initial volume and the surface temperature, and the contact diameter is significantly affected by surface temperature. As the surface temperature and the droplet volume increases, the variation mode of contact angle is abundant. Evaporation of sessile droplets on superhydrophobic surface is more prone to contact line slip phenomenon than that on hydrophobic surface, which restrains the droplets evaporation to a certain extent. The evaporation of droplets on (super)hydrophobic surfaces also follows the V2/3 law.

Interaction and motion of two neighboring Leidenfrost droplets on oil surface

Evaporation of droplets on a hot oil surface is a natural phenomenon. However, most of existing studies focus on the evaporation of a single droplet, and the evaporation of multiple droplets is insufficiently understood. Here, we explore the Leidenfrost evaporation of two identical FC-72 droplets on the surface of a hot oil bath. The oil temperature ranges from 73.6 to 126.6 ℃, and the evaporation of droplets each with an initial diameter of 1.5 mm is recorded by an infrared thermographer and a high-speed camera. The shallow oil depth keeps the oil temperature uniform relatively in the slot compared with that in the deep liquid pool due to the larger ratio of the surface area for copper-oil contact to the slot volume. We find that the neighboring droplets evaporate in three stages: non-coalescing, bouncing, and separating. The radius of neighboring Leidenfrost droplets follows the power law <i>R</i>(<i>t</i>)~(1−<i>t</i>/<i>τ</i>)<sup><i>n</i></sup>, where <i>τ</i> is the characteristic droplet lifetime and <i>n</i> is an exponent factor. Moreover, the diffusion-mediated interaction between the neighboring droplets slows down the evaporation process compared with the action of isolated Leidenfrost droplet and leads to an asymmetric temperature field on the droplet surface, thereby breaking the balance of the forces acting on the droplets. A simple dual-droplet evaporation model is developed which considers four forces acting horizontally on the droplet, namely, the Marangoni force resulting from the non-uniform droplet temperature, the gravity component, the lubrication-propulsion force, and the viscous drag force. Scale analysis shows that the Marangoni force and gravity component dominate dual-droplet evaporation dynamics. In the non-coalescence stage, the gravity component induces the droplets to attract each other, while the vapor film trapped between droplets prevents them from directly contacting. When the droplets turn smaller, the gravity component is insufficient to overcome the Marangoni force. Hence, the droplets separate in the final evaporation stage. Finally, we conclude that the competition between Marangoni force and gravitational force is the origin of the bounce evaporation by comparing the theoretical and experimental transition times at distinct stages. This study contributes to explaining the complex Leidenfrost droplet dynamics and evaporation mechanism.

Investigation of supercritical transition and evaporation process of a hydrocarbon droplet under diesel engine-relevant conditions

Fuel spray supercritical transition in the diesel engine has been concerned widely, especially supercharged ones, however, whether the transition can significantly affect the fuel–air mixing under diesel engine-relevant conditions is an issue that still needs evaluating carefully. Thus, the transition and evaporation of individual n-heptane droplets in a nitrogen environment under various ambient conditions were investigated through a high-pressure evaporation model. The influence of ambient pressures on droplet lifetime (τlife) and evaporation rate constant (K) varies with ambient temperature. When Tr < 0.98, with increasing pressure, τlife first increases and then decreases, and K decreases. When 0.98 < Tr < 1.28, as pressure increases, τlife would increase even if K increases, because the increment of initial heat-up periods exceeds the decrease in quasi-steady evaporation periods. When Tr > 1.28, with increasing pressure, τlife decreases, and K increases. As the droplet heats up, the droplet interface is where the binary mixture would reach the critical mixing state if ambient conditions are high enough. Increasing ambient density decreases the minimum ambient temperature (Tm) required for the transition to occur. Above the Tm, transitions occur earlier with the increasing energy density of ambient gas. Increasing ambient density decreases the minimum ambient temperature required for diffusive fuel-gas mixing modes to dominate the mixing process. For droplets large enough to ignore gas–liquid interfacial thickness, the Tm required for the attainment of critical mixing states is almost independent of droplet diameter.

Numerical investigation of AdBlue film formation and NH3 conversion in generic SCR system using Eulerian stochastic fields method

In this paper, the Eulerian stochastic fields (ESF) method in a LES framework is applied to a generic selective catalytic reduction (SCR) configuration in order to retrieve seamlessly the effect of turbulence–chemistry interaction on the NH3-conversion and AdBlue film formation. The ESF method is based on the transport equation of a joint scalar filtered density function. The injection of AdBlue and relevant spray dynamics are modeled by the Lagrange-particle description where a computational parcel represents a cluster of real AdBlue droplets. Owing to the low injection pressure, weak atomization of AdBlue leads to intense droplet wall-impingement and film formation on the SCR duct wall and/or on the mixer elements, if present. A 2-D thin film approach is adopted to model the film formation and dynamics in combination with multi-regimes droplet–wall interaction model. After verification tasks against the reference data for simple gas phase reactive cases, the verified ESF method is coupled with a one-equation based LES turbulence model together with the Lagrange particle-tracking which is combined with the 2-D thin film approach to simulate the AdBlue injection and the film formation in the generic SCR configuration. At first, the assessment of the adopted LES mesh is carried out in terms of the so-called LES quality of index to determine the optimal mesh resolution. Next, to ensure the convergence with respect to the number of required ESF and mesh resolution, SCR simulations are performed by using various (2, 4, 6, 8, 12 and 16) stochastic fields and also with refined mesh. Thereby, the sensitivity of the predictive capability of the numerical tool is evaluated for the production of NH3 and HNCO with and without ESFs. This clearly indicates the importance of an accurate description of the turbulence–chemistry interaction to retrieve reliably the conversion of NH3. Finally, the film dynamics especially the evolution of the film thickness described with six required ESF and without ESF is compared with experimental data from the generic SCR configurations. To further demonstrate the potential of suggested numerical approach, a detailed numerical analysis is provided in terms of spray-impingement dynamics, scalar uniformity index, droplet life time and characteristics of the droplets prone to form solid deposits in the monolith channel.

Jetting Dynamics of Burning Gel Fuel Droplets.

Jetting in burning gel fuel droplets is an important process which, in addition to pure vaporization, enables the convective transport of unreacted fuel vapors from the droplet interior to the flame envelope. This aids in accelerating the fuel efflux and enhancing the mixing of the gas phase, which improves the droplet burn rates. In this study, Schlieren imaging was used to characterize different jetting dynamics that govern the combustion behavior of organic-gellant-laden ethanol gel fuel droplets. To initiate jetting, the gellant shell of the burning gel fuel droplet was subjected to either oscillatory bursting or isolated bursting, or both. However, irrespective of the jetting mode, the jets interacted with the flame envelope in one of three possible ways. Based on the velocity and the degree to which a jet disrupts the flame envelope, it is classified as either a flame distortion, a fire ball outside the flame or a pin hole jet (localized flame extinction), where the pin hole jets have the highest velocity (1000-1550 mm/s), while the flame distortion events have the lowest velocity (500-870 mm/s). Subsequently, the relative number of the three types of jetting events during the droplet lifetime was analyzed as a function of the type of organic gellant. It was demonstrated that the combustion behavior of gel fuels (hydroxypropyl methylcellulose: HPMC at 3 wt.%) that tend to form thin-weak-flexible shells is dominated by low-velocity flame distortion events, while the gel fuels (methylcellulose: MC at 9 wt.%) that facilitate the formation of thick-strong-rigid shells are governed by high-velocity fire ball and pin hole jets. Overall, this study provides critical insights into the jetting behavior and its characterization, which can help us to tune the droplet gasification and the gas phase mixing to achieve an effective combustion control strategy for gel fuels.

A new model for puffing and micro-explosion of single titanium(IV) isopropoxide/p-xylene precursor solution droplets

In flame spray pyrolysis, the precursor solution droplet undergoes heating and evaporation, followed by gas phase combustion and generation of nanoparticles. Depending on the precursor solution, the precursor/solvent droplet may experience puffing and micro-explosion. A new one-dimensional model is developed to describe these processes in the spherically symmetric droplet interior with emphasis on the puffing and the micro-explosion. After the initial droplet heating, the preferential evaporation of the higher volatile component causes the accumulation of the lower volatile precursor at the droplet surface and thus forms a liquid shell that hinders the evaporation process. In the droplet interior away from the liquid shell, the higher volatile component accumulates and when the temperature reaches the boiling point of that liquid, puffing occurs. If the pressure inside the droplet exceeds the ambient pressure, the droplet undergoes micro-explosion. The present study concerns single precursor/solvent droplets of titanium(IV) isopropoxide (TTIP) in p-xylene at room temperature in hot convective air at atmospheric pressure. A parameter study is performed to show the dependence of the puffing and the micro-explosion on the initial precursor loading, the initial droplet size, the ambient gas temperature, and the relative velocity between the droplet and the ambience. There are situations where micro-explosion follows the puffing or puffing repeats until the end of the droplet lifetime with no micro-explosion. This is the first model to successfully describe the puffing in precursor solution droplets and the micro-explosion which may follow the puffing.

Effect of ambient temperature and water content on emulsified heavy fuel oil droplets evaporation: Evaporation enhancement by droplet puffing and micro-explosion

In this study, the evaporation characteristics of emulsified heavy fuel oil (HFO) with water content from 0 to 30 % at the ambient temperatures of 673, 773, and 873 K are experimentally studied. Three distinguishing phases can be clarified and summarized, including initial heating, fluctuation evaporation, and steady evaporation phases. The fluctuation evaporation phase involves the two sub-phases of heat equilibrium and secondary heating, which are first discovered in the evaporation of emulsified HFO. And the heat equilibrium sub-phase is the distinguishing feature of emulsified HFO from other blended fuels. The duration of the initial heating phase and the fluctuation evaporation phase decreases with the elevated ambient temperature but increases with rising water content. However, the variation of micro-explosion intensity with ambient temperature and water content is the opposite. The time consumed in the fluctuation evaporation phase determines the droplet lifetime. The difference between the evaporation characteristics of HFO and emulsified HFO is that the fluctuation evaporation phase of HFO is not distinct, which is due to the violent and frequent micro-explosion occurring in the emulsified HFO droplet. Consequently, the droplet lifetime of emulsified HFO is significantly shorter than that of HFO. Eventually, a regression formula for the dependences of droplet lifetime from temperature and water content is proposed, which can provide a reference for the practical application of emulsified HFO.

Control of Dy164 Bose-Einstein condensate phases and dynamics with dipolar anisotropy

We investigate the quench dynamics of quasi-one and two dimensional dipolar Bose-Einstein condensates (dBEC) of $^{164}$Dy atoms under the influence of a fast rotating magnetic field. The magnetic field thus controls both the magnitude and sign of the dipolar potential. We account for quantum fluctuations, critical to formation of exotic quantum droplet and supersolid phases in the extended Gross-Pitaevskii formalism, which includes the so-called Lee-Huang-Yang (LHY) correction. An analytical variational ansatz allows us to obtain the phase diagrams of the superfluid and droplet phases. The crossover from the superfluid to the supersolid phase and to single and droplet arrays is probed with particle number and dipolar interaction. The dipolar strength is tuned by rotating the magnetic field with subsequent effects on phase boundaries. Following interaction quenches across the aforementioned phases, we monitor the dynamical formation of supersolid clusters or droplet lattices. We include losses due to three-body recombination over the crossover regime, where the three-body recombination rate coefficient scales with the fourth power of the scattering length ($a_s$) or the dipole length ($a_{dd}$). For fixed values of the dimensionless parameter, $\epsilon_{dd} = a_{dd}/a_s$, tuning the dipolar anisotropy leads to an enhancement of the droplet lifetimes.

Effect of temperature and surfactants on evaporation and contact line dynamics of sessile drops

Researches on droplet evaporation has a vast range of applications in different fields. The mechanisms of droplets evaporation have been a topic of investigation for decades. The droplet after spreading on the substrate exhibits constant evaporation rate for a constant base diameter and decreasing contact angle; followed by another evaporation mode with constant contact angle and decreasing base diameter; and finally, evaporation with decreasing of both contact angle and base diameter. In the present study, the evaporation rate and mechanisms influenced by surfactant and temperature effects are analysed. The evaporation was demonstrated to be related to the dynamics of the sessile droplet spreading ability and the depinning event. It remains complex to predict the evaporation of the sessile droplet due to the continuous change in surfactant local concentration. Moreover, the temperature change modifies the surface tension and the induced inner flows. These two questions find answers in the triple line dynamic which controls the evaporation rate. The forces controlling the depinning with surfactant was quantified. A clear constant force was found following depinning. Such dynamic equilibrium demonstrates also the effect of the friction, surface roughness and moreover the limit of surfactant concentration in the triple line direct vicinity. The evaporation rate shows two particularities: one is the classical decrease of the evaporation rate after depinning; the second in case of surfactant, there is an enhancement appearing at later droplet lifetime, which could be a consequence of film evaporation contributing to the evaporation near the triple line. The local evaporation does not change significantly despite the change in the contact angle with respect to surfactant concentration. The substrate temperature enhances evaporation rate. The coupled temperature and surfactant effect show a local evaporation rate decreasing with surfactant concentration increase at ambient temperature (around 20 °C) and enhancement with surfactant concentration at higher temperature.

Droplet impact and Leidenfrost dynamics on a heated post

This study experimentally explores fluid breakup and Leidenfrost dynamics for droplets impacting a heated millimetric post. Using high-speed optical and infrared imaging, we investigate the droplet lifetime, breakup and boiling modes, as well as the cooling performance of different substrates. The post substrate leads to a shorter droplet lifetime and a 20 °C higher Leidenfrost temperature compared to a flat substrate, attributed to mixed boiling modes along the height of the post and additional pinning. For temperatures below the Leidenfrost point, in the nucleate boiling regime, the post substrate also provides a larger maximum temperature drop than its flat counterpart. The enhanced cooling capacity can be attributed to better droplet pinning and an enlarged droplet-substrate contact area. The post's superior cooling performance becomes especially clear for impact on an inclined surface, where the post successfully prevents the rolling and bouncing of the droplet, providing a 51% to 180% increase in the maximum local temperature drop. Interestingly, at temperatures slightly above the Leidenfrost point and for a relatively narrow range of Weber numbers, droplets on the post substrate rebound rather than break up due to a complex interplay of inertial, capillary, and bubble (thin film) dynamics. Overall, the findings show that a large-scale structure increases the Leidenfrost temperature and enhances droplet breakup and interfacial heat transfer efficiency during non-isothermal droplet impact.

Combustion characteristics of a 1-butanol gel fuel droplet in elevated pressure conditions

The aim of this study was to determine the combustion characteristics of a 1-butanol gel fuel droplet under elevated temperature and pressure conditions. (1)-Butanol is an excellent candidate for replacing conventional fossil fuels because it is sustainable and has a higher heating value than ethanol and methanol. A 1-butanol gel fuel was produced by adding 10 wt% of distilled water and 2.5 and 3 wt% of hydroxypropyl methylcellulose. The combustion of 1-butanol gel fuel can be divided into three stages: droplet heating, 1-butanol combustion, and crust combustion. Puffing was frequent at 1 bar, but it decreased as the ambient pressure increased. An increase in the ambient temperature reduced the ignition delay and droplet lifetime but slightly increased the effective burning rate owing to droplet inflation at 600 °C. The elevation of the ambient pressure did not significantly affect the ignition delay and effective burning rate, except at 1 bar, because the gel layer around the droplet surface dominated the combustion process by suppressing the evacuation of fuel vapor. The ignition was delayed as the gellant concentration increased when the ambient temperature was 600 °C; however, the gap diminished when the ambient temperature was increased to 700 °C. The effective burning rates were similar in all cases. The overall results demonstrated that the combustion behavior of the gel droplet was insensitive to pressure compared with that of the neat liquid fuel because of the effect of the gel layer at the droplet surface.

Molecular dynamics simulation of droplet evaporation in a one-dimensional standing wave acoustic field

In this paper, a molecular dynamics (MD) simulation has been adopted to investigate impacts of one-dimensional standing wave acoustic field on the droplet evaporation. Evaporation processes of single and multi-component droplets in nitrogen are simulated by considering acoustic wave amplitude and frequency impacts. Results reveal that the droplet evaporation rate is apparently enhanced in a pressure oscillation environment. The larger oscillation amplitude and acoustic field frequency would reduce the droplet lifetime. Besides, the instantaneous evaporation rate and droplet temperature show a periodic oscillation characteristic when the oscillation amplitude is relatively large. These simulation results indicate that the instant droplet evaporation rate would be increased by the larger pressure oscillation frequency. Moreover, water molecules in the droplet would display a stronger response to the acoustic field by comparing with the ethanol case.

Evaporation of Leidenfrost droplet on thin soluble liquid bath with thermal non-equilibrium effect

Leidenfrost droplet evaporation on a liquid bath exhibits unique features such as ultra-low resistance to sample transition and low-temperature operation; however, the physical mechanisms responsible for these phenomena are incompletely understood. Droplet size and temperature are two key parameters influencing Leidenfrost droplet evaporation. We report herein the thermal non-equilibrium process of an FC-72 droplet over a thin oil layer. We show that the Leidenfrost droplet radius follows the power law R(t) ∼ (1 − t/τ)n, where τ is the characteristic droplet lifetime and n ranges from 0.63 to 0.91. Based on experimental results and theoretical predictions, the remarkable nonmonotonic variation of droplet temperature departs from the saturation-temperature assumption. For lower oil superheating, a cold (subcooled) droplet can sustain evaporation until it disappears. For higher oil superheating, the droplet goes through both subcooled and superheating stages. This phenomenon is well described by sensible heat absorption and release throughout droplet evaporation. These results are helpful for applications such as drug delivery, wherein a cold droplet can float on a liquid bath, thereby extending the lifetime of the biological sample in a high-temperature environment via a localized, low-temperature system.

Experimental Investigation Of Disruptive Burning Phenomena On Nanofuel Droplets

The use of sustainable and green fuels in the aeronautical industry has been implemented due to the environmental concerns and depletion of fossil fuels. The introduction of biofuels, a renewable energy source in the transportation sector, has shown advantages in terms of pollutant reduction. Recently, the addition of nanoparticles in the combustion of biofuel has been studied with the purpose of enhancing its combustion characteristics. Consequently, the present work evaluates nanofuel single droplet in a falling droplet method. In this way, the fiber suspension effect was neglected, and droplets with a diameter of 250 μm were evaluated. A comparison between pure biofuel and a nanofuel at two furnace temperatures (T = 800 ºC and T = 1000 ºC) was performed. Nanofuels can be defined as a novel class of fuel, where energetic or not energetic nanoparticles are stably suspended in liquid fuel. The preparation of nanofuels demands a methodic procedure to accomplish stable, long-term stability. Therefore, in the present work, the liquid fuel is a biofuel named NEXBTL (HVO) from NESTE. The nanofuel is composed of HVO and aluminum nanoparticles of 40 nm in a particle concentration of 1.0 wt.%. The combustion of pure biofuel and nanofuel was evaluated in a drop tube furnace. This droplet combustion facility consists of an electrically heated drop tube furnace (DTF), an illumination set, an image acquisition system, and an injector device. To follow the burning process from the injector tip until the micro-explosion, a CMOS high–speed camera (CR600×2, Optronics), coupled with a high magnification lens, was used. The image acquisition was pursued with 1000 fps with a resolution of 1200×500 pixels and an exposure time of 1/13000 s. Moreover, a Photron FASTCAM mini UX50, a high-speed camera with 1.3 Megapixel was used to acquire micro-explosions with more detail, and a high magnification lens was coupled to the camera. The results reveal that disruptive burning phenomena occur when aluminum nanoparticles are added to the biofuel. Consequently, a micro-explosion determines the end of the droplet lifetime, mainly affected by the furnace temperature. Disruptive burning phenomena identified in the experimental study are difficult to acquire since its duration is brief and occurs at the end of the droplet lifetime. Due to this, different camera operating ranges were considered. Several frame rates from 2000 to 5000 fps and exposure times of 1/16000s and 1/20000s were considered. The image data processing was performed in ImageJ and MATLAB software. First, the droplet size evolution was evaluated using ImageJ based on the droplet outline through the brightness gradient. Then, the bright spots provided from the micro-explosions were identified and counted by an algorithm developed in MATLAB. It was concluded that adding aluminum combustion in a proper particle concentration can potentially improve the biofuel combustion performance. Moreover, no puffing or micro-explosions were detected in the pure HVO. In contrast, nanofuels displayed puffing and micro-explosions at the end of the droplet lifetime. In addition, the furnace temperature affects the disruptive process, being more intense at T = 1000 ºC. It was observed that a higher number of fragments are ejected at the highest furnace temperature. In addition, the fragments after the explosion are mainly on the lower side, however more studies focusing on this topic should be performed since these disruptive events are unpredictable. The image data acquisition provided the visualization and description of micro-explosions in nanofuel droplets, as well as comprehension of its phenomenology.

Experimental study on the micro-explosion characteristics of biodiesel/1-pentanol and biodiesel/ methanol blended droplets

Alcohols and biodiesel are promising alternative biofuels for their excellent physicochemical properties and renewable property. Compared with low-carbon alcohols, high-carbon 1-Pentanol has better fuel characteristics. The objective of the present study was to reveal the effects of different 1-pentanol addition ratios on the micro-explosion characteristics of blended droplets. Single droplet suspension experiment under atmospheric pressure and 700 °C was carried out, and results were compared with that of methanol. The droplet had three initial diameters (1.281, 1.451, and 1.537 mm). The droplet evaporation images were recorded at 500 fps with a resolution of 1024 × 1024 pixels by a high-speed video camera, and the corresponding droplet diameter in the evaporation process was obtained by a MATLAB program. A new dimensionless micro-explosion intensity was proposed. The results showed that when the 1-pentanol volume content was 50% (B50P50), the micro-explosion number and total micro-explosion intensity reached the maximum. With the increase of initial droplet diameter, the micro-explosion number, total micro-explosion intensity and droplet lifetime increased. Compared with biodiesel/1-pentanol blended droplets, biodiesel/methanol ones had better micro-explosion characteristics and shorter droplet lifetime. White fog formed in the evaporation process of blended droplets and pure biodiesel ones. In short, the addition of methanol and 1-pentanol caused micro-explosions, changed the evaporation behavior of biodiesel, and reduced droplet lifetime, which could be considered as an alternative fuel blended with biodiesel.

On the lifetimes of two-dimensional droplets on smooth wetting patterns

We study the evolution and lifetime of droplets evaporating on a smooth chemical pattern, which is characterised by a spatially varying contact angle. We formulate a model that combines the evaporation rate of the droplet for a given volume with the static stability of the droplet as the volume changes in time quasi-statically. We derive an exact equation for the evaporation rate that is studied analytically under the limiting cases of nearly neutral wetting and highly hydrophilic conditions. We find that the evaporation rate of the droplet is highly dependent on the size and shape of the fictitious infinity where a far-field boundary conditions needs to be applied. We also study how the droplet’s lifetime depends on the averaged contact angle and strength varepsilon of the chemical pattern, observing that the lifetime of the droplet is maximised for a droplet with average contact angle pi /2 and with varepsilon lesssim 0.1.

Understanding lifetime and dispersion of cough-emitted droplets in air.

To understand the exact transmission routes of SARS-CoV-2 and to explore effects of time, space and indoor environment on the dynamics of droplets and aerosols, rigorous testing and observation must be conducted. In the current work, the spatial and temporal dispersions of aerosol droplets from a simulated cough were comprehensively examined over a long duration (70min). An artificial cough generator was constructed to generate reliably repeatable respiratory ejecta. The measurements were performed at different locations in front (along the axial direction and off-axis) and behind the source in a sealed experimental enclosure. Aerosols of 0.3-10µm (around 20% of the maximum nuclei count) were shown to persist for a very long time in a still environment, and this has a substantial implication for airborne disease transmission. The experiments demonstrated that a ventilation system could reduce the total aerosol volume and the droplet lifetime significantly. To explain the experimental observations in more detail and to understand the droplet in-air behaviour at various ambient temperatures and relative humidity, numerical simulations were performed using the Eulerian-Lagrangian approach. The simulations show that many of the small droplets remain suspended in the air over time instead of falling to the ground.

Simulation of the evaporation/boiling transition for the vaporization of a bi-component droplet under wide-temperature environments

A boiling model was proposed and coupled with a quasi-dimensional evaporation model to simulate the vaporization of a bi-component droplet under wide-temperature environments. Different from previous droplet evaporation models, the transition between droplet evaporation and boiling was especially focused on in this study. In the boiling model, the bubble point temperature of the bi-component mixture was employed to determine the critical condition for the occurrence of the boiling phenomenon. The mass rate of the boiling vaporization of the droplet was calculated by combining the heterogeneous nucleation model, bubble growth model, and bubble fragmentation model. Based on the measurements of the vaporization of an n-heptane/n-hexadecane droplet, the results indicate that the present vaporization model can satisfactorily reproduce the droplet lifetime and diameter evolution under wide ambient temperatures. The simulation results of the vaporization process are greatly affected by the boiling mass rate as the droplet enters the boiling stage. The heterogeneous nucleation rate is the main factor affecting the boiling mass rate, and the decrease of effective thermal conductivity slows down the overall droplet vaporization rate.

Airborne lifetime of respiratory droplets

We formulate a model for the dynamics of respiratory droplets and use it to study their airborne lifetime in turbulent air representative of indoor settings. This lifetime is a common metric to assess the risk of respiratory transmission of infectious diseases, with a longer lifetime correlating with higher risk. We consider a simple momentum balance to calculate the droplets' spread, accounting for their size evolution as they undergo vaporization via mass and energy balances. The model shows how an increase in the relative humidity leads to higher droplet settling velocity, which shortens the lifetime of droplets and can, therefore, reduce the risk of transmission. Emulating indoor air turbulence using a stochastic process, we numerically calculate probability distributions for the lifetime of droplets, showing how an increase in the air turbulent velocity significantly enhances the range of lifetimes. The distributions reveal non-negligible probabilities for very long lifetimes, which potentially increase the risk of transmission.