Pure Water Droplets Research Articles

The Aqueous Surface as an Efficient Transient Stop for the Reactivity of Gaseous NO2 in Liquid Water.

The heterogeneous reaction of NO2 with water on diverse surfaces is broadly considered as a possible source of atmospheric HONO in dark conditions, but the associated mechanisms are not fully understood. We report data from first-principles simulations showing that the lifetime of the putative reactive NO2 dimer on the surface of pure water droplets is too small to host the whole process. One infers from our results that the hydrolysis of NO2 in clouds must be catalyzed by organic or inorganic species adsorbed on the droplets.

Secondary Ice Production upon Freezing of Freely Falling Drizzle Droplets

AbstractIce multiplication processes are known to be responsible for the higher concentration of ice particles versus ice nucleating particles in clouds, but the exact secondary ice formation mechanisms remain to be quantified. Recent in-cloud observations and modeling studies have suggested the importance of secondary ice production upon shattering of freezing drizzle droplets. In one of our previous studies, four categories of secondary ice formation during freezing of supercooled droplets have been identified: breakup, cracking, jetting, and bubble bursts. In this work, we extend the study to include pure water and an aqueous solution of analog sea salt drizzle droplets moving at terminal velocity with respect to the surrounding cold humid air. We observe an enhancement in the droplet shattering probability as compared to the stagnant air conditions used in the previous study. Under free-fall conditions, bubble bursts are the most common secondary ice production mode in sea salt drizzle droplets, while droplet fragmentation controls the secondary ice production in pure water droplets.

Electrowetting diminishes contact line friction in molecular wetting

We use large-scale molecular dynamics to study dynamics at the three-phase contact line in electrowetting of water and electrolytes on no-slip substrates. Under the applied electrostatic potential the line friction at the contact line is diminished. The effect is consistent for droplets of different sizes as well as for both pure water and electrolyte solution droplets. We analyze the electric field at the contact line to show how it assists ions and dipolar molecules to advance the contact line. Without an electric field, the interaction between a substrate and a liquid has a very short range, mostly affecting the bottom, immobilized layer of liquid molecules which leads to high friction since mobile molecules are not pulled towards the surface. In electrowetting, the electric field attracts charged and polar molecules over a longer range which diminishes the friction.

Evaporation of sessile ethanol-water droplets on a critically inclined heated surface

The evaporation of a binary sessile ethanol-water droplet on an inclined substrate is studied experimentally just below the critical sliding angles for different substrate temperatures. A customized goniometer equipped with a CMOS camera and an infrared (IR) camera is used. The droplet is observed to remain pinned in the advancing side during the evaporation process, while the receding side contracts. The asymmetry in the advancing and receding contact angles of the droplet on inclined substrate results in complex thermo-solutal Marangoni convection that is captured through IR images. The droplet exhibits two distinct oscillatory water-rich cold regions around the advancing contact line during the early stage of evaporation, while the more volatile and lighter ethanol creates a hotter and rapidly evaporating cell near the receding side. As ethanol evaporates away, the ethanol rich cells collapse producing thermal pulsations along the incline. Subsequently, the thermal patterns become similar to that of the pure-water droplet. It is also observed that the thermo-solutal driven oscillatory convection increases with increasing substrate temperature. Despite the complexity in convection dynamics, the evaporation rate exhibits a universal behavior in the normalized time at different substrate temperatures which can be represented by piecewise linear fits at the early and late stages of evaporation.

Modeling the role of respiratory droplets in Covid-19 type pandemics.

In this paper, we develop a first principles model that connects respiratory droplet physics with the evolution of a pandemic such as the ongoing Covid-19. The model has two parts. First, we model the growth rate of the infected population based on a reaction mechanism. The advantage of modeling the pandemic using the reaction mechanism is that the rate constants have sound physical interpretation. The infection rate constant is derived using collision rate theory and shown to be a function of the respiratory droplet lifetime. In the second part, we have emulated the respiratory droplets responsible for disease transmission as salt solution droplets and computed their evaporation time, accounting for droplet cooling, heat and mass transfer, and finally, crystallization of the dissolved salt. The model output favourably compares with the experimentally obtained evaporation characteristics of levitated droplets of pure water and salt solution, respectively, ensuring fidelity of the model. The droplet evaporation/desiccation time is, indeed, dependent on ambient temperature and is also a strong function of relative humidity. The multi-scale model thus developed and the firm theoretical underpinning that connects the two scales—macro-scale pandemic dynamics and micro-scale droplet physics—thus could emerge as a powerful tool in elucidating the role of environmental factors on infection spread through respiratory droplets.

Cooling of porous metal surfaces by droplet impact

An experimental study was conducted on the impact of droplets of pure water and n-heptane on impervious and porous (5 µm and 100 µm pore size) stainless steel surfaces. Surface porosity, roughness and thermal conductivity were measured. Droplet diameter (2.5 mm) and impact velocity (0.9 m/s) were kept constant while surface temperature was varied from room temperature to 300 °C. Droplet impact was observed using a high-speed video camera. Droplet evaporation times were measured by recording the change in weight of the surface on which the droplet was deposited and surface temperature variation during droplet impact measured using a fast response surface thermocouple.Droplets spread out on porous surfaces during impact to form a thin film that was then drawn into pores by capillary forces. The rate of droplet spread decreased with greater surface roughness. The Leidenfrost temperature of n-heptane droplets increased with surface porosity while water droplets did not go into film boiling on the porous surfaces. A one-dimensional heat conduction model was used to estimate heat transfer coefficients between the impacting droplets and substrates. Heat transfer coefficients increased with surface temperature until sufficient vapor was generated at the liquid-solid interface to inhibit contact. Heat transfer coefficients then decreased with further increases in temperature.

Effect of Substrate Conductivity on the Transient Thermal Transport of Hygroscopic Droplets during Vapor Absorption.

In all kinds of liquid desiccant dehumidification systems, the temperature increase of the desiccant solution due to the effect of absorptive heating is one of the main reasons of performance deterioration. In this study, we look into the thermal effects during vapor absorption into single hygroscopic liquid desiccant droplets. Specifically, the effect of substrate conductivity on the transient heat and mass transfer process is analyzed in detail. The relative strength of the thermal effect and the solutal effect on the rate of vapor absorption is investigated and compared to the thermal effect by evaporative cooling taking place in pure water droplets. In the case of liquid desiccants, results indicate that the high thermal conductivity of copper substrates ensures more efficient heat removal, and the temperature at the droplet surface decreases more rapidly than that on Polytetrafluoroethylene (PTFE) substrates. As a result, the initial rate of vapor absorption on copper substrates slightly outweighs that on PTFE substrates. Further analysis by decomposing the vapor pressure difference indicates that the variation of vapor pressure caused by the temperature change during vapor absorption is much weaker than that induced by the concentration change. The conclusions demonstrate that a simplified isothermal model can be applied to capture the main mechanisms during vapor absorption into hygroscopic droplets even though it is evidenced to be unreliable for droplet evaporation.

A comparative analysis of shielding of thermal radiation of fires using mist curtains containing droplets of pure water or sea water

The paper is focused on comparative computational modelling of the attenuation of fire radiation by water mists of pure water or sea water. The use of sea water in fire protection could be a more convenient and practical choice in coastal areas, on offshore installations or transported ships. The spectral absorption and scattering properties of both water droplets and salt particles formed by evaporation of sea water droplets are considered. A combined heat transfer problem is based on a combination of the spectral radiative transfer in a mist curtain, the kinetics of water evaporation, and convective heat transfer along the curtain. The developed computational model is used to analyze the radiative heating and evaporation of droplets of pure water and more complex multi-phase processes in droplets of sea water at all stages of the process. The numerical results for the case problem indicate sufficiently good shielding quality of a sea-water mist curtain. The suggested approach is expected to be useful for important engineering applications in fire protection.

Influence of the viscosity and charge mobility on the shape deformation of critically charged droplets

In this work, we model and simulate the shape evolution of critically charged droplets, from the initial spherical shape to the charge emission and back to the spherical shape. The shape deformation is described using the viscous correction for viscous potential flow model, which is a potential flow approximation of the Navier-Stokes equation for incompressible Newtonian fluids. The simulated shapes are compared to snapshots of experimentally observed drop deformations. We highlight the influence of the dimensionless viscosity and charge carrier mobility of the liquid on the shape evolution of droplets and discuss the observed trends. We give an explanation as to why the observed deformation pathways of positively and negatively charged pure water droplets differ and give a hint as to why negatively charged water droplets emit more charge during charge breakup than positively charged ones.

Microscopic Mechanisms of N2O5 Hydrolysis on the Surface of Water Droplets.

Reactions of N2O5, in particular heterogeneous hydrolysis, play a vital role in determining the chemistry of the atmosphere. The N2O5 heterogeneous hydrolysis reaction has been the subject of extensive research for decades, yet the physicochemical details of the mechanism have not been established. In this study, we show that this reaction can occur on the surface of a pure water droplet. We compute a relevant transition state for a nano-size model system and follow its evolution in time by means of ab initio molecular dynamics. This transition state, where N2O5 has a strong ion-pair character, leads directly to HNO3. Both electrophilic and nucleophilic mechanisms take place. It is suggested that corresponding simulations for hydrolysis in the bulk are desirable.

Preparations of superhydrophobic surfaces using the one-step spin coating method and characterizations of their anti-icing behavior

AbstractSuperhydrophobic surfaces with an excellent anti-icing performance were prepared on an aluminum substrate using a simple one-step spin coating method. The wettability, morphology, and surface compositions of the prepared surfaces were characterized using the measuring instrument for contact angle and sliding angle, scanning electron microscopy, and Fourier transform infrared spectroscopy, respectively. The contact angle of the as-prepared superhydrophobic surfaces was as high as 165 ± 1.5°, and the sliding angle was less than 5° for a 4 μL pure water droplet, indicating excellent superhydrophobicity and low adhesion. The effects of addition of ZnO powders in different amounts on the morphology and wettability were further analyzed. Moreover, the anti-icing performance of the superhydrophobic surfaces was investigated using a simple lab-made icing monitoring apparatus, and the results are discussed using the one dimensional heat transfer and classical nucleation theory. It was found that the theoretical icing time of the superhydrophobic sample was about five times longer than that of the reference sample whereas for the untreated aluminum, the contact angle was 72 ± 1.5°, which was nearly consistent with the experimental results. The present study demonstrates that the prepared superhydrophobic surface can delay the icing time and decrease the icing temperature, and could be found potential applications in various industries.

Ion hydration free energies and water surface potential in water nano drops: The cluster pair approximation and the proton hydration Gibbs free energy in solution.

We estimate both single ion hydration Gibbs free energies in water droplets, comprising from 50 to 1000 molecules, and water/vacuum surface potentials in pure water droplets comprising up to 10 000 molecules. We consider four ions, namely, Li+, NH4 +, F-, and Cl-, and we model their hydration process and water/water interactions using polarizable force fields based on an induced point dipole approach. We show both ion hydration Gibbs free energies and water surface potentials to obey linear functions of the droplet radius as soon as droplets comprising a few hundred water molecules. Moreover, we also show that the differences in anion/cation hydration Gibbs free energies in droplets obey a different regime in large droplets than in small clusters comprising no more than six water molecules, in line with the earlier results computed from standard additive point charge force fields. Hence, both point charge and more sophisticated induced point dipole molecular modeling approaches suggest that methods considering only the thermodynamical properties of small ion/water clusters to estimate the absolute proton hydration Gibbs free energy in solution are questionable. In particular, taking into account the data of large ion/water droplets may yield a proton hydration Gibbs free energy in solution value to be shifted by several kBT units compared to small clusters-based approaches.

Experimental investigation of the evaporation behavior of urea‐water‐solution droplets exposed to a hot air stream

AbstractThe behavior of droplets of urea‐water‐solution (UWS) evaporating under the influence of a hot stream of air was investigated experimentally, under temperatures ranging from 100°C to 400°C. The droplets were suspended on a glass microfiber to minimize the influence of heat conduction, through the fiber, on the evaporation rate of the droplet. The flow rate of air, under all experimental conditions, was measured and these data were used to estimate the average velocity of air around the droplet. Experiments were also conducted on droplets of pure water and the results were compared. The initial mass fraction of urea, in the solution, did not appear to have a significant effect on the evaporation constant, but it did affect a few essential aspects of the evaporation behavior. The evaporation of water droplets was in accordance with the d2 law at all temperatures, whereas the evaporation of UWS droplets was ambient‐temperature dependent.

Evaporation and dispersion of exhaled droplets in stratified environment

Exhaled droplets of an infected person can spread diseases with the expiratory flows in indoor environments. This work focuses on how the thermal stratification affects the evaporation and dispersion of exhaled droplets by using a theoretical integral model. Pure water droplets with different diameters (10-240μm) are exhaled and disperse with different RHs (0, 30%, 50% and 60%). Results show that for medium droplets (50-170μm), the thermal stratification can obviously weaken the evaporation of droplets due to less heat and mass transfer between droplets and ambient air. The droplets were similarly influenced when the ambient RH increases from 0 to 50%, particularly, the 50μm droplets showed an obvious fluctuation trend with the jet flow at the lockup height in the stratified environment. When RH increased to 60%, a possible condensation phenomenon occurred on droplets, increasing the suspending time of droplets in the air. This theoretical model will be useful to control infectious diseases in rooms, especially when the thermal stratification exists.

Surface Tension Measurement of Water Surface Under IPA Vapor and the Impact of Its Concentration Dependence on the Marangoni Convection in Wafer Drying Process

As higher integration of semiconductor devices has progressed recently, mechano-chemical polishing process is playing a very important role. At the same time, cleaning and drying technologies after the polishing process have been more and more important in order to ensure the surface cleanliness of the wafer products. As a drying process after the cleaning, there is a method of supplying pure water and IPA (Iso-Propyl Alcohol) vapor simultaneously to the surface of the water meniscus region on a rotating wafer (which is called Rotagoni drying method). This technique causes a surface tension difference on the liquid surface and then gives rise to the convection by the Marangoni effect, which is thought to efficiently remove impure particles on the wafer surface. In this method, IPA vapor impinges on the vicinity of the liquid meniscus on the wafer. It is very difficult to measure the surface tension of the liquid surface in that part in situ, and the details about the convection are not well understood. As this technique further advances in near future in order to remove ultra-fine foreign particles on the wafer surface more efficiently, observation and understanding of the Marangoni convection in the meniscus region are necessary for the future progress of the technique. In this study, in order to consider the actual situation of the drying process, not only the measurement of the surface tension of the droplet of the IPA aqueous solution, but also the measurement by exposing pure water droplet to various concentrations of the IPA vapor were performed, and the relationship between the concentration of IPA vapor and the surface tension of the droplet was investigated. The latter situation is much closer to actual drying situation. Specifically, in measurements, a small pure water droplet was placed in a glass container, and the container was filled with a constant concentration of IPA vapor. At the time when it was judged that the IPA adsorption to the liquid surface had reached equilibrium by confirming that the shape of the droplet became unchanged, the image of the droplet shape was taken from the side of the container with digital camera. It is known that the static shape of a droplet which is flattened under the influence of gravity can be analytically expressed by a mathematical equation in consideration of surface tension and mechanical equilibrium. In this study, the surface tension was deduced by assuming that a uniform concentration of IPA and a uniform surface tension over the whole droplet surface. In another project of the authors’ group, physical simulation using a two-dimensional PDMS flow channel was performed to reproduce a situation of impinging IPA vapor to the meniscus region, and PIV (Particle Image Velocimetry) method was applied to obtain the flow velocity of the Marangoni convection. The measurement of IPA concentration on water surface during the actual drying process is very difficult, and moreover, the in-situ measurement of surface tension is almost impossible. From the data of surface tension as a function of IPA concentration and the measured velocity data with a simple analytical flow model, the estimation of surface tension and IPA concentration on the droplet surface will be discussed. END Figure 1

Simulation of desulphurization wastewater evaporation through flue gas

As an advanced treatment of desulphurization wastewater, spray evaporation technology has received significant attention and broad research interests recently. However, the results of previous studies were based on the simplification that droplets of desulphurization wastewater were regarded as droplets of pure water. To effectively reveal the evaporation mechanism of desulphurization wastewater, the evaporation model of droplets with different salt contents was used. The effects of parameters such as nozzle arrangement, droplet diameter, temperature of flue gas and salt content were obtained by numerical simulation. And the operation situations of evaporation equipment under different working conditions were analyzed. Numerical results show that the symmetrical arrangement of nozzles should be adopted since there is no scaling or corrosion in flue. The evaporation time of droplets could be shortened by adopting uniform diameter distribution. Droplets with small diameter under high flue gas temperature require less time and shorter distance for complete evaporation. The optimum diameter and temperature of flue gas are found to be 54 μm and 403 K, respectively. The increase of salt content of droplets could lead to the increase of mean distance and time which are needed for complete evaporation of droplets. And the droplets cannot completely evaporate under 75% THA (turbine heat acceptance) in which the evaporation equipment should not be turned on.

Single droplet with or without additives impacting on high-temperature burning liquid pool

This paper is a follow-up study for the phenomenon of vapor explosion for single water droplet impacting on a hot liquid pool. To explore the effects of vapor explosion on the dynamics of single droplet interacting with burning liquid pool, a series of experiments of pure water, water droplet with 5% NaCl, water droplet with 3% AFFF, and HFE-7100 droplet impacting on high-temperature burning oil were performed. For the cases of pure water, water droplet with 5% NaCl, and water droplet with 3% AFFF, vapor explosion can occur when the oil temperature reaches 210 °C. For the case of HFE-7100 droplet, however, the vapor explosion can only appear when the oil temperature ranges from 180 to 190 °C. Moreover, the vapor explosion has a significant effect on flame expansion. The flame expansion intensity is quantified by calculating the flame area and volume. Flame expansion intensity can rank as pure water droplet > droplet with 5% NaCl > droplet with 3% AFFF.

Application of projection lithography using a gradient-index lens array and wet etching to texturing for control of the hydrophobic properties of stainless-steel plates

The effectiveness of line-and-space (L&S) textures grooved onto stainless-steel SUS 304 plates for improving hydrophobic properties was investigated. The textures were grooved using a scan-projection exposure system with a gradient-index lens array as a projection lens and wet chemical etching in ferric chloride aqueous solution. The lithography system was applied because it was developed for printing rough patterns of larger than 15 μm in a large exposure field of 50 mm square. On the other hand, it was expected that textured protrusions with considerably sharper peaks might be formed by the undercut phenomenon during wet etching. When the contact angles of 2 μl pure-water droplets which had been dropped down onto 20 μm L&S textures were measured, it was found that contact angles measured by observing a droplet from the L&S direction were larger than those measured from the direction perpendicular to the L&S, and contact angles were changeable by grooving textures of different sizes.

Mass, charge, and radius of droplets in a linear quadrupole electrodynamic balance

Single particle levitation is a key tool in the analysis of the physicochemical properties of aerosol particles. Central to these techniques is the ability to determine the size of the confined particle or droplet, usually achieved via optical methods. While some of these methods are extremely accurate, they are not suitable for all applications and sample types, such as solid or optically absorbing particles. In this work, measurements of the radius, mass, and charge of droplets in a linear quadrupole electrodynamic balance (LQ-EDB) are reported. Using the elastic light scattering pattern produced by laser illumination, a method to determine the radius is described, with an accuracy of as good as ±60 nm and a sensitivity to changes on the order of 10 nm. The effect of refractive index on these measurements is explored by application of the technique to simulated data using Mie theory. In addition to radius, the relative and absolute mass and charge of droplets in the trap is measured from the voltage required to stabilize their vertical position. These measurements are facilitated by stacking multiple droplets in the LQ-EDB and solving the force balance equations to yield both parameters. These approaches are demonstrated through measurements of the evaporation of pure ethylene glycol and pure water droplets, the change in density of an aqueous glycerol solution as water evaporates, and the mass and charge of pure glycerol droplets.Copyright © 2019 American Association for Aerosol Research

Wetting and Spreading of Commercially Available Aqueous Surfactants on Porous Materials

The wetting properties of aqueous solutions of a commercially available surfactant at various concentrations on porous media are investigated using the KRUSS DSA100 shape analyzer and the ADVANCED software to process the data. Time evolution of both the contact angle and drop base diameter at each surfactant concentration after deposition were monitored. Three different porous substrates (sponges) were examined. The sponges used were a car sponge, dish sponge and audio sponge. The sponges were investigated both dry and at different degrees of saturation, that is, the amount of water absorbed into the sponge. It was found that pure distilled water droplets deposited on the dry porous media showed non-wetting. However, if droplets of surfactant solutions were deposited, then a change to a complete wetting case was found at all surfactant concentrations used. It has been observed that for all sponges, no matter the degree of saturation, they display a minimum contact angle after which the droplet is rapidly absorbed into the porous media.