Large Water Droplets Research Articles

Wetting Behavior between Droplets and Dust

The wetting behavior of dust by droplets is investigated by experiments and numerical simulation methods. Experimental observation reveals that the surface of a coal slice is hydrophilic in nature, while surfaces of coal dust stacks are hydrophobic. We show that water droplets settle on these surfaces following the Cassie—Baxter wetting model, as supported by theoretical, numerical analyses and experimental observations, i.e. water droplets only wet the first layer of coal dust. Our numerical simulation results also show that a water droplet could not enclose any coal dust inside it and many coal dust particles are adhered with a hexagonal close packing on a large water droplet. Based on these results, we conclude that the surface area of water droplets is an important factor on their wetting and capturing coal dust, and producing smaller water droplets can improve the efficiency of settling dust.

Numerical Simulation of Effects of Cloud Top Temperatures and Generating Cells on Secondary Ice Production in Stratiform Clouds with a Detailed Microphysical Model

This paper outlines a one-dimensional, heightdependent bin model with detailed microphysical processes in which ice splinters are produced by a riming process. The model is then applied to simulate the shift of particle size distribution effected by the secondary ice production process within clouds with different generating cells and cloud top temperatures. The result of model simulations reveals the general effects of cloud updrafts on increasing ice particle concentration by extending the residence time of ice particles in clouds and providing sufficiently large supercooled water droplets. The rimesplintering mechanism is more effective in clouds with lower ice seeding rates than those with higher rates. Evolutions of hydrometeor size distribution triggered by the rime-splintering mechanism indicate that the interaction between large ice particles and supercooled water drops adds a “second maximum” to the primary ice spectra.

Flash Atomization: A New Concept to Control Combustion Instability in Water-Injected Gas Turbines

The objective of this work is to explore methods to reduce combustor rumble in a water-injected gas turbine. Attempts to use water injection as a means to reduce NOXemissions in gas turbines have been largely unsuccessful because of increased combustion instability levels. This pulsation causes chronic fretting, wear, and fatigue that damages combustor components. Of greater concern is that liberated fragments could cause extensive damage to the turbine section. Combustion instability can be tied to the insufficient atomization of injected water; large water droplets evaporate non-uniformly that lead to energy absorption in chaotic pulses. Added pulsation is amplified by the combustion process and acoustic resonance. Effervescent atomization, where gas bubbles are injected, is beneficial by producing finely atomized droplets; the gas bubbles burst as they exit the nozzles creating additional energy to disperse the liquid. A new concept for effervescent atomization dubbed “flash atomization” is presented where water is heated to just below its boiling point in the supply line so that some of it will flash to steam as it leaves the nozzle. An advantage of flash atomization is that available heat energy can be used rather than mechanical energy to compress injection gas for conventional effervescent atomization.

Enzymatic synthesis of biodiesel from fatty acids. Kinetics of the reaction measured by fluorescent response of Nile Red

Production of biodiesel (B) from free fatty acids (F) was investigated. Different amounts of F dissolved in B were esterified by methanol or ethanol with help of lipase Novozym 435. The kinetic model included (i) steady-state scheme; (ii) reversible inhibition of the enzyme by alcohols; and (iii) aggregation of water W + W ↔ WW. The aggregated form WW imitated large water droplets with low chemical activity. Forward and backward reactions were recorded using the calibrated fluorescent signal from Nile Red. The relevant rate constants were calculated and used in computer simulations. The model demonstrated that content of F could be decreased below the specification level of 0.25% by means of the enzymatic conversion exclusively. It was found that the reaction could be accomplished in one step starting from content of F ≤ 1%, water = 100 ppm and MeOH = 6% or EtOH = 13%. The higher levels of F (≥4%) would require three cycles where MeOH (≥4%) or EtOH (≥7%) are added at the beginning of each step, and water is dried to 100 ppm between the steps. The designed kinetic scheme is a part of the general biodiesel reaction model, which is currently under construction.

Investigation of performance characteristics and water flooding in the cathode flow channel of a polymer electrolyte membrane fuel cell

This study was experimentally carried out to investigate the relationship between fuel-cell performance and the water flooding phenomena at the cathode, which is an important research area of water management in the polymer electrolyte membrane fuel cell. A transparent fuel cell was fabricated to observe the water flooding phenomenon in the cathode channel. Images of the flooding phenomena were obtained by digital and high-speed cameras under various operating conditions involving cell temperature, cathode flowrate, humidifying temperature, cathode backpressure, and oxygen concentration. At the same time, performance was measured at the potentiostatic mode. In this study, investigations for the effects of operating parameters on the water flooding phenomenon, as well as water droplet generation and removal in the cathode flow channel were made. As time elapsed, it was seen that small water droplets formed and then grew into larger water droplets, when the flooding phenomena occurred. It was also observed that when the cell temperature increased the occurrence of flooding phenomena decreased due to evaporation, and performance was significantly improved. The water droplets in the cathode channel were found to be almost zero at high cathode flowrates, and the instances of the flooding phenomenon were decreased by evaporation as the humidified temperature was increased. On account of increasing cathode backpressure, the flooding phenomenon due to high partial pressure was reduced and as compared to air when oxygen was used as an oxidant, the water distribution was increased. Based on this experimental study, it is seen that the flooding phenomenon is closely related to fuel-cell performance.

Coupling of the microphysical and optical properties of an Arctic nimbostratus cloud during the ASTAR 2004 experiment: Implications for light‐scattering modeling

Airborne measurements in an Arctic mixed‐phase nimbostratus cloud were conducted in Spitsbergen on 21 May 2004 during the international Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign. The in situ instrument suite aboard the Alfred Wegener Institute Polar 2 aircraft included a polar nephelometer (PN), a cloud particle imager (CPI), a Nevzorov probe, and a standard PMS 2DC probe to measure the cloud particle single‐scattering properties (at a wavelength of 0.8 μm), and the particle morphology and size, as well as the in‐cloud partitioning of ice/water content. The main objective of this work is to present a technique based on principal component analysis and light‐scattering modeling to link the microphysical properties of cloud particles to their optical characteristics. The technique is applied to the data collected during the 21 May case study where a wide variety of ice crystal shapes and liquid water fractions were observed at temperatures ranging from −1°C to −12°C. CPI measurements highlight the presence of large supercooled water droplets with diameters close to 500 μm. Although the majority of ice particles were found to have irregular shapes, columns and needles were the prevailing regular habits between −3°C and −6°C while stellars and plates were observed at temperatures below −8°C. The implementation of the principal component analysis of the PN scattering phase function measurements revealed representative optical patterns that were consistent with the particle habit classification derived from the CPI. This indicates that the synergy between the CPI and the PN can be exploited to link the microphysical and shape properties of cloud particles to their single‐scattering characteristics. Using light‐scattering modeling, we have established equivalent microphysical models based on a limited set of free parameters (roughness, mixture of idealized particle habits, and aspect ratio of ice crystals) that reproduce the main optical features assessed for cloud regions with different particle geometries and liquid water fractions. However, the retrieved bulk microphysical parameters can substantially differ from the measurements (by several times for the effective size and up to 3 orders of magnitude for the number concentration). Several possible explanations for these discrepancies are discussed. The retrievals show that the optical contribution of small particles with sizes lower than 50 μm (droplets and ice crystals) is significant, always exceeding 50% of the total scattering signal, and thus needs to be more accurately quantified. The shattering of large ice crystals on the shrouded inlet of the PN could also strongly affect the retrieved microphysical parameters.

Interaction between gigawatt laser pulses and liquid media: Part 1. Explosive boiling of large individual water droplets

The results of experiments on femtosecond GW laser pulse interaction with isolated millimeter-size water droplets are presented. The temporal and spatial dynamics of optical breakdown in a liquid particle is investigated. The mechanical fragmentation of the droplet as a result of the evaporation and explosion of superheated areas is also discussed. The spectral characteristics of water particle glow during its explosive boiling are investigated.

Numerical simulation of macro- and micro-structures of intense convective clouds with a spectral bin microphysics model

By use of a three-dimensional compressible non-hydrostatic convective cloud model with detailed microphysics featuring spectral bins of cloud condensation nuclei (CCN), liquid droplets, ice crystals, snow and graupel particles, the spatial and temporal distributions of hydrometeors in a supercell observed by the (Severe Thunderstorm Electrification and Precipitation Study) STEPS triple-radar network are simulated and analyzed. The bin model is also employed to study the effect of CCN concentration on the evolution characteristics of the supercell. It is found that the CCN concentration not only affects the concentration and spectral distribution of water droplets, but also influences the characteristics of ice crystals and graupel particles. With a larger number of CCN, more water droplets and ice crystals are produced and the growth of graupel is restrained. With a small quantity of CCN the production of large size water droplets are promoted by initially small concentrations of water droplets and ice crystals, leading to earlier formation of small size graupel and restraining the recycling growth of graupel, and thus inhibiting the formation of large size graupel (or small size hail). It can be concluded that both the macroscopic airflow and microphysical processes influence the formation and growth of large size graupel (or small size hail). In regions with heavy pollution, a high concentration of CCN may restrain the formation of graupel and hail, and in extremely clean regions, excessively low concentrations of CCN may also limit the formation of large size graupel (hail).

Broadband emission spectrum dynamics of large water droplets exposed to intense ultrashort laser radiation

We report on experiments on the interaction of a gigawatt femtosecond laser pulse train with hanging isolated millimeter-sized water droplets. A transparent droplet experienced explosive boiling-up and emitted light in the visible spectrum as a result of laser-induced plasma formed inside the droplet volume. The droplet emission spectra showed remarkable broadening, depending on the laser power. The role of pulse self-phase modulation in measured spectral broadening when the pulse propagates through the droplet is discussed.

Uniform Beads with Controllable Pore Sizes for Biomedical Applications

Uniform, porous poly(D, L-lactide-co-glycolide) (PLGA) beads with controllable pore sizes were prepared using an unstable water-in-oil (W-O) emulsion and a simple fluidic device. Optical micrographs revealed that the unstable emulsion phase-separated into two phases: the top layer rich in large water droplets and the bottom layer rich in small water droplets. When a syringe with a luer tip offset from the barrel was used, we could selectively introduce each layer of the phase-separated emulsion as a discontinuous phase into the fluidic device while a poly(vinyl alcohol) (PVA) solution was used as the continuous phase. The resultant water-in-oil-in-water (W-O-W) droplets evolved into PLGA beads with pores both in the interior and on the surface after the organic solvent had evaporated. Porous beads prepared using the top layer exhibited larger pores and windows interconnecting the pores than the beads obtained from the bottom layer. To show the effect of pore size on cell growth, we cultured fibroblasts in the beads with small and large pores. Fluorescence micrographs and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay confirmed a high density of viable cells in the beads with large pores. These results suggest that the beads with large pores could provide a more favorable environment for cells and thus be potentially useful for tissue engineering and cell delivery applications.

Detection of chemical agents in the atmosphere by open-path FT-IR spectroscopy under conditions of background interference: II. Fog and rain

Open-path FT-IR spectra of low-concentration releases of diethyl ether were measured both when a glycol fog was passed into the infrared beam and when large water droplets from a lawn sprinkler were sprayed into the beam. It was shown that the glycol fog, for which the droplet size was much less than the wavelength of the infrared radiation, gave rise to a significant interference such that partial least squares (PLS) regression would only yield reasonable values for the ether concentration if background spectra in which the glycol fog was present were included in the calibration set. On the other hand, target factor analysis (TFA) allowed the presence of the ether to be recognized without precalibration. When large water droplets were present in the beam, any infrared radiation entering the droplet was completely absorbed, so that both PLS and TFA would yield accurate results.

Magnetic levitation of large water droplets and mice

We report successful levitation of large water droplets and mice using a newly built variable gravity simulator. The simulator consists mainly of a superconducting magnet with a room temperature accessible experimental levitating space. The superconducting magnet generates a field and field gradient product that is large enough to levitate water and many other common liquids. The warm bore of the magnet has a diameter of 66 mm, large enough to levitate small mammals. We demonstrate that water drops up to 50 mm in diameter and young mice can be levitated in the system. The capability of levitating large water drops and biological systems offers new opportunities for conducting detailed and in-depth study of properties of fluids and biological systems in reduced gravity environments.

Demulsification of water-in-oil emulsions by permeation through Shirasu-porous-glass (SPG) membranes

To investigate the effect of the droplet/pore size ratio on membrane demulsification, water-in-oil (W/O) emulsions with uniform-sized droplets was demulsified by permeation through Shirasu-porous-glass (SPG) membranes with a narrow pore size distribution at mean droplet/pore diameter ratios of 0.52–5.75. At transmembrane pressures above a critical pressure, the water droplets larger than the membrane pore size were demulsified, where the SPG membrane acted as a coalescer because the hydrophilic membrane surface had a high affinity for the water droplets. By contrast, at transmembrane pressures below the critical pressure, the larger water droplets were all retained by the membrane due to the sieving effect of the uniform-sized pores. When a W/O emulsion with a mean droplet diameter of 2.30 μm was allowed to permeate through a membrane with a mean pore diameter of 0.86 μm, the demulsification efficiency increased with increasing transmembrane pressure, to a maximum value of 91% at a transmembrane pressure of 392 kPa, and then decreased, while the transmembrane flux increased almost linearly with increasing transmembrane pressure. The demulsification efficiency was higher for higher water phase content and lower concentration of the surfactant, tetraglycerin condensed ricinoleic acid ester, in the emulsions due to the reduction of the emulsion stability.

Sources of Error in Dual-Wavelength Radar Remote Sensing of Cloud Liquid Water Content

Abstract Dual-wavelength ratio (DWR) techniques offer the prospect of producing high-resolution mapping of cloud microphysical properties, including retrievals of cloud liquid water content (LWC) from reflectivity measured by millimeter-wavelength radars. Unfortunately, noise and artifacts in the DWR require smoothing to obtain physically realistic values of LWC with a concomitant loss of resolution. Factors that cause inaccuracy in the retrieved LWC include uncertainty in gas and liquid water attenuation coefficients, Mie scattering due to large water droplets or ice particles, corruption of the radar reflectivities by noise and nonatmospheric returns, and artifacts due to mismatched radar illumination volumes. The error analysis presented here consists of both analytic and heuristic arguments; it is illustrated using data from the Mount Washington Icing Sensors Project (MWISP) and from an idealized simulation. In addition to offering insight into design considerations for a DWR system, some results suggest methods that may mitigate some of these sources of error for existing systems and datasets.

The effect of spray head sprinklers with different deflector plates on irrigation uniformity, runoff and sediment yield in a Mediterranean soil

Low-pressure centre-pivot systems are usually equipped with spray head sprinklers that can have different types of deflector plates and thus produce different drop sizes causing different effects on the infiltration process. Field tests using low-pressure (140 kPa) spray head sprinklers with smooth and medium groove deflector plates and maximum water applications rates between 99 and 125 mm/h were carried out in a Mediterranean soil. These tests made it possible to study the effect of the deflector plate on irrigation uniformity, runoff and sediment yield. Medium groove deflectors produce lower irrigation uniformity, but apply water with larger water droplets which are less affected by wind and can lead to less evaporation and wind drift losses. However, these droplets have higher impact energy over the soil surface, increasing surface sealing and crust formation, which reduce infiltration and increase runoff. Higher runoff amounts and soil particles detachment produced by grooved deflectors also lead to more sediment yield and soil erosion.

Phase Transitions and Microstructure of Emulsion Systems Prepared with Acylglycerols/Zinc Stearate Emulsifier

The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil phase formed ternary systems, such as water-in-oil (W/O) emulsions, oil-in-water (O/W) dispersions, and unstable oil-water mixtures, were investigated in order to characterize the progressive transformations of the dispersed systems. The type, structure, and phase transitions of the systems were found to be determined by temperature and water phase content. Crystallization of the emulsifier caused the destabilization and subsequent phase inversion of the emulsions studied, at a temperature of 60-61 degrees C. The observed destabilization was temporary and led, at lower temperature, to W/O emulsions, "O/W + O" systems, or O/W dispersions, depending on the water content. Simultaneous emulsification and cooling of 20-50 wt % water systems resulted in the formation of stable W/O emulsions that contained a number of large water droplets with dispersed oil globules inside them ("W/O + O/W/O"). In water-rich systems (60-80 wt % of water), crystallization of the emulsifier was found to influence the formation of crystalline vesicle structures that coexisted, in the external water phase, with globules of crystallized oil phase. Results of calorimetric, rheological, and light scattering experiments, for the O/W dispersions obtained, indicate the possible transition of a monostearoylglycerol-based alpha-crystalline gel phase to a coagel state, in these multicomponent systems.

A method for on‐line measurement of water‐soluble organic carbon in ambient aerosol particles: Results from an urban site

An instrument for on‐line continuous measurement of the water‐soluble organic carbon (WSOC) component of aerosol particles is described and results from an urban site in St. Louis are presented. A Particle‐into‐Liquid Sampler impacts ambient particles, grown to large water droplets, onto a plate and then washes them into a flow of purified water. The resulting liquid is filtered and the carbon content quantified by a Total Organic Carbon analyzer providing continuous six‐minute integral measurements with a detection limit of 0.1 μg C/m3. Summer and fall measurements of WSOC and organic carbon (OC) indicated WSOC/OC ratio typically ranged from 0.40 to 0.80. A diurnal variation in WSOC/OC that correlated with ozone was observed over extended periods in June; however, other periods in August had no correlation. The results suggested that WSOC was composed of a complex mixture of compounds that may contain a significant fraction from secondary organic aerosol formation.

Effects of an Amphiphilic Graft Copolymer on an Oil-Continuous Microemulsion. Molecular Self-Diffusion and Viscosity

Mixtures of an amphiphilic graft copolymer in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/cyclohexane oil-continuous microemulsions are studied by the PGSE NMR technique and by viscometry. The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Water, surfactant, and polymer self-diffusion coefficients can be measured individually by PGSE NMR. The water and the surfactant self-diffusion coefficients are equal, and both monitor the overall diffusion of the water droplets of the microemulsion. The graft copolymer enhances the microemulsion viscosity (relative increase = 2000), and large viscosifying effects are promoted by increased degrees of grafting, increased polymer concentration, and large water droplets. Systems with high viscosities always display slow polymer self-diffusion. The droplet diffusion is always much more rapid than the polymer diffusion; thus, the polymer molecules do not immobilize the droplets. The results...

Fusion of emulsion particles to the polarized 1,2-dichloroethane ∣ water interface

Current–potential curves at the interface between the 1,2-dichloroethane (DCE) solution of Aerosol-OT (AOT, sodium diisooctyl sulfosuccinate, DOSS −Na +) and the aqueous solution of LiCi (W) show unusual current spikes, which are distinctly different from the curves predicted from the diffusion-controlled transfer of Na + ions across the interface. After the initial negative current, presumably corresponding to the transfer of Na + ions from DCE to W, the current reduces to the level of that of the base solution and concomitantly whitish emulsions become visible only in the DCE side of the interface. Then intermittent current spikes appear. The quantity of electric charge transferred by a single current spike far exceeds the charge that one emulsion particle bears, suggesting the avalanche-type transfer of many emulsion particles across the interface. Video-imaging of the interface demonstrates that the current spikes are triggered by the fusion to the interface of a large water droplet probably formed by the coalescence of W/O emulsion particles in the bulk of the DCE phase.

Water-in-CO2 Microemulsions Studied by Small-Angle Neutron Scattering

Aggregate structures in water-in-CO2 microemulsions were studied by high-pressure small-angle neutron scattering (SANS). With liquid CO2 at 15 °C, the partially fluorinated, di-chain surfactant bis(1H,1H,5H-octafluoro-n-pentyl) sodium sulfosuccinate (di-HCF4) stabilized single-phase microemulsions at pressures above ∼400 bar. The maximum water loading (w) investigated was 30 ([water]/[di-HCF4]), representing formation of relatively large water droplets in the microemulsion. Between w = 5 and 30, the SANS data were consistent with a model for attractive polydisperse spherical droplets. A linear relationship between the water droplet radius (Rc) and w was found, which gave an apparent head group area for the surfactant of 87 Å2 at the water−CO2 interface. In Winsor II type microemulsions the value of Rc, measured in the presence of excess water, increased with pressure from 36 Å at 400 bar to 56 Å at 550 bar.