Chemical reaction and mixing inside a coalesced droplet after a head-on collision

We investigated the phenomena of a chemical reaction inside a coalesced droplet after a direct (head-on) collision. A droplet containing an alkaline solution collided with a droplet containing a pH indicator on a surface with a wettability gradient. We used a high-speed camera to observe the color-changing reaction inside the coalesced droplet. Compared with a traditional dye-mixing test, the chemical reaction inside the coalesced droplet facilitated the mixing of two counter-reactive fluids and was more than 100 times as efficient as for unreactive fluids mixing inside the coalesced droplet. Instead of mere mixing, a chemical reaction inside a coalesced droplet is valuable for applications in a digital microfluidic open system. In droplet coalescence, the characteristics of the fluids and the ratio of volumes of two droplets caused a varied profile of the droplet coalescence, especially the neck curvature that affects the shape of the material interface between the two droplets at an initial phase. We observed the evolution of the chemical reaction with a varying radius of neck curvature inside the coalesced droplet. For the case of a small radius of neck curvature, the small interfacial area between two reactive fluids accumulated an intense heat of reaction and induced a rapid growth of the fingers. For the case of a large radius of neck curvature, the growth of fingers was slight and the interface was uniform across the large interfacial area. Our work illustrates a correlation between the rate of chemical reaction and the profile of a coalesced droplet, which is a significant reference in droplet-based microfluidic systems for biochemical applications.     

Flow-induced particle migration in microchannels for improved microfiltration processes

Microchannels can be used to induce migration phenomena of micron sized particles in a fluid. Separation processes, like microfiltration, could benefit from particle migration phenomena. Currently, microfiltration is designed around maximum flux, resulting in accumulation of particles in and on the membrane. In this paper it is shown that starting the design at the particle level will result in a new microfiltration process. The behaviour of suspensions between 9 and 38 volume% was studied by confocal scanning laser microscopy; migration as a result of shear-induced diffusion was observed in a rectangular microchannel with nonporous walls. Particles segregated on size within the first 10 cm of the channel. To illustrate this, at 20 volume% of small (1.53 μm) and large (2.65 μm) particles each, the larger particles migrated to the middle of the channel, while the small particles had high concentrations near the walls. The small particles could then be collected from their position close to the permeable walls, e.g. membranes, where the pore size of the membrane is no longer the determining factor for separation. Guidelines for using this phenomenon in a microfiltration process were derived and the selectivity of the process was experimentally evaluated. The small droplets could be removed from the mixtures with a membrane having pores 3.7 times larger than the droplets, thereby minimizing accumulation of droplets in and on the membrane. As long as the process conditions are chosen appropriately, no droplet deposition takes place and high fluxes (1.7 × 10³ L h^?1 m^?² bar^?1) can be maintained.     

Fluid dynamic behavior of dispensing small droplets through a thin liquid film

This paper presents a technology for dispensing droplets through thin liquid layers. The system consists of a free liquid film, which is suspended in a frame and positioned in front of a piezoelectric printhead. A droplet, generated by the printhead, merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. The technology allows the dispensing, mixing and ejecting of picolitre liquid samples in a single step. This paper overviews the concept, potential applications, experiments, results and a numerical model. The experimental work includes studying the flight of ink droplets, which ejected from an inkjet print head, fly through a free ink film, suspended in a frame and positioned in front of the printhead. We experimentally observed that the minimum velocity required for the 80 pl droplets to fly through the 75 ± 24 μm thick ink film was of 6.6 m s^?1. We also present a numerical simulation of the passage of liquid droplets through a liquid film. The numerical results for different initial speeds of droplets and their shapes are taken into account. We observed that during the droplet–film interaction, the surface energy is partially converted to kinetic energy, and this, together with the impact time, helps the droplets penetrate the film. The model includes the Navier–Stokes equations with continuum-surface-tension force derived from the phase-field/Cahn–Hilliard equation. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after their passage through the thin liquid film well matches the measured velocity from the experiment.     

Numerical prediction of three-dimensional time-dependent viscoelastic extrudate swell using differential and algebraic models

The dynamic behavior of a droplet on a solid surface is simulated by the lattice Boltzmann method (LBM) for two-phase fluids with large density differences; the wetting boundary condition on solid walls is incorporated in this simulation. By using the method, the dynamic behavior of a droplet impinging on a horizontal wall is investigated in terms of various Weber numbers. The dynamic contact angle, the contact line velocity, and the wet length are calculated, and found to be in good agreement with available experimental data. In addition, the method is applied to simulations of the collision of a falling droplet with a stationary droplet on a solid surface. The behavior of the droplets and the mixing process during their collision are simulated in terms of various impact velocities and several static contact angles on the solid surface. It is seen that mixing occurs around the rim of the coalescent droplet due to the circular flows. Also, the relationship between the mixing rate of the primary coalescent droplet and Weber number is investigated.     

Variations of surface boundary layer parameters associated with Cyclone Gonu over the Arabian Sea using QuikSCAT data

This study attempted to quantify the variations of the surface marine atmospheric boundary layer (MABL) parameters associated with the tropical Cyclone Gonu formed over the Arabian Sea during 30 May–7 June 2007 (just after the monsoon onset). These characteristics were evaluated in terms of surface wind, drag coefficient, wind stress, horizontal divergence, and frictional velocity using 0.5° × 0.5° resolution Quick Scatterometer (QuikSCAT) wind products. The variation of these different surface boundary layer parameters was studied for three defined cyclone life stages: prior to the formation, during, and after the cyclone passage. Drastic variations of the MABL parameters during the passage of the cyclone were observed. The wind strength increased from 12 to 22 m s^?1 in association with different stages of Gonu. Frictional velocity increased from a value of 0.1–0.6 m s^?1 during the formative stage of the system to a high value of 0.3–1.4 m s^?1 during the mature stage. Drag coefficient varied from 1.5 × 10^?3 to 2.5 × 10^?3 during the occurrence of Gonu. Wind stress values varied from 0.4 to 1.1 N m^?2. Wind stress curl values varied from 10 × 10^?7 to 45 × 10^?7 N m^?3. Generally, convergent winds prevailed with the numerical value of divergence varying from 0 to –4 × 10^?5 s^?1. Maximum variations of the wind parameters were found in the wall cloud region of the cyclone. The parameters returned to normally observed values in 1–3 days after the cyclone passage.     

Study of flow behaviors of droplet merging and splitting in microchannels using Micro-PIV measurement

Droplet merging and splitting are important droplet manipulations in droplet-based microfluidics. However, the fundamental flow behaviors of droplets were not systematically studied. Hence, we designed two different microstructures to achieve droplet merging and splitting respectively, and quantitatively compared different flow dynamics in different microstructures for droplet merging and splitting via micro-particle image velocimetry (micro-PIV) experiments. Some flow phenomena of droplets different from previous studies were observed during merging and splitting using a high-speed microscope. It was also found the obtained instantaneous velocity vector fields of droplets have significant influence on the droplets merging and splitting. For droplet merging, the probability of droplets coalescence (η) in a microgroove is higher (50% < η < 92%) than that in a T-junction microchannel (15% < η < 50%), and the highest coalescence efficiency (η = 92%) comes at the two-phase flow ratio e of 0.42 in the microgroove. Moreover, compared with a cylinder obstacle, Y-junction bifurcation can split droplets more effectively and the droplet flow during splitting is steadier. The results can provide better understanding of droplet behaviors and are useful for the design and applications of droplet-based microfluidics.     

Quantitative analysis of microfluidic mixing using microscale schlieren technique

In this study, we discuss the employment of microscale schlieren technique to facilitate measurement of inhomogeneities in a micromixer. By mixing dilute aqueous ethanol and water in a T-microchannel, calibration procedures are carried out to obtain the relation between the concentration gradients and grayscale readouts under various incident illuminations, concentrations of aqueous ethanol solution, and knife-edge cutoffs. We find that to broaden measuring range with minimal error, the luminous exitance should be tuned to have a reference background with an average grayscale readout of 121, and dilute aqueous ethanol solution with a mass fraction of 0.05 should be used along a 50 % cutoff. For concentration gradients greater than 6.8 × 10^?3 or below ?2.5 × 10^?2 μm^?1, the calibration curves show great linearity. Correspondingly, the discernable limit of our microscale schlieren system is 2.3 × 10^?5 μm^?1 for a positive refractive index gradient and ?8.6 × 10^?5 μm^?1 for a negative refractive index gradient. Once the relation between concentration gradients and grayscale readouts is known, the concentration distribution in a microfluidic can be reconstructed by integrating its microscale schlieren image with appropriate boundary conditions. The results prove that the microscale schlieren technique is able to provide spatially resolved, noninvasive, full-field measurements. Since the microscale schlieren technique is directly linked to the measurement of a refractive index gradient, the present method can be easily extended to other scalar quantifications that are related to the variation of refractive index.     

Particulate organic carbon in the surface waters of the North Atlantic: spatial and temporal variability based on satellite ocean colour

We have examined the 16-year time series of particulate organic carbon (POC) concentration in the surface waters of the North Atlantic derived from SeaWiFS and MODIS-Aqua data. The annual mean POC concentrations are the highest in the northern North Atlantic, reaching 120 mg m^?3. Moving south, the mean annual POC concentrations decrease to minimum values of about 30 mg m^?3 at around 30° N and increase in the equatorial region to about 70 mg m^?3. The seasonal amplitude of POC concentration in the northern North Atlantic region is larger when compared to other regions. The annual mean surface POC concentrations in the entire North Atlantic basin show a statistically significant trend with an average decrease of 0.79 mg m^?3 year^?1. Regionally averaged 16-year mean POC biomass integrated over the optical depth, euphotic depth, and mixed-layer depth is estimated at about 1.27, 4.34, and 4.59 g m^?2, respectively. Even larger biomass of 6.26 g m^?2 is estimated if one chooses to use in the calculations the greatest from the daily values of the estimates listed above at each pixel of the satellite data. Comparisons of POC biomass with primary productivity allowed us to assess temporal and spatial patterns of POC losses.     

Bare surface soil moisture retrieval from the synergistic use of optical and thermal infrared data

Land surface soil moisture (SSM) is crucial to research and applications in hydrology, ecology, and meteorology. To develop a SSM retrieval model for bare soil, an elliptical relationship between diurnal cycles of land surface temperature (LST) and net surface shortwave radiation (NSSR) is described and further verified using data that were simulated with the Common Land Model (CoLM) simulation. In addition, with a stepwise linear regression, a multi-linear model is developed to retrieve daily average SSM in terms of the ellipse parameters x₀ (horizontal coordinate of the ellipse centre), y₀ (vertical coordinate of the ellipse centre), a (semi-major axis), and θ (rotation angle), which were acquired from the elliptical relationship. The retrieval model for daily average SSM proved to be independent of soil type for a given atmospheric condition. Compared with the simulated daily average SSM, the proposed model was found to be of higher accuracy. For eight cloud-free days, the root mean square error (RMSE) ranged from 0.003 to 0.031 m³ m^?3, while the coefficient of determination (R²) ranged from 0.852 to 0.999. Finally, comparison and validation were conducted using simulated and measured data, respectively. The results indicated that the proposed model showed better accuracy than a recently reported model using simulated data. A simple calibration decreased RMSE from 0.088 m³ m^?3 to 0.051 m³ m^?3 at Bondville Companion site, and from 0.126 m³ m^?3 to 0.071 m³ m^?3 at the Bondville site. Coefficients of determination R² = 0.548 and 0.445 were achieved between the estimated daily average SSM and the measured values at the two sites, respectively. This paper suggests a promising avenue for retrieving regional SSM using LST and NSSR derived from geostationary satellites in future developments.     

Electrowetting droplet microfluidics on a single planar surface

Electrowetting refers to an electrostatically induced reduction in the contact angle of an electrically conductive liquid droplet on a surface. Most designs ground the droplet by either sandwiching the droplet with a grounding plate on top or by inserting a wire into the droplet. Washizu and others have developed systems capable of generating droplet motion without a top plate while allowing the droplet potential to float. In contrast to these designs, we demonstrate an electrowetting system in which the droplet can be electrically grounded from below using thin conductive lines on top of the dielectric layer. This alternative method of electrically grounding the droplet, which we refer to as grounding-from-below, enables more robust droplet translation without requiring a top plate or wire. We present a concise electrical-energy analysis that accurately describes the distinction between grounded and non-grounded designs, the improvements in droplet motion, and the simplified control strategy associated with grounding-from-below designs. Electrowetting on a single planar surface offers flexibility for interfacing to liquid-handling instruments, utilizing droplet inertial dynamics to achieve enhanced mixing of two droplets upon coalescence, and increasing droplet translation speeds. In this paper, we present experimental results and a number of design issues associated with the grounding-from-below approach.     

Contact dynamics of nanodroplets in carbon nanotubes: effects of electric field,tube radius,and salt ions

We report contact dynamics of nanodroplets in carbon nanotubes using molecular dynamics simulations. The effects of electric field, nanotube radius, and salt ions included in the nanodroplets are explored in more detail. For the cases without applied electric field, the droplet fills the cross section of carbon nanotubes with small radius completely. When the tube radius becomes larger, the droplet retracts towards the surface of the nanotube to minimize the surface tension of the droplet and shows wider extension along the axial direction. When an electric field perpendicular to the axial direction of the carbon nanotubes is applied, the position and shape of the droplets are changed which is also related to the tube radius and whether the droplet contains salt ions. Unlike a planar surface, the nanotube limits spreading of the droplets along the radial direction. The variation of the center of mass of the droplets indicates a significant confinement to the position of the droplets in the electric field. For the salty water droplets, a strong electric field induces ejection of small water clusters from the droplet in a nanotube with large radius. As a consequence, the droplet and water clusters are separated and moved to two opposite sides of the nanotube by the electric field.     

Nebulization of water/glycerol droplets generated by ZnO/Si surface acoustic wave devices

Efficient nebulization of liquid sessile droplets (water and water/glycerol mixtures) was investigated using standing waves generated using ZnO/Si surface acoustic wave (SAW) devices under different RF powers, frequencies and liquid viscosity (varied glycol concentrations in water). At such high RF powers, there are strong competitions between vertical jetting and nebulization. At lower SAW frequencies of 12.3 and 23.37 MHz, significant capillary waves and large satellite droplets were generated before nebulization could be observed. At frequencies between 23.37 and 37.2 MHz, spreading, displacement or occasionally jetting of the parent sessile droplet was frequently observed before a significant nebulization occurred. When the SAW frequencies were increased from 44.44 to 63.3 MHz, the minimum RF power to initiate droplet nebulization was found to increase significantly, and jetting of the parent droplet before nebulization became significant, although the average size of the nebulized particles and ejected satellite droplets appeared to decrease with an increase in frequency. With the increase of glycerol concentration in the test sessile droplets (or increase in liquid viscosity), nebulization became difficult due to the increased SAW damping rate inside the liquid. Acoustic heating effects were characterized to be insignificant and did not show apparent contributions to the nebulization process due to silicon substrate’s natural effect as an effective heat sink and the employment of a metallic holder beneath the ZnO/Si SAW device substrates.     

A numerical investigation of central binary collision of droplets

The paper presents a numerical investigation of the central collision of two equal-sized droplets in a gaseous phase. The investigation is based on the numerical solution of the Navier-Stokes equations in their axi-symmetric form using the finite volume technique. The Volume of Fluid Method (V.O.F) is employed for tracking the liquid-gas interface. An adaptive local grid refinement technique developed recently is used in order to increase the resolution around the interface. By using two V.O.F indicator functions the identity of each droplet is preserved and can be detected after droplet contact until coalescence. The results are compared with available experimental data and provide a very detailed picture of the collision process, the ligament formation and dimensions, the pinch off mechanism and the creation of the satellite droplet. The conversion of the droplet’s kinetic energy to the surface energy and vise versa, the energy viscous dissipation as well as the maximum deformation of the droplets are also evaluated.     

A superposable double-gradient droplet array chip for preparation of PEGDA microspheres containing concentration-gradient drugs

This article describes a superposable double-concentration-gradient droplet array chip, which allows a variety of concentration combinations of two components to be formed simultaneously. The concentration gradients generated from two layers of the chip could be arbitrarily superimposed by adjusting the center angle between the two bonding layers. With the aqueous phase flow rate of 1.0 μL min^?1 and the oil phase flow rate of 30.0 μL min^?1, the droplets about 58 μm in diameter were produced, and the coefficients of variation were below 6.0% for single channel and 5.7% for all the channels. Using a dual-32-channel superposable gradient droplet array chip, poly(ethylene glycol) diacrylate (PEGDA) microspheres containing concentration-gradient combinations of rhodamine B and fluorescein were fabricated to demonstrate the capability of PEGDA for encapsulating hydrophilic and hydrophobic substances, as well as the proper concentration-gradient distribution. Furthermore, PEGDA microspheres loaded with two anticancer drugs, hydrophilic doxorubicin hydrochloride and hydrophobic paclitaxel, of 17 concentration combinations were simultaneously prepared. The drug-induced apoptosis of human uterine cervix cancer cells was investigated using the dual-drug-loaded PEGDA microspheres. The optimum synergistic concentration combination of the two drugs was 12.5 μg mL^?1 for doxorubicin hydrochloride and 43.75 μg mL^?1 for paclitaxel according to the preliminary screening. The superposable double-gradient droplet array generator was demonstrated to be a promising platform for screening multiple drug combination in microcarriers.     

Optical measurement of flow field and concentration field inside a moving nanoliter droplet

Droplets-based method has been employed to enhance mixing in microfluidic systems. This paper presents experimental studies of the recirculating flow field inside a moving droplet and the characterization of the mixing of two aqueous droplets. In the first part, the velocity field inside the moving water droplet was measured using the micro-particle image velocimetry (micro-PIV) technique. The PIV measurements showed that recirculation flow exists inside the droplet. However, the findings suggested that the outer layer of droplets move at a faster velocity than the central part. The result is different from what is reported by other researchers. In the second part, two water droplets, a de-ionized (DI) water droplet and another DI water droplet with fluorescent dye, were brought together by the carrier fluid to form a bigger droplet. The mixing between the two aqueous droplets was characterized by the fluorescent dye concentration distribution.     

Mapping growing stock volume and biomass carbon storage of larch plantations in Northeast China with L-band ALOS PALSAR backscatter mosaics

ABSTRACT

Reliable spatial information on growing stock volume (GSV) and biomass is critical for creating management strategies for plantation forests. This study developed empirical models to map the GSV and biomass of larch plantations (LPs) in Northeast China (1.25 million km² total area) by integrating L-band synthetic aperture radar (SAR) data with ground-based survey data. The best correlation model was used to map the GSVs and biomasses of LPs. The total GSV and biomass carbon storage were estimated at 224.3 ± 59.0 million m³ and 113.0 ± 29.7 × 10¹² g C with average densities of 85.1 m³ ha^?1 and 42.9 10⁶ g × C ha^?1, respectively, over a total area of 2.64 million ha. The saturation effect of SAR was determined beyond 260 m³ ha^?1, which was expected to influence the estimations for a small proportion of the study area. The accuracy of the estimations has limitations mainly due to the uncertainties in the GSV inventories, discrimination of natural larch and the SAR dataset. Based on the mapping results of the GSVs of LPs, a planning strategy for multipurpose management was tentatively proposed. This study can inform policies and management practices to assure broader and sustainable benefits from plantation forests in the future.     

High throughput micro droplet generator array controlled by two-dimensional dynamic virtual walls

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. The nozzle density of this compact droplet generator can be five to ten times higher than that of commercial inkjet printheads in one-dimensional formats. The volume and initial speed of the generated droplets was 3.6–5.7 pL and 14–15 m/s, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise meniscus control. The analyzed data shows the meniscus undergoes a “push–pull–push” progress which effectively cuts the liquid column short. The refilling time of the innovative micro droplet generator was estimated to be 0.296 μs from the simplified chamber model, and it was one-tenth of the commercial printheads. In addition, the frequency response was estimated to be higher than 20 kHz by observing the meniscus fluctuation condition. Finally, a 3 × 5 heater array was used to generate two droplets simultaneously, which shows that the crosstalk problem can be eliminated by precise time-delayed control. An interlacing operation was also proposed to address the large array control algorithm. To summarize, a 330-dpi monolithic micro droplet generator prototype has been proposed for high speed and large 2D format printing.     

Upper ocean response to Super Typhoon Tembin (2012) explored using multiplatform satellites and Argo float observations

Using multiplatform satellites and in situ Argo float observations, this study systematically examined the upper ocean response to Super Typhoon Tembin (2012) in the western north pacific, and the interaction between typhoon and a pre-existing cold core eddy (CCE) was particularly focused on. Significant sea surface temperature (SST) cooling and sea surface height anomaly (SSHA) decrease was detected along track after typhoon, with the maximum SST cooling and SSHA decrease reaching 4.0°C and 25 cm, respectively. The pre-existing CCE was located to the left of the typhoon track, resulting in an intriguing leftward bias of SST cooling. The maximum SST cooling appeared at about 25 km to the left of the typhoon track, with SST cooling to the left of the track 40–100% larger than that to the right. After typhoon, the CCE was expanded by 50% due to the typhoon’s cyclonic wind stress. The thermocline was uplifted by 15–25 m by the typhoon-induced upwelling. Typhoon-enhanced vertical mixing was inferred from high-resolution Argo float data based on the Gregg–Henyey–Polzin parameterization method. The diapycnal diffusivity reached 9 × 10^?4 m² s^?1 after typhoon, which was more than 10 times larger than that before typhoon.     

The effect of monomolecular surface films on the microwave brightness temperature of the sea surface

Abstract

Airborne microwave radiometer measurements at 1·43 and 2·65 GHz over a sea surface covered with a monomolecular oleyl alcohol surface film and over adjacent slick sea surfaces are presented. The measurements show that at 2·65 GHz the brightness temperature T _B is not affected by the slick, while at 1·43 GHz it drops from 93 K to a minimum value of almost O K. This implies that at 1·43 GHz the emissivity of the slick-covered sea surface is extremely small, similar to a metallic layer, and that this resonant-type phenomenon is confined to a narrow frequency band of width δ?/ ?<0·6.

The theoretical implications of these experimental findings are discussed in the framework of the Debye relaxation theory of polar liquids. It is conjectured that a thin layer of water molecules polarized by the surface film gives rise to an anomalous dispersion, which causes the large decrease in brightness temperature at 1·43 GHz.

The modulus of the relative dielectric constant ε? is estimated to be ≥ 5·2 × 10^?4 and the thickness of the emitting layer ≤1·9 × 10^?4 m for 1·43 GHz. Furthermore, the film-induced surface activation energy is calculated to be 9·18 × 10^?21 J. These values seem reasonable in the light of the theories on the physicochemical structure of surface layers.     

Impact of thermal contact resistances on micro-gap heat losses for microthermionic power generators

Thermionic power generation is a safe and clean energy source that allows for converting heat into electrical energy using thermionic electrons. The miniaturization is an advantage of this technology that led to the recent development of micro-gap thermionic power generators. In this work, thermal contact resistances between the micro-gap insulators and the emitter as well as between the micro-gap insulators and the collector are measured. A thermal resistance of 48.6 K/W is obtained by downsizing the insulators until 60 × 45 μm² of contact area with the emitter, demonstrating a high impact for decreasing the micro-gap conduction heat loss density from the emitter to the collector from 28 W/cm² (theoretical value obtained without considering contact resistances) to 5.6 W/cm². Downsizing the contact area between the insulators and the emitter from 320 × 300 to 60 × 45 μm² leads to an increase of the power conversion efficiency from 9.1 × 10^?5 until 1.5 × 10^?3.