Monodisperse Water Droplets Research Articles

Generating nano/microliter droplets with on-demand repetition rates by femtosecond filament-fabricated superhydrophobic bevel needle nozzles

Nano- and micro-droplets provide a platform for microanalysis of a variety of biochemical reactions with less consumption of reagents and large-extent elimination of cross-contamination, but stable generation of such monodisperse droplets with an on-demand size and repetition rate should be first established. Here, we present a simple and robust approach to generate nano/microliter (nL/μL) droplets with predetermined size and repetition rate by using femtosecond laser filament-fabricated stainless bevel needle nozzles (BNNs). We demonstrate that hierarchically heterogeneous micro/nanostructures can be directly formed on both the inner and outer surfaces of BNNs by femtosecond laser filament processing, and that after a heat treatment the fabricated BNNs show an excellent superhydrophobic property with the water contact angle of ∼160° and the rolling-off angle of ∼0.4°. With the superhydrophobic BNNs (19–27 gauge), we produce monodisperse water droplets (2.5 μL–87 nL) with nearly the same sizes as the needle diameters, and dispense them repeatedly with on-demand repetition rates from sub-1 Hz to more than 100 Hz over several orders of the flow rate. The results provide a new way for generating microdroplets on demand for various droplet-based applications.

Connecting finite-time Lyapunov exponents with supersaturation and droplet dynamics in a turbulent bulk flow.

The impact of turbulent mixing on an ensemble of initially monodisperse water droplets is studied in a turbulent bulk that serves as a simplified setup for the interior of a turbulent ice-free cloud. A mixing model was implemented that summarizes the balance equationsof water vapor mixing ratio and temperature to an effective advection-diffusion equationfor the supersaturation field s(x,t). Our three-dimensional direct numerical simulations connect the velocity and scalar supersaturation fields in the Eulerian frame of reference to an ensemble of cloud droplets in the Lagrangian frame of reference. The droplets are modeled as point particles with and without effects due to inertia. The droplet radius is subject to growth by vapor diffusion. We report the dependence of the droplet size distribution on the box size, initial droplet radius, and the strength of the updraft, with and without gravitational settling. In addition, the three finite-time Lyapunov exponents λ_{1}≥λ_{2}≥λ_{3} are monitored which probe the local stretching properties along the particle tracks. In this way, we can relate regions of higher compressive strain to those of high local supersaturation amplitudes. For the present parameter range, the mixing process in terms of the droplet evaporation is always homogeneous, while it is inhomogeneous with respect to the relaxation of the supersaturation field. The probability density function of the third finite-time Lyapunov exponent, λ_{3}<0, is related to the one of the supersaturation s by a simple one-dimensional aggregation model. The probability density function (PDF) of λ_{3} and the droplet radius r are found to be Gaussian, while the PDF of the supersaturation field shows sub-Gaussian tails.

Dual-wavelength extinction rainbow refractometry for in-situ characterization of colloidal droplets

Considerable interest and demand exists in multi-parameter characterization of colloidal droplets. However, an optical in-situ characterization of such droplets is a particular challenge due to its heterogeneous and multi-scale nature, motivating the investigation of a dual-wavelength extinction rainbow refractometry. An analytical model is proposed, in which dual-wavelength extinction effect of the inclusions on the amplitude of the primary rainbow peak allows measurement of inclusion concentration and particle size. The rainbow pattern itself yields the refractive index and diameter of the host droplet. Validation experiments using deionized monodisperse water droplets containing standard polystyrene nanoparticles with different concentrations (0%, 0.1%, 0.2%, 0.3%) and particle sizes (210 nm, 530 nm) were conducted. Measured refractive indices and droplet diameters agree well with the known values. Measurement errors of inclusion concentration and size were analyzed with and without a prior knowledge of the inclusion size. Measurement limits of this approach are also discussed.

Reaction front development from ignition spots in n-heptane/air mixtures: Low-temperature chemistry effects induced by ultrafine water droplet evaporation

Effects of low-temperature chemistry induced by ultrafine water droplet evaporation on reaction front development from an ignition spot with temperature gradient are studied in this work. The Eulerian–Eulerian method is used to simulate the gas–liquid two-phase reactive flows, and the physical model is one-dimensional spherical reactor with stoichiometric gaseous n-heptane/air mixture and ultrafine monodisperse water droplets (initial diameter 5 μm). Homogeneous ignitions of two-phase mixtures are first simulated. The water droplets can completely evaporate in the reactor prior to ignition, and hence pronouncedly reduce gas temperature, which may induce the low-chemistry reactions. It is found that the turnover temperature for negative temperature coefficient range increases with droplet volume fraction. Three-stage ignitions are present when the volume fraction is beyond a critical value, that is, low-temperature, intermediate-temperature, and high-temperature ignitions. The chemical explosive mode analysis also confirms the low-chemistry reactions induced by the evaporation of ultrafine water droplets. Then, reaction front development from an ignition spot with temperature gradient in two-phase mixtures is analyzed based on one-dimensional simulations. Different modes for reaction front origin in the spot are identified, based on the initial gas temperature and lower turnover temperature. Specifically, the reaction front can be initiated at the left and right ends of the ignition spot, and inside it. Detailed reaction front developments corresponding to the above three modes are discussed. In addition, the pressure wave from high-temperature ignition is important, compared to those from low and intermediate chemistries. The reaction front propagation speed and thermal states of fluid particles corresponding to different reaction front initiation modes are analyzed. Moreover, autoignition modes are summarized in the diagrams of normalized temperature gradient vs normal acoustic time and droplet volume fraction. The detonation limits of two-phase mixtures highly depend on the droplet volume fraction and are not regularly peninsular-shaped, like those for purely gaseous mixtures.

Numerical analysis on behavior of dilute water droplets in detonation

Two-dimensional numerical simulations are conducted based on the Eulerian-Lagrangian method to model a gaseous detonation laden with monodispersed water droplets. The premixed mixture is a slightly diluted stoichiometric hydrogen oxygen mixture at low pressure. The outcome of the interactions of the droplet breakup with the cellular instabilities and the non-uniform flow behind the leading front is analyzed. The simulation results are also analyzed using instantaneous flow fields and Favre average profiles for water droplets. Breakup occurs mainly near the detonation front. The mean final diameter of the water droplets at the end of the breakup process is the same regardless of the initial strength of the leading shock or whether it is lower or greater than the Chapman-Jouguet value. The polydispersity comes from local phenomena behind the leading shock, such as forward jets coming from triple point collisions, transverse waves and a combination of both. The total breakup time is longer than that estimated from post-shock conditions and the present finding is in line with the previous experimental results on spray detonation.

Mammatus cloud formation by settling and evaporation

Abstract

Effects of aerosol modulation transfer function on target identification

Atmospheric aerosol effects are often overlooked in target acquisition studies. Typically, performance models only consider extinction and turbulence within the prediction processes. The aerosol modulation transfer function (MTF) is included in range acquisition algorithms to determine how scattering and absorption effects change the target identification predictions. We modeled the aerosols as monodisperse water droplets comparable to a tenuous fog or mist. Integrating the aerosol MTF into the system MTF gives the opportunity to utilize the night vision integrated performance model to predict the target identification range with aerosol contributions. The aerosol MTF is a function of range, water droplet composition, wavelength, and aperture size. The analysis focuses on these variables with an emphasis on wavelength dependence to characterize mid-wave and long-wave performance. Results show that the mid-wave systems have a substantial diffraction advantage over long-wave systems. Only in the limit of increasing optical depths do the mid-wave and long-wave performance models begin to converge, verifying that the aerosols can be the limiting factor for target identification.

Mask-Free Laser Lithography for Rapid and Low-Cost Microfluidic Device Fabrication.

Microfluidics has become recognized as a powerful platform technology associated with a constantly increasing array of applications across the life sciences. This surge of interest over recent years has led to an increased demand for microfluidic chips, resulting in more time being spent in the cleanroom fabricating devices using soft lithography-a slow and expensive process that requires extensive materials, training and significant engineering resources. This bottleneck limits platform complexity as a byproduct of lengthy delays between device iterations and affects the time spent developing the final application. To address this problem, we report a new, rapid, and economical approach to microfluidic device fabrication using dry resist films to laminate laser cut sheets of acrylic. We term our method laser lithography and show that our technique can be used to engineer 200 μm width channels for assembling droplet generators capable of generating monodisperse water droplets in oil and micromixers designed to sustain chemical reactions. Our devices offer high transparency, negligible device to device variation, and low X-ray background scattering, demonstrating their suitability for real-time X-ray-based characterization applications. Our approach also requires minimal materials and apparatus, is cleanroom free, and at a cost of around $1.00 per chip could significantly democratize device fabrication, thereby increasing the interdisciplinary accessibility of microfluidics.

Dynamics of single droplet impact on cylindrically-curved superheated surfaces

Liquid droplet impact on superheated symmetric surfaces such as flat and spherical geometries has been extensively investigated in the past. However, the effect of a superheated asymmetric curvature on the collision dynamics remains unknown. In this study, experiments are performed to elucidate the thermo-hydrodynamic behavior of milli-metric water droplets impacting on superheated cylindrical surfaces having convex and concave profiles. These geometries offer an asymmetric zone for droplets to expand and retract. Mono-dispersed water droplets are impinged on convex and concave surfaces maintained in the temperature range of 125–290 °C. The impact Weber number is varied between 5 and 65 and droplet evolution is visualized using the high-speed imaging technique. It is observed that, droplet morphology and spreading characteristics are entirely different for convex and concave surfaces compared to a flat substrate due to momentum anisotropy. The maximum spread factor and residence time of the water droplet are significantly influenced by the impact Weber number while the surface superheat has negligible effect. Importantly, the asymmetric distribution of momentum facilitated 16% and 24% reduction in contact time for convex and concave surfaces respectively. The preferential extension of fluid in the azimuthal direction assisted by the gravity causes greater spreading of droplets on a convex surface. In contrary, these conditions enforced a reduction in contact time for droplets on a concave topography.

An optical backscatter probe for time resolved droplet measurements in turbomachines

The presence of particles in the flow path of turbomachines can result in undesirable engine operation. In order to improve the efficiency of turbomachines and guarantee their safe operation, the flow mechanisms that govern the particles’ need to be studied and associated with the main aerodynamic flow field. This paper describes a newly developed optical backscatter probe for droplet diameter and speed measurements in turbomachines. The miniature probe has a tip diameter of 5 mm and is capable of resolving droplets from 40 to 110 μm in diameter that travel up to 200 m s−1. The calibration of the novel probe is performed with a droplet generator capable of producing monodispersed water droplets. In addition, the probe is calibrated for droplet speed measurements in the same calibration facility. The paper conducts a detailed uncertainty analysis and describes the post processing code. In the final part of this paper the probe is used in an axial turbine with an installed spray generator to perform droplet measurements under two different operating conditions. Measurements have shown that the part load condition results in larger droplet diameters and higher relative droplet speeds. As a consequence higher erosion rates at the rotor leading edge suction side will occur when operating at part load condition.

Investigation of monodisperse droplet generation in liquids by inkjet

The principle of the generation of liquid droplets in liquids by inkjet was discussed in this paper. As a liquid/liquid droplet generator, inkjet was used to produce monodisperse water droplets in oil (W/O) and oil droplets in water (O/W). Strategies for regulating droplet generation in various liquids were experimentally explored. The diameter of the generated monodisperse droplet also could be accurately tuned by varying the driving voltage and pulse width exerted on the miniature piezo ceramic slide of inkjet device. Experimentally, limitations of generation monodisperse droplets were discussed for both W/O and O/W. The work could be a guidance to generate monodisperse droplets in liquids by inkjet aiming at wide applications of simple droplets generation in liquids.

Secondary breakup of a drop at moderate Weber numbers

We present volume of fluid based numerical simulations of secondary breakup of a drop with high density ratio (approx. 1000) and also perform experiments by injecting monodisperse water droplets in a continuous jet of air and capture the breakup regimes, namely, bag formation, bag-stamen, multibag and shear breakup, observed in the moderate Weber number range (20–120). We observe an interesting transition regime between bag and shear breakup for We =80, in both simulations as well as experiments, where the formation of multiple lobes, is observed, instead of a single bag, which are connected to each other via thicker rim-like threads that hold them. We show that the transition from bag to shear breakup occurs owing to the rim dynamics which shows retraction under capillary forces at We =80, whereas the rim is sheared away with flow at We =120 thus resulting in a backward facing bag. The drop characteristics and timescales obtained in simulations are in good agreement with experiments. The drop size distribution after the breakup shows bimodal nature for the single-bag breakup mode and a unimodal nature following lognormal distribution for higher Weber numbers.

High velocity impingement of single droplets on a dry smooth surface

The vertical impact of single, mono disperse water droplets on a dry smooth surface was studied experimentally by means of shadowgraphy. A glass substrate was mounted on a rotating wheel to obtain high impact velocities. The droplets were generated on demand. While the Ohnesorge number was kept constant, Weber number and Reynolds number were varied by adjusting the impact velocity. In all performed experiments, splashing was observed. The distinction of the different measurement series was done by the use of the Weber number. The different Weber numbers were, 3,500, 5,000 and 10,000. Phase-locked images were taken and the temporal evolution of the impact was reconstructed by means of the nondimensional impingement time. The outcome of the measurement was analysed by digital image processing to quantify the distribution of the diameter of the resulting secondary droplets in size and time as well as their velocity, and the total deposited mass fraction remaining on the surface after the impingement. In all cases, the greater part of the impinging primary droplet remained on the substrate.

Prediction of droplet evaporation characteristics of nebuliser based humidification and drug delivery devices

This study reports on a model for the evaporation of mono-disperse water droplets into air that can be used to aid the design of nebuliser based humidification systems for respiratory treatments and also for predicting droplet size distributions for pulmonary drug delivery devices under either isothermal or adiabatic conditions. The governing equations are developed from the coupled mass and energy balances for the droplets and for the surrounding air. An adjustment to one of these equations allows for the modelling of either isothermal or adiabatic conditions. Additionally, the proposed model allows for the variation of fluid transport and thermodynamic properties during the solution of the governing equations. The model is compared to and validated against an overall mass and energy balance and the literature. Confirmation of the general evaporation characteristics of water droplets evaporating into air is gained from the model results and, its accuracy, usability and applicability are improved over existing models. The proposed model indicates that the droplet evaporation process in humidification devices is largely unaffected by the initial water temperature and also indicates advantages of adiabatic conditions when using cascade impactor technology for droplet sizing.

Deformation and Breakup of Micro- and Nanoparticle Stabilized Droplets in Microfluidic Extensional Flows

Using a microfluidic flow-focusing device, monodisperse water droplets in oil were generated and their interface populated by either 1 μm or 500 nm amine modified silica particles suspended in the water phase. The deformation and breakup of these Pickering droplets were studied in both pure extensional flow and combined extensional and shear flow at various capillary numbers using a microfluidic hyperbolic contraction. The shear resulted from droplet confinement and increased with droplet size and position along the hyperbolic contraction. Droplet deformation was found to increase with increasing confinement and capillary number. At low confinements and low capillary numbers, the droplet deformation followed the predictions of theory. For fully confined droplets, where the interface was populated by 1 μm silica particles, the droplet deformation increased precipitously and two tails were observed to form at the rear of the droplet. These tails were similar to those seen for surfactant covered droplets. At a critical capillary number, daughter droplets were observed to stream from these tails. Due to the elasticity of the particle-laden interface, these drops did not return to a spherical shape, but were observed to buckle. Although increases in droplet deformation were observed, no tail streaming occurred for the 500 nm silica particle covered droplets over the range of capillary numbers studied.

Effects of Fine Water Mist on a Confined Blast

A computational, multi-phase, model has been developed to study the interactions between water droplets and radial expansion of a gas cloud in a spherical chamber. Initial conditions for the gas cloud are specified based on chemical equilibrium calculations for the detonation of a high explosive (RDX). Mono-dispersed water droplets are injected at uniform concentration into the chamber prior to the expansion. A Lagrangian model is used to track the breakup of the parent drops near the shock front to form child drops, which are extremely small. The Navier–Stokes solutions show that the child droplets accumulate near the shock front and evaporate at 100 times higher rate than the parent droplets. Latent heat absorption is the dominant mechanism followed by the sensible heat absorption by the water vapor (and droplets), and momentum absorption from the high velocity gases by the child droplets. The simulations also show that the water vapor formed by the evaporation increases the gas density at the shock front. The increased density and reduced gas temperature (cooling) have opposite effects on the pressure at the shock front. This leads to only a modest suppression in the pressure. At realistic droplet concentrations (0.08 kg droplets/m3 of air), the water mist is shown to evaporate completely in a short time (2.42 ms) prior to shock reflection at the chamber wall mainly due to the breakup at the shock front. High concentrations of mist may be desirable, but are difficult to achieve in practice at the total flooding conditions.

Interplay of kinetics and interfacial interactions in breath figure templating – A phenomenological interpretation

Breath Figure BF templating is an attractive technique for the production of polymer films with controlled porosity: upon the exposure to a moist airflow, monodisperse water droplets condense on the surface of evaporating polymer solution into ordered arrays, acting as a template for the final film structure. This work has been focused on polylactic acid (PDLLA), a biocompatible biodegradable polymer, and aims to provide a phenomenological interpretation of BF process based on the role of kinetics and the influence of interfacial tension. The effects of physical and chemical properties of solvents have been evaluated; the contribution of kinetics has been assessed by modifying the rate of film hardening from seconds to minutes, via different experimental set-up. Our experimental observations indicated that the interplay of kinetics and interfacial tension defines the structure of BF films: interfacial forces govern the physical interactions among water, solvent and polymer, but the dynamics of the process and thus the final pore structure is regulated by kinetics.

STUDIES OF THE FORMATION OF MICROPOROUS POLYMER FILMS IN "BREATH FIGURE" CONDENSATION PROCESSES

We report studies of the formation of ordered microporous polymer films by the evaporation of polymer solutions following exposure to a humid atmosphere. High-speed microphotographic (HSMP) studies of the formation process reveal that near the surface of the polymer solution, vapor condensation produces near monodisperse water droplets which crystallize to form a close-packed array. Following the evaporation of the solvent, characterization of the solid by Atomic Force Microscopy, confocal microscopy and white light interferometry reveals that the surface of the polymer film features extensive regions of hexagonally close-packed microscopic pores, whose spatial arrangement replicates that of the initial droplet monolayer. Defects recorded by HSMP in the packing of the colloidal monolayer of liquid droplets formed above the surface of the polymer solution are found to correspond to those transferred into the eventual solid film, providing the first direct evidence of the structure templating role of the droplet monolayer.

Ultra-Fine Water Mist Extinction Dynamics of a Co-Flow Diffusion Flame

Computations are performed to examine the effectiveness of mono-disperse water droplets in extinguishing a co-flow, propane diffusion flame by injecting the droplets into the air stream. The calculations show that the droplets entrained into the reaction kernel at the flame base are crucial for extinction. The reaction kernel detaches from the burner rim and blows-off when the droplet concentration is increased to a critical value (extinction concentration). At the critical value, the maximum chain-branching reaction (H2 + O = OH + H) rate in the reaction kernel was found to be reduced by a factor of 5 in our computations. A large decrease in the reaction rate indicates that the maximum heat generations rate and Damkholer number are too low to sustain the flame, and cause the flame blow-off. Large drops are more effective than small drops, and the extinction concentration of water increases from 10.5% to 15% by mass as the size is reduced from 32 to 4 μm. This is because of competition between the degree of penetration and the rate of evaporation of the water drops. The large drops penetrate the reaction kernel at the flame base better than the small drops, which evaporate completely before reaching the 600 K isotherm located well outside the reaction kernel as shown by our computations. As the droplet diameter is increased further (> 32 μm), the trend will reverse as the evaporation rates get too small, despite increased penetration of flame core by the drops.

Flow-Focusing Generation of Monodisperse Water Droplets Wrapped by Ionic Liquid on Microfluidic Chips: From Plug to Sphere

Generating droplets via microfluidic chips is a promising technology in microanalysis and microsynthesis. To realize room-temperature ionic liquid (IL)-water two-phase studies in microscale, a water-immiscible IL was employed as the continuous phase for the first time to wrap water droplets (either plugs or spheres) on flow-focusing microfluidic chips. The IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), could wet both hydrophilic and hydrophobic channel surfaces because of its dual role of hydrophilicity/hydrophobicity and extremely high viscosity, thus offering the possibility of wrapping water droplets in totally hydrophilic (THI), moderately hydrophilic (MHI), and hydrophobic (HO) channels. The droplet shape could be tuned from plug to sphere, with the volume from 6.3 nL to 65 pL, by adding an orifice in the focusing region, rendering the hydrophilic channel surface hydrophobic, and suppressing the Uw/UIL ratio below 1.0. Three different breakup processes were defined and clarified, in which the sub-steady breakup and steady breakup were essential for the formation of plugs and spheric droplets, respectively. The influences of channel hydrophilicity/hydrophobicity on droplet formation were carefully studied by evaluating the wetting abilities of water and IL on different surfaces. The superiority of IL over water in wetting hydrophobic surface led to the tendency of forming small, spheric aqueous droplets in the hydrophobic channel. This IL-favored droplet-based system represented a high efficiency in water/IL extraction, in which rhodamine 6G was extracted from aqueous droplets to [BMIM][PF6] in the hydrophobic orifice-included (HO-OI) channel in 0.51 s.