Micrometer-sized Water Droplets Research Articles

Water Microdroplets in Air: A Hitherto Unnoticed Natural Source of Nitrogen Oxides.

Water microdroplets are widespread in the atmosphere. We report a striking observation that micron-sized water droplets obtained from zero-volt spray sources (sonic spray, humidifier, spray bottle, steamer, etc.) spontaneously generate nitrogen oxides. The mechanistic investigation through the development of custom-designed sampling sources combined with mass spectrometry and isotope labeling experiments confirmed that air nitrogen reacts with the water at the air-water interface, fixing molecular nitrogen to its oxides (NO, NO2, and N2O) and acids (HNO2 and HNO3) at trace levels without any catalyst. These reactions are attributed to the consequence of an experimentally detected feeble corona discharge (breakdown of air) at the air-water interface, likely driven by the high intrinsic electric field at the surface of water microdroplets. The extent of this corona discharge effect varies depending on the pH, salinity/impurity, size, speed, and lifetime of microdroplets in the air. Thus, this study discloses that the air-water interface of microdroplets breaks the strong chemical bond of nitrogen (N2), producing nitrogen oxides in the environment, while lightning strikes and microbial processes in soil are considered their dominant natural sources. As nitrogen oxides are toxic air pollutants, their spontaneous formation at the air-water interface should have important implications in atmospheric reactions, requiring further investigations.

The transport dynamics of tens of micrometer-sized water droplets in RF atmospheric pressure glow discharges

Charging of particles having diameters of tens of microns has been extensively studied at atmospheric pressure in the context of, for example, electrostatic precipitators where the focus was on unipolar charging. The ambipolar charging of particles in atmospheric pressure plasmas, and of droplets in particular, has received less attention. The plasma activation of droplets is of interest for water purification, fertilizer production and materials synthesis, all of which depend on the transport of the droplets through the plasma, which in turn depends on their charging. In this paper, we report on the transport dynamics of water droplets, tens of microns in diameter, carried by the gas flow through an atmospheric pressure radiofrequency glow discharge sustained in helium. The droplets pass through the plasma with minimal evaporation and without reaching the Rayleigh limit. The droplet trajectory in the presence and absence of the plasma provides insights on the forces acting on the droplet. The measurements were analyzed using results from a three-dimensional fluid model and a two-dimensional plasma hydrodynamics model. We found that the transport dynamics as the droplet enters and leaves the plasma are due to differential charging of the droplet in the plasma gradients of the bounding sheaths to the plasma.

Fast Fabrication of WS2/Bi2Se3 Heterostructures for High-Performance Photodetection.

Two-dimensional (2D) material heterostructures have attracted considerable attention owing to their interesting and novel physical properties, which expand the possibilities for future optoelectronic, photovoltaic, and nanoelectronic applications. A portable, fast, and deterministic transfer technique is highly needed for the fabrication of heterostructures. Herein, we report a fast half-wet poly(dimethylsiloxane) (PDMS) transfer process utilizing the change of adhesion energy with the help of micron-sized water droplets. Using this method, a vertical stacking of the WS2/Bi2Se3 heterostructure with a straddling band configuration is successfully assembled on a fluorophlogopite substrate. Thanks to the complementary band gaps and high efficiency of interfacial charge transfer, the photodetector based on the heterostructure exhibits a superior responsivity of 109.9 A W-1 for a visible incident light at 473 nm and 26.7 A W-1 for a 1064 nm near-infrared illumination. Such high photoresponsivity of the heterostructure demonstrates that our transfer method not only owns time efficiency but also ensures high quality of the heterointerface. Our study may open new pathways to the fast and massive fabrication of various vertical 2D heterostructures for applications in twistronics/valleytronics and other band engineering devices.

Dynamics and thermodynamics of bimolecular reactions at heterogeneous biological interfaces.

The behaviors of biomolecules at heterogeneous biological aqueous-nonaqueous interfaces markedly differ from those in bulk water. We found that various biomolecular reactions, including protein unfolding, protein-ligan binding, chlorophyll demetallation, redox reactions, and crystallization, are accelerated by a factor of 103 or higher at the interface of micron-sized water droplets (microdroplets) (PNAS, 2015; QRevPhys, 2015 and 2017). The FRET analysis revealed that molecules are highly localized at the nanoscale regime near the water surface, which is in part responsible for the observed accelerated dynamics (JPCB, 2020). Besides, the thermodynamics of biomolecular reactions are also altered near the water surface. We found that molecules are in highly ordered states near the aqueous-nonaqueous interface (Sci Adv, 2020, Sci Rep, 2020). This minimized entropy lowers entropic barriers so that thermodynamically unfavorable reactions such as phosphorylation and RNA formation can proceed without any enzyme or ATP energy source at room temperature (PNAS 2017, 2018). Moreover, the charge exchange between water molecules and biomolecules spontaneously occurs at the water interface, leading to the oxidation of water molecules and the reduction of biomolecules in the Krebs cycle (JACS, 2019; Nat Commun, 2018; PNAS, 2018, 2019; Chem Sci, 2019). Vibrational Stark spectroscopy coupled with stimulated Raman excited fluorescence microscopy showed the presence of a strong intrinsic interfacial electric field formed at the surface of the microdroplet (JPCL, 2020). This strong electric field, combined with the partial desolvation effect, can be responsible for the unusual behaviors of biomolecules at the aqueous-nonaqueous interface (Annu Rev Phys Chem, 2020). These findings suggest that the dynamics and thermodynamics of biomolecular reactions occurring in cells with various aqueous-nonaqueous interfaces and compartments may significantly differ from those in bulk water.

Providing evidence for two remarkably stable liquid states in water

Examining the second glass transition without pressure-induced ice amorphization via micron-sized water droplets

Pressure-annealed high-density amorphous ice made from vitrified water droplets: A systematic calorimetry study on water's second glass transition.

In previous work, water's second glass transition was investigated based on an amorphous sample made from crystalline ice [Amann-Winkel et al., Proc. Natl. Acad. Sci. U. S. A. 110, 17720 (2013)]. In the present work, we investigate water's second glass transition based on the genuine glassy state of high-density water as prepared from micron-sized liquid water droplets, avoiding crystallinity at all stages. All the calorimetric features of water's second glass transition observed in the previous work are also observed here on the genuine glassy samples. This suggests that the glass transition indeed thermodynamically links amorphous ices continuously with deeply supercooled water. We proceed to extend the earlier study by investigating the effect of preparation history on the calorimetric glass transition temperature. The best samples prepared here feature both a lower glass transition temperature Tg,2 and a higher polyamorphic transition temperature Tons, thereby extending the range of thermal stability in which the deeply supercooled liquid can be observed by about 4K. Just before the polyamorphic transition, we observe a spike-like increase of heat capacity that we interpret in terms of nucleation of low-density water. Without this spike, the width of water's second glass transition is 15K, and the Δcp amounts to 3 ± 1JK-1 mol-1, making the case for the high-density liquid being a strong liquid. We suggest that samples annealed at 1.9GPa to 175K and decompressed at 140K to ≥0.10GPa are free from such nuclei and represent the most ideal high-density amorphous glasses.

An integrated electrical condensation particle counter for compact and low-cost ultrafine particle measurement system

In this study, we developed a compact and low-cost measurement system that can measure the real-time number concentration of airborne ultrafine particles (UFPs, particles smaller than 100 nm). Our system is based on the condensation nucleation method; however, it counts particles based on their electrostatic properties (i.e., dielectric constant) to overcome the limitations of complex optical systems that have traditionally been employed. The new system consists of two components: a microfluidic particle growth channel to grow UFPs into micron-sized water droplets and a capacitive particle counter to count the grown droplets. To effectively reduce the size and cost of the system, each system was fabricated into a single module using a semiconductor manufacturing process prior to integration. Super-hydrophilic micropillar wicks were monolithically integrated with Printed Circuit Board substrate channels to use water as the condensing working fluid. A glass substrate interdigitated electrode chip with a 3 μm gap was fabricated and placed on the impaction plate of an inertial impactor to be used as a sensor. Numerical calculations were performed to verify the dimensions of the channel required for efficient supersaturated vapor generation. Quantitative experiments using NaCl, (NH4)2SO4, and Ag2O UFPs showed that our system could grow particles larger than 50 nm into micron-sized droplets and count up to 10,300 N cm−3 particles with linear electrostatic properties.

Collisions of micron-sized charged water droplets in still air

We show that fixed points and their stable manifolds determine collision outcomes of oppositely charged water droplets settling in still air. For droplets with large enough opposite charges, the stable manifold of a saddle forms a separatrix that divides colliding trajectories from those that do not collide. A consequence of our findings is that for large charges, collision outcomes are not sensitive to the breakdown of hydrodynamics at small distances.

Wood-Inspired Compressible Superhydrophilic Sponge for Efficient Removal of Micron-Sized Water Droplets from Viscous Oils.

Efficient micron-sized droplet separation materials have become a new demand for environmental protection and economic development. However, existing separation methods are difficult to be effectively used for micron-sized water droplets surrounded by viscous oil, and common materials have difficulty maintaining hydrophilicity underoil. Here, inspired by the microstructure of tree xylem, we report a cellulose-polyurethane sponge (CP-Sponge) with wood-like pores and underoil superhydrophilicity using directional freeze-casting. The CP-Sponge has an excellent selective water absorption capacity underoil and compression resilience. This preparation strategy can flexibly control the sponge's dimensional morphology. The designed cylindrical CP-Sponge can be easily installed in the silicone tube of a peristaltic pump. During pump operation, with a simple absorption, compression, and recovery process, the CP-Sponge continuously and effectively removes micron-sized water from crude oil and lubricating oil, reducing residual water in the oil to less than 2 ppm. The absorption-saturated sponge can be dried to continue recycling. Eco-friendly, recyclable, and sustainable artificial porous sponges provide new ideas and inspiration for the practical application of deep dehydration of viscous oils.

Heterogeneous Ice Growth in Micron-Sized Water Droplets Due to Spontaneous Freezing

Understanding how ice nucleates and grows into larger crystals is of crucial importance for many research fields. The purpose of this study was to shed light on the phase and structure of ice once a nucleus is formed inside a metastable water droplet. Wide-angle X-ray scattering (WAXS) was performed on micron-sized droplets evaporatively cooled to temperatures where homogeneous nucleation occurs. We found that for our weak hits ice grows more cubic compared to the strong hits that are completely hexagonal. Due to efficient heat removal caused by evaporation, we propose that the cubicity of ice at the vicinity of the droplet’s surface is higher than for ice formed within the bulk of the droplet. Moreover, the Bragg peaks were classified based on their geometrical shapes and positions in reciprocal space, which showed that ice grows heterogeneously with a significant population of peaks indicative of truncation rods and crystal defects. Frequent occurrences of the (100) reflection with extended in-planar structure suggested that large planar ice crystals form at the droplet surface, then fracture into smaller domains to accommodate to the curvature of the droplets. Planar faulting due to misaligned domains would explain the increased cubicity close to the droplet surface.

Supercritical CO2-assisted atomization for deposition of cellulose nanocrystals: an experimental and computational study

Nanoparticle spray deposition finds numerous applications in pharmaceutical, electronics, manufacturing, and energy industries and has shown great promises in engineering the functional properties of the coated parts. However, current spray deposition systems either lack the required precision in controlling the morphology of the deposited nanostructures or do not have the capacity for large-scale deposition applications. In this study, we introduce a novel spray system that uses supercritical CO2 to assist the atomization process and create uniform micron-size water droplets that are used as cellulose nanocrystal (CNC) carriers. CNCs are selected in this study as they are abundant, possess superior mechanical properties, and contain hydroxyl groups that facilitate interaction with neighboring materials. We fundamentally investigate the effect of different process parameters, such as injection pressure, gas-to-liquid ratio, the axial distance between the nozzle and substrate, and CNC concentration on the final patterns left on the substrate upon evaporation of water droplets. To this end, we show how tuning process parameters control the size of carrier droplets, dynamics of evaporation, and self-assembly of CNCs, which in turn dictate the final architecture of the deposited nanostructures. We will particularly investigate the morphology of the nanostructures deposited after evaporation of micron-size droplets that has not been fully disclosed to date. Different characterization techniques such as laser diffraction, polarized microscopy, and high-resolution profilometry are employed to visualize and quantify the effect of each process parameter. Numerical simulations are employed to inform the design of experiments. Finally, it is shown that the fabricated nanostructures can be engineered based on the size of the carrier droplets controlled by adjusting spray parameters and the concentration of nanoparticles in the injected mixture. Process parameters can be selected such that nanoparticles form a ring, disk, or dome-shaped structure. Moderate operational conditions, simplicity, and time efficiency of the process, and use of abundant and biodegradable materials, i.e., water, CNCs, and CO2 promote the scalability and sustainability of this method.

Effectual removal of indoor ultrafine PM using submicron water droplets

Exposure to ultrafine airborne particulate matter (PM1.0) poses a significant risk to human health and well-being. Examining the effect of submicron water droplets on the removal of ultrafine PM is timely and important for mitigating indoor ultrafine PM, which is difficult to filter out from incoming air. In this study, submicron water droplets were made by using a nanoporous membrane and an ultrasonic module of a commercial household ultrasonic humidifier (UH) for effectual ultrafine PM removal. The effect of water droplet size on indoor PM removal was experimentally investigated. Variations in the normalized PM concentration, removal efficiency and deposition constants were evaluated by analyzing the temporal variation in PM concentration inside a test chamber. The measured PM deposition constants were compared with the results of other previous studies. As a result, submicron water droplets of 800 nm in mean diameter were generated by ultrasonic module combined passive nanoporous membrane, and PM1.0 concentration decreased by 30% in the initial 30 min. Compared with micron-sized water droplets, PM1.0 removal efficiency improved by approximately two times higher. Moreover, the substitution of the experimental results into a theoretical model ascertained that PM collection efficiency is increased by approximately 103 levels as the size of water droplets decreases. These results would be utilized in the development and implementation of effective strategies for indoor PM removal.

Influence of fat crystallization in W/O emulsions on the water droplet size determination by NMR diffusometry

HypothesisIt is expected that low resolution (LR) NMR diffusometry enables (more) accurate water droplet size determination for solid-fat based water-in-oil (W/O) emulsions with (sub)-micron size water droplets in comparison to liquid-oil based W/O emulsions due to hindered extra-droplet water diffusion. ExperimentsW/O emulsions with a volume-weighed mean diameter of about 1 µm and a solid fat content (SFC) ranging from 0% to 74% were produced. The aqueous phase contained the ionic marker tetraphenylphosphonium chloride (TPPCl). The water droplet size was estimated using LR and high resolution (HR) NMR diffusometry. FindingsHR-NMR diffusometry showed that the diffusion behavior of water and TPPCl was different, indicating water diffusion beyond the droplet’s interfacial boundaries. From a certain SFC onwards, a slower echo decay was observed for the water molecules, thus decreasing the overestimation of the water droplet size in (sub)micron W/O emulsions. For those emulsions, the solid fat matrix is believed to hinder extra-droplet water diffusion, which is most likely to be related to the increased tortuosity of the diffusive path in the porous fat crystal network. Using LR-NMR, it can be verified whether the water echo attenuation is mono-exponential or bi-exponential by increasing the gradient pulse duration for the maximum gradient strength, which is more convenient for routine analysis compared to HR-NMR.

Dilatational rheology of water-in-diesel fuel interfaces: effect of surfactant concentration and bulk-to-interface exchange.

Micrometer-sized water droplets dispersed in diesel fuel are stabilized by the fuel's surface-active additives, such as mono-olein and poly(isobutylene)succinimide (PIBSI), making the droplets challenging for coalescing filters to separate. Dynamic material properties found from interfacial rheology are known to influence the behavior of microscale droplets in coalescing filters. In this work, we study the interfacial dilatational properties of water-in-fuel interfaces laden with mono-olein and PIBSI, with a fuel phase of clay-treated ultra-low sulphur diesel (CT ULSD). First, the dynamic interfacial tension (IFT) is measured using pendant drop tensiometry, and a curvature-dependent form of the Ward and Tordai diffusion equation is applied for extracting the diffusivity of the surfactants. Additionally, Langmuir kinetics are applied to the dynamic IFT results to obtain the maximum surface concentration (Γ∞) and ratio of adsorption to desorption rate constants (κ). We then use a capillary pressure microtensiometer to measure the interfacial dilatational modulus, and further extract the characteristic frequency of surfactant exchange (ω0) by fitting a model assuming diffusive exchange between the interface and bulk. In this measurement, 50-100 μm diameter water droplets are pinned at the tip of a glass capillary in contact with the surfactant-containing fuel phase, and small amplitude capillary pressure oscillations over a range of frequencies from 0.45-20 rad s-1 are applied to the interface, inducing changes in interfacial tension and area to yield the dilatational modulus, E*(ω). Over the range of concentrations studied, the dilatational modulus of CT ULSD with either mono-olein or PIBSI increases with a decrease in bulk concentration and plateaus at the lowest concentrations of mono-olein. Characteristic frequency (ω0) values extracted from the fit are compared with those calculated using equilibrium surfactant parameters (κ and Γ∞) derived from pendant drop tensiometry, and good agreement is found between these values. Importantly, the results imply that diffusive exchange models based on the equilibrium relationships between surfactant concentration and interfacial tension can be used to infer the dynamic dilatational behavior of complex surfactant systems, such as the water-in-diesel fuel interfaces in this study.

Optical Manipulation of Water Droplets in Air by a Double-beam Laser Trapping Technique

When micrometer-sized water droplets are levitated in air, thermodynamic metastable states are stable for a long time because they are not affected by the solid surface that provides a scaffold for nucleation. Therefore, it is expected that the use of aerosol droplets as a microreactor would allow chemical processes that are different from those under thermal equilibrium to proceed. The purpose of this study is to construct a double-beam laser trapping system to simultaneously manipulate two water droplets in air, and then to contact and fuse the droplets at an arbitrary timing. Using this system, the effect of surface charge was investigated when water droplets coalesce in air.

Condensing water vapor to droplets generates hydrogen peroxide

It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A, 116, 19294-19298 (2019)] that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water-air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.

Asphaltene-induced spontaneous emulsification: Effects of interfacial co-adsorption and viscoelasticity

Asphaltenes are a class of high molecular weight aromatic compounds found in crude oil. They adsorb onto toluene-water interfaces and induce a spontaneous emulsification phenomenon, whereby stable water-in-oil emulsions form without the need of an external energy input. This work aims to control and understand the factors affecting spontaneous droplet formation in the presence of asphaltene adsorption. This is particularly useful for crude oil refining, where the presence of a stable emulsion hampers the efficiency of downstream processing operations. We explore the effect of the addition of copolymers designed as crude oil flow improvers as a means to control the extent of emulsion formation. We find that the polymers competitively adsorb onto the toluene-water interface and diminish spontaneous emulsification. We also conduct fluorescence microscopy experiments and measurements of the interfacial energy to determine the mechanism of spontaneous emulsification in asphaltene systems. We conclude that an emulsion forms via the diffusion of molecular water into the oil phase and subsequent binding with asphaltene aggregates, leading to the nucleation of micrometer-sized water droplets. We find that the polymer forms complexes with the dissolved asphaltenes, possibly hampering the ability of diffused water to bind to the asphaltenes and reducing the extent of spontaneous emulsification. Finally, we investigate the role of interfacial shear and dilatational viscoelasticity to better understand which fundamental interfacial properties are important in the emulsification of asphaltene-laden systems. We find that the rate of formation of an interfacial microstructural network is inversely correlated with the extent and rate of spontaneous emulsification.

Spraying Small Water Droplets Acts as a Bacteriocide.

Disinfectants are important for arresting the spread of pathogens in the environment. Frequently used disinfectants are often incompatible with certain surfaces, expensive and can produce hazardous by-products. We report that micron-sized water droplets can act as an effective disinfectant, which were formed by spraying pure bulk water with coaxial nebulizing airflow. Spraying for 20 min onto Escherichia coli and Salmonella typhimurium on stainless-steel discs caused inactivation of over 98% of the bacteria. Control experiments resulted in less than 10% inactivation (water stream only and gas only) and 55% inactivation with 3% hydrogen peroxide. Experiments have shown that cell death results from cell wall destruction. We suggest that the combined action of reactive oxygen species present in water droplets (but not in bulk water) along with the droplet surface charge is responsible for the observed bactericidal activity.

Laminated superwetting aerogel/membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions

Although superwetting filtration membranes have been widely used, they suffer from the disadvantages of low flux and membrane fouling caused by the nanoscale pore size. Herein, a laminated cellulose aerogel/membrane composite with large pore size is fabricated via combining facile freeze-drying and hydrophobic modification using cellulose fibers as building blocks. An ultrathin silanization coating layer on cellulose fibers surface is constructed via directly coupling alkoxysilane and endows the composite with excellent superhydrophobicity (161°) and superoleophilicity (0°). Upon contact with a water-in-oil emulsion, the aerogel layer causes the micrometer-sized water droplets to coalesce. The coalesced water droplets are then repelled by the membrane layer with large pore size, achieving ultrafast gravity-driven separation with high separation efficiency (99.5%) and separation flux (12890 L m−2 h−1). Moreover, the composite exhibits ease to cycle and good usability. The outstanding performance of composite highlights its potential applications in the field of oil related industry.

Micrometer-Sized Water Droplets Induce Spontaneous Reduction.

Bulk water serves as an inert solvent for many chemical and biological reactions. Here, we report a striking exception. We observe that in micrometer-sized water droplets (microdroplets), spontaneous reduction of several organic molecules occurs, pyruvate to lactate, lipoic acid to dihydrolipoic acid, fumarate to succinate, and oxaloacetate to malate. This reduction proceeds in microdroplets without any added electron donors or acceptors and without any applied voltage. In three of the four cases, the reduction efficiency is 90% or greater when the concentration of the dissolved organic species is less than 0.1 μM. None of these reactions occurs spontaneously in bulk water. One example demonstrating the possible broad application of reduction in water microdroplets to organic molecules is the reduction of acetophenone to form 1-phenylethanol. Taken together, these results show that microdroplets provide a new foundation for green chemistry by rendering water molecules to be highly electrochemically active without any added reducing agent or applied potential. In this manner, aqueous microdroplets might have provided a route for abiotic reduction reactions in the prebiotic era, thereby providing organic molecules with a reducing power before the advent of biotic reducing machineries.