Droplets

Nanoparticles have been a research focus for more than two decades. The advent of microfluidic technology provides alternative strategies for the synthesis of nanoparticles in reactors having a micrometer length scale. The integration of nanoparticle and microreactor technologies delivers enormous opportunity and impetus for the further development of novel materials and reactors. In this article, recent achievements in the synthesis of nanoparticles in microfluidic reactors are reviewed. A variety of strategies for synthesizing nanoparticles are presented and compared, including continuous flow, gas-liquid segmented flow and droplet-based microreactors. Different categories of nanoparticles manufactured in microreactors are summarized to highlight the wide application of microfluidic systems in the development of novel nanomaterials. The control of nanoparticle synthesis is also briefly discussed.

A new kinetic model for heating and evaporation of Diesel fuel droplets is suggested. The model is based on the introduction of the kinetic region in the immediate vicinity of the heated and evaporating droplets, where the dynamics of molecules are described in terms of the Boltzmann equations for vapour components and air, and the hydrodynamic region. The effects of finite thermal conductivity and species diffusivity inside the droplets and inelastic collisions in the kinetic region are taken into account. Diesel fuel is approximated by n-dodecane or a mixture of 80% n-dodecane and 20% p-dipropylbenzene. In both cases, the evaporation coefficient is assumed equal to 1. The values of temperature and vapour density at the outer boundary of the kinetic region are inferred from the equirement that both heat flux and mass flux of vapour (or vapour components) in the kinetic and hydrodynamic regions in the vicinity of the interface between these regions should be equal. Initially, the heat and mass fluxes in the hydrodynamic region are calculated based on the values of temperature and vapour density at the surface of the droplet. Then the values of temperature and vapour density at the outer boundary of the kinetic region, obtained following the above-mentioned procedure, are used to calculate the corrected values of hydrodynamic heat and mass fluxes. The latter in their turn lead to new corrected values of temperature and vapour density at the outer boundary of the kinetic region etc. It is shown that this process quickly converges for the cases analysed in the paper, and it leads to self-consistent values for both heat and mass fluxes. The model is applied to the analysis of heating and evaporation of Diesel fuel droplets with initial radii and temperature equal to 5 lm and 300 K, immersed into gas with temperatures in the range 800–1200 K and pressure
equal to 30 bar. It is shown that in all cases the kinetic effects lead to a decrease in droplet surface temperature and an increase in the evaporation time. The kinetic effects on the droplet evaporation time are shown to increase with increasing gas temperatures.

In this work the effect of flash boiling on liquid break up and atomization is first reviewed and then described in detail for the sprays produced by two high pressure multistream gasoline direct injectors. They were operated at 120bar and 200bar pressure with a fuel injection duration of 0.8ms into sub-atmospheric pressures between 1 and 0.1bar and four temperatures between 20°C and 100°C. The sprays were injected into an optically accessed chamber whose temperature and pressure were independently controlled to recreate the conditions found in an engine likely to cause flash boiling. Shadowgraph imaging with a single shot CCD camera was applied to obtain images of the spray morphology under a matrix of the above conditions. The interaction between spray streams is particularly complex under flash boiling conditions being a function of the nature of the fuel and the geometry of the injector. In this investigation, three different fuels, regular RON-95 gasoline and single components fuels, n-heptane and iso-octane were used with two different injectors, one with a 60 degree and the other with a 90 degree nominal external cone angle. In total, five variables were investigated: fuel pressure, ambient pressure, ambient temperature, fuel composition and injector geometry. Once processed, the recorded images allowed measurement of spray tip 2 penetration and cone angle. Qualitative data on the change in shape of the spray was also available. The results showed that flash boiling has potential to reduce droplet diameters and improve fuel vaporisation, however, the associated change in spray shape must be taken into account to avoid problems with spray impingement.

This study deals with the diagenetic evolution of dolostones of the Romaine Formation, from cores and outcrops on Anticosti and Mingan islands, and reports evidence for hydrocarbon migration in secondary porosity generated by high temperature / hydrothermal alteration. Four types of calcite (grouped into early (C1) and late (C2) assemblages) and six types of dolomite (grouped into 3 different (D1, D2 and D3) assemblages) are distinguished. According to fluid-inclusion and stable C-O isotope data, the early calcite group, which consists of sedimentary micrite, replacement microspar and early pore-filling calcite records near surface diagenetic environments. Early and pervasive replacement dolomites (RD1 and RD2 in the first dolomite (D1) assemblage) were formed during early burial, and are not related to porosity generation. Later replacement dolomites (RD3 and RD4 in the second dolomite (D2) assemblage) and pore-filling dolomite cements (PD1 and PD2 in the third dolomite (D3) assemblage) as well as minor sphalerite were formed from warm, saline fluids, which were likely hydrothermal in origin. Vuggy porosity was produced through brecciation and fracturing, and through some dissolution of the D1 dolomite assemblage by these hydrothermal fluids. Intercrystalline porosity was also created during formation of the D2 dolomite assemblage. The vugs were partly occluded by late-stage pore filling dolomite and calcite cements, pyrite and barite. Bitumen droplets in vugs together with hydrocarbon inclusions recorded in C2 calcite and in contemporaneous barite indicate a hydrocarbon migration event after the porositygenerating processes.

There are many processes in nature and technology in which the time needed for fluid fragmentation or coalescence is comparable to the characteristic time of the investigated macroscopic physical phenomenon. Thus methods and models are needed to generalize laboratory observations, in order to adequately model fragmentation and coalescence dynamics and their influence on system behavior of flows.

The effect of an electric field on the formation of a drop of an inviscid, perfectly conducting liquid from a capillary which protrudes from the top plate of a parallel-plate capacitor into a surrounding dynamically inactive, insulating gas is studied computationally. This free boundary problem which is comprised of the surface Bernoulli equation for the transient drop shape and the Laplace equation for the velocity potential inside the drop and the electrostatic potential outside the drop is solved by a method of lines incorporating the finite element method for spatial discretization. The finite element algorithm employed relies on judicious use of remeshing and element addition to a two-region adaptive mesh to accommodate large domain deformations, and allows the computations to proceed until the thickness of the neck connecting an about to form drop to the rest of the liquid in the capillary is less than 0.1% of the capillary radius. The accuracy of the computations is demonstrated by showing that in the absence of an electric field predictions made with the new algorithm are in excellent agreement with boundary integral calculations (Schulkes, R. M. S. M. J. Fluid Mech. 278, 83 (1994)) and experimental measurements on water drops (Zhang, X., and Basaran, O. A. Phys. Fluids 7(6), 1184 (1995)). In the presence of an electric field, the algorithm predicts that as the strength of the applied field increases, the mode of drop formation changes from simple dripping to jetting to so-called microdripping, in accordance with experimental observations (Cloupeau, M., and Prunet-Foch, B. J. Aerosol Sci. 25(6), 1021 (1994); Zhang, X., and Basaran, O. A. J. Fluid Mech. 326, 239 (1996)). Computational predictions of the primary drop volume and drop length at breakup are reported over a wide range of values of the ratios of electrical, gravitational, and inertial forces to surface tension force. In contrast to previously mentioned cases where both the flow rate in the tube and the electric field strength are nonzero, situations are also considered in which the flow rate is zero and the dynamics are initiated by impulsively changing the field strength from a certain value to a larger value. When the magnitude of the step change in field strength is small, the results of the new transient calculations accord well with those of an earlier stability analysis (Basaran, O. A., and Scriven, L. E. J. Colloid Interface Sci. 140(1), 10 (1990)) and thereby provide yet another testament to the accuracy of the new algorithm.

We numerically investigated liquid droplet impact behavior onto a dry and flat surface. The numerical method consists of a coupled level set and volume-of-fluid framework, volume/surface integrated average based multimoment method, and a continuum surface force model. The numerical simulation reproduces the experimentally observed droplet behavior quantitatively, in both the spreading and receding phases, only when we use a dynamic contact angle model based on experimental observations. If we use a sensible simplified dynamic contact angle model, the predicted time dependence of droplet behavior is poorly reproduced. The result shows that precise dynamic contact angle modeling plays an important role in the modeling of droplet impact behavior.

Applying a thin, protective coating of a nontoxic, chemically resistant epoxy to the interior of existing pipes is an alternative method to pipe replacement. In order to find the controlling parameters in this method, in this study, viscous epoxy was propelled by compressed air through clear polyvinyl chloride (PVC) pipes. Epoxy flow was annular, and it hardened to form a thin, uniform coating on the inner pipe surface. A video camera was employed to record fluid motion, and the thickness of the coating was measured using an image analysis program named ImagJ. Tests were done with varying air temperature, airflow rate, piping configuration, and epoxy temperature. A one-dimensional numerical algorithm was developed to model fluid flow, heat transfer, and epoxy curing. Heating the epoxy makes it move faster because liquid viscosity decreases with increasing temperature. The coating was significantly thicker at the bottom of a horizontal pipe than at the top due to sagging of the epoxy coating after it had been applied, resulting in flow from the top to the bottom of the pipe. Sagging could be reduced by maintaining airflow until curing was almost complete and the epoxy had hardened enough to prevent it from moving easily. The combination of the experimental results and numerical modeling showed that the most important parameters controlling the speed of the epoxy and coating thickness were the air flow rate and temperature, since they determine the shear forces on the epoxy layer and the rate at which the epoxy cures. Raising air temperature increases the reaction rate and therefore decreases the time required for the epoxy to cure inside the pipe. The results of the simulation showed a very good agreement with the experimental results in pipes with 1-in diameter or less.

Although a lot of research has been done in modeling the oil recovery from fractured reservoirs by countercurrent imbibition, less attention has been paid to the effect of the fracture fluid velocity upon the rate of oil recovery. Experiments are conducted to determine the effect of fracture flow rate upon countercurrent imbibition. A droplet detachment model is proposed to derive the effective water saturation in a thin boundary layer at the matrix-fracture interface. This effective boundary water saturation is a function of fluid properties, fluid velocity in the fracture and fracture width. For a highly water-wet porous medium, this model predicts an increase in the boundary water saturation with increase in fracture fluid velocity. The increase in boundary water saturation, in turn, increases the oil recovery rate from the matrix, which is consistent with the experimental results. The model also predicts that the oil recovery rate does not vary linearly with the boundary water saturation.

Based on the fact that water can be collected from fog under favorable topographical and atmospheric conditions. The small water droplets present in the fog precipitate when they come in contact with solid objects. The frequent fogs that occur in the southwestern region of Kingdom of Saudi Arabia have the potential to provide an alternative source of freshwater in this otherwise dry region if harvested through the use of simple and low-cost collection systems known as fog collectors. A fog collection project has been carried out in Asir region of Saudi Arabia. Three Standard Fog Collectors (SFC) were designed and manufactured with locally available materials and imported mesh. These SFCs were installed in April 2006. Three different sites were chosen based on topography and altitude and data from April 2006 to April 2007 were obtained. Measurements with the SFCs were made for regions with 2,260 to 3,200 m elevation. The results indicate that at highest altitudes (at Alsooda), it is feasible to obtain an average water production of 6.215 L/m 2 day over the studied period, and in the lower altitudes, which are in Abha city, it is possible to collect more than 3.3 L/m 2 day. The highest water collection was 24 L/m 2 day and recorded in Dec. 20, 2006 at Al-Sahab park station, and furthermore for the same site, the best average water production of 11.5 L/m 2 day was obtained in winter 3 months period namely (December, November and January). Data about the weather conditions has been collected for the studied region. An analysis of fog collection effectiveness was carried out and compared with international standards. The obtained results indicate that fog can be a useful source of water supply in the southwest region of Saudi Arabia.

A new predictive model for collisional interactions between liquid droplets, which is valid for moderate to high Weber numbers (>40), has been developed and validated. Four possible collision outcomes, viz., bouncing, coalescence, reflexive separation and stretching separation, are considered. Fragmentations in stretching and reflexive separations are modeled by assuming that the interacting droplets form an elongating ligament that either breaks up by capillary wave instability, or retracts to form a single satellite droplet. The outcome of a collision, number of satellites formed from separation processes and the post-collision characteristics such as velocity and drop-size are compared with available experimental data. The comparisons include colliding mono-and poly-disperse streams of droplets of different fuels under atmospheric conditions, and the results agree reasonably well.

Abstract A study of heat transfer and film dynamics caused by single and multiple streams of
impinging droplets using HFE-7100 as the cooling liquid under constant heat flux conditions
is presented. Single and multiple streams of mono-dispersed droplets were produced using
a piezoelectric droplet generator with the ability to adjust parameters such as droplet
impingement frequency, droplet diameter, droplet velocity, and spacing between adjacent
impinging droplet streams.

Recently developed approaches to droplet and spray modeling in computational fluid dynamics (CFD) codes are reviewed. It is emphasized that the modeling of spray primary break-up needs to take into account the transient nature of sprays, while the modeling of droplet heating and evaporation needs to take into account a number of factors, including recirculation and finite thermal conductivity of liquid, semi-transparency of droplets and the kinetic effects. This modeling needs to take place in realistic three-dimensional enclosures, which makes it essential to find a compromise between the accuracy of the models and their CPU efficiency. Evaporation 6 S. SAZHIN Periodica 4, 2009, 1 droplet break-up, heating and evaporation in engineering computational fluid dynamics (CFD) codes. Some of the new models have been implemented into the customized version of the KIVA 2 computational fluid dynamics (CFD) code. The implementation of these models into other CFD codes seems to be a straightforward task.

The main results of the aerothermodynamic hypersonic characterization of the intermediate experimental vehicle, by means of both computational fluid dynamics simulations and wind-tunnel measurements, have been reported and analyzed in this work, developed in the frame of ESA Future Launcher Preparatory Program. A test campaign has been carried out at the von Kármán Institute Free Piston Longshot wind tunnel in a cold hypersonic regime. The numerical simulations, rebuilding the von Kármán Institute wind-tunnel conditions, have been performed by the Italian Aerospace Research Center, the goal being to support the procedure of extrapolation-to-flight of the measurements and the general aerothermal characterization. A detailed comparison of all measured and predicted hypersonic relevant phenomena and aerothermodynamic parameters, such as surface pressure and heat flux, is reported in the paper, together with a detailed description of the configuration, freestream conditions, and model attitude effects. The reproducibility of flight conditions in the von Kármán Institute Longshot test campaign is qualitatively very good, both in terms of flow features and surface properties as indicated by wind-tunnel numerical simulations. The comparisons of numerical and experimental results indicate a good agreement in terms of external flow structure, i.e., bow shock, shock layer, expansion at the junction on windward side, shock-wave boundary-layer interaction in the flap area, and multiple-shock interaction above the flap. Sideslip, angle-of-attack, and flapdeflection effects are correctly reproduced by computational fluid dynamics in terms of pressure coefficient and heat flux.

It is shown here that hygroscopic seeding requires two orders of magnitude more hygroscopic agent than can be delivered by flare technology for producing raindrop embryos in concentrations to detect by cloud physics aircraft the microphysical signature of rain initiation. An alternative method of finely milled salt powder is shown to be capable of achieving this goal. During field experiments the use of a sulfur hexafluoride (SF 6 ) gas tracer to identify the exact seeded cloud volume and to quantify dilution of the seeding agent showed that the seeding agent dilutes to the order of 10 210 of its released concentration in updrafts at a height of $1 km above cloud base. This means that the theoretically expected changes in the cloud drop size distribution (DSD) would not be detectable with a cloud droplet spectrometer in a measurement volume collected during the several seconds that the seeded volume is traversed by an aircraft. The actual measurements failed to identify a clear microphysical seeding signature from the burning of hygroscopic flares within the seeded convective clouds. This uncertainty with respect to hygroscopic flare-seeding experiments prompted an experimental and theoretical search for optimal hygroscopic seeding materials. This search culminated in the production of a salt powder having 2-5-mm-diameter particle sizes that are optimal according to model simulations, and can be distributed from a crop duster aircraft. Such particles act as giant cloud condensation nuclei (GCCN). Any potential broadening of the DSD at cloud base by the competition effect (i.e., when the seeded aerosols compete with the natural ambient aerosols for water vapor) occurs when the seeding agent has not been substantially diluted, and hence affects only a very small cloud volume that dilutes quickly. Therefore, the main expected effect of the GCCN is probably to serve as raindrop embryos. The salt powderseeding method is more productive by two orders of magnitude than the hygroscopic flares in producing GCCN that can initiate rain in clouds with naturally suppressed warm rain processes, because of a combination of change in the particle size distribution and the greater seeding rate that is practical with the powder. Experimental seeding of salt powder in conjunction with the simultaneous release of an SF 6 gas tracer produced strong seeding signatures, indicating that the methodology works as hypothesized. The efficacy of the accelerated warm rain processes in altering rainfall amounts may vary under different conditions, and requires additional research that involves both observations and simulations.

The steady-state and transient shear flow dynamics of polymer drops in a microchannel are investigated using the dissipative particle dynamics (DPD) method. The polymer drop is made up of 10% DPD solvent particles and 90% finite extensible non-linear elastic (FENE) bead spring chains, with each chain consisting of 16 beads. The channel's upper and lower walls are made up of three layers of DPD particles, respectively, perpendicular to Z-axis, and moving in opposite directions to generate the shear flow field. Periodic boundary conditions are implemented in the X and Y directions. With FENE chains, shear thinning and normal stress difference effects are observed. The "colour" method is employed to model immiscible fluids according to Rothman-Keller method; the χ-parameters in Flory-Huggins-type models are also analysed accordingly. The interfacial tension is computed using the Irving-Kirkwood equation. For polymer drops in a steady-state shear field, the relationship between the deformation parameter (D def ) and the capillary number (Ca) can be delineated into a linear and nonlinear regime, in qualitative agreement with experimental results of Guido et al. [J. Rheol. 42 ]. In the present study, Ca < 0.22, in the linear regime. As the shear rate increases further, the drop elongates; a sufficiently deformed drop will break up; and a possible coalescence may occur for two neighbouring drops. Dynamical equilibrium between break-up and coalescence results in a steady-state average droplet-size distribution. In a shear reversal flow, an elongated and oriented polymer drop retracts towards a roughly spherical shape, with a decrease in the first normal stress difference. The polymer drop is found to undergo a tumbling mode at high Schmidt numbers. A stress analysis shows that the stress response is different from that of a suspension of solid spheres. An overshoot in the strain is observed for the polymer drop under extension due to the memory of the FENE chains. .sg (N. Phan-Thien).

1] The influences of the aerosol type and concentration and relative humidity (RH) on cumulus clouds have been investigated using a two-dimensional spectral-bin cloud model. Three simulations are conducted to represent the polluted continental, clean continental, and marine aerosol types. Under the same initial dynamic and thermodynamic conditions, the maritime aerosol case results in more intensive radar reflectivity in both developing and mature stages than the continental aerosol cases, because of enhanced warm rain by collisions and ice processes by deposition growth due to larger droplet sizes and higher supersaturation, respectively. The considerable delay in convective development due to reduced droplet condensation is responsible for the longer cloud lifetime in the marine aerosol case. For the continental case, the most noticeable effects of increasing aerosol number concentrations (with 15 different initial values) are the increases of the cloud droplet number concentration and cloud water content but a decrease in the effective droplet radius. More latent heat release from increasing condensation results in stronger convection and more melting precipitation at the higher aerosol concentrations. Melting precipitation and secondary clouds primarily contribute to enhanced precipitation with increasing aerosols. The precipitation, however, decreases with increasing aerosol in the extremely high aerosol cases (over 5 Â 10 4 cm À3 ) due to suppression of convection from depleted water vapor and inefficient coalescence. When the initial aerosol concentration exceeds a critical level, most of the cloud properties become less sensitive to aerosols, implying that the aerosol effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effect on the cloud properties is strongly dependent on RH. As the surface RH increases from 40 to 70%, the cloud changes from shallow warm to deep convective types due to a significant increase of convective available potential energy (CAPE). The aerosol effects on the cloud microphysical properties and precipitation are negligible in dry air (40% surface RH), but much more significant in humid air (60-70% surface RH). The rain delay is sensitive to RH, but not to aerosols under similar initial thermodynamic conditions.

Most materials of practical interest are neither completely wetting nor completely non-wetting. Through careful design, it is possible to alter the natural wettability of a surface to be more omniphilic or omniphobic. This is principally realized by modifying the physics and/or the chemistry of the surface. In this work, the effects of external uniform magnetic fields on the behavior of the static contact angle of different oil-based ferrofluid droplets were examined for a handful of horizontal surfaces of varying wettability. Experimental results show that the static contact angle of ferrofluid droplets in a magnetic field is not constant around the periphery of the droplet on a horizontal surface, rather it varies in a rich, yet predictable way with the azimuthal angle and magnetic field strength. In addition, the spontaneous motion of two oil-based ferrofluid droplets on a wedge-shaped surface tension gradient was studied for two distinct surface tension gradient orientations in the presence of different external uniform magnetic fields (0-250G). Various wedge angles from 10° to 17° were studied. It was observed that for a given ferrofluid droplet size, larger wedge angles generally produced larger overall surface tension driving forces. As a result, higher droplet velocities were generally achieved in this case, but also shorter overall travel distances. Eventually, for each ferrofluid that was studied, a magnetic field was found above which the droplet was "pinned" on the surface, and droplet movement was stopped despite the underlying presence of the surface tension gradient force.

Powder particles are projected from a thermal spray gun towards substrates to generate protective coatings. A clear understanding of the dynamic impingement when droplets make contact with substrates is critical for controlling and optimizing the thermal spray process. A droplet impingement model is developed to simulate the transient flow dynamics during impact, spreading and solidification. The volume of fluid surface tracking technique is employed within a fixed Eulerian structured mesh. The numerical model is validated with experimental data from tin droplet measurements. The results prove that thermal contact resistance is the key element in characterizing the substrate surface roughness for impingement modelling. It is found that spreading, solidification and air entrapment are closely related to surface roughness.

We discuss the stability of bubbles to coalescence when undergoing a pressure drop and their stability to disproportionation under quiescent conditions, studied using previously established 'single bubble layer experimental' techniques, focussing on the effects on stability of the inclusion of a low volume fraction (0.25%) of stable oil droplets. Detailed measurements of the surface dilatational elasticity (3 s ) and surface shear viscosity (h s ) of systems in the presence and absence of oil droplets have been performed. The surface rheology and stability have been measured as a function of adsorption time and pH, between pH 4.5 and 7, by including glucono-d-lactone (GDL) as an acidification agent. Commercial sodium caseinate (SC) and purified b-lactoglobulin (b-L) were used at 1 wt% bulk concentration as bubble stabilizing agents. The emulsion oil droplet phase was n-tetradecane (TD) or 1-bromohexadecane (BHD), with a mean droplet size of (d 43 ) ¼ 0.59 and 0.67 mm, respectively. The emulsion droplets were completely stable to coalescence and did not enter or spread at the air-water (A-W) interface. With SC at all pH values the values of h s were markedly higher in the presence of TD droplets than in their absence, but particularly when the pH was lowered to pH 5.5 such that the SC started to aggregate. The increase in h s correlated with the increase in coalescence stability under the same conditions. With neutrally buoyant BHD droplets the increase in h s was not as great, but h s was still significantly higher than in the absence of droplets, indicating that the rise to, and packing of, TD droplets at the A-W interface due to gravity was not solely responsible for their observed effects. The values of 3 s did not increase much at all for either b-L or SC as the pH was lowered and/or TD droplets were added, except at very low pH values, when the effects with SC were obscured by the tendency for the bulk SC to gel. In agreement with the relatively insignificant changes in 3 s as the pH was lowered or droplets were added, the resistance to disproportionation of bubbles did not change very much either.

The one-step synthesis of barium titanate (BaTiO 3 ) nanoparticles was studied by employing a low-pressure spray pyrolysis (LPSP) method. The effects of temperature, pressure, and the addition of urea to the precursor were investigated experimentally. The results were compared with the experimental data of the conventional (atmospheric) spray pyrolysis method. It was shown that the BaTiO 3 nanoparticles could be synthesized by the low-pressure method, while only spherical hollow particles with smooth surfaces could be produced by the conventional spray method. The addition of urea greatly improved the crystal growth and particle breakup due to extra heat supplied during the combustion reaction coupled with the evolution of gases. The dispersity of nanoparticles increased with the quantity of urea and with a decrease in pressure. The possible mechanism of the formation of BaTiO 3 nanoparticles in the LPSP process was also proposed.

The pore-scale effects of seismic stimulation on two-phase flow are modeled numerically in random 2D grain0pack geometries. Seismic stimulation aims to enhance oil production by sending seismic waves across a reservoir to liberate immobile patches of oil. For seismic amplitudes above a welldefined ͑analytically expressed͒ dimensionless criterion, the force perturbation associated with the waves indeed can liberate oil trapped on capillary barriers and get it flowing again under the background pressure gradient. Subsequent coalescence of the freed oil droplets acts to enhance oil movement further because longer bubbles overcome capillary barriers more efficiently than shorter bubbles do. Poroelasticity theory defines the effective force that a seismic wave adds to the background fluid-pressure gradient. The lattice-Boltzmann model in two dimensions is used to perform pore-scale numerical simulations. Dimensionless numbers ͑groups of material and force parameters͒ involved in seismic stimulation are defined carefully so that numerical simulations can be applied to field-scale conditions. Using the analytical criteria defined in the paper, there is a significant range of reservoir conditions over which seismic stimulation can be expected to enhance oil production.

Nanoparticles have been a research focus for more than two decades. The advent of microfluidic technology provides alternative strategies for the synthesis of nanoparticles in reactors having a micrometer length scale. The integration of nanoparticle and microreactor technologies delivers enormous opportunity and impetus for the further development of novel materials and reactors. In this article, recent achievements in the synthesis of nanoparticles in microfluidic reactors are reviewed. A variety of strategies for synthesizing nanoparticles are presented and compared, including continuous flow, gas-liquid segmented flow and droplet-based microreactors. Different categories of nanoparticles manufactured in microreactors are summarized to highlight the wide application of microfluidic systems in the development of novel nanomaterials. The control of nanoparticle synthesis is also briefly discussed.

This work deals with the modeling and the direct numerical simulation of the absorption of a gas component into a spherical liquid droplet in free fall, where the absorbed component takes part to a chemical reaction. This study is realized by computing the flow fields and the concentration fields simultaneously in the gas and liquid phases. The time evolution of the droplet global mass absorption rate is studied for various regimes characterized by the Reynolds and the Hatta numbers. The monitoring of the time evolution of the concentration fields in the droplet enables the understanding of the interactions between the diffusive and convective mass transports and the chemical reaction. The phenomena controlling the mass transfer rate and how they evolve during the mass absorption process can then be identified. In a first stage, the case of the physical absorption is studied. Moreover, analytical expressions are developed to correlate the time evolution of the Sherwood number with the Reynolds number and the absorption time. In a second stage, the influence of the coupling with a chemical reaction is studied. It is observed that several rate-limiting mechanisms successively control the global absorption rate, enlightening the complex interactions between convective and diffusive mass transport and the chemical reaction.

Surface microstructures of solids play a significant role in producing superhydrophobic surfaces. In the present paper, the Cassie-Baxter and Wenzel models on a rough substrate are examined from the viewpoints of geometry and energy. The result shows that if the air beneath a droplet on a sinusoidal substrate is open to the atmosphere, the superhydrophobic state can exist only when the substrate is hydrophobic, and that the geometric parameters of the microstructure have a great influence on the wetting behavior. Two mechanisms that may lead to a superhydrophobic property from a hydrophilic substrate are addressed. Firstly, for closed or airproof microstructures (e.g. honeycomb structures), a negative Laplace pressure difference caused by the trapped air under the drop can keep the balance of the liquid/vapor interface. Secondly, some special topologies of surface structures satisfying a certain geometric condition may also lead to the formation of a Cassie-Baxter state even if the microstructures are open to the air. Therefore, some surface morphologies may be designed to obtain superhydrophobic properties on hydrophilic surfaces. The present study is also helpful to understand some superhydrophobic phenomena observed in experiments and in nature.

In this work the effect of flash boiling on liquid break up and atomization is first reviewed and then described in detail for the sprays produced by two high pressure multistream gasoline direct injectors. They were operated at 120bar and 200bar pressure with a fuel injection duration of 0.8ms into sub-atmospheric pressures between 1 and 0.1bar and four temperatures between 20°C and 100°C. The sprays were injected into an optically accessed chamber whose temperature and pressure were independently controlled to recreate the conditions found in an engine likely to cause flash boiling. Shadowgraph imaging with a single shot CCD camera was applied to obtain images of the spray morphology under a matrix of the above conditions. The interaction between spray streams is particularly complex under flash boiling conditions being a function of the nature of the fuel and the geometry of the injector. In this investigation, three different fuels, regular RON-95 gasoline and single components fuels, n-heptane and iso-octane were used with two different injectors, one with a 60 degree and the other with a 90 degree nominal external cone angle. In total, five variables were investigated: fuel pressure, ambient pressure, ambient temperature, fuel composition and injector geometry. Once processed, the recorded images allowed measurement of spray tip 2 penetration and cone angle. Qualitative data on the change in shape of the spray was also available. The results showed that flash boiling has potential to reduce droplet diameters and improve fuel vaporisation, however, the associated change in spray shape must be taken into account to avoid problems with spray impingement.

Centre-of-mass motions of two coupled spherical-cap droplets are considered. A model with surface tension and inertia that accounts for finite-amplitude deformations is derived in closed form. Total droplet volume λ and half-length L of the tube that connects the droplets are the control parameters. The model dynamics reside in the phase-plane. For lens-like droplets λ < 1, and for any L there is a single steady state about which the droplets vibrate with limit-cycle behaviour. For λ > 1, the symmetric state loses stability (saddle point) and new antisymmetric steady states arise about which limit-cycle oscillations occur. These mirror states -big-droplet up or bigdroplet down -are also stable. In addition, there are large finite-amplitude 'looping' oscillations corresponding to limit cycles that enclose both steady states in the phaseplane. All three kinds of oscillations are documented in an experiment that sets the system into motion by 'kicking' one of the droplets with a prescribed pressure-pulse. Model predictions for frequencies are consistent with observations. Small-amplitude predictions are placed in the wider context of constrained Rayleigh vibrations. A model extension to account for the small but non-negligible influence of viscosity is also presented.

The characteristic conditions causing spreading or splashing after drop impact on solid surfaces are considered together with the underlying mechanisms. To this end, the results of the various studies published over the past few years that have addressed the issue of splashing after droplet impact, specifically in terms of the definition of a splashing threshold, are critically compared and synthesized. The discussion aims at clarifying some of the conflicting findings. Information drawn from these considerations is used to distinguish between various splashing thresholds and it is shown that there exists a distinct difference between splashing on smooth and on rough surfaces, both in terms of the splashing thresholds and in terms of the mechanisms. Finally, a physical mechanism akin to air entrainment in dynamic wetting is proposed that may be of primary importance for the inception of splashing as well as fingering on smooth surfaces. M. Rein (B)

Drop size distributions (DSDs) were continuously and simultaneously measured by laser raindrop-sizing instruments (LD gauges) in an open site and in three forest stands consisting of Japanese cypress (CY: Chamaecyparis obtusa), Japanese cedar (CD: Cryptomeria japonica), and sawtooth oak (SO: Quercus acutissima), during three rainfall events in Tokyo, Japan. Drop data during the whole observation period were used in an hourly based data set and divided into three meteorological condition groups: calm, heavy rain, and strong wind. Evaluating the influence of canopy species and meteorological factors on the D 50 and DSD difference revealed some throughfall DSD characteristics. First, throughfall had different DSDs among canopy species under conditions of little canopy vibration with low rainfall intensity and wind speed; D 50 values were 2.00, 2.93, and 3.60 mm in CY, CD, and SO, respectively. Second, throughfall contained smaller drops under conditions of severe canopy vibration, with high rainfall intensity and/or high wind speed, than under calm meteorological conditions. Vibration of the canopy could reduce water coalescence and increase spattered rainwater from canopies. Third, the influence of meteorological factors was different between canopy species; SO was readily influenced but CY was not. Moreover, results from this study implied that throughfall consisted of three drop components -free throughfall, drips, and splash droplets -and suggested a process for generating throughfall DSD that could explain the variations in throughfall DSDs between canopy species and that influenced by meteorological factors.

The paper describes a numerical method for the prediction of condensing steam¯ow within compressible boundary layers. The method is based on a simple stream function technique, which enables straightforward integration of the nucleation and droplet growth equations in a Lagrangian frame of reference. Calculations show how viscous dissipation and reduced expansion rate within a typical boundary layer in¯uence nucleation and growth, leading to droplet radii and size distributions that dier substantially from those predicted in inviscid¯ow. The impact of condensation on temperature and velocity pro®les, and the implications for thermodynamic loss are also considered. Ó

We report the results of a comparative study of microfluidic emulsification of liquids with different viscosities. Depending on the properties of the fluids and their rates of flow, emulsification occurred in the dripping and jetting regimes. We studied the characteristic features and typical dependence of the size and of the size distribution of droplets in each regime. For each liquid, we identified a range of hydrodynamic conditions promoting generation of highly monodisperse droplets. Viscosity played an important role in emulsification: highly viscous liquids were emulsified into larger droplets with lower polydispersity. Although it was not possible to provide a unified scaling for the volumes of the droplets, our results suggest that the break-up dynamics of the lower viscosity fluids resembles the rate-of-flow-controlled break-up, as reported earlier for the formation of bubbles in flow-focusing geometries [Garstecki P, Stone HA, Whitesides GM (2005) Phys Rev Lett 94:164501]. The results of this study can be helpful for a rationalized selection of liquids for the controlled formation of droplets with a predetermined size and with a narrow distribution of sizes.

It is of both fundamental and practical interest to study the flow physics in the manipulation of droplets. In this paper, we investigate complex flow in liquid droplets actuated by a linear gradient of wettability using dissipative particle dynamics simulation. The wetting property of the substrate ranging from hydrophilic to hydrophobic is achieved by adjusting the conservative solid-liquid interactions which results in a variation of solid-liquid surface tension. The internal three-dimensional velocity field with transverse flow in droplet is revealed and analyzed in detail. When the substrate is hydrophobic, it is found that there is slight deformation but strong flow circulation inside the droplet, and the droplet rolling is the dominant mechanism for the movement. However, large deformation of the droplet is generated after the droplet reaches the hydrophilic surface, and a mechanism combining rolling and sliding dominates the transportation of the droplet. Another interesting finding is that the thermal fluctuation can accelerate the spontaneous motion of a liquid droplet under a wetting gradient. The effects of the steepness of wetting gradient and the size of droplet on the translation speed are studied as well.

The three-dimensional, moving mesh interface tracking (MMIT) method coupled with local mesh adaptations by Quan and Schmidt [S.P. Quan, D.P. Schmidt, A moving mesh interface tracking method for 3D incompressible two-phase flows, J. Comput. Phys. 221 (2007) [761][762][763][764][765][766][767][768][769][770][771][772][773][774][775][776][777][778][779][780] demonstrated the capability to accurately simulate multiphase flows, to handle large deformation, and also to perform interface pinch-off for some specific cases. However, another challenge, i.e. how to handle interface merging (such as droplet coalescence) has not been addressed. In this paper, we present a mesh combination scheme for interface connection and a more general mesh separation algorithm for interface breakup. These two schemes are based on the conversion of liquid cells in one phase to another fluid by changing the fluid properties of the cells in the combination or separation region. After the conversion, the newly created interface is usually ragged, and a local projection method is employed to smooth the interface. Extra mesh adaptation criteria are introduced to handle colliding interfaces with almost zero curvatures as the distance between the interfaces diminishes. Simulations of droplet pair collisions including both head-on and off-center coalescences show that the mesh adaptations are capable of resolving very small length scales, and the mesh combination and mesh separation schemes can handle the topological transitions in multiphase flows. The potential of our method to perform detailed investigations of droplet coalescence and breakup is also displayed.

Applicability of two kinds of computational-fluid-dynamics method adopting Cahn-Hilliard (CH) and Allen-Cahn (AC)-type diffuse-interface advection equations based on a phase-field model (PFM) is examined to simulation of motions of microscopic incompressible two-phase fluid on solid surface. A capillarity-driven gas-liquid motion in rectangular channel is simulated by use of a PFM method for solving Navier-Stokes (NS) equations and a CH equation, whereas an immiscible liquid-liquid flow in a microchannel with T-junction and square cross section is simulated by use of another PFM method proposed in this study, which adopts a lattice-Boltzmann method based on fictitious particles kinematics as numerical scheme for solving NS equations and an AC equation that is modified to improve volume-of-fluid conservation. The major findings are as follows: (1) effect of capillary force on the dynamic two-phase fluid system with a high density ratio is well predicted for cross-sectional aspect ratio of the channel = 1 and 2; (2) mono-dispersed slug flow pattern transition is reproduced in good agreement with experimental observations in terms of variation in length and interval of droplets as increasing their volumetric flow rates at a constant flow rate ratio = 1. These results prove that the PFM methods can be used for analyzing two-phase fluid motions in various microfluidic devices and micro fabrication processes.

Drop size distributions (DSDs) were continuously and simultaneously measured by laser raindrop-sizing instruments (LD gauges) in an open site and in three forest stands consisting of Japanese cypress (CY: Chamaecyparis obtusa), Japanese cedar (CD: Cryptomeria japonica), and sawtooth oak (SO: Quercus acutissima), during three rainfall events in Tokyo, Japan. Drop data during the whole observation period were used in an hourly based data set and divided into three meteorological condition groups: calm, heavy rain, and strong wind. Evaluating the influence of canopy species and meteorological factors on the D 50 and DSD difference revealed some throughfall DSD characteristics. First, throughfall had different DSDs among canopy species under conditions of little canopy vibration with low rainfall intensity and wind speed; D 50 values were 2.00, 2.93, and 3.60 mm in CY, CD, and SO, respectively. Second, throughfall contained smaller drops under conditions of severe canopy vibration, with high rainfall intensity and/or high wind speed, than under calm meteorological conditions. Vibration of the canopy could reduce water coalescence and increase spattered rainwater from canopies. Third, the influence of meteorological factors was different between canopy species; SO was readily influenced but CY was not. Moreover, results from this study implied that throughfall consisted of three drop components -free throughfall, drips, and splash droplets -and suggested a process for generating throughfall DSD that could explain the variations in throughfall DSDs between canopy species and that influenced by meteorological factors.

This study is aimed at investigating the forces responsible for the detachment of molten metal droplets formed at an electrode tip, which imminently and eventually drop into the weld pool during the welding process. The Equations used by Kim and Eagar in 1993 were applied to this study. It was found that the different detaching forces which are the gravitational force, the electromagnetic force, and the drag force, were determined to be 7.154 x 10 -6 N, 0.05N and 1.736N respectively. Whereas, the primary retaining force, which is the surface tension force, was calculated to be 0.0195N. From the findings, since the combination of the detaching forces taken together is greater than the retaining force, detachment of the molten metal droplet must inevitably occur. The combined effect of these forces on the behaviour of molten metal during the droplet detachment process was adequately investigated in this study.

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 m in radius has not been fully explained. In this paper, the authors investigate the growth of cloud droplets by collisioncoalescence, 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 hydrodynamicalgravitational 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.

Hydrophobic surfaces with microscale roughness can be rendered ultrahydrophobic by the addition of sub-micron-scale roughness. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. Using this method the complexities generally associated with the fabrication of two-tier roughness structures are eliminated. Three two-tier roughness surfaces with different roughness parameters are fabricated and tested. The surfaces are characterized in terms of the static contact angle and roll-off angle, and are compared with surfaces consisting of only single-tier microscale roughness. The evaporation characteristics of a sessile droplet on the hierarchical surfaces is also assessed relative to comparable single-roughness (SR) surfaces. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces exhibit very high contact angle and lower contact angle hysteresis compared to the SR surfaces and are more resistant to wetting. The energy loss during impact on the surfaces is quantified in terms of the coefficient of restitution for droplets bouncing off the surface.

The rates of the reduction of Cr(VI) with S(IV) were measured in deaerated NaCl solution as a function of pH, temperature and ionic strength. The rates of the reaction were found to be first order with respect to Cr(VI) and second order with respect to S(IV), in agreement with previous results obtained at concentrations two order higher than the present study. The reaction also showed a first-order dependence of the rates on the concentration of the proton and a small influence of temperature with an apparent energy of activation DH app of 22.8 F 3.4 kJ/mol. The rates were independent of ionic strength from 0.01 to 1 M. The rate of Cr(VI) reduction is described by the general expression (M. Pettine).

A large eddy simulation (LES) coupled with a Lagrangian stochastic model has been applied to the study of droplet dispersion in a turbulent boundary layer. Droplets are tracked in a Lagrangian way. The velocity of the fluid particle along the droplet trajectory is considered to have a large-scale part and a small-scale part given by a modified three-dimensional Langevin model using the filtered subgrid scale (SGS) statistics. An appropriate Lagrangian correlation timescale is considered in order to include the influences of gravity and inertia. Two-way coupling is also taken into account. The interdroplet collision has been introduced as the main mechanism of secondary breakup. A stochastic model for breakup has been generalized for coalescence simulation, thereby two phenomena, coalescence and breakup are simulated in the framework of a single stochastic model. The parameters of this model, selectively for coalescence and for breakup, are computed dynamically by relating them to the local resolved properties of the dispersed phase compared to the main fluid. The model is validated by comparison with an agglomeration model and with experimental results on secondary breakup. The LES coupled with Lagrangian particle tracking and the model for droplet coalescence and breakup is applied to the study of the atmospheric dispersion of wet cooling tower plumes. The simulations are done for different droplet size distributions and volume fractions. We focused on the influence of these parameters on mean concentration, concentration variance and mass flux profiles.