Mean Droplet Radius Research Articles

Modeling of subgrid-scale cloud-clear air turbulent mixing in Large Eddy Simulation of cloud fields

This paper presents computational approach to locally predict the homogeneity of subgrid-scale turbulent mixing between a cloud and its environment in large-eddy simulation of warm (ice-free) shallow convective clouds applying a double-moment bulk microphysics scheme. The term homogenity of mixing refers to the change of the mean droplet size associated with evaporation of cloud water due to entrainment. The two contrasting limits are the homogeneous mixing, where the radius of all droplets is reduced and the concentration does not change during microscale homogenization, and the extremely inhomogeneous mixing, where the microscale homogenization leads to complete evaporation of some droplets and does not affect the rest. The novel approach is applied to simulations of shallow convective cloud field. The results show that locally the homogeneity of mixing can vary significantly because of the spatial variability of the intensity of turbulence and the mean droplet radius. On average, however, the mixing becomes more homogeneous with height because of higher turbulence intensities and larger droplet sizes aloft.

Fabrication, characterization and lipase digestibility of food-grade nanoemulsions

The behavior of nanoemulsion-based delivery systems within the gastrointestinal tract determines their functional performance. In this study, the influence of particle radius (30–85 nm) on the in vitro digestion of nanoemulsions containing non-ionic surfactant stabilized lipid (corn oil) droplets was examined using simulated small intestine conditions. Nanoemulsions were prepared by a combination of high-pressure homogenization and solvent (hexane) displacement. Lipid droplets with different sizes were prepared by varying the oil-to-solvent ratio in the disperse phase prior to homogenization. The fraction of free fatty acids (FFA) released from emulsified triacylglycerols (TG) during digestion was measured by an in vitro model (pH-Stat titration). Nanoemulsions exhibited a lag-period before any FFA were released, which was explained by inhibition of lipase adsorption to the oil–water interface by free surfactant. After the lag-period, the digestion rate increased with decreasing oil droplet diameter (increasing specific surface area). The total amount of FFA released from the emulsions increased from 61% to 71% as the mean droplet radius decreased from 86 nm to 30 nm. The incomplete digestion of the emulsified lipids could be explained by inhibition of lipase activity by the release of fatty acids and/or by interactions between lipase and surfactants molecules.

A study of shallow cumulus cloud droplet dispersion by large eddy simulations

Cloud droplet dispersion is an important parameter in estimating aerosol indirect effect on climate in general circulation models (GCMs). This study investigates droplet dispersion in shallow cumulus clouds under different aerosol conditions using three-dimensional large eddy simulations (LES). It is found that cloud droplet mean radius, standard deviation, and relative dispersion generally decrease as aerosol mixing ratio increases from 25 mg−1 (clean case) to 100 mg−1 (moderate case), and to 2000 mg−1 (polluted case). Under all the three simulated aerosol conditions, cloud droplet mean radius and standard deviation increase with height. However, droplet relative dispersion increases with height only in the polluted case, and does not vary with height in the clean and moderate cases.

Homogeneous nucleation of a homologous series of n-alkanes (CiH2i+2, i=7–10) in a supersonic nozzle

Homogeneous nucleation rates of the n-alkanes (C(i)H(2i+2); i=7-10) were determined by combining information from pressure trace measurements and small angle x-ray scattering (SAXS) experiments in a supersonic Laval nozzle. The condensible vapor pressure p(J max), the temperature T(J max), the characteristic time Deltat(J max), and supersaturation S(J max) corresponding to the peak nucleation rate J(max) were determined during the pressure trace measurements. These measurements also served as the basis for the subsequent SAXS experiments. Fitting the radially averaged SAXS spectrum yielded the mean droplet radius r, 5<r/nm<31, the width of the aerosol size distribution sigma, 2<sigma/nm<6, and the particle number density N, 7x10(10)<N/cm(-3)<2.2x10(12). The nucleation rates for the n-alkanes J(max), 4x10(15)<J(max)/cm(-3) s(-1)<2x10(18), vary by almost three orders of magnitude as the temperature T(Jmax) decreases from approximately 200 K to as low as 150 K. At the lowest temperatures, the supersaturations S(Jmax) are on the order of 10(5). In spite of these extreme operating conditions, we find good agreement between the current experimental results and those available in the literature using Hale's scaling formalism [Phys. Rev. A 33, 4156 (1986); Metall. Trans. A 23, 1863 (1992)] and the scaling parameters reported by Rusyniak and El-Shall [J. Phys. Chem. B 105, 11873 (2001)]. Comparing the experimental nucleation rates with the predictions of classical nucleation theory, we find that our experimental nucleation rates are 4.5-8 orders of magnitude higher than the predictions.

Classical Nucleation Theory and Its Application to Condensing Steam Flow Calculations

The paper discusses the classical theory of the homogeneous nucleation of water droplets from supersaturated vapour and its application in predicting condensation in steam nozzles. The first part consists of a review of classical nucleation theory, focusing on the many modifications made to the original Becker-Döring theory and providing some new insights into recent developments. It is concluded that the predictive accuracy required for engineering calculations is not yet attainable with a theory derived from first principles. The areas that require most attention relate to the properties of small molecular clusters and the energy transfer processes in the non-isothermal theory. Experiments in converging-diverging nozzles provide the best means for validation at the very high nucleation rates of interest, but measurements of pressure distribution and the Sauter mean droplet radius are insufficient to provide independent checks on the separate theories of nucleation and droplet growth. Nevertheless, a judicious choice for the nucleation rate equation, in combination with a standard droplet growth model and a suitable equation of state for steam, can provide accurate predictions over a wide range of conditions. The exception is at very low pressures where there is evidence that the droplet growth rate in the nucleation zone is underestimated.

Sterically stabilized emulsion polymerization of styrene: Pseudo‐semicontinuous approach

AbstractThe sterically stabilized emulsion polymerization of styrene initiated by a water‐soluble initiator at different temperatures has been investigated. The rate of polymerization (Rp) versus conversion curve shows the two non‐stationary‐rate intervals typical for the polymerization proceeding under non‐stationary‐state conditions. The shape of the Rp versus conversion curve results from two opposite effects—the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two‐phase or three‐phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer‐swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the “highly monomer” swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40–50% conversion. The monomer droplets and/or large highly monomer‐swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804–820, 2003

Pressure-induced structural phase transition of dense droplet microemulsions studied by small-angle x-ray scattering

A small-angle x-ray scattering (SAXS) study of dense water-in-oil droplet microemulsions composed of water, decane, and AOT [sodium bis(2-ethylhexyl) sulfosuccinate] was performed in order to clarify phase behavior with applied pressure and the corresponding structural phase transitions. SAXS spectra were collected for pressures between ambient pressure (0.1 MPa) and 80 MPa and droplet volume fraction, φ, from 0.40 to 0.70. With increasing φ, the mean radius of droplets decreased slightly and polydispersity increased. With increasing pressure, the droplet structure transformed to a two-phase system with coexistence of lamellar and droplet structures, independent of the droplet volume fraction. These results suggest that, with increasing pressure, the increasing inter-droplet attractive force controls the pressure variation of the structure.

Prediction of emulsion color from droplet characteristics: dilute monodisperse oil-in-water emulsions

The influence of droplet characteristics on the transmission of light through dilute monodisperse oil-in-water emulsions was investigated in the presence and absence of dye. The turbidity spectra and tristimulus color coordinates ( L*, a*, b*) of a series of n-hexadecane oil-in-water emulsions with different disperse phase volume fractions ( φ=0–0.02 wt%), mean droplet radii ( r=0.1–1 μm) and red dye concentration (0 and 0.002 wt%) were measured. The dependence of the optical properties of colored and non-colored emulsions on droplet size and concentration could be qualitatively described by light scattering theory. Nevertheless, there was relatively poor quantitative agreement between theory and experiment because of the influence of the measurement system on the optical properties of emulsions. The information presented in this paper will facilitate the formulation of dilute food and beverage emulsions with specific optical characteristics.

Ultrasonic Spectroscopy Study of Salad Dressings

ABSTRACT:Ultrasonic velocity and attenuation spectra (1 to 100 MHz) of salad dressings with different disperse phase volume fractions (φ= 0 to 0.394) and mean droplet radii (0.3 to 0.6 μm) were measured at 25 °C. There were significant differences between the experimental measurements and theoretical predictions due to droplet flocculation. The measured attenuation coefficient was lower than expected at low frequencies because of thermal overlap effects, but it was greater than expected at high frequencies because of scattering. These deviations could be accounted for using an effective medium theory. Results suggest that ultrasonic velocities at 14 MHz were independent of droplet size and could be used to measure droplet concentration of salad dressings.

Cloud liquid water, mean droplet radius, and number density measurements using a Raman lidar

A new technique for measuring cloud liquid water, mean droplet radius, and droplet number density is outlined. The technique is based on simultaneously measuring Raman and Mie scattering from cloud liquid droplets using a Raman lidar. Laboratory experiments on liquid microspheres have shown that the intensity of Raman scattering is proportional to the amount of liquid present in the spheres. This fact is used as a constraint on calculated Mie intensity assuming a gamma function particle size distribution. The resulting retrieval technique is shown to give stable solutions with no false minima. It is tested using Raman lidar data where the liquid water signal was seen as an enhancement to the water vapor signal. The general relationship of retrieved average radius and number density is consistent with traditional cloud physics models. Sensitivity to the assumed maximum cloud liquid water amount and the water vapor mixing ratio calibration are tested. Improvements to the technique are suggested.

Continental Stratus Clouds: A Case Study Using Coordinated Remote Sensing and Aircraft Measurements

Abstract A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350–600 m deep, slightly supercooled (2.0° to −2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm−3, and a maximum liquid water content of 0.8 g m−3 and mean droplet radius of 9 μm near cloud top. Lidar data indicate that the Ka...

Crystallization of condensation droplets on a liquid surface

Highly ordered microporous two-dimensional membranes have been obtained from polymer solutions (Widawski et al. (1994) Nature 369: 397–399). Recently, a mechanism for the formation of such membranes was proposed, involving water vapour condensation (induced by the rapid evaporation of the volatile solvent) onto the surface of solutions and the formation of floating water droplets. Unfortunately, the droplets growth process was not observed, and consequently only qualitative information was reported. In the present paper, results of light-scattering experiments with this system are reported. The formation of water droplets growing at the surface of the solution has been observed and the evolution with time of the mean droplet radius has been found to be described by a power law with an exponent of 1/3, proving that no coalescence processes occur. This particular behaviour is attributed to the precipitation of the polymer at the water/solution interface and to the formation of a mechanically resistant polymer layer encapsulating each droplet. In this way, water droplets behave like solid particles, allowing compact sheets to be formed. The presence of important surface currents is believed to promote the formation of “polycrystal” and “monocrystal” patterns.

Influence of Thermal Overlap Effects on the Ultrasonic Attenuation Spectra of Polydisperse Oil-in-Water Emulsions

This study examines the influence of thermal overlap effects on the ultrasonic attenuation of polydisperse emulsions. The ultrasonic attenuation coefficient of a series of corn oil-in-water emulsions was measured as a function of droplet concentration (1−50 wt %), volume-surface mean droplet radius (r32 = 0.12 and 0.58 μm), and frequency (1−120 MHz). There was good agreement between these measurements and predictions of the traditional ultrasonic scattering theory (UST) for the emulsions containing 0.58 μm droplets, but the UST greatly overestimated the attenuation of the emulsions containing 0.12 μm droplets, especially at higher droplet concentrations, due to thermal overlap effects. We show that the UST can be extended to account for these effects using a core−shell model and highlight the range of experimental conditions over which the existing UST can be successfully used.

Effect of temperature on the ultrasonic properties of oil-in-water emulsions

The influence of temperature on the ultrasonic properties of oil-in-water emulsions was investigated. The ultrasonic velocity and attenuation coefficient of a series of corn oil-in-water emulsions with different disperse phase volume fractions ( φ=0 to 0.5) and mean droplet radii ( r=0.1 to 0.5 μm) were measured as a function of temperature (5 to 50°C). These measurements were in reasonable agreement with predictions made using ultrasonic scattering theory. The ultrasonic velocity of the emulsions was particularly sensitive to their composition, temperature and droplet size. Around 15°C, the ultrasonic velocity was fairly insensitive to oil concentration. Below this temperature, it increased with oil concentration, whilst above this temperature it decreased. The ultrasonic velocity increased with droplet size. The attenuation coefficient of the emulsions was much more sensitive to composition and droplet size, rather than temperature. It increased with oil concentration and decreased with temperature. The implications of these results for the use of ultrasound for determining the size distribution and concentration of droplets in emulsions are investigated.

The acoustic–electric coalescence and the intensification of precipitation radar echoes in clouds

Droplet motion and electrical effects produced by lightning within clouds may cause a rapid and effective drop coalescence process. The problem of spectrum evolution in warm convective clouds is investigated using a one dimensional axial-symmetric time-dependent model. It is analyzed with the influence of acoustic waves on rain gushes as well as the effects of the rearrangement of charge deposited along the lightning channel on the coalescence of droplets. The numerical results for different cloud conditions (liquid water content, mean droplet radius, energy density of lightning channel, values of droplet charge intensity and distribution of charged droplets) show that acoustic–electric coalescence for less than 1 s can increase the mean volume radius of the initial spectra by 10 to 20% and the radar reflectivity by about 2 dBZ. That increase of droplet size could be important for further gravitational growth, which suggests the idea that lightning has the potential to initiate rain gushes and gives an additional explanation for intensification of radar echoes.

Model for an investigation of radiative transfer in cloudy atmosphere

Parameterization of droplet size distributions using a composition of three gamma functions is suggested to simulate a wide spectrum of cloud droplets including drizzle and rain. It should be stressed out that this parameterization is based on extensive material (accumulated in the Russian Central Aerological Observatory) of aircraft observations in clouds and dependent on the cloud type, temperature and other factors. Effective computation technique using the geometric optics approach for the calculations of the parameters of electromagnetic scattering on spherical polydispersions and the database containing these parameters for a wide set of aerosol and cloud mediums are described. The parameterization, computation technique and the database are applied to create different optical atmospheric models with scattering properties rather close to reality for theoretical investigations of the atmospheric radiative transfer. Some examples of cloud radiation forcing studies are also considered in this paper. Their principal feature was the use of the original effective line-by-line and Monte-Carlo codes to take into account rigorously both the cloud/molecular scattering and a detailed spectral structure of water vapor, ozone, carbon dioxide and oxygen. It has been found: (a) variations of the mean droplet radius depending on the cloud temperature and other factors lead to essential changes in shortwave fluxes (tens of W/m 2), cloud radiation absorption (tens of W/m 2) and the Liquid Water Path (LWP) (by several times); (b) In continental clouds short wave fluxes mainly depend on small droplets. The existence of large (drizzle) drops perturbs slightly the fluxes and rather strongly affects the liquid water path and the cloud radiation absorption; (c) The solar radiation absorption and scattering by cloud media are generally defined only by the extinction coefficient and the effective radius and are slightly affected by other details of cloud size droplet distributions. These statements are in good agreement with other investigations. Thus, these parameterization and database should be good enough for the cloud optical properties simulation in the majority of practical applications. Moreover, the developed codes may be recommended for exact simulation of atmospheric radiative transfer in satellite and full-scale experiments, for testing and improving the radiation codes being used in climate models, etc.

Experimental validation of condensing flow theory for a stationary cascade of steam turbine blades

The paper describes a detailed experimental and theoretical study of non-equilibrium condensing steam flow in a stationary cascade of turbine blades operating transonically. Instrumentation was installed for obtaining colour schlieren photographs of the shock wave structure, the blade surface static pressure distribution, the pitchwise variation of the mean droplet radius downstream of the cascade and the stagnation pressure loss across the cascade. Only one blade profile was tested but a comprehensive set of measurements was acquired covering a wide range of inlet steam conditions and exit Mach numbers. By careful interpretation of the data, it was possible, for the first time, to infer the thermodynamic loss due to irreversible condensation directly from experimental measurements. An elaborate comparison of the experimental data with condensing flow theory was also undertaken using a two-dimensional inviscid time-marching calculation scheme, simulating both steady and unsteady flows. Excellent agreement was obtained throughout and it can be stated with some confidence that the theory and calculation procedures used reproduce accurately all the main features of steady transonic condensing flow in stationary cascades.

Water-in-Alkyd-Resin Emulsions: Droplet Size and Interfacial Tension

Ultrasound and dynamic shear measurements were used to investigate nonintrusively a water-in-alkyd-resin emulsion in which the resin was a typical highly viscous component of an offset-ink vehicle. The aging of the emulsion was studied over 40 days and an increase in the size of the suspended water droplets was observed. The value of the mean radius R of these droplets was calculated from ultrasonic attenuation data at three stages of the emulsion aging, in the frequency range 600 kHz to 2.5 MHz. At each stage, the value of the ratio between the interfacial tension γ and the mean droplet radius R was obtained from dynamic shear measurements data in the frequency range 10-230 Hz. The values ofR and γ were found to be in good agreement with the corresponding results obtained by conventional techniques. The merits of the concomitant application of the two nonintrusive techniques are emphasized in terms of the possibility of characterizing quantitatively the features of the highly viscous emulsions encountered in lithography.

Effects of long-range repulsive interactions on Ostwald ripening.

The effects of a long-range repulsive interaction on the phenomena of Ostwald ripening are studied. Beginning with a Langevin description, interface equations describing both the growth and motion of droplets are derived and solved numerically. Two noncoarsening and one coarsening regimes are found. In the noncoarsening regimes, if the system is allowed to reach hexagonal order, the droplet distribution function approaches a \ensuremath{\delta} function; if hexagonal order is somehow precluded, the system becomes kinetically frozen. In the coarsening regime, the mean radius of droplets grows in time. In both the kinetically frozen and coarsening regimes, the system is disordered and polydisperse with a strong coupling between the position and the size of the droplets.

Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes

Cloud microphysical processes are introduced in the precipitation parameterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipitation: autoconversion of droplets, collection of droplets by falling raindrops, and evaporation of raindrops falling in clear sky. The mean droplet radius, r, is calculated from the cloud water mixing ratio, which is computed in the model, and the cloud droplet number concentration, N, which is prescribed. The autoconversion rate is set to zero for r &lt; r0, a prescribed threshold mean droplet radius. We investigate the model sensitivity to r0 and to N, the cloud droplet concentration, which is linked to the concentration of cloud condensation nuclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In further experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the precipitation parameterization in order to bring more consistency between the different schemes. We again investigate the model sensitivity to r0 and to N and we find that an increase in N significantly enhances cloud albedo.