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Surfaces designed so that drops do not adhere to them but instead bounce off have received substantial attention because of their ability to stay dry (1-4), self-clean (5-7) and resist icing (8-10). A drop striking a non-wetting surface of this type will spread out to a maximum diameter (11-14) and then recoil to such an extent that it completely rebounds and leaves the solid material (15-18). The amount of time that the drop is in contact with the solid--the 'contact time'--depends on the inertia and capillarity of the drop1, internal dissipation (19) and surface-liquid interactions (20-22). And because contact time controls the extent to which mass, momentum and energy are exchanged between drop and surface (23), it is often advantageous to minimize it. The conventional approach has been to minimize surface-liquid interactions that can lead to contact line pinning (20-22); but even in the absence of any surface interactions, drop hydrodynamics imposes a minimum contact time that was conventionally assumed to be attained with axisymmetrically spreading and recoiling drops (21,24). Here we demonstrate that it is possible to reduce the contact time below this theoretical limit by using superhydrophobic surfaces with a morphology that redistributes the liquid mass and thereby alters the drop hydrodynamics. We show theoretically and experimentally that this approach allows us to reduce the overall contact time between a bouncing drop and a surface below what was previously thought possible., Our experiments involve releasing a water drop (radius R = 1.33 mm, velocity U = 1.2 m [s.sup.-1]) onto a superhydrophobic surface and filming the bounce dynamics with high-speed cameras [...]

The chemical history of dust particles in the atmosphere is crucial for assessing their impact on both the Earth's climate and ecosystem. So far, a number of studies have shown that, in the vicinity of strong anthropogenic emission sources, Ca-rich dust particles can be converted into aqueous droplets mainly by the reaction with gaseous HN[O.sub.3] to form Ca[(N[O.sub.3]).sub.2]. Here we show that other similar processes have the potential to be activated under typical remote marine atmospheric conditions. Based on field measurements at several sites in East Asia and thermodynamic predictions, we examined the possibility for the formation of two highly soluble calcium salts, Ca[(N[O.sub.3]).sub.2] and Ca[Cl.sub.2], which can deliquesce at low relative humidity. According to the results, the conversion of insoluble CaC[O.sub.3] to Ca[(N[O.sub.3]).sub.2] tends to be dominated over urban and industrialized areas of the Asian continent, where the concentrations of HN[O.sub.3] exceed those of HCl ([HN[O.sub.3]/HCl] > ~1). In this regime, Ca[Cl.sub.2] is hardly detected from dust particles. However, the generation of Ca[Cl.sub.2] becomes detectable around the Japan Islands, where the concentrations of HCl are much higher than those of HN[O.sub.3] ([HN[O.sub.3]/ HCl] < ~0.3). We suggest that elevated concentrations of HCl in the remote marine boundary layer are sufficient to modify Ca-rich particles in dust storms and can play a more important role in forming a deliquescent layer on the particle surfaces as they are transported toward remote ocean regions. heterogeneous reactions | marine atmosphere doi/ 10.1073/pnas.1008235107

We describe gravimetric methods for measuring the mass of droplets generated by a drop-on-demand (DOD) microdispenser. Droplets are deposited, either continuously at a known frequency or as a burst of known number, into a cylinder positioned on a submicrogram balance. Mass measurements are acquired precisely by computer, and results are corrected for evaporation. Capabilities are demonstrated using isobutyl alcohol droplets. For ejection rates greater than 100 Hz, the repeatability of droplet mass measurements was 0.2%, while the combined relative standard uncertainty ([u.sub.c]) was 0.9%. When bursts of droplets were dispensed, the limit of quantitation was 72 [micro]g (1490 droplets) with [u.sub.c] : 1.0%. Individual droplet size in a burst was evaluated by high-speed videography. Diameters were consistent from the tenth droplet onward, and the mass of an individual droplet was best estimated by the average droplet mass with a combined uncertainty of about 1%. Diameters of the first several droplets were anomalous, but their contribution was accounted for when dispensing bursts. Above the limits of quantitation, the gravimetric methods provided statistically equivalent results and permit detailed study of operational factors that influence droplet mass during dispensing, including the development of reliable microassays and standard materials using DOD technologies. 10.1021/ac901563j

We have developed a method for high-throughput isothermal amplification of single DNA molecules in a droplet-based microfluidic system. DNA amplification in droplets was analyzed using an intercalating fluorochrome, allowing fast and accurate 'digital' quantification of the template DNA based on the Poisson distribution of DNA molecules in droplets. The clonal amplified DNA in each 2 pL droplet was further analyzed by measuring the enzymatic activity of the encoded proteins after fusion with a 15 pL droplet containing an in vitro translation system.

The imaging and quantification of droplet sizes in sprays is a challenging task for optical scientists and engineers. Laser sheet dropsizing (LSDS) combines the two-dimensional information of two different optical processes, one that is proportional to the droplet volume and one that depends on the droplet surface, e.g., Mie scattering. Besides Mie scattering, here we use two-dimensional Raman scattering as the volume-dependent measurement technique. Two different calibration strategies are presented and discussed. Two-dimensional droplet size distributions in a spray have been validated in comparison with the results of point-resolved phase Doppler anemometry (PDA) measurements. OCIS codes: 110.4190, 120.3940, 280.1100, 290.5850, 290.5860.

The measured rain rate, raindrop size distribution (DSD), and the ITU-R model over the frequency range from 1-100 GHz have been used to elucidate the cumulative rainfall rate and the variability of rain attenuation at Kototabang. Rain rate and DSD are recorded from ground-based optical rain gauge and disdrometer measurements, respectively. Considerable differences between the recorded data and the ITU-R model are observed at small time percentage. The specific rain attenuation obtained from the DSD measurement shows diurnal variation with the largest attenuation observed in the morning hours. This characteristic is due to the raindrop spectra of rain events in this period containing more small-sized drops ( Index Terms--Kototabang (KT), rain attenuation, raindrop size distribution (DSD).

We have developed a molten-solder ejection method by which molten-solder droplets from a 13- (1 pL) to 450-[micro]m (48 nL) diameter could be realized. This method has been developed as a technique to supply small solder balls. The authors have developed a newly designed solder ejection head and confirmed that solder balls with almost no surface oxidation can be formed by using this method. The diameter of the molten-solder droplets using an advanced head is 13 [micro]m, which is much smaller than the 35 [micro]m of a conventional minimum droplet. As the relative positioning accuracy, a standard deviation of 0.81 [micro]m was achieved when the diameter of the solder droplets was 13 [micro]m. Furthermore, by changing the drive waveform of the conventional head, we enlarged the diameter of the solder droplets to 450 [micro]m, which was much larger than 200 [micro]m of a conventional maximum droplet. In this case, the standard deviation of 26 [micro]m as the relative positioning accuracy was also achieved when the diameter of the solder droplets was 450 [micro]m. [2008-0068] Index Terms--Lead-free solder, molten-solder ejection, packaging, soldering.

This paper describes a method to control the volume and the velocity of drops generated in a flow-focusing device dynamically and independently. This method involves simultaneous tuning of the temperature of the nozzle of the device and of the flow rate of the continuous phase; the method requires a continuous phase liquid that has a viscosity that varies steeply with temperature. Increasing the temperature of the flow-focusing nozzle from 0 to 80 [degrees]C increased the volume of the drops by almost 2 orders of magnitude. Tuning both the temperature and the flow rate controlled the drop volume and the drop velocity independently; this feature is not possible in a basic flow-focusing device. This paper also demonstrates a procedure for identifying the range of possible drop volumes and drop velocities for a given flow-focusing device and shows how to generate drops with a specified volume and velocity within this range. This method is easy to implement in on-chip applications where thermal management is already incorporated in the system, such as DNA amplification using the polymerase chain reaction and nanoparticle synthesis.

Droplet-based microfluidics has raised a lot of interest recently due to its wide applications to screening biological/chemical assays with high throughput. Despite the advances on droplet-based assays involving cells, gene delivery methods that are compatible with the droplet platform have been lacking. In this report, we demonstrate a simple microfluidic device that encapsulates cells into aqueous droplets and then electroporates the encapsulated cells. The electroporation occurs when the cell-containing droplets (in oil) flow through a pair of microelectrodes with a constant voltage established in between. We investigate the parameters and characteristics of the electroporation. We demonstrate delivering enhanced green fluorescent protein (EGFP) plasmid into Chinese hamster ovary (CHO) cells. We envision the application of this technique to high-throughput functional genomics studies based on droplet microfluidics.

A microfluidic method to rapidly measure the octanol--water partition coefficient in thousands of individual picoliter drops is described. AT-junction microfluidic chip is used to generate a segmented flow of monodisperse, fluorescein-laden water in octanol carder fluid. The partitioning of individual drops reaches equilibrium in less than 2 s. Epifluorescence microscopy is used measure the partition coefficient of fluorescein as a function of pH. Results compare well with previous measurements using traditional shake-flask methods. The methods presented here are rapid, provide detailed statistics, and can be run in parallel, enabling the simultaneous partitioning of thousands of compounds for various applications such as drug development, environmental testing, and combinatorial chemistry. Microfluidic partitioning and extraction in picoliter drops may be useful for studying molecules and particles away from their equilibrium state and in cases with limited samples.

Pattern formation in evaporating colloidal droplets is an important phenomenon that is commonly observed in several solute--solvent systems; this is also an emerging technique for obtaining fine patterning through controlled conditions of drying. After evaporation of the solvent a ring-like pattern remains on the solid substrate under the condition of contact line pinning. We have used a new analytical technique, time-resolved infrared imaging, to investigate the formation of patterned structures with droplet drying, which is a typical time-dependent phenomenon. We have coupled the technique with optical imaging to follow the evolution of the droplet shape and dimension in correspondence with the chemical images. The main advantage of the technique is represented by the possibility to have simultaneous spatial and time-resolved information; we have applied the method to a water-methylene blue system that has been studied during drying. We have monitored the droplet profile change, in terms of water and methylene blue variation with time and space, at the droplet edge. The analysis has allowed a detailed reconstruction of the evaporating droplet profile with a micrometer scale resolution and of the change in concentration of the dye as a function of evaporating time. A uniform ring-like pattern, after evaporation in controlled relative humidity, is observed. The data are consistent with a constant evaporation model, whose conditions are realized when a constant evaporation is achieved along the entire droplet. The technique has allowed elucidating the evaporation phenomenon close to the contact line in a dye solute--solvent system, which is very difficult to study with other techniques.

Atmospheric glories are caused by backscattering of sunlight from spherical droplets of water (e.g., from fog or clouds). But what would glories look like if they were caused by scattering from more exotic substances, such as clouds of ethane as found on Titan? Examining backscattering as a function of the refractive index n of spherical droplets leads to the surprising conclusion that a glory's appearance is almost independent of n (at least for 1.03 < n < 1.7)--unlike the colors of rainbows, which are critically dependent on the variation of n across the visible spectrum. OCIS codes: 010.1290, 010.1350, 240.6690, 350.1260.

The atmospheric glory caused by backscattering of sunlight from clouds usually has circular colored rings. However, glories with noncircular rings are frequently observed, especially along the edges of clouds. Noting that the angular radius of the rings of glories is a sensitive indicator of the size of the water droplets in clouds, several images of glories have been examined in an attempt to explain the formation of noncircular glories. OCIS codes: 010.1290, 010.1350.

The nonlinear behavior of water repellency (WR) with respect to soil water content ([[theta].sub.g]) may be described by curve parameters derived from two widespread methodologies such as the water drop penetration time (WDPT) and molarity of an ethanol droplet (MED) tests. While the former measures WR persistence in terms of the infiltration time of a water drop sitting on the soil surface, the latter quantifies the repellency degree as contact angle or 90[degrees] surface tension, [[gamma].sub.90o]. From a hydrologic point of view, the WDPT test may be considered more closely related to the natural infiltration process of the soil, but the infiltration times become prohibitively large (on the order of hours) in highly repellent soils; thus, the MED test is more convenient. This study was conducted to determine whether the spatial interpolation of time-consuming WDPT measurements may be complemented with less time-demanding WR parameters derived from the MED test. Measurements from saturation to oven dryness using both MED and WDPT were performed on 140 topsoil samples from a humid evergreen forest watershed of the Garajonay National Park (Canary Islands). We found useful links between curve parameters derived from the MED and WDPT tests. The area below the WR-[[theta].sub.g] curve measured with the WDPT test ([S.sub.WDPT]) was found to be correlated with that measured with the MED test ([S.sub.MED]) and with the [[theta].sub.g] at which repellency is triggered ([[theta].sub.g-min]). By contrast, previously published relations that established a logarithmic relationship between 1/WDPT and [[gamma].sub.90o] were valid only in the dry soil state, but must be ruled out if soil water content is taken into account. We took advantage of the existence of such parameter correlations to show how cokriging geostatistical tools may be applied to improve the spatial interpolation of [S.sub.WDPT] with [S.sub.MED] or [[theta].sub.g-min] as subsidiary variables. In general, cokriging was superior to kriging in terms of mean variance minimization with little extra labor cost. A minimal-effort spatial sampling strategy may be designed taking into account these considerations. Abbreviations: IRDI, integrative repellency dynamic index; MED, molarity of an ethanol droplet; SOM, soil organic matter; WDPT, water drop penetration time; WR, water repellency.

Recent research has demonstrated the value of polarimetric measurements for the correction of rain-path attenuation at X-band radar frequency and the estimation of rain parameters including drop-size distributions (DSD). The issue this paper is concerned with is to what degree uncertainties in attenuation correction can affect the estimation of DSD. Since attenuation-correction uncertainty enhances with rain path, our hypothesis is that DSD retrieval uncertainty at X-band may deteriorate with range. In this paper, we evaluate the relative accuracy of X-band DSD retrieval against DSD estimates from S-band radar observations and in situ disdrometer spectra. We present comparisons of various techniques for estimating DSD model parameters from attenuation-corrected X-band dual-polarization radar data. Coincident X-band polarimetric-radar (XPOL) and S-band polarimetric-radar dual-polarized radar measurements from the International [H.sub.2]O Project experiment as well as coincident XPOL (MP-X) measurements over disdrometer during a typhoon storm case in Japan are used to assess the accuracy of the different DSD retrieval algorithms applied to X-band radar measurements. Index Terms--Attenuation correction, drop-size distribution (DSD) retrievals, polarimetric radar, S-band radar, X-band radar.

A large data set of raindrop size distribution (RSD) measurements collected with the Joss--Waldvogel disdrometer (JWD) and the 2-D video disdrometer (2DVD) in the U.K., Greece, Japan, and the U.S. are analyzed and modeled. This work extends a previous effort devoted to the exploitation of U.K. data and the design of a stochastic procedure to randomly generate synthetic RSD intermittent time series. This study seeks to: 1) explore the differences of RSD-derived moments for distinct hydroclimate regions, ranging from tropics to subtropics and mid and northern latitudes; 2) compare the governing parameters of the normalized gamma RSD for both stratiform and convective events and perform a sensitivity analysis by using different best fitting techniques; 3) exploit the time-correlation structure of the estimated RSD parameters as the input of a vector autoregressive stationary model used to simulate time series (or horizontal profiles) of RSDs and, consequently, its moments as the rain rate and concentration; and 4) characterize the distribution of the inter-rain duration and rain duration to design a semi-Markov chain to represent the intermittency feature of the rainfall process in a climatological framework. This climatological analysis and the related stochastic RSD generation model may find useful applications within both hydrometeorology and radio propagation. Index Terms--Autoregressive process, climatology, disdrometer data, Markov chain, modeling, radar applications, radar meteorological factors, radio propagation, rain, raindrop size distribution (RSD), signal synthesis, weather radar.

In the present paper we first present a derivation based on the time-dependent perturbation theory to develop the dynamical equations which can be applied to model the response of a droplet quartz crystal microbalance (QCM) in contact with a single viscoelastic media. Moreover, the no-slip boundary condition across the device-viscoelastic media interface has been relaxed in the present model by using the Ellis-Hayward slip length approach. The model is then used to illustrate the characteristic changes in the frequency and attenuation of the QCM with and without the boundary slippage due to the changes in viscoelasticity as the coated media varies from Newtonian liquid to solid. To complement the theory, experiments have been conducted with microliter droplets of aqueous glycerol solutions and silicone oils with a viscosity in the range of 50~10 000 cS. The results have confirmed the Newtonian characteristics of the glycerol solutions. In contrast, the acoustic properties of the silicones oils as reflected in the impedance analysis are different from the glycerol solutions. More importantly, it was found that for the silicone oils the frequency steadily increased for several hours and even exceeded the initial value of the unloaded crystal as reflected in the positive frequency shift. Collaborative effects of interfacial slippage and viscoelasticity have been introduced to qualitatively interpret the measured frequency up-shifts for the silicone oils. The present work shows the potential importance of the combined effects of viscoelasticity and interracial slippage when using the droplet QCM to investigate the rheological behavior of more complex fluids.

Rapid kinetic measurements are important in understanding chemical interactions especially for biological molecules. Herein, we present a droplet-based microfluidic platform to study streptavidin--biotin binding kinetics with millisecond time resolution. With integration of a confocal fluorescence detection system, individual droplets can be monitored and characterized online to extract kinetic information. Using this approach, binding kinetics between streptavidin and biotin were observed via fluorescence resonance energy transfer. The binding rate constant of streptavidin and biotin was found to be in a range of 3.0 x [10.sup.6] - 4.5 x [10.sup.7] [M.sup.-1] [s.sup.-1].

Raindrop collision and breakup is a stochastic process that affects the evolution of drop size distributions (DSDs) in precipitating clouds. Low and List have remained the obligatory reference on this matter for almost three decades. Based on a limited number of drop sizes (10), Low and List proposed generalized parameterizations of collisional breakup across the raindrop spectra that are standard building blocks for numerical models of rainfall microphysics. Here, recent laboratory experiments of drop collision at NASA's Wallops Island Facility (NWIF) using updated high-speed imaging technology with the objective of assessing the generality of Low and List are reported. The experimental fragment size distributions (FSDs) for the collision of selected drop pairs were evaluated against explicit simulations using a dynamical microphysics model (Prat and Barros, with parameterizations based on Low and List updated by McFarquhar). Oneto-one comparison of the FSDs shows similar distributions; however, the model was found to underestimate the fragment numbers observed in the smallest diameter range (e.g., D < 0.2 mm), and to overestimate the number of fragments produced when small drops (diameter DS [greater than or equal to] 1mm) and large drops (diameter DL [greater than or equal to] 3mm) collide. This effect is particularly large for fragments in the 0.5-1.0-mm range, and more so for filament breakup (the most frequent type of breakup observed in laboratory conditions), reflecting up to 30% uncertainty in the left-hand side of the FSD (i.e., the submillimeter range). For coalescence, the NWIF experiments confirmed the drop collision energy cutoff ([E.sub.[tau]]) estimated by Low and List (i.e., [E.sub.T] > 5.0 [micro]J). Finally, the digital imagery of the laboratory experiments was analyzed to determine the characteristic time necessary to reach stability in relevant statistical properties. The results indicate that the temporal separation between particle (i.e., single hydrometeor) and population behavior, that is, the characteristic time scale to reach homogeneity in the NWIF raindrop populations, is 160 ms, which provides a lower bound to the governing time scale in population-based microphysical models.

The freezing of supercooled water droplets in the atmosphere, with an emphasis on the entropic aspects of the problem, is examined. Supercooled water is a metastable state and, therefore, the associated phase transition must be irreversible. Temperature-dependent heat capacities of supercooled water and ice are used to calculate the entropy difference. That difference is then used to establish a lower bound on the amount of latent heat that can be liberated by the freezing droplets. The calculation is compared with tabulated values of the latent heat of fusion with surprising results. Based on a novel physical picture of the freezing process, the authors suggest a simple estimate for the effective latent heat that is suitable for heat budget calculations of glaciating clouds. In addition, the authors arrive at a quadratic dependence on supercooling, [([DELTA]T).sup.2], for the irreversible contribution to heat exchange during the freezing process. The proportionality factor is estimated as -0.3 J [mol.sup-1] [K.sup.-2].

To study the kinetics of drop nucleation in clouds, the integro-differential equation for integral water supersaturation in cloud is derived and analyzed. Solving the supersaturation equation with an algebraic form of the cloud condensation nuclei (CCN) activity spectrum, analytical expressions are obtained for the time [t.sub.m] of CCN activation process, the maximum supersaturation [S.sub.m], and droplet concentration [N.sub.dr]([s.sub.m]), limited by the total aerosol concentration at high supersaturations. All three quantities are expressed as functions of vertical velocity and characteristics of the CCN size spectra: mean geometric radius, dispersion, and parameters of solubility. A generalized power law for the drop activation, [[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], is formulated that is similar in form to the Twomey power law, but both the coefficient C([s.sub.m]) and index k([s.sub.m]) are functions of supersaturation expressed analytically in terms of vertical velocities and CCN microphysical parameters. A simple and economical numerical solution was developed that describes all of these characteristics without conducting numerous simulations using parcel models. An extended series of numerical experiments was performed, in which the dependencies of [t.sub.m], [s.sub.m], [N.sub.dr]([s.sub.m]), C([s.sub.m]), k([s.sub.m]), and several other important characteristics of activation process were studied as functions of vertical velocity and physicochemical properties of the aerosol. In particular, it is shown that a decrease in the condensation coefficient [[alpha].sub.c] leads to slower CCN activation and higher maximum supersaturation and droplet concentration. Uncertainties in [[alpha].sub.c] may prevent correct estimates of the direct and indirect aerosol effects on climate. The solutions and expressions for the parameters presented here can be used for parameterization of the drop activation process in cloud and climate models.

The influence of incomplete liquid fuel prevaporization on the emissions of nitric oxides in a swirl stabilized model gas turbine combustor is investigated experimentally and numerically. The design of the model combustor enables the variation of the degree of prevaporization. This is achieved by using two liquid fuel injectors. One injector is located far upstream of the combustor and generates a fully prevaporized and premixed air fuel mixture. The second injector is located at the combustor inlet. Consequently, the liquid fuel mass flow split between the two injectors determines the fraction of nonprevaporized fuel present in the reaction zone. The NO/N[O.sub.2] measurements were performed with a chemoluminescence analyzer. In accordance to the findings of other researchers, the present experimental study revealed that the influence of prevaporization on nitric oxide emissions is of significance for practical applications. The experimental studies were accompanied by numerical studies of partially prevaporized lean combustion in an abstracted configuration. The purpose of this numerical study is to gain a detailed understanding of the influence of droplet slip on droplet flame position and peak temperature. The droplet slip velocity was found to have a significant impact on the peak temperature of the droplet flame and, therefore, NO formation rates within the droplet flame. The combustion system used for the experimental investigation was characterized regarding droplet slip velocities with an extended laser Doppler anemometry technique. The comparison between numerical and experimental results shows that the droplet slip velocities in the macroscopic reaction zone are within the transition range from an envelope to a wake flame. It is concluded that small-scale mixing effects play a significant role in the formation of nitric oxides in spray combustion systems with incomplete prevaporization. [DOI: 10.1115/1.2906185]

Two overlapping droplets impacting on a solid surface coalesce and recoil so that the edges of the droplets are drawn back, a phenomenon called drawback. A series of experiments were conducted on the merging of two overlapping wax droplets deposited on an aluminum drum to characterize the drawback process between the two droplets. Drum temperature, droplet overlap ratio, and the time interval between impacts of droplets were varied. Wax bumps, formed by coalescence of two droplets on the drum surface, were photographed and their length and width measured. An aspect ratio and dimensionless drawback index, quantifying the extent of drawback, were calculated from these measurements. When drum temperature is increased, or the time interval between impacts of the two droplets is reduced, there is more drawback and the ink bumps become round, since the cooling rate of droplets is slower and droplets have a longer time to change shape due to surface tension. A simple heat transfer model was developed to predict changes in droplet-cooling rate with changes in droplet overlap, substrate temperature, or time interval (deposition frequency). Experiments were also conducted on the formation of lines by depositing 20 droplets. Measurements on the drawback of two droplets were used to predict conditions under which broken lines are obtained. [DOI: 10.1115/1.2952821] Keywords: drawback, droplet deposition, inkjet printing, overlap, solidification, cooling time

The objective of this study was to develop an experimentally verified computational model that accurately predicts evolution of shear-thinning liquid jets. A secondary objective was to investigate the formation of satellite drops and to determine conditions under which their diameter can be controlled. The model employs the Galerkin finite/ element approach to solve the complete two-dimensional set of axisymmetric governing equations and the corresponding kinematic and dynamic boundary conditions at the free surface. The effect of shear-thinning behavior on breakup was studied in detail for the case of an infinitely long non-Newtonian jet. It was found that shear-thinning behavior may be useful in controlling satellite drop sizes. (We observe that increasing the shear-thinning behavior at Re ~ 5 leads to an initial increase in the satellite drop size, followed by a subsequent decrease.) Comparison of model predictions with experimental data is presented for the case of a shear-thinning non-Newtonian jet. The experimental liquid was pumped through a capillary and drop shapes obtained using a high speed camera. The experimentally obtained shapes were compared to those predicted by the model and found to be in good agreement. [DOI: 10.1115/1.2956612]

The classical method of images is used to construct closed form exact solutions for the two-dimensional (2D) perturbed flow fields in the presence of a 2D vapor-liquid compound droplet in the limit of low-Reynolds number. The geometry of the multiphase droplet is composed of two overlapping infinitely long cylinders [C.sub.a] and [C.sub.b] of radii a and b, respectively, intersecting at a vertex angle [pi]/2. The composite inclusion has the shape resembling a 2D snowman type of object with a vapor cylinder [C.sub.a] partly protruded into the cylinder [C.sub.b] filled with another fluid whose viscosity is different from that of the host fluid. The mathematical problem with this inclusion in the Stokes flow environment is formulated in terms of Stokes stream function with mixed boundary conditions at the boundary of the hybrid droplet. General expressions for the perturbed stream functions in the two phases are obtained in a straighforward fashion using Kelvin's inversion together with shift and reflection properties of biharmonic functions. Application of our method to other related problems in creeping flow and possible further generalizations are also discussed. The general results are then exploited to derive singularity solutions for the hybrid droplet embedded in (i) a centered shear flow, (ii) a quadratic potential flow, and (iii) an extensional flow past the 2D vapor-liquid compound droplet. The image singularities in each case depend on the two radii of the cylinders, the center-to-center distance, and the viscosity ratio. The exact solutions are utilized to plot the flow streamlines and they show some interesting patterns. While the flow fields exterior to the droplet exhibit symmetrical topological structures, the interior flow fields show existence of free eddies--enclosed in a figure-eight separatrix--and stagnation points (hyperbolic points). The flow characteristics are influenced by the viscosity and radii ratios. Furthermore, the asymptotic analysis leads to a rather surprising conclusion that there is a (subdominant) uniform flow far away from the droplet in all cases. The existence of an origin, the natural center of the drop of the composite geometry, which neutralizes the uniform flow for a particular choice of the physical parameters, is illustrated. This reveals the sensitivity of the geometry in 2D Stokes flow. The present results may be of some interest in models involving a combination of stick and slip boundaries. Moreover, the method discussed here can be useful both as a teaching tool and as a building block for further calculations. [DOI: 10.1115/1.2948351]

A novel trajectory ensemble model of a stratocumulus cloud is described. In this model, the boundary layer (BL) is fully covered by a great number of Lagrangian air parcels that during their motion can contain either wet aerosols or aerosols and droplets. The diffusion growth of aerosols and droplets, as well as drop collisions, is accurately described in each parcel. Droplet sedimentation is taken into account, which allows simulation of precipitation formation. The Lagrangian parcels are advected by the velocity field generated by the turbulent-like flow model obeying turbulent correlation laws. The output of the numerical model includes droplet and aerosol size distributions and their moments, such as droplet concentration, droplet spectrum width, cloud water content, drizzle content, radar reflectivity, etc., calculated in each parcel. Horizontally averaged values are calculated as well. Stratocumulus clouds observed during two research flights (RF01 and RF07) in the Second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS II) field campaign are simulated. A good agreement between the dynamical and microphysical structures simulated by the model with observations is obtained even in the nonmixing limit. A crucial role of sedimentation for the creation of a realistic cloud microphysical structure is demonstrated. In sensitivity studies, the statistical stability of the model is analyzed. Applications of the model for the investigation of droplet size distribution and drizzle formation are discussed, as is the possible utilization of the model for remote sensing applications.

The effervescent atomization of viscoelastic liquids is reported. A total of 23 fluids, formulated from a 60 wt % glycerine/40 wt % water solvent to which were added varying concentrations (0.001-0.5 wt %) of poly(ethylene oxide) polymers whose molecular weights ranged from 12,000 to 900,000, were sprayed through a conventional effervescent atomizer Mean drop sizes were measured using a forward light scattering instrument. The drop size ([D.sub.32]) data show the expected decrease with an increase in air-liquid ratio by mass (ALR), the expected increase with an increase in polymer concentration, plus an increase with an increase in polymer molecular weight for most cases. However, no significant change in [D.sub.32] was observed for polymer solutions whose molecular weights ranged from 12,000 to 35,000, suggesting the presence of a critical molecular weight below which spray performance is unaltered. This argues for two different factors controlling drop size: Polymer molecular weight is most influential at the highest polymer concentrations while polymer concentration is most influential at the lowest polymer concentrations. Analysis of the spray formation process was carried out using a ligament formation model previously developed for the effervescent atomization of Newtonian liquids coupled with a linear stability model for the breakup of viscoelastic liquid jets. The jet breakup model assumes that an unrelaxed axial tension exists within the fluid. A comparison of model predictions and experimental data indicates that the model predicts the observed dependencies of mean drop size on ALR, polymer concentration, and polymer molecular weight. Quantitative agreement is within 10-50% of experimental values in all cases. Finally, a shortcoming of the model is noted and a means of avoiding this limitation reported. [DOI: 10.1115/1.2917430]

Predicting the runback threshold for liquid drops in aerodynamic boundary layers is a challenging problem with numerous applications including aircraft icing simulations. The critical parameters that govern drop runback are investigated in this experiment by using a wind tunnel that provides a turbulent accelerated flow similar to flows near an unswept wing's leading edge. The experiments feature water drops on aluminum with a contact angle of 70 [+ or -] 5 deg. Results show that significant water/air interface unsteadiness precedes drop runback. This is likely due to air-flow separation in the drop wakes. For displacement-thickness-scaled Reynolds numbers ranging from 348 to 429, a constant-Weber-number runback threshold We =3.45 [+ or -] 0.09 is found to adequately correlate the runback results. [DOI: 10.1115l/1.2917429] Keywords: drop, runback, boundary layer

The formation of microscale single- and double-emulsion droplets with various sizes is crucial for a variety of industrial applications. In this paper, we report a new microfluidic device which can actively fine-tune the size of single- and double-emulsion droplets in liquids by utilizing controllable moving-wall structures. Moreover, various sizes of external and internal droplets for double emulsions are also successfully formed by using this device. Three pneumatic side chambers are placed at a T-junction and flow-focusing channels to construct the controllable moving-wall structures. When compressed air is applied to the pneumatic side chambers, the controllable moving-wall structures are activated, thus physically changing the width of the microchannels. The size of the internal droplets at the intersection of the T-junction channel is then fine-tuned due to the increase in the shear force. Then, the internal droplets are focused into a narrow stream hydrodynamically and finally chopped into double-emulsion droplets using another pair of moving-wall structures downstream. For single emulsions, oil-in-water droplets can be actively fine-tuned from 50.07 to 21.80 [micro]m under applied air pressures from 10 to 25 psi with a variation of less than 3.53%. For a water-in-oil single emulsion, droplets range from 50.32 to 14.76 [micro]m with a variation of less than 4.62% under the same applied air pressures. For double emulsions, the sizes of the external and internal droplets can be fine-tuned with external/ internal droplet diameter ratios ranging from 1.69 to 2.75. The development of this microfluidic device is promising for a variety of applications in the pharmaceutical, cosmetics, and food industries. [2007-0230] Index Terms--Double emulsion, flow focusing, microdroplet, microelectromechanical systems (MEMS), microfluidics, moving wall, T-junction.

We describe the development of an enzyme assay inside picoliter microdroplets. The enzyme alkaline phosphatase is expressed in Escherichia coli cells and presented in the periplasm. Droplets act as discrete reactors which retain and localize any reaction product. The catalytic turnover of the substrate is measured in individual droplets by monitoring the fluorescence at several time points within the device and exhibits kinetic behavior similar to that observed in bulk solution. Studies on wild type and a mutant enzyme successfully demonstrated the feasibility of using microfluidic droplets to provide time-resolved kinetic measurements.

A high-throughput single copy genetic amplification (SCGA) process is developed that utilizes a microfabricated droplet generator ([micro]DG) to rapidly encapsulate individual DNA molecules or cells together with primer functionalized microbeads in uniform PCR mix droplets. The nanoliter volume droplets uniquely enable quantitative high-yield amplification of DNA targets suitable for long-range sequencing and genetic analysis. A hybrid glass--polydimethylsiloxane (PDMS) microdevice assembly is used to integrate a micropump into the [micro]DG that provides uniform droplet size, controlled generation frequency, and effective bead incorporation. After bulk PCR amplification, the droplets are lysed and the beads are recovered and rapidly analyzed via flow cytometry. DNA targets ranging in size from 380 to 1139 bp at single molecule concentrations are quantitatively amplified using SCGA. Long-range sequencing results from beads each carrying ~100 amol of a 624 bp product demonstrate that these amplicons are competent for achieving attomole-scale Sanger sequencing from a single bead and for advancing pyrosequencing read-lengths. Successful single cell analysis of the glyceraldehyde 3 phosphate dehydrogenase (GAPDH) gene in human lymphocyte cells and of the gyr B gene in bacterial Escherichia coil K12 cells establishes that SCGA will also be valuable for performing high-throughput genetic analysis on single cells.

We present a purely hydrodynamic method for the high-throughput encapsulation of single cells into picoliter droplets, and spontaneous self-sorting of these droplets. Encapsulation uses a cell-triggered Rayleigh-Plateau instability in a flow-focusing geometry, and self-sorting puts to work two extra hydrodynamic mechanisms: lateral drift of deformable objects in a shear flow, and sterically driven dispersion in a compressional flow. Encapsulation and sorting are achieved on-flight in continuous flow at a rate up to 160 cells per second. The whole process is robust and costeffective, involving no optical or electrical discrimination, active sorting, flow switching, or moving parts. Successful encapsulation and sorting of 70-80% of the injected cell population into drops containing one and only one cell, with 10,000-fold enrichment as compared with the initial mix. The method can be implemented in simple 'soft lithography' chips, allowing for easy downstream coupling with microfluidic cell biology or molecular biology protocols. droplets | Rayleigh-Plateau | size-fractionnation | digital microfluidics

This paper presents the development and application of a binned approach to cloud-droplet riming within a bulk microphysics model. This approach provides a more realistic representation of collision-coalescence that occurs between ice and cloud particles of various sizes. The binned approach allows the application of specific collection efficiencies, within the stochastic collection equation, for individual size bins of droplets and ice particles; this is in sharp contrast to the bulk approach that uses a single collection efficiency to describe the growth of a distribution of an ice species by collecting cloud droplets. Simulations of a winter orographic cloud event reveal a reduction in riming when using the binned riming approach and, subsequently, larger amounts of supercooled liquid water within the orographic cloud.

Raindrop size distribution (DSD) parameters are retrieved from dual-frequency (UHE and VHF) wind profiler measurements made at Gadauki, India, in a summer monsoon season. The convoluted UHF spectra are first corrected for vertical air motion and spectral broadening (using VHF measurements) and later are used for deriving DSD parameters. Two distinctly different case studies, a mesoscale convective system and a pure stratiform precipitation system, have been considered for a detailed study. DSD parameters obtained in these case studies reveal systematic variations of DSD from case to case and also from one rain regime to another within the same precipitating system. A statistical study has been carried out using the profiler data collected during the passage of 16 rain events. The retrieved DSD profiles are divided into separate rain regimes (stratiform and convection), based on reflectivity, to examine salient microphysicat characteristics and the vertical variability of DSD in different precipitation regimes. The distribution of DSD parameters is, in general, wider in the convective rain regime than in the strafiform regime, particularly below 2.4 kin. The vertical variation of the gamma parameter distribution in the stratiform rain regime is minimal, indicating that the microphysical processes (growth and decay), which alter the rain DSD, may be in equilibrium. On the other hand, the distribution in the convective rain regime appears to be more complex, with the mean profile of the shape parameter varying significantly with height. The observed vertical variability of the gamma parameters and the median volume diameter in the convective rain regime is attributed to two major microphysical processes: evaporation and breakup. The role of other processes, like drop sorting and collision-coalescence, in altering the DSD parameters is also discussed. The present statistics, representing continental monsoon rainfall, are compared with the existing statistics at Darwin, Australia, and the results are discussed in light of DSD differences in oceanic and continental monsoon precipitation.

We present two approaches that use the environmental conditions in order to exceed the classical Abbe's limit of resolution of an aperture-limited imaging system. At first we use water drops in order to improve the resolving capabilities of an imaging system using a time-multiplexing approach. The limit for the resolution improvement capabilities is equal to the size of the rain drops. The rain drops falling close to the imaged object act as a sparse and random high-resolution mask attached to it. By applying proper image processing, the center of each falling drop is located, and the parameters of the encoding grating are extracted from the captured set of images. The decoding is done digitally by applying the same mask and time averaging. In many cases urban environment includes periodic or other high-resolution objects such as fences. Actually urban environment includes many objects of this type since from an engineering point of view they are considered appealing. Those objects follow well known standards, and therefore their structure can be a priori known even without being fully capable of imaging them. We show experimentally how we use such objects in order to superresolve the contour of moving targets passing in front of them. OCIS codes: 100.6640, 110.4850.

An open question in cloud physics is how rain forms in warm cumulus as rapidly as it is sometimes observed. In particular, the growth of cloud droplets across the size gap from 10 to 50[micro]m in radius has not been fully explained. In this paper, the authors investigate the growth of cloud droplets by collision-coalescence, taking into account both the gravitational mechanism and several enhancements of the collision-coalescence rate due to air turbulence. The kinetic collection equation (KCE) is solved with an accurate bin integral method and a newly developed parameterization of turbulent collection kernel derived from direct numerical simulation of droplet-laden turbulent flows. Three other formulations of the turbulent collection kernel are also considered so as to assess the dependence of the rain initiation time on the nature of the collection kernel. The results are compared to the base case using the Hall hydrodynamical--gravitational collection kernel. Under liquid water content and eddy dissipation rate values typical of small cumulus clouds, it is found that air turbulence has a significant impact on the collection kernel and thus on the time required to form drizzle drops. With the most realistic turbulent kernel, the air turbulence can shorten the time for the formation of drizzle drops by about 40% relative to the base case, applying measures based on either the radar reflectivity or the mass-weighted drop size. A methodology is also developed to unambiguously identify the three phases of droplet growth, namely, the autoconversion phase, the accretion phase, and the larger hydrometeor self-collection phase. The important observation is that even a moderate enhancement of collection kernel by turbulence can have a significant impact on the autoconversion phase of the growth.

High-resolution numerical simulations of the aerosol-cloud feedbacks are performed with a mesoscale model. The multimodal aerosol species, added to the model, and the cloud species were represented by two spectral moments. The aerosol sources include particle activation, solute transfer between drops due to collision and coalescence of drops, and particle regeneration. A summertime case was simulated consisting of a cold frontal cloud system and a postfrontal stratus. Experiments with both simple and mechanistic activation parameterization of aerosol and with one and two aerosol modes were performed. Verification was made of the stratus properties against measurements taken during the Radiation Aerosol and Cloud Experiment (RACE). The results demonstrate a significant sensitivity of the stratus and of the frontal system to the aerosol and a moderate impact on the particle spectrum of drop collision-coalescence. The stratus simulation with mechanistic activation and unimodal aerosol showed the best agreement of droplet concentration with the observations, and the simulations with mechanistic activation and a bimodal aerosol and with simple activation underestimated the droplet concentration. A similar high sensitivity was found for the frontal precipitation intensity. Drop collision-coalescence in the frontal system was found to have an impact on the particle mean radius whose magnitude amounted to 10% and 15% for one and multiple cloud cycles, respectively. This impact was also found to be highly variable in space. The modified particle spectrum, following activation in clouds, was found to increase droplet concentration.

The feedbacks between aerosols, cloud microphysics, and cloud chemistry are investigated in a mesoscale model. A simple bulk aqueous-phase sulfur chemistry scheme was fully coupled to the existing aerosol and microphysics schemes. The representation of aerosol and microphysics follows the explicit bulk doublemoment approach. A case of summertime stratocumulus cloud system is simulated at high resolution (3-km grid spacing), and the evolution of an observed continental aerosol spectrum that changes during the course of the simulation as a result of cloud processing is examined. The results demonstrate that the bulk approach to the aerosol and droplet spectra correctly represents the feedbacks in the coupled system. The simulations capture the characteristic bimodal aerosol size spectrum resulting from cloud processing, with the first mode consisting of particles that did not participate as cloud condensation nuclei and the second mode, in the region of 0.08-0.12-[micro]m radii, comprising the particles that were affected by processing. New information is revealed about the impact of the two main processing pathways and about the spatial distribution of the processed aerosol. One cycle of physical processing produced a relatively modest impact of 3%-5% on the processed particle mean radius of the order that was comparable to the impact of chemical processing, while continuous physical recycling produced a much larger impact as high as 30%-50%. A strong constraint on the chemical processing was found to be the initial chemistry input and the assumption of bulk chemical composition. Simple tests with a more slowly depleting primary oxidant ([H.sub.2][O.sub.2]) and including the droplet chemical heterogeneity effect favor stronger sulfate production, by, respectively, the [H.sub.2][O.sub.2] and [O.sub.3] oxidation reaction, and both show a larger impact on the processed particle mean radius of similar magnitude, 10%-20%. Spatially, the impact of processing is found initially in the downdraft regions below cloud and at later times at substantial distances downwind. It is shown that cloud processing can either enhance or suppress the number of activated drops in subsequent cycles.

We have demonstrated a 2-D droplet manipulation platform allowing fully optical manipulation of droplets on a photosensitive surface. Optically controlled injection, transport, separation, and multiple droplet manipulation have been achieved for nanoliter-size droplets. These functions are realized by sandwiching the droplets between two optoelectrowetting (OEW) surfaces. Optical illumination on OEW surfaces changes the surface wettability locally through the electrowetting mechanism. Optical illumination turns the initially Teflon-coated surface from hydrophobic to hydrophilic. This process is reversible and can be controlled in real time. We have achieved a maximum transport speed of 78 mm/s for a 100-nL droplet using a scanning laser beam. We have also demonstrated a fully decoupled 2-D multidroplet manipulation on the OEW surfaces. Potentially, the OEW mechanism can be scaled up to process a large number of liquid droplets using projected optical images for high throughput applications. [2007-0054] Index Terms--Electrowetting, laboratory-on-a-chip, optoelectrowetting (OEW), surface tension.

Hydrometeorological and radio propagation applications can benefit from the capability to model the time evolution of raindrop size distribution (RSD). A new stochastic vector autoregressive semi-Markov model is proposed to randomly synthesize (generate) the temporal series of the three driving parameters of a normalized Gamma RSD. Rainfall intermittence is reproduced through a discrete semi-Markov process, modeled from disdrometer measurements using two-state analytical statistics of rain and dry period duration. The overall model is set up by means of a large set of disdrometer measurements, collected from 2003 to 2005 at Chilbolton, U.K. The driving parameters of the retrieved RSD are estimated using three approaches: the Gamma moment method and the 1-D and 3-D maximum-likelihood methods. Interestingly, these methodologies lead to quite different results, particularly when one is interested in evaluating RSD higher order moments such as the rain rate. The accuracy of the proposed RSD time-series generation technique is evaluated against available disdrometer measurements, providing excellent statistical scores. Index Terms--Autoregressive process, disdrometer, estimation techniques, intermittent rain process, raindrop size distribution (RSD), semi-Markov chain.

The first lab-on-chip system for picoliter droplet generation and PCR amplification with real-time fluorescence detection has performed PCR in isolated droplets at volumes [10.sup.6] smaller than commercial real-time PCR instruments. The system utilized a shearing T-junction in a silicon device to generate a stream of monodisperse picoliter droplets that were isolated from the microfluidic channel walls and each other by the oil-phase carrier. An off-chip valving system stopped the droplets on-chip, allowing them to be thermally cycled through the PCR protocol without droplet morion. With this system, a 10-pL droplet, encapsulating less than one copy of viral genomic DNA through Poisson statistics, showed real-time PCR amplification curves with a cycle threshold of ~18, 20 cycles earlier than commercial instruments. This combination of the established real-time PCR assay with digital microfluidics is ideal for isolating single-copy nucleic acids in a complex environment.

Ambient temperature strongly influences gas turbine power output causing a reduction of around 0.50% to 0.90% for every 1[degrees]C of temperature rise. There is also a significant increase in the gas turbine heat rate as the ambient temperature rises, resulting in an increased operating cost. As the increase in power demand is usually coincident with high ambient temperature, power augmentation during the hot part of the day becomes important for independent power producers, cogenerators, and electric utilities. Evaporative and overspray fogging are simple, proven, and cost effective approaches for recovering lost gas turbine performance. A comprehensive review of the current understanding of the analytical, experimental, and practical aspects including climatic and psychrometric aspects of high-pressure inlet evaporative fogging technology is provided. A discussion of analytical and experimental results relating to droplets dynamics, factors affecting droplets size, and inlet duct configuration effects on inlet evaporative fogging is covered in this paper. Characteristics of commonly used fogging nozzles are also described and experimental findings presented. [DOI: 10.1115/1.2364003] Key words: evaporative fogging, power augmentation, droplets dynamics, fogging nozzle design

This paper describes the use of an optical vortex trap for the transport and fusion of single femtoliter-volume aqueous droplets. Individual droplets were generated by emulsifying water in acetophenone with SPAN 80 surfactant. We demonstrate the ability of optical vortex traps to position trapped droplets precisely while excluding surrounding aqueous droplets from entering the trap, thereby preventing unwanted cross contamination by other nearby droplets. Additionally, the limitation of optical vortex traps for inducing droplet fusion is illustrated, and a remedy is provided through modulation of the spatial intensity profile of the optical vortex beam. Spatial modulation was achieved by translating the computer-generated hologram (CGH) with respect to the input Gaussian beam, thereby shifting the location of the embedded phase singularity (dark core) within the optical vortex beam. We present both simulated and experimentally measured intensity profiles of the vortex beam caused by translation of the CGH. We further describe the use of this technique to achieve controlled and facile fusion of two aqueous droplets.

In this study the range of visible refractive indices and single-scattering albedos that can be obtained using various mean field formulations for mixtures of two components, where one component absorbs in the visible, is explored. For calculations of global radiative forcing and climate response, such formulations provide a convenient way to take into account internal mixing of aerosol components without the computational burden of a more exact theory, provided that the limits of their applicability are kept in mind. Within those limits, the results show that for water and sulfate drops containing dust and soot inclusions, the degree of interaction between the electric fields of neighboring inclusions is not important. However, the shape and size of the inclusions can cause a variation in the effective imaginary refractive index of up to a factor of 3.4 and a variation in the single-scattering albedo of up to 0.18 in absolute value.

High-temperature annealing of gallium arsenide in vacuum causes excess evaporation of arsenic, with accumulation of gallium as liquid droplets on the surface. Using real-time in situ surface electron microscopy, we found that these droplets spontaneously run across the crystal surface. Running droplets have been seen in many systems, but they typically require special surface preparation or gradient forces. In contrast, we show that noncongruent evaporation automatically provides a driving force for running droplets. The motion is predicted and observed to slow and stop near a characteristic temperature, with the speed increasing both below and above this temperature. The same behavior is expected to occur during the evaporation of similar III-V semiconductors such as indium arsenide.

Experimental and computational studies were conducted to understand the head-on collision dynamics of two identical droplets. The information obtained from the simulated collision images could be used to study the collision flow field including the energy budget and the fundamental differences between the soft and hard collisions that lead to merging.

A theoretical approach is developed for analyzing the droplet dynamics in a microchannel subjected to electrocapillary actuation mechanisms. The analysis has indicated that a threshold limit is present on the value of the interelectrode pitch, beyond which the overall microfluidic transport rates are reduced.

The geometry of drops hanging on a circular capillary can be determined by numerically solving a dimensionless differential equation that is independent on any material properties, which enables one to follow the change of the height, surface area, and contact angle of drops hanging on a particular capillary. The results show that the application of drop tensionmeters for the determination of surface tension is on the basis of the calculated drop shapes and experimental functions.