Nucleation Of Water Droplets Research Articles

Porous Spindle-Knot Fiber by Fiber-Microfluidic Phase Separation for Water Collection and Nanopatterning.

Porous spindle-knot structures have been found in many creatures, such as spider silk and the root of the soybean plant, which show interesting functions such as droplet collection or biotransformation. However, continuous fabrication of precisely controlled porous spindle-knots presents a big challenge, particularly in striking a balance among good structural controllability, low-cost, and functions. Here, we propose a concept of a fiber-microfluidics phase separation (FMF-PS) strategy to address the above challenge. This FMF-PS combines the advantages of a microchannel regulated Rayleigh instability of polymer solution coated onto a fiber with the nonsolvent-induced phase separation of the polymer solution, which enables continuous and cost-effective production of porous spindle-knot fiber (PSKF) with well-controlled size and porous structures. The critical factors controlling the geometry and the porous structures of the spindle-knot by FMF-PS have been systematically investigated. For applications, the PSKF exhibited faster water droplet nucleation, growth, and maximum water collection capability, compared to the control samples, as revealed by in situ water collection growth curves. Furthermore, high-level fabrics of the PSKFs, including a two-dimensional network and three-dimensional architecture, have been demonstrated for both large-scale water collection and art performance. Finally, the PSKF is demonstrated as a programmable building block for surface nanopatterning.

Observation of Capillary Condensation and Pattern Bending Phenomena in Si Nanopillars Using <i>In Situ</i> TEM

Capillary condensation, a ubiquitous phenomenon involving the heterogeneous nucleation of liquid droplets, has significant implications in various industrial, biological, and atmospheric processes. Strong capillary forces induced by highly curved menisci of condensates can have potentially significant impact on the structural integrity and functionality of nanodevices. While the influence of surface properties on the nucleation and growth of water droplets has been extensively studied at microscale, our understanding of water condensation at the nanoscale remains limited due to experimental challenges in imaging liquids at nanometer scales. In this study, we employ in situ liquid phase TEM imaging and for the first time present real-time observations of water condensation dynamics on arrays of vertical silicon (Si) nanopillars. Experimental and simulation results show that nucleation of water droplets occurs at the edges of the nanopillars and substrate, followed by the growth of an interfacial layer resembling a corona around the nanopillars. Subsequently, the formation of bridges between adjacent growing coronas leads to the development of symmetric and asymmetric bridged nanopillar geometries. Importantly, we find that the formation of bridges can induce bending and collapse of the nanopillars, depending on their aspect ratios. Overall, this study provides valuable insights into the nanoscale dynamics of capillary condensation and paves the way for advanced engineering applications and optimization of various technological processes.

Homogeneous water nucleation in argon, nitrogen, and nitrous oxide as carrier gases.

Experimental results for homogeneous nucleation of water droplets in the presence of argon, nitrogen, and, for the first time, nitrous oxide as carrier gases are reported. The main objective of the study was to investigate the effects of different carrier gases on the nucleation rate. These gases differ in the number of atoms comprising the molecule, which affects the expansion ratio required to reach the target nucleation temperature. The experiments cover nucleation temperatures from 220 to 260K, nucleation pressures from 40 to 73kPa, supersaturations of 6.5-20, and nucleation rates ranging from about 4 × 105 to 4 × 109cm-3s-1. The new data obtained with a modernized expansion chamber utilizing the nucleation pulse method show good agreement with the available literature data. Another important point of our work is the application of real gas corrections to calculate the nucleation temperature and supersaturation. The results show that the influence of carrier gases on nucleation rate in the investigated pressure range is modest (less than a factor of 7), while without the real gas correction, the apparent effect of carrier gas is significant, with differences as high as a factor of 90.

Few-layer graphene on nickel enabled sustainable dropwise condensation

Condensation is critical for a wide range of applications such as electrical power generation, distillation, natural gas processing, dehumidification and water harvest, and thermal management. Compared with “filmwise” mode of condensation (FWC) prevailing in industrial-scale systems, dropwise condensation (DWC) can provide an order of magnitude higher heat transfer rate owing to drastically reduced thermal resistance from the formation of discrete and mobile droplets. In the past, promoting DWC by controlling surface wetting has attracted wide attention, but DWC highly relies on non-wetting surfaces and only lasts days under practical conditions due to the poor reliability of coatings. Here, we developed nanostructured graphene coatings on nickel (Ni) substrates that we can control and enhance the nucleation of water droplets on graphene grain boundaries. Surprisingly, this enables DWC even under normal “wetting” conditions. This is contradictory to the widely accepted DWC mechanism. Moreover, the Ni-graphene surface enables exceptional long-term condensation from days to more than 3 years under practical or even more aggressive testing environments.

Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger

Air conditioning energy consumption accounts for most building energy consumption, indoor dehumidification is the main cause of air conditioning energy consumption. Optimize the dehumidification methods of air conditioning systems have great significance to the development of green buildings and people’s pursuit of comfort. Improvement of fins on air conditioning heat exchangers is a hot topic of current research and has achieved considerable results in terms of indoor dehumidification and energy saving compared to traditional air conditioners. This paper reviews two kinds of heat exchangers modified by coating, including desiccant-coated heat exchangers and hydrophobic/hydrophilic coated heat exchangers. For desiccant-coated heat exchangers, the preparation methods of advanced desiccant materials and the possibilities of using this material to achieve excellent energy efficiencies were presented, and the operating parameters that affect thermal performance and dehumidification are determined, including airflow temperature, air velocity, inlet air relative humidity, and regeneration temperature. For hydrophobic/hydrophilic coated heat exchangers, different kinds of hybrid hydrophobic-hydrophilic surfaces are highlighted for they are a high water droplet nucleation rate and surface heat transfer efficiency. In addition, the challenges and future works are explained at last. This paper will provide a valuable reference for the follow-up research, which will be helpful for indoor humidity control and reducing the energy consumption of air conditioning.

Studying the Bulk and Contour Ice Nucleation of Water Droplets via Quartz Crystal Microbalances.

Due to the stochastic and time-dependent character of the ice embryo formation and growth (i.e., a process that can be analyzed statistically, but cannot be predicted precisely), the heterogeneous ice nucleation on atmospheric aerosols or macroscopic solid surfaces is still shrouded in mystery, regardless of the extremely active research and exponential progress within this scientific field. For instance, whether the icing appears from outside-in or inside-out is a subject of intense controversy, with practicability in designing passive icephobic coatings or improving the effectiveness of the cryopreservation technologies. Here, we propose an artful technique for quantitative analysis of the different modes of water freezing using super-nonwettable soot-coated quartz crystal microbalances (QCMs). To achieve this goal, a set of 5 MHz QCMs are loaded one at a time with a 50 μL droplet, whose bulk or contour solidification is detected in real-time. The obtained experimental results show that our sensor devices recognize explicitly if the ice nuclei form predominantly at the liquid–solid interface or spread along the droplet’s entire outer shell by triggering individual reproducible responses in terms of the direction of signal evolution in time. Our results may serve as a foundation for the future incorporation of QCM devices in different freezing assays, where gaining information about the ice adhesion forces and ice layer’s thickness is mandatory.

Cellulose-Based Superhydrophobic Surface Decorated with Functional Groups Showing Distinct Wetting Abilities to Manipulate Water Harvesting.

Inspired by the distinct functions of desert beetles with efficient droplet nucleation and lotus leaves with excellent droplet removal, an integrated method is presented for the design of a superhydrophobic surface decorated with hydrophilic groups that can efficiently nucleate and remove water droplets. We constructed a cellulose-based superhydrophobic surface containing numerous olefin terminal groups by solvent exchange and spray coating. This surface is different from most of the reported biomimicking water harvesting surfaces that rely on complicated lithography and micropatterning techniques requiring special instruments. The obtained superhydrophobic surface was further modified using various thiol compounds via a thiol-ene reaction to manipulate the water harvesting property. The modified surfaces containing hydrophobic groups (e.g., 1-octadecanethiol and 1H,1H,2H,2H-perfluorodecanethiol) or a strong hydrophilic group (e.g., 3-mercaptopropionic acid and 6-mercapto-1-hexanol) exhibited insufficient fog collecting abilities due to poor water droplet nucleation or strong water adhesion. By contrast, the modified surface decorated with moderately hydrophilic amino groups combines the advantages of biological surfaces with distinct wetting features (such as fog-harvesting beetles and water-repellent lotus leaves), resulting in accelerated water nucleation and less compromise of the water removal efficiency. Molecular dynamic simulations revealed that the efficient droplet nucleation is attributed to the hydrophilic amino groups whereas the rapid droplet removal is due to the maintained superhydrophobicity of the amino group-modified surface. This strategy of decorating a superhydrophobic surface with moderately hydrophilic functional groups provides insight into the manipulation of droplet nucleation and removal for water collection efficiency.

Aerosol Dynamics Modeling With Chemkin-Pro Surface-Kinetics User-Routines

Abstract The Chemkin-Pro Application Programming Interface (API) was used to implement surface-kinetics user-routines to expand current aerosol dynamics models. Phase change mechanisms were expanded to include homogeneous nucleation in supersaturated environments, and particle size-dependent vapor condensation and evaporation. Homogeneous nucleation of water droplets was modeled with classical nucleation theory (CNT) and a modified form of nucleation theory published by Dillmann, A., and Meier, G. E. A. (1991, “A Refined Droplet Approach to the Problem of Homogeneous Nucleation From the Vapor-Phase,” J. Chem. Phys., 94(5), pp. 3872–3884). The Chemkin-Pro homogeneous nucleation module, developed in this work, was validated against published data for nucleation fluxes at varying pressures, temperatures, and vapor concentrations. A newly released feature in Chemkin-Pro enabled particle size-dependent surface reaction rates. A Chemkin-Pro vapor condensation and evaporation module was written and verified with the formulation published in Hinds, W. C. (1999, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, Wiley, New York). Lastly, Chemkin-Pro results for coagulation in the transition regime were verified with the semi-implicit method developed by Jacobson, M. Z. (1999, Fundamentals of Atmospheric Modeling, Cambridge University Press, New York, NY). Good performance was observed for all three Chemkin-Pro modules. This work illustrates the utility of the Chemkin-Pro API, and the flexibility with which models can be developed using surface-kinetics user-routines. This paper illustrates that Chemkin-Pro can be developed to include more physically representative aerosol dynamics processes where rates are defined based on physical and chemical parameters rather than Arrhenius rates. The methods and modules developed in this work can be applied to industrial problems like material synthesis (e.g., powder production), processes involving phase change like heat exchangers, as well as more fundamental scientific processes like cloud physics.

Sequential Evaporation–Induced Formation of Polymeric Surface Microdents via Ouzo Effect

AbstractMicrodents fabricated on solid substrates have numerous applications, such as containers for cell growth, reactors for chemical reactions, and nucleation sites for growing photonic crystals. In this study, a new method is developed to fabricate microdents on polystyrene (PS) film deposited on a solid surface. The process relied on Ouzo effect in a ternary solution consisting of three solvents exhibiting different volatilities. In the ternary solution, precursor water droplets are first nucleated when a good solvent of water evaporated. The nucleated water droplets then act as templates for microdent formation during PS film deposition. After water droplets eventually evaporates, microdents are generated on the PS film. The detailed process of microdent formation is experimentally revealed. By controlling the nucleation of water droplets and PS concentration, the lateral size and the depth of microdents are systematically tuned. Moreover, it is demonstrated that the microdent decorated films obtained with the proposed method can be used to fabricate lubricating surfaces, which exhibit a low contact angle hysteresis of 2°.

Theoretical analysis of crystallization by homogeneous nucleation of water droplets.

We propose a novel method for analyzing the crystallization process from supercooled water droplets. The method, which is based on nucleation theory, simultaneously evolves homogeneous ice nucleation and crystal growth in the cooling process and obtains the crystallization temperature and the number of crystal nuclei in the droplets. The model can reproduce not only the crystallization of water but also the vitrification process. The model well replicated the results of previous laboratory experiments, especially, the different responses of the crystallization temperatures of the micrometer- and nanometer-sized particles as a function of cooling rates. For particle sizes ranging from 1 to 1000 μm and cooling rates below 104 K s-1, the crystallization temperature was 230-240 K. At cooling rates above 104 K s-1, the crystallization temperature decreased rapidly. On the other hand, the crystallization temperature of 10 nm particles was 200-230 K at cooling rates below 104 K s-1. When describing the interfacial tension by σ = 29.1 + 0.1(T - 273.15) erg cm-2 (where T is the water droplet temperature in K), the analyses explained well the previously reported crystallization temperatures of droplets sized from a few nm to 100 μm under various cooling conditions. Our model also predicts the critical cooling rate for vitrification of the liquid water droplets. The critical cooling rate of vitrification is predicted to be 107-108 K s-1, consistent with the experimental rates. These analyses are useful not only for comprehensively understanding the ice nucleation process but also for predicting the crystallization processes in various environments such as cirrus clouds, which are difficult to reproduce in experiments.

How Nanostructures Affect Water Droplet Nucleation on Superhydrophobic Surfaces

Nucleation is the first stage of phase change phenomena, including condensation on nanostructured superhydrophobic surfaces. Despite plenty of theoretical studies on the effect of nanostructure density and shape on water droplet nucleation, not many experimental investigations have been reported. Here, we show both experimentally and theoretically that a moderate increase in the nanostructure density can lead to an increase in the nucleation density of water droplets because of the decreased energy barrier of nucleation in cavities formed between the nanostructures. Specifically, we observed droplets aligned in regions with denser nanostructures. The number and average volume of the aligned droplets in these regions were larger than that of the droplets in the surrounding areas. However, nucleation in cavities subsequently caused initial pinning of the droplet base within the nanostructures, forming a balloonlike, slightly elongated droplet shape. The dewetting transition of the pinned droplets from the Wenzel state to the unpinned Cassie state was predicted by quantifying the aspect ratio of droplets ranging from 3 to 30 μm. Moreover, the coalescence-jumping of droplets was followed by a new cycle of droplet condensation in an aligned pattern in an emptied area. These findings offer guidelines for designing enhanced superhydrophobic surfaces for water and energy applications.

Energetic and entropic components of the Tolman length for mW and TIP4P/2005 water nanodroplets

The surface free energy of a droplet is approximately γ(R)=γ(∞)(1-2δ/R), with R being the droplet radius and δ being the Tolman length. Here we use the mitosis method to compute δ=-0.56±0.1Å at 300 K for mW water, indicating that γ(R) increases as the droplet size decreases. The computed Tolman length agrees quite well with a previous study of TIP4P/2005 water. We also decompose the size-dependent surface free energy into energetic and entropic contributions for the mW and TIP4P/2005 force fields. Despite having similar Tolman lengths, the energy-entropy decompositions are very different for the two force fields. We discuss critical assumptions which lead to these findings and their relation to experiments on the nucleation of water droplets. We also discuss surface broken bonds and structural correlations as possible explanations for the energetic and entropic contributions.

The upside-down water collection system of Syntrichia caninervis.

Desert plants possess highly evolved water conservation and transport systems, from the root structures that maximize absorption of scarce ground water(1-5), to the minimization of leaf surface area(6) to enhance water retention. Recent attention has focused on leaf structures that are adapted to collect water and promote nucleation from humid air(7-9). Syntrichia caninervis Mitt. (Pottiaceae) is one of the most abundant desert mosses in the world and thrives in an extreme environment with multiple but limited water resources (such as dew, fog, snow and rain), yet the mechanisms for water collection and transport have never been completely revealed. S. caninervis has a unique adaptation: it uses a tiny hair (awn) on the end of each leaf to collect water, in addition to that collected by the leaves themselves. Here we show that the unique multiscale structures of the hair are equipped to collect and transport water in four modes: nucleation of water droplets and films on the leaf hair from humid atmospheres; collection of fog droplets on leaf hairs; collection of splash water from raindrops; and transportation of the acquired water to the leaf itself. Fluid nucleation is accomplished in nanostructures, whereas fog droplets are gathered in areas where a high density of small barbs are present and then quickly transported to the leaf at the base of the hair. Our observations reveal nature's optimization of water collection by coupling relevant multiscale physical plant structures with multiscale sources of water.

Role of the electric double layer in the ice nucleation of water droplets under an electric field

Figuring out the mechanism of ice nucleation on charged aerosols or in thunderstorms is of fundamental importance in atmospheric science. However, findings on whether the electric field promotes or suppresses heterogeneous ice nucleation are conflicting. In this work, we design an apparatus and test the influence of the electric field on ice nucleation by freezing a series of deionized water droplets resting on solid surfaces with an electric field perpendicular to the substrates. Results show that ice nucleation is obviously promoted under the electric field and is independent of the field direction. Theoretic analyses show that the promotion is due to the reduction of Gibbs free energy which can be partially rationalized by the electric field sustained in the electric double layer at the solid-water interface, with strength about two orders higher than that of the external electric field. Moreover, water-droplet deformation under the electric field is not expected to be the cause of the ice-nucleation promotion.

Thermokinetics of heterogeneous droplet nucleation on conically textured substrates.

Within the framework of the classical theory of heterogeneous nucleation, a thermokinetic model is developed for line-tension-associated droplet nucleation on conical textures considering growth or shrinkage of the formed cluster due to both interfacial and peripheral monomer exchange and by considering different geometric configurations. Along with the principle of free energy extremization, Katz kinetic approach has been employed to study the effect of substrate conicity and wettability on the thermokinetics of heterogeneous water droplet nucleation. Not only the peripheral tension is found to have a considerable effect on the free energy barrier but also the substrate hydrophobicity and hydrophilicity are observed to switch over their roles between conical crest and trough for different growth rates of the droplet. Besides, the rate of nucleation increases and further promotes nucleation for negative peripheral tension as it diminishes the free energy barrier appreciably. Moreover, nucleation inhibition can be achievable for positive peripheral tension due to the enhancement of the free energy barrier. Analyzing all possible geometric configurations, the hydrophilic narrower conical cavity is found to be the most preferred nucleation site. These findings suggest a physical insight into the context of surface engineering for the promotion or the suppression of nucleation on real or engineered substrates.

Thermodynamic losses of hetero and homogeneous condensation in steam turbine cascades

The approach used in this paper for the condensation of flowing steam is based on the homogeneous condensation of steam and also on the heterogeneous (binary) nucleation of water droplets with a representative chemical impurity NaCl. The physical and mathematical models are briefly described. The homogeneous and binary nucleation numerical models are then used for the calculation of steam flow with condensation in the two-dimensional nozzle-blade cascade of the first wet stage of the low-pressure part of a fossil-fuelled condensing 200-MW steam turbine and in a similar cascade of the low-pressure part of a nuclear 1000-MW turbine. Results of the calculations of the flow in the cascades and of the thermodynamic loss are presented and analysed. It is recommended to situate the salt solution zone of NaCl upstream of a turbine nozzle cascade, to use a lower expansion rate on the cascade, and to realise there, primarily, a heterogeneous process (binary nucleation of steam and NaCl) for the initial condensation.

Investigating the significance of zero-point motion in small molecular clusters of sulphuric acid and water

The nucleation of particles from trace gases in the atmosphere is an important source of cloud condensation nuclei, and these are vital for the formation of clouds in view of the high supersaturations required for homogeneous water droplet nucleation. The methods of quantum chemistry have increasingly been employed to model nucleation due to their high accuracy and efficiency in calculating configurational energies; and nucleation rates can be obtained from the associated free energies of particle formation. However, even in such advanced approaches, it is typically assumed that the nuclei have a classical nature, which is questionable for some systems. The importance of zero-point motion (also known as quantum nuclear dynamics) in modelling small clusters of sulphuric acid and water is tested here using the path integral molecular dynamics method at the density functional level of theory. The general effect of zero-point motion is to distort the mean structure slightly, and to promote the extent of proton transfer with respect to classical behaviour. In a particular configuration of one sulphuric acid molecule with three waters, the range of positions explored by a proton between a sulphuric acid and a water molecule at 300 K (a broad range in contrast to the confinement suggested by geometry optimisation at 0 K) is clearly affected by the inclusion of zero point motion, and similar effects are observed for other configurations.

Improved coarse-grained model for molecular-dynamics simulations of water nucleation

We developed a new coarse-grained (CG) model for water to study nucleation of droplets from the vapor phase. The resulting potential has a more flexible functional form and a longer range cutoff compared to other CG potentials available for water. This allowed us to extend the range of applicability of coarse-grained techniques to nucleation phenomena. By improving the description of the interactions between water molecules in the gas phase, we obtained CG model that gives similar results than the all-atom (AA) TIP4P model but at a lower computational cost. In this work we present the validation of the potential and its application to the study of nucleation of water droplets from the supersaturated vapor phase via molecular-dynamics simulations. The computed nucleation rates at T = 320 K and 350 K at different supersaturations, ranging from 5 to 15, compare very well with AA TIP4P simulations and show the right dependence on the temperature compared with available experimental data. To help comparison with the experiments, we explored in detail the different ways to control the temperature and the effects on nucleation.

Water condensation behavior on the surface of a network of superhydrophobic carbon fibers with high-aspect-ratio nanostructures

We have explored the condensation behavior of water on a superhydrophobic carbon fiber (CF) network with high-aspect-ratio hair-like nanostructures. Nanostructures ranging from nanopillars to hairy shapes were grown on CFs by preferential oxygen plasma etching. Superhydrophobic CF surfaces were achieved by application of a hydrophobic siloxane-based hydrocarbon coating, which increased the water contact angle from 147° to 163° and decreased the contact angle hysteresis from 71° to below 5°, sufficient to cause droplet roll-off from the surface. Water droplet nucleation and growth on the superhydrophobic CF were significantly retarded due to the high-aspect-ratio nanostructures under super-saturated vapor conditions. CFs are observed to wet with condensation between fibers of the pristine surface under super-saturated vapor conditions, which eventually leads to flooding. However, dropwise condensation became dominant in the superhydrophobic CF network, allowing for easy removal of the condensed droplets, which largely allowed the interstitial spaces of the fiber network to remain dry. It is implied that superhydrophobic CF can provide a passage for vapor or gas flow in wet environments such as a gas diffusion layer requiring the effective water removal in the operation of proton exchange membrane fuel cell.

How ambient pressure influences water droplet nucleation at tropospheric conditions

[1] Nucleation theories typically neglect the influence of the ambient pressure on the condensation of droplets from a supersaturated vapor, despite increasing experimental evidence. We have applied a recently presented model that incorporates this effect to the homogeneous nucleation rates of water at tropospheric conditions. We measured experimental pressure dependent nucleation rates of water in helium at low to intermediate pressures (70–200 kPa) and at temperatures from 240–270 K with the laminar flow diffusion chamber. The observed pressure effect shows a clear positive effect (increasing nucleation rate with increasing pressure) at 270 K and a weaker effect with lower temperatures, consistent with the theory. The experimental pressure effect was more pronounced than predicted by theory. The same principle of the pressure effect should also hold for heterogeneous nucleation of water, which implies that water vapor removal by droplet nucleation of water may be suppressed at tropospheric conditions.