Droplet Lifetime Research Articles

Deviations from classical droplet evaporation theory.

In this article, we show that significant deviations from the classical quasi-steady models of droplet evaporation can arise solely due to transient effects in the gas phase. The problem of fully transient evaporation of a single droplet in an infinite atmosphere is solved in a generalized, dimensionless framework with explicitly stated assumptions. The differences between the classical quasi-steady and fully transient models are quantified for a wide range of the 10-dimensional input domain and a robust predictive tool to rapidly quantify this difference is reported. In extreme cases, the classical quasi-steady model can overpredict the droplet lifetime by 80%. This overprediction increases when the energy required to bring the droplet into equilibrium with its environment becomes small compared with the energy required to cool the space around the droplet and therefore establish the quasi-steady temperature field. In the general case, it is shown that two transient regimes emerge when a droplet is suddenly immersed into an atmosphere. Initially, the droplet vaporizes faster than classical models predict since the surrounding gas takes time to cool and to saturate with vapour. Towards the end of its life, the droplet vaporizes slower than expected since the region of cold vapour established in the early stages of evaporation remains and insulates the droplet.

Selective evaporation rate modeling of volatile binary mixture droplets

Despite numerous studies about droplet evaporation dynamics, the development of an accurate model to estimate the evaporation rate has been under debate for several decades. In particular, the selective evaporation characteristics of binary mixture droplets (BMD) have not been investigated well, in spite of its importance in many applications. The present study examines the selective evaporation characteristics of BMDs and develops a corresponding evaporation rate model. Surface plasmon resonance imaging (SPRi) and long-working distance microscopic imaging methods are applied to simultaneously measure ethanol concentration and droplet geometry, respectively, over the droplet lifetime. The evaporation rates of each BMD component are calculated based on concentration measurement. In this study, the Rayleigh number is employed to analyze the effect of free convection on evaporation dynamics. A new evaporation rate model is suggested as a function of the Rayleigh number by involving diffusive and convective evaporation effects. This model shows a strong correlation with the observed selective evaporation behavior over the droplet lifetime.

From the environment to the cells: An overview on pivotal factors which affect spreading and infection in COVID-19 pandemic

Several factors ranging from environmental risks to the genetics of the virus and that of the hosts, affect the spread of COVID-19. The impact of physicochemical variables on virus vitality and spread should be taken into account in experimental and clinical studies. Another avenue to explore is the effect of diet and its interaction with the immune system on SARS-CoV-2 infection and mortality rate. Past year have witnessed extensive studies on virus and pathophysiology of the COVID-19 disease and the cellular mechanisms of virus spreading. However, our knowledge has not reached a level where we plan an efficient therapeutic approach to prevent the virus entry to the cells or decreasing the spreading and morbidity in severe cases of disease. The risk of infection directly correlates with the control of virus spreading via droplets and aerosol transmission, as well as patient immune system response. A key goal in virus restriction and transmission rate is to understand the physicochemical structure of aerosol and droplet formation, and the parameters that affect the droplet-borne and airborne in different environmental conditions. The lifetime of droplets on different surfaces is described based on the contact angle. Hereby, we recommend regular use of high-quality face masks in high temperature and low humidity conditions. However, in humid and cold weather conditions, wearing gloves and frequently hand washing, gain a higher priority. Additionally, social distancing rules should be respected in all aforementioned conditions. We will also discuss different routes of SARS-CoV-2 entry into the cells and how multiple genetic factors play a role in the spread of the virus. Given the role of environmental and nutritional factors, we discuss and recommend some strategies to prevent the disease and protect the population against COVID-19. Since an effective vaccine can prevent the transmission of communicable diseases and abolish pandemics, we added a brief review of candidate SARS-CoV-2 vaccines.

Isolated moving charged droplet evaporation characteristics in electrostatic field with highly volatile R134a

The evaporation characteristics of a charged moving droplet with high volatility are the key during electrostatic spray cooling, which determines cooling capability. Fundamental investigations of the charged droplet evaporation are helpful in enhancing the cooling capability and advancing the electrostatic spray model, whereas the corresponding data with the highly volatile R134a are very scarce. In this work, an improved evaporation model that accounted for the effects of the corona wind, Coulombic fission, and electric force under the electric field was developed and validated to evaluate the importance of the electric field on the moving charged R134a droplet evaporation for the first time. Results demonstrated that the electrostatic field can improve the charged droplet evaporation through heat and mass transfer enhancement by the corona wind and counteracting the drag force from air. The droplet lifetime was shortened by 20.8% (i.e., from 105.6 ms to 83.6 ms) when the applied voltage increased from 0 kV to 10 kV. The increase in the ambient pressure has a damping effect on the charged droplet evaporation because of the decreasing driving force for the heat and mass transfer, which should be avoided in electrostatic spray cooling. Exponential decay in the charged droplet lifetime when the applied voltage increased was observed, and the corresponding results with different initial droplet diameters and velocities provide the first-hand fundamental data for the modeling and simulation of electrostatic spray cooling with the highly volatile R134a.

Combustion characteristics of a single droplet of hydroprocessed vegetable oil blended with aluminum nanoparticles in a drop tube furnace

This study examines the burning characteristics and disruptive burning phenomena of single droplets of aluminum nanoparticles (n-Al) stably suspended in a biofuel (HVO). The biofuel used in the present work is a promising alternative fuel already tested in the aviation sector to reduce greenhouse gas and pollutant emissions. Experiments were conducted with two particle sizes (40nm and 70nm) and two particle concentrations (0.5 wt.% and 1.0 wt.%) to study its influence when added to the biofuel. The effect of size and concentration of the aluminum nanoparticles was studied at 1100 °C in a drop tube furnace. This experimental facility allows the study of combustion characteristics of falling droplets, ensuring there is no influence of the supporting fiber on the burning rate and disruptive burning phenomena occurrence. A CMOS high - speed camera coupled with a high magnification lens was used to evaluate the droplet size, burning rate, and micro-explosions. Based on this procedure, pure biofuel droplets were compared with those of biofuel blended with nanoparticles. The results suggest that the combustion characteristics of pure HVO can be enhanced with the addition of aluminum nanoparticles. Furthermore, by decreasing the particle size, a slight increase in the burning rate of nanofuels was noticed. Additionally, an increase in the particle concentration leads to a pronounced increase in the burning rate. The particle concentration also influences the delay and intensity of micro-explosions, disruptive burning phenomena detected at the end of the droplet lifetime.

EXAMINING EVAPORATION CHARACTERISTICS OF TYPICAL AVIATION FUELS INCLUDING JP-4, JP-5, JP-7, RT, Jet A-1 AND TS-1

This work numerically studies the evaporation characteristics of 6 typical gas turbine fuels (JP-4, JP-5, JP-7, RT, Jet A-1, and TS-1, respectively). In this work, the influence of ambient temperature and liquid properties on the droplet lifetime is examined. At the same initial conditions (e.g. fuel temperature Ts0 = 300 K, droplet diameter D0 = 1 mm and atmospheric pressure p = 101.33 kPa) and low ambient temperature (e.g. 500 K), significant differences in evaporation characteristics among these fuels are observed. JP-4 has the longest droplet lifetime while JP-7’s is the shortest. However, at high ambient temperature (e.g. 2000 K), the droplet lifetimes of the fuels are almost identical. Under critical temperature, it is shown that the fuel properties remarkably impact the evaporation rate.

Motility Plays an Important Role in the Lifetime of Mammalian Lipid Droplets.

The lipid droplet is a kind of organelle that stores neutral lipids in cells. Recent studies have found that in addition to energy storage, lipid droplets also play an important role in biological processes such as resistance to stress, immunity, cell proliferation, apoptosis, and signal transduction. Lipid droplets are formed at the endoplasmic reticulum, and mature lipid droplets participate in various cellular processes. Lipid droplets are decomposed by lipase and lysosomes. In the life of a lipid droplet, the most important thing is to interact with other organelles, including the endoplasmic reticulum, mitochondria, peroxisomes, and autophagic lysosomes. The interaction between lipid droplets and other organelles requires them to be close to each other, which inevitably involves the motility of lipid droplets. In fact, through many microscopic observation techniques, researchers have discovered that lipid droplets are highly dynamic organelles that move quickly. This paper reviews the process of lipid droplet motility, focusing on explaining the molecular basis of lipid droplet motility, the factors that regulate lipid droplet motility, and the influence of motility on the formation and decomposition of lipid droplets. In addition, this paper also proposes several unresolved problems for lipid droplet motility. Finally, this paper makes predictions about the future research of lipid droplet motility.

Auto-selection of quasi-components/components in the multi-dimensional quasi-discrete model

A new algorithm for the auto-selection of quasi-components and components (QC/Cs) in the ‘multi-dimensional quasi-discrete’ model is suggested. This algorithm is applied to the analysis of heating and evaporation of multi-component fuel droplets. It allows one to automatically select QC/Cs and update the initial selection during droplet evaporation. The new algorithm is expected to be applicable to the analysis of a wide range of fuels and fuel blends. It can be directly implemented into CFD codes with minimal intervention by end-user. Using this algorithm, the effects of transient diffusion of species on droplet lifetimes are investigated for mixtures of Diesel and E85 (85% vol. ethanol and 15% vol. gasoline) fuels. It is shown that the new algorithm can reduce the analysis of the E85-Diesel fuel droplets, taking into account the contributions of up to 119 components at the initial stage of heating and evaporation, to that based on 5 QC/Cs, near the end of droplet evaporation, with up to 1.9% errors in predicted droplet temperatures and radii. The CPU time needed to perform calculations using the new algorithm is shown to be 80% less than that when considering the full composition of fuel.

Experimental Study on Evaporation Characteristics of Light Cycle Oil Droplet under Various Ambient Conditions

The authors conducted droplet evaporation experiments of light cycle oil (LCO) at various ambient temperatures and pressures. Five kinds of LCO and three kinds of arranged fuels were used. We investigated the evaporation characteristics of LCO and the relationships between the evaporation characteristics and the cetane index. In addition to that, a surrogate fuel composed of four chemical species, which can simulate the droplet evaporation characteristics of LCO, was suggested. Experimental results show that the differences in droplet lifetime between fuel species become larger with decreasing ambient temperature. This is because the low volatile component made the evaporation rate outstandingly slow at a low ambient temperature. It was found that the relationship between droplet lifetime and the late-stage distillation temperature becomes stronger at low ambient temperature and high ambient pressure. By an analysis employing the properties of chemical species in LCO surrogate fuel, it is clarified that the mass evaporation rate becomes smaller than the internal diffusion, which is the condition similar to that in the distillation test. Finally, the relationship between the droplet lifetime and the cetane index was investigated. It can be concluded that the droplet lifetime is independent of the cetane index under all conditions tested in this study. The experimental data obtained by this research can be utilized for the validation of multicomponent fuel droplet evaporation models in the future.

An Experimental Investigation of Top-of-Line Corrosion in a Static CO2 Environment

This paper presents an experimental and theoretical investigation into water condensation and corrosion under noncorrosion product forming conditions at the top of the line in a static, CO2 environment. An experimental test cell is developed to measure droplet lifetimes, condensation rates, as well as in situ and integrated corrosion rates (using miniature electrodes and mass loss specimens, respectively) as a function of the surface and gas temperatures when the gas flow is dominated by natural convection. Experimental results show clearly that the water condensation rate is not very influential on the corrosion rate at low surface temperatures (Ts) (particularly below 25°C) but becomes much more important at higher surface temperatures (>40°C). These findings are summarized in a new empirical correlation for the top-of-line corrosion rate as a function of the condensation rate and surface temperature. A model for condensation at the top of line for static, buoyancy-driven conditions is also presented and is shown to predict dropwise condensation rates accurately for a range of experimental conditions. The developed miniature electrodes for in situ electrochemical measurement are shown to provide an accurate interpretation of the transient response in general corrosion behavior by giving real-time corrosion rates to complement the mass loss measurement.

Effect of Surface Textures and Wettability on Droplet Impact on a Heated Surface

A liquid droplet can hover over a solid surface that is heated above the Leidenfrost point (LFP), at which an insulating vapor layer is formed that acts as a heat transfer barrier. Recent studies have reported that hierarchical micro- and nanoscale textures provide high wettability and significant LFP enhancement. However, such textures are often difficult and expensive to fabricate. Therefore, this study aimed to experimentally demonstrate LFP enhancement through the use of low-cost hierarchical textures. Surface textures were fabricated by coating SiO2 nanoparticles on stainless steel wire meshes. The droplet lifetime method was used to determine the LFP in a temperature range of 200 °C–490 °C. High-speed imaging (4000–23,000 fps) was performed for visualizing the impact behavior of a droplet. The LFP value of the nanocoated mesh surface was found to be greater than 490 °C. This enhanced LFP was 178 °C higher than that of a stainless steel surface and 38 °C higher than that of a single-layer textured surface. Furthermore, with respect to the LFP enhancement, the explosive impact behavior of a droplet can be observed on nanocoated mesh surfaces.

Collisions of water droplets in the high-temperature air

The paper presents experimental research findings on the integral characteristics of the interaction of water droplets in a gas heated to high temperatures. An induction heater with an internal volume of about 0.13 m3 was used. It was fitted with quartz glass observation windows to record the characteristics of droplet motion before and after collisions. Air was used as a gas medium in the inductor. The parameters were varied in the following ranges: the air medium temperature 20–700 °С, the initial radius of droplets 0.3–0.9 mm, their velocity 0.1–7 m/s and impact angle 0–90°. At high gas temperatures, droplets were two-phase objects, because vapor bubbles formed in their near-surface layer. Frames with four collision regimes (coalescence, separation, disruption and bounce) were recorded. There were significant differences in the transformation of the water droplet surface and in the interaction of droplets with each other at various gas temperatures in the heating chamber. It has been shown that the growth of the air temperature increases the droplet lifetime and causes them to deviate from the spherical shape. The effect of the gas temperature on the position of droplet collision boundaries was determined with due consideration of the relative linear interaction parameter and the Weber number. When the gas temperature increases, two areas may form, corresponding to the bounce of droplets on the collision regime map. Differences were established in the number and dimensions of secondary water droplets formed from the collision of two initial ones. The total surface areas of liquid before and after droplet collisions were calculated.

Insights into the evaporation characteristics of saliva droplets and aerosols: Levitation experiments and numerical modeling

Understanding the transmission phenomena of SARS-CoV-2 by virus-laden droplets and aerosols is of paramount importance for controlling the current COVID-19 pandemic. Detailed information about the lifetime and kinematics of airborne droplets of different size is relevant in order to evaluate hygiene measures like wearing masks but also social distancing and ventilation concepts for indoor environments. However, the evaporation process of expiratory droplets and aerosols is not fully understood. Consequently, the main objective of this study is to present evaporation characteristics of saliva droplets. An acoustic levitator is utilized in conjunction with microscopic imaging for recording the temporal evolution of the evaporation of saliva droplets under well-defined ambient conditions. Following the evaporation of the water content, a saliva droplet reaches a final size, which remains stable in the timescale of hours. By investigating numerous droplets of different size, it was found that the final droplet diameter correlates well to 20% of the initial diameter. This correlation is independent of the ambient conditions for a temperature range from 20°C to 29°C and a relative humidity from 6% to up to 65%. The experimentally obtained evaporation characteristics are implemented into a numerical model, which is based on one-dimensional droplet kinematics and a rapid mixing evaporation model. By taking into account the evaporation-falling curve as presented by Wells, the significance of the experimental results for predicting the lifetime of saliva droplets and aerosols is demonstrated. The numerical predictions may be used to determine the impact of the droplet size and the ambient conditions on the transmission risks of infectious diseases like COVID-19.

Extended Lifetime of Respiratory Droplets in a Turbulent Vapor Puff and Its Implications on Airborne Disease Transmission.

To quantify the fate of respiratory droplets under different ambient relative humidities, direct numerical simulations of a typical respiratory event are performed. We found that, because small droplets (with initial diameter of 10 μm) are swept by turbulent eddies in the expelled humid puff, their lifetime gets extended by a factor of more than 30 times as compared to what is suggested by the classical picture by Wells, for 50% relative humidity. With increasing ambient relative humidity the extension of the lifetimes of the small droplets further increases and goes up to around 150 times for 90% relative humidity, implying more than 2m advection range of the respiratory droplets within 1sec. Employing Lagrangian statistics, we demonstrate that the turbulent humid respiratory puff engulfs the small droplets, leading to many orders of magnitude increase in their lifetimes, implying that they can be transported much further during the respiratory events than the large ones. Our findings provide the starting points for larger parameter studies and may be instructive for developing strategies on optimizing ventilation and indoor humidity control. Such strategies are key in mitigating the COVID-19 pandemic in the present autumn and upcoming winter.

Dissolution dynamics of a vertically confined sessile droplet.

We experimentally investigate the dissolution of microscale sessile alcohol droplets in water under the influence of impermeable vertical confinement. The introduction of confinement suppresses the mass transport from the droplet to bulk medium in comparison with the nonconfined counterpart. Along with a buoyant plume, flow visualization reveals that the dissolution of a confined droplet is hindered by a mechanism called levitated toroidal vortex. The morphological changes in the flow due to the vortex-induced impediment alters the dissolution rate, resulting in enhancement of droplet lifetime. Further, we have proposed a modification in the key nondimensional parameters [Rayleigh number Ra^{'} (signifying buoyancy) and Sherwood number Sh^{'} (signifying mass flux)] and droplet lifetime τ_{c}^{'}, based on the hypothesis of linearly stratified droplet surroundings (with revised concentration difference ΔC^{'}), taking into account the geometry of the confinements. We show that experimental results on droplet dissolution under vertical confinement corroborate scaling relations Sh^{'}∼Ra^{'}^{1/4} and τ_{c}^{'}∼ΔC^{'}^{-5/4}. We also draw attention to the fact that the revised scaling law incorporating the geometry of confinements proposed in the present work can be extended to other known configurations such as droplet dissolution inside a range of channel dimensions, as encountered in a gamut of applications such as microfluidic technology and biomedical engineering.

Evaporation-induced alterations in oscillation and flow characteristics of a sessile droplet on a rose-mimetic surface.

Strategic control of evaporation dynamics can help control oscillation modes and internal flow field in an oscillating sessile droplet. This article presents the study of an oscillating droplet on a bio-inspired "sticky" surface to better understand the nexus between the modes of evaporation and oscillation. Oscillation in droplets can be characterized by the number of nodes forming on the surface and is referred to as the mode of oscillation. An evaporating sessile droplet under constant periodic perturbation naturally self-tunes between different oscillation modes depending on its geometry. The droplet geometry evolves according to the mode of evaporation controlled by substrate topography. We use a bio-inspired, rose patterned, "sticky" hydrophobic substrate to perpetually pin the contact line of the droplet in order to hence achieve a single mode of evaporation for most of the droplet's lifetime. This allows the prediction of experimentally observed oscillation mode transitions at different excitation frequencies. We present simple scaling arguments to predict the velocity of the internal flow induced by the oscillation. The findings are beneficial to applications which seek to tailor energy and mass transfer rates across liquid droplets by using bio-inspired surfaces.

Chemically striped surface accelerate the droplet evaporation

The chemically striped surface contains both hydrophilic and super-hydrophobic stripes. During the droplet spreading on the striped surface, the inconsistent retraction of liquids on hydrophilic stripes and super-hydrophobic stripes causes the droplet to split out satellite droplets. The generation of the satellite droplet can significantly increase the liquid-solid contact area and the length of the three-phase contact line. The increase of the contact line length broadens the range of higher heat flux, and the rise of the contact area enlarges the heat transfer area. Therefore, the chemically striped surface can promote droplet evaporation and increase the heat transfer 41.51 % higher than the hydrophilic surface. Due to different sizes, the satellite droplets enter the CCA evaporation mode earlier than the main droplet. Therefore, the droplet will experience a CCR−CCA mixed mode. During this mode, the contact line length and contact area will decrease rapidly. The satellite droplets on the chemical surface with thinner stripes are smaller than those on the chemical surface with wider stripes, leading the droplet on the former surface to enter the CCR−CCA mixed mode earlier than the droplet on the latter surface. Therefore, the lifetime of droplets on the chemical surface with wider stripes is shorter than that of droplets on the chemical surface with thinner stripes. Besides, the lower impact velocity is not conducive to droplet splitting and, therefore, evaporation.

Analytical and experimental study on boiling vaporization and multi-mode breakup of binary fuel droplet

In this paper, the boiling vaporization and breakup of single droplets of n-butanol, n-hexadecane and their binary mixtures with 10-50 w.t.% n-butanol have been studied. The experiments of droplet evaporation with an elevated ambient temperature range of 537-609 K were conducted through the pendant drop method and high-speed camera technology. The mathematical method was also proposed to predict the heterogeneous superheat limit and nucleation rate within the droplet. The experimental results indicate that mono-component droplets only go through the transient heating and stable evaporation phases. While for bi-component droplets, the characteristics show strong two-phase flow instability. With increasing n-butanol concentration or ambient temperature, the heterogeneous nucleation will occur inside the blended droplet, which experiences fluctuation evaporation between the transient heating and stable evaporation phases. The droplet vaporization characteristics gradually change from normal evaporation to partial rupture or passive breakup and then to micro explosion which reduces the droplet lifetime considerably. The novelty of this work is that various ambient temperature conditions and compositions are adopted to study the droplet evaporation to identify the conditions that favor flash boiling, and a new analytical model of heterogeneous nucleation is proposed, which can be used to estimate the temperature at the instant of bubble nucleation.

Distinguishing Carbohydrate Isomers with Rapid Hydrogen/Deuterium Exchange-Mass Spectrometry.

Carbohydrates play key roles in facilitating cellular functions, yet characterizing their structures is analytically challenging due to the presence of epimers, regioisomers, and stereoisomers. In-electrospray-hydrogen/deuterium exchange-mass spectrometry (in-ESI HDX-MS) is a rapid HDX method that samples solvated carbohydrates with minimal instrument modification. When applied to proteins, HDX is often measured after multiple time points to sample the dynamics of structures. Herein, we alter the HDX reaction time by modifying the spray-solvent conductivity, which changes the initial size of ESI droplets, and thus, the droplet lifetimes. We show that this change in droplet lifetime alters the magnitude of HDX for carbohydrate-metal adducts. Furthermore, we illustrate how monitoring HDX at multiple time points enables three trisaccharide isomers (melezitose, maltotriose, and isomaltotriose) to be distinguished. This work illustrates the feasibility of this method for characterizing solvated carbohydrates, including isomeric species which differ only by linkage.

Dynamics of respiratory droplets carrying SARS-CoV-2 virus in closed atmosphere

From the epidemiological point of view, the lifetime of cough and sneeze droplets in the ambient atmosphere plays a significant role in the transmission rate of Coronavirus. The lifetime of indoor respiratory droplets, per se, is a function of droplet size, ambient temperature, and humidity. In the attempt to explore the effective factors of droplet lifetime, sufficient knowledge of atomic-scale interactions and dynamics of the droplet with themselves, as well as the airflow molecules in the room space, is necessary. In this study, the vertical traveling of a wide range (100 nm–10 μm) of representative carrier droplets is studied in three ambient temperatures of 258, 298, and 318 K using all-atom molecular dynamics simulation. Our obtained results confirm that by increasing the room temperature, the suspending time of aerosol (suspended droplets carrying virus particles) increases due to the higher dynamics of air and evaporated water molecules in room space. In fact, by increasing the indoor temperature, the collision rate of aerosol and ambient atmosphere molecules increases significantly. Our result shows this higher rate of collision could have a dual effect on the lifetime of aerosol considering the fact of faster deposition of larger (heavier) droplet due to the gravitational force. On one hand, in higher temperatures, the higher collision can split the droplets to smaller ones with a semi-permanent suspension period. On the other hand, the higher dynamics of ambient molecules can lead to meet and coalesce of smaller cough/sneeze droplets making larger (heavier) droplets with faster sediment times. So, the role of indoor humidity to fuel the probability of coalescence phenomenon and lifetime of droplets becomes more determinant in the warmer spaces.