Ice Droplets Research Articles

Ice accretion on circular cylinder in relation to its diameter

Numerical study of atmospheric ice accretion on circular cylinder is carried out to understand the relation between cylinder diameter and resultant ice accretion. To validate the numerical model, results are compared with the experimental data and a good agreement is found. Numerical study shows that change in cylinder diameter significantly affects the air and droplet behavior that results a change in ice growth along cylinder. A detailed parametric study is carried out for different droplet sizes and cylindrical diameters. Results show a decrease in droplet collision efficiency and ice growth with the increase in cylinder diameter.

On Localized Vapor Pressure Gradients Governing Condensation and Frost Phenomena.

Interdroplet vapor pressure gradients are the driving mechanism for several phase-change phenomena such as condensation dry zones, interdroplet ice bridging, dry zones around ice, and frost halos. Despite the fundamental nature of the underlying pressure gradients, the majority of studies on these emerging phenomena have been primarily empirical. Using classical nucleation theory and Becker-Döring embryo formation kinetics, here we calculate the pressure field for all possible modes of condensation and desublimation in order to gain fundamental insight into how pressure gradients govern the behavior of dry zones, condensation frosting, and frost halos. Our findings reveal that in a variety of phase-change systems the thermodynamically favorable mode of nucleation can switch between condensation and desublimation depending upon the temperature and wettability of the surface. The calculated pressure field is used to model the length of a dry zone around liquid or ice droplets over a broad parameter space. The long-standing question of whether the vapor pressure at the interface of growing frost is saturated or supersaturated is resolved by considering the kinetics of interdroplet ice bridging. Finally, on the basis of theoretical calculations, we propose that there exists a new mode of frost halo that is yet to be experimentally observed; a bimodal phase map is developed, demonstrating its dependence on the temperature and wettability of the underlying substrate. We hope that the model and predictions contained herein will assist future efforts to exploit localized vapor pressure gradients for the design of spatially controlled or antifrosting phase-change systems.

Numerical simulation of ice accretion under mixed-phase conditions

Ice crystal ingestion at high altitude is a menace to the safe operation of jet engines. Because the core airflow in jet engine has higher temperature, ice crystals may partially melt into droplets when they enter the core airflow. A mixed-phase condition is seen consisting of both water droplets and ice crystals, which will cause ice accretion on both the static surfaces and rotating components in a compressor. This ice accretion may give rise to compressor surge or even mechanical damage of jet engine. In order to analyze this in depth, a numerical method of mixed-phase icing was developed. The Reynolds-averaged Navier–Stokes equations were used for the airflow solution. The Lagrangian method was employed for determining the trajectories of ice crystals and droplets. An ice crystal impingement model was created, in which the breakup and rebounding of ice crystals and splashing of film were considered. A thermodynamic model was proposed for ice crystals and droplets on the basis of the first law of thermodynamics. An icing simulation was developed under mixed-phase conditions with different liquid water contents and ice water contents, and the results were compared with experimental data from the literature. The comparison showed a fairly good correlation, which supports the validity and rationality of the method of mixed-phase icing.

Nanostructure-induced icephobic sol–gel coating for glass application

We present a study on materials formulation, self-assembled surface structure, and the correlation between room temperature water contact angle and contact angle hysteresis (CAH) and the icephobic property at −20 °C. Coating materials are based on TiO2-modified polysiloxane (PDMS) incorporated in the inorganic–organic hybrid (sol–gel) materials with a self-assembled surface structure induced by fluoroalkylsilane (FAS) additive and nanoparticle dispersion. A home-made single-drop ice tester is used to record the ice formation and ice sliding process on the tilted surfaces with adjustable chamber temperatures below 0 °C. It is found that a CAH less than 10°, surface roughness in the range of 10–80 nm, and a two-scale surface structure with nanospikes are the essential features for automatic ice sliding at −20 °C at a tilting angle of 30°. We achieve such features by a proper formulation of sol–gel matrix containing PDMS and 2 % FAS, with incorporation of 0.5–1 wt% alumina nanoparticle dispersion in water/solvent 1:1 ratio. Preferred particle size is about 100 nm with single peak distribution as measured by a Marvin Zetasizer. Surface morphologies of coatings are measured by atomic force microscopy technique. Coatings are applied by an air atomizing spraying process and thermally cured. Chemical analyses by XPS, FTIR, and SEM–EDX evidenced the designed composition with PDMS and fluoro-group on the coated surface. Accelerated weathering test proved the stability of the coating for at least 1000 h. Home-made single-drop ice tester (a) and the video-captured image of ice droplets on uncoated glass (b) and icephobic-coated glass during ice sliding down (c) at −20 °C.

Combined wet chemical etching and anodic oxidation for obtaining the superhydrophobic meshes with anti-icing performance

The superhydrophobic state of surface of AMG2 aluminum alloy meshes was obtained by wet chemical etching in mixture of HCl and HF, anodic oxidation in phosphoric acid solution, and chemisorption of fluorooxysilane. Testing of fabricated coatings against long term contact with high-octane gasoline and water indicates very high chemical resistance of superhydrophobic coatings. The anti-icing potential of fabricated superhydrophobic meshes with respect to deionized and salt water was estimated by analysing the work of adhesion of supercooled droplets, the adhesion of iced droplets to the superhydrophobic mesh, and the freezing statistics. The detected freezing delay for both deionized and salt water amounts several hours allowing the removal of the droplets before freezing due to their rolloff under wind or vibration loads.

Comparação das Propriedades Microfísicas de Diferentes Tipos de Nuvens em Duas Regiões Tropicais com Ecossistemas Distintos Usando Dados do Satélite CloudSat

The approach used in this work’s methodology looks to represent the average microphysical structure of different types of clouds formed in two different ecosystems. The selected regions were located in the Midwest of Brazil (coordinates: 12-18 ° S and 46-58 ° W) and the South Atlantic Ocean (coordinates: 12-25 ° S and 9-30 ° W). In order to accomplish the proposed goal, data from CloudSat satellite from a period of approximately five years are used (from June 2006 to April 2011). The results show that there is little variation in microphysical structure between continental and maritime clouds in the studied regions. The mean droplet (ice particle) effective radius obtained in all the cloud types analyzed was 12.4 μm (71.9 μm) for maritime clouds and 11.0 μm (72.9 μm) for continental clouds. Particle size distribution (concentration as a function of size) was the most efficient parameter to distinguish cloud types, making it possible to identify particularities of each type. The results showed that even in the period with the highest concentration of aerosols observed in the Central-West region of Brazil (spring), there is a significant difference in the cloud particle size distribution in the regions studied. These results indicate that other external agents (besides the total concentration of aerosol integrated in the atmospheric column) are acting more significantly to modulate the particles distribution within the clouds in the Central-W est region.

To Help Fix the Hole in the Ozone Layer, Just Add Ice

Computer simulations show that adding tiny droplets of ice to the atmosphere during the spring could help eliminate chlorofluorocarbons and repair the hole in the ozone layer.

Droplet freezing and ice adhesion strength measurement on super-cooled hydrophobic surfaces

ABSTRACTThis report investigates the freezing of water droplets on hydrophobic surfaces and reports a means to quantitatively measure the ice-adhesion strength at micro-patterned hydrophobic surfaces. Water droplets were placed on a super-cooled surface where they froze following the heterogeneous nucleation phase-transition. The freezing occurred with two distinct transitions: (a) an initial snapping transition freezing of the external shell of the droplet, and (b) a subsequent bulk freezing of the droplet with a characteristic formation of peaked apex protruding upwardly. To measure the ice-adhesion strength, frozen droplets were sheared from the surface using a Universal Materials Tester (UMT) measuring the resulting knock-off forces. Ice formation and adhesion was investigated on three variations of hydrophobic polydimethylsiloxane (PDMS) surfaces: regularly patterned with high and low surface densities of micro-pillar arrays and non-patterned flat surfaces. Patterning the surfaces was found to accelerate freezing transitions but decrease the measured ice-adhesion strength in correlation with a reduction of total interfacial contact area. However, a more concentrated local stress was observed for the less dense surface structures, indicating a potential area of concern for more fragile systems.

CFD Modeling of Mixed-Phase Icing

AbstractIce crystal ingestion at high altitude has been reported to be a threat for safe operation of aero-engine in recently. Ice crystals do not accrete on external surface because of cold environment. But when they enter the core flow of aero-engine, ice crystals melt partially into droplets due to higher temperature. Air-droplets-ice crystal is the mixed-phase, which will give rise to ice accretion on static and rotating components in compressor. Subsequently, compressor surge and engine shutdowns may occur. To provide a numerical tool to analyze this in detail, a numerical method was developed in this study. The mixed phase flow was solved using Eulerian–Lagrangian method. The dispersed phase was represented by one-way coupling. A thermodynamic model that considers mass and energy balance with ice crystals and droplets was presented as well. The icing code was implemented by the user-defined function of Fluent. The method of ice accretion under mixed-phase conditions was validated by comparing the results simulated on a cylinder with experimental data derived from literature. The predicted ice shape and mass agree with these data, thereby confirming the validity of the numerical method developed in this research for mixed-phase conditions.

Spectral Characteristics of Tropical Anvils Obtained by Combining TRMM Precipitation Radar with Visible and Infrared Scanner Data

The spectral characteristics of anvils in tropical areas (25°S–25°N) have been investigated on the basis of data from the tropical rainfall measuring mission’s (TRMM) precipitation radar (PR) and a visible and infrared scanner (VIRS), from 1998 to 2007. The anvils’ vertical structures were captured by TRMM PR and categorized into two subtypes: ice anvils with an echo base of ≥6 km and mixed anvils with an echo base between 3 and 6 km. Visible and infrared signals for the anvils, which are from reflectance at 0.63 and 1.6 μm (hereafter referred to as RF1 and RF2, respectively) and the equivalent brightness temperatures of a black body at 3.7, 10.8, and 12.0 μm (hereafter referred to as TB3, TB4, and TB5, respectively), were derived simultaneously by use of TRMM VIRS. The findings reveal that the normalized frequency distribution (i.e., probability density functions, PDF) of anvil tops and bases have a bimodal distribution whereas that of anvil thickness has a single-peak curve. For visible signals, the PDF distribution of RF1 (RF2) for anvils, mixed anvils, and ice anvils has an approximately symmetric distribution with a tropics-wide averages of 0.74, 072, and 0.80 (0.21, 0.21, and 0.20), respectively. It can be concluded that ice anvils are optically thicker and contain many more ice-cloud droplets at the cloud top than mixed anvils. RF1 of anvils is usually lower over land than over ocean, by ~0.1, whereas RF2 of anvils is usually higher over land than over ocean, by ~0.3. This implies that anvil clouds have thinner optical depth and their cloud tops consist of many more small ice droplets over land than over ocean. For infrared signals, TB4 is regarded as a representative channel. The PDF distribution of TB4 for anvils and mixed anvils is broad, with tropics-wide averages of 229.2 and 232 K, respectively. They contain two peaks and the secondary peak lies at a much lower value. For ice anvils, the PDF distribution of TB4 is a single-peak curve with a tropics-wide average of 219.5 K. Mean TB4 of anvils, mixed anvils, and ice anvils is usually lower over land than over ocean, by ~3.6 K, ~1.6 K, and 5 K, respectively. In addition, land–ocean differences between RF1-to-RF2 reflectance ratios are more obvious than those between TB4 and TB5 brightness temperatures. Furthermore, these spectral signals have been proved to be a potentially useful way of revealing the presence of anvils over land and ocean, and especially for separating anvils into the mixed and ice subtypes. Finally, long-term mean values reveal that the spectral signals of anvils and of stratiform and convective precipitating clouds are indeed different, and that the difference varies with different geographical location. In real time, however, uncertainty still exists when only spectral signals are used to discriminate anvils from stratiform and convective precipitating clouds, which means active satellite observations are indispensable.

Ice and water droplets on graphite: a comparison of quantum and classical simulations.

Ice and water droplets on graphite have been studied by quantum path integral and classical molecular dynamics simulations. The point-charge q-TIP4P/F potential was used to model the interaction between flexible water molecules, while the water-graphite interaction was described by a Lennard-Jones potential previously used to reproduce the macroscopic contact angle of water droplets on graphite. Several energetic and structural properties of water droplets with sizes between 10(2) and 10(3) molecules were analyzed in a temperature interval of 50-350 K. The vibrational density of states of crystalline and amorphous ice drops was correlated to the one of ice Ih to assess the influence of the droplet interface and molecular disorder on the vibrational properties. The average distance of covalent OH bonds is found 0.01 Å larger in the quantum limit than in the classical one. The OO distances are elongated by 0.03 Å in the quantum simulations at 50 K. Bond distance fluctuations are large as a consequence of the zero-point vibrations. The analysis of the H-bond network shows that the liquid droplet is more structured in the classical limit than in the quantum case. The average kinetic and potential energy of the ice and water droplets on graphite has been compared with the values of ice Ih and liquid water as a function of temperature. The droplet kinetic energy shows a temperature dependence similar to the one of liquid water, without apparent discontinuity at temperatures where the droplet is solid. However, the droplet potential energy becomes significantly larger than the one of ice or water at the same temperature. In the quantum limit, the ice droplet is more expanded than in a classical description. Liquid droplets display identical density profiles and liquid-vapor interfaces in the quantum and classical limits. The value of the contact angle is not influenced by quantum effects. Contact angles of droplets decrease as the size of the water droplet increases which implies a positive sign of the line tension of the droplet.

Role of water vapor desublimation in the adhesion of an iced droplet to a superhydrophobic surface.

The study of the adhesion of solid and liquid aqueous phases to superhydrophobic surfaces has become an attractive topic for researchers in various fields as a vital step in the design of icephobic coatings. The analysis of the available results shows that the experimentally measured values of adhesion strength for superhydrophobic substrates, which in some cases are quite small, are still essentially higher than might be expected from the portion of the actual wetted area. In this study we have considered the peculiarities of the three-phase contact zone between sessile supercooled water or ice droplets and a superhydrophobic coating at negative temperatures (below 0 °C) and during the water-ice phase transition. Two types of superhydrophobic coatings with very different textures were used to analyze the evolution of shape parameters of a sessile water droplet during droplet cooling and freezing. It was shown that the evolution of the contact angle and droplet contact diameter of a water droplet deposited on a superhydrophobic surface does not undergo essential changes when the droplet is cooled simultaneously with the substrate and the surrounding environment, and the humidity is maintained close to 100% during the cooling process. However, the phase transition from supercooled water to ice droplets leads to the growth of a metastable iced meniscus and a frost halo in the vicinity of the three-phase contact zone. The meniscus effectively increases the area of adhesive contact between the droplet and the substrate. This phenomenon is intrinsically related to the release of the heat of crystallization and is responsible for the enhancement of adhesion to a superhydrophobic substrate upon droplet transition from supercooled water to ice. At the same time, it was shown that the metastable state of the above meniscus leads to its spontaneous sublimation during exposure at negative temperatures.

Inner convective system cloud-top wind estimation using multichannel infrared satellite images

Knowledge of deep convective system cloud processes and dynamic structures is a key feature in climate change and nowcasting. However, the horizontal inner structures at the cloud tops of deep convective systems are not well understood due to lack of measurements and the complex processes linked to dynamics and thermodynamics. This study describes a new technique to extract inner cloud-top dynamics using brightness temperature differences. This new information could help clarify ring and U or V shape structures in deep convection and be potentially useful in nowcasting applications. Indeed, the use of high-resolution numerical weather prediction (NWP) models, which now include explicit microphysical processes, requires data assimilation at very high resolution as well. A standard atmospheric motion vector tracking algorithm was applied to a pair of images composed of combinations of Spinning Enhanced Visible and Infra-red Imager (SEVIRI) channels. Several ranges of channel differences were used in the tracking process, such intervals being expected to correspond to specific cloud-top microphysics structures. Various consistent flows of motion vectors with different speeds and/or directions were extracted at the same location depending on the channel difference intervals used. These differences in speed/direction can illustrate local wind shear situations, or correspond to expansion or dissipation of cloud regions that contain high concentrations of specific kinds of ice crystals or droplets. The results from this technique were compared to models and ancillary data to advance our discussion and inter-comparisons. Also, the technique proposed here was evaluated using SEVIRI images simulated by the radiative transfer model RTTOV with input data from the UK Met Office Unified Model. A future application of the new data is exemplified by showing the relationship between wind divergence calculated from the new atmospheric motion vector and convective cloud top intensification.

Size Effect on Nucleation Rate for Homogeneous Crystallization of Nanoscale Water Film

The nucleation rate from classical nucleation theory is independent of sample size. In the past decades, several experimental and theoretical studies argued that the homogeneous nucleation rate of ice in supercooled droplets increases when the drop size is decreased. In this paper, we investigate the nucleation of ice in nanoscale water films using molecular dynamics simulations. We found that the nucleation rate of ice actually decreases when the film thickness decreases in the nanoscale regime. A theoretical model is presented to interpret the mechanism of nucleation rate decrease, which agrees well with the simulation results. The model divides films into the near-surface and the middle regions that are characterized by relatively low and high nucleation rates, respectively. The middle region dominates the nucleation process of films, whereas its effect is continuously weakened when increasing volume fraction of the near-surface region by decreasing the film size, leading to a decrease of the total nucleation rate. The structural and thermodynamic analyses indicate that the high stress induced by the surface layering slows down the diffusion and increases the nucleation barrier in the near-surface region, which is responsible for the low nucleation rate and eventually the decrease of the total nucleation rate.

Design of self-dispersible dry nanosuspension through wet milling and spray freeze-drying for poorly water-soluble drugs

The purpose of the present research is to establish a novel nanosizing technique starting from wet nano-milling, named “dry nanosuspension” technique for poorly water-soluble drugs. The spray freeze-drying (SFD) method was applied instead of the spray-drying one previously developed. Drug particles were milled in the aqueous solution of dispersing agents using an oscillating beads-milling apparatus. The milled nanosuspension was sprayed to the surface of liquid nitrogen, and the resultant iced droplets were freeze-dried to obtain the powdery product. The loading ratio of a dispersing agent was investigated to enhance its redispersing property. Dry nanosuspension, which could be spontaneously dispersed into original nanosuspension in water, was obtained by SFD process. It was assumed that self dispersion property would be attributed to its structure with porous network, in which the primary milled drug crystals were embedded. Such unique structure contributed greatly to immediate release behaviors of the drug in gastrointestinal buffered media. These pharmaceutical properties were enhanced by increasing the ratio of the dispersing agent to the drug and the solid content in suspension to be sprayed. The present technique via wet milling and spray freeze-drying processes would be a novel dissolution-enhanced technology for poorly water-soluble drugs.

Thermal Conductivity Measurements of Thin Biological Tissues Using a Microfabricated 3-Omega Sensor

Thermal Conductivity Measurements of Thin Biological Tissues Using a Microfabricated 3-Omega Sensor

Ice nucleation at the nanoscale probes no man’s land of water

At a given thermodynamic condition, nucleation events occur at a frequency that scales with the volume of the system. Therefore at the nanoscale, one may expect to obtain supercooled liquids below the bulk homogeneous nucleation temperature. Here we report direct computational evidence that in supercooled water nano-droplets ice nucleation rates are strongly size dependent and at the nanoscale they are several orders of magnitude smaller than in bulk water. Using a thermodynamic model based on classical nucleation theory, we show that the Laplace pressure is partially responsible for the suppression of ice crystallization. Our simulations show that the nucleation rates found for droplets are similar to those of liquid water subject to a pressure of the order of the Laplace pressure within droplets. Our findings aid the interpretation of molecular beam experiments and support the hypothesis of surface crystallization of ice in microscopic water droplets in clouds.

Efficient Preparation of Giant Vesicles as Biomimetic Compartment Systems with High Entrapment Yields for Biomacromolecules

The 'lipid-coated ice-droplet hydration method' was applied for the preparation of milliliter volumes of a suspension of giant phospholipid vesicles containing in the inner aqueous vesicle pool in high yield either calcein, α-chymotrypsin, fluorescently labeled bovine serum albumin or dextran (FITC-BSA and FITC-dextran; FITC=fluorescein isothiocyanate). The vesicles had an average diameter of ca. 7-11 μm and contained 20-50% of the desired molecules to be entrapped, the entrapment yield being dependent on the chemical structure of the entrapped molecules and on the details of the vesicle-formation procedure. The 'lipid-coated ice droplet hydration method' is a multistep process, based on i) the initial formation of a monodisperse water-in-oil emulsion by microchannel emulsification, followed by ii) emulsion droplet freezing, and iii) surfactant and oil removal, and replacement with bilayer-forming lipids and an aqueous solution. If one aims at applying the method for the entrapment of enzymes, retention of catalytic activity is important to consider. With α-chymotrypsin as first model enzyme to be used with the method, it was shown that high retention of enzymatic activity is possible, and that the entrapped enzyme molecules were able to catalyze the hydrolysis of a membrane-permeable substrate which was added to the vesicles after their formation. Furthermore, one of the critical steps of the method that leads to significant release of the molecules from the water droplets was investigated and optimized by using calcein as fluorescent probe.

Prediction of Ice Accretion on Wind Turbine with Numerical Method

A three dimensional numerical method and its computer codes, which are suitable to predict the process of horizontal axis wind turbine icing, are presented. The method is composed of the Multiple Reference Frame (MRF) method to calculate flowfield of air, an Eulerian method to compute collection efficiency and a three dimensional icing model companying with an iterative arithmetic for solving the model. Ice accretion on a 1.5 MW horizontal axis wind turbine is then computed with the numerical method, and characteristics of droplet collection efficiency and ice shape/type are obtained. The results show that ice on the hub and blade root is slight and it can be neglected comparing with ice near blade tip. From blade tip to root, ice becomes thinner and glaze ice may changes into rime ice.

Spray Freeze-Dried Porous Microparticles of a Poorly Water-Soluble Drug for Respiratory Delivery

Particles of poorly water-soluble drugs were prepared to develop a dry powder inhaler (DPI). Spray freeze-drying (SFD) technique using a four-fluid nozzle (4N), which has been developed by authors, was applied in this research. Ciclosporin and mannitol were used as a poorly water-soluble model drug and a dissolution-enhanced carrier, respectively. The organic solution of ciclosporin and aqueous solution of mannitol were separately and simultaneously atomized through the 4N, and the two solutions were collided with each other at the tip of the nozzle edge. The spray mists were immediately frozen in liquid nitrogen to form a suspension. Then, the iced droplets were freeze-dried to prepare the composite particles of the drug and carrier. tert-Butyl alcohol (t-BuOH) was used as the organic spray solvent due to its relatively high freezing point. The resultant composite particles with varying drug content were characterized depending on their morphological and physicochemical properties. The particles contained amorphous ciclosporin and δ-crystalline mannitol. The characteristic porous structure of SFD particles potentially contributed to their good aerodynamic performance. A series of particles with a similar size distribution and different drug content revealed that the incorporation of mannitol successfully improved the cohesive behavior of ciclosporin, leading to enhanced aerosol dispersion. The dissolution test method using low-volume medium was newly established to simulate the release process from particles deposited on the surface of the bronchus and pulmonary mucosa. The composite with hydrophilic mannitol dramatically improved the in vitro dissolution behavior of ciclosporin in combination with the porous structure of SFD particles.