Droplet Speed Research Articles

Impact of Droplets on Liquid Films in the Presence of Surfactant

The impact of droplets on liquid films is ubiquitous in natural and industrial processes, and surfactants can significantly alter the impact process by changing the local surface tension. Here we study the impact of droplets on liquid films in the presence of surfactant using high-speed photography, and reveal the flow pattern by dye-tracing. The effects of the droplet size and speed, and the initial film thickness on the impact process are elucidated. The results show that the flow is significantly affected by adding surfactant to the droplet, the liquid film, or to both phases. In particular, the film dye patterns form concentric circles and flower-shaped structures at low and high droplet Weber numbers, respectively. We also show how surfactant-induced Marangoni stresses modify these flow patterns, and alter the characteristics of the phenomena associated with the impact process, such as the propagation of capillary waves, the evolution of the crown, and the formation of secondary droplets. During the impact of surfactant droplets on thin water films, the Marangoni stresses can be sufficiently strong so as to drive film dewetting.

Identification of Droplet-Flow-Induced Electric Energy on Electrolyte–Insulator–Semiconductor Structure

Recently, various energy transducers driven by the relative motion of solids and liquids have been demonstrated. However, in relation to the energy transducer, a proper understanding of the dynamic behavior of ions remains unclear. Moreover, the energy density is low for practical usage mainly due to structural limitations, a lack of material development stemming from the currently poor understanding of the mechanisms, and the intermittently generated electricity given the characteristics of the water motion (pulsed signals). Here, we verify a hypothesis pertaining to the ion dynamics which govern the operation mechanism of the transducer. In addition, we demonstrate enhanced energy transducer to convert the mechanical energy of flowing water droplets into continuous electrical energy using an electrolyte-insulator-semiconductor structure as a device structure. The output power per droplet mass and the ratio of generated electric energy to the kinetic energy of water drops are 0.149v2 mW·g-1·m-2·s2 and 29.8%, respectively, where v is the speed of the water droplet.

Pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss interferometry in a viscous granular source model**Supported by National Natural Science Foundation of China (11675034, 11275037)

We examine the evolution of quark-gluon plasma (QGP) droplets with viscous hydrodynamics and analyze the pion transverse-momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in a granular source model consisting of viscous QGP droplets. The shear viscosity of the QGP droplet speeds up and slows down the droplet evolution in the central and peripheral regions of the droplet, respectively. The effect of the bulk viscosity on the evolution is negligible. Although there are viscous effects on the droplet evolution, the pion momentum spectrum and elliptic flow change little for granular sources with and without viscosity. On the other hand, the influence of viscosity on HBT radius Rout is considerable. It makes Rout decrease in the granular source model. We determine the model parameters of granular sources using the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii together, and investigate the effects of viscosity on the model parameters. The results indicate that the granular source model may reproduce the experimental data of pion transverse-momentum spectrum, elliptic flow, and HBT radii in heavy-ion collisions of Au-Au at and Pb-Pb at in different centrality intervals. The viscosity of the droplet leads to an increase in the initial droplet radius and a decrease of the source shell parameter in the granular source model.

Self-Propelled Motion of Monodisperse Underwater Oil Droplets Formed by a Microfluidic Device.

We evaluated the speed profile of self-propelled underwater oil droplets comprising a hydrophobic aldehyde derivative in terms of their diameter and the surrounding surfactant concentration using a microfluidic device. We found that the speed of the oil droplets is dependent on not only the surfactant concentration but also the droplet size in a certain range of the surfactant concentration. This tendency is interpreted in terms of combination of the oil and surfactant affording spontaneous emulsification in addition to the Marangoni effect.

CFD simulation of the behavior of droplet rising in water column with the effect of surfactant

Surfactant decreases the oil-water interfacial tension and thus oil droplets breakup into smaller ones and are dispersed into the water column. This paper presents the analysis of surfactant (premixed with oil droplet and bulk separately) on the oil droplet behavior in the water column. We implemented additional equations to account for the oil-water interfacial tension due to the presence of surfactant and those equations were incorporated into FLUENT by using UDF (User Defined Function). The streamlines around the oil droplet show major recirculation both inside and outside the oil droplet. It is noticed that the surfactant concentration is higher at the bottom and edges of oil droplet due to the shear effect. The presence of surfactant affects the deformation process of oil droplets of different diameters: Oil droplet with larger diameter (e.g., 4 mm in diameter) deforms into flat shape and breaks up. Tip-streaming is noticed and the droplet continue to rise up in jelly-fish like shape. However, much smaller droplets (e.g., 20 μm in diameter) only deform into oval shape and do not continue to breakup under the condition of same initial surfactant concentration. The presence of surfactant also deceases the rising speed of oil droplet compared to the terminal velocity of clean oil droplet. The results may be valuable to help us to apply surfactant to oil spill in a more efficient way.

A direct contact type ice generator for seawater freezing desalination using LNG cold energy

In receiving terminal, liquid natural gas (LNG) needs to be re-gasified by the ambient heat generally supplied by seawater. Unfortunately, a great amount of LNG cold energy is wasted during this process, resulting in energy inefficiency and environmental implications. Seawater freezing desalination (SFD) should be an effective way to use cold energy of LNG in terms of energy match and operation cost. According to the properties of LNG, this work aims to develop a direct contact type ice generator, which integrating some special designs such as jacketed cylinder, nozzle arrangement and gas floatation, in order to improve heat transfer and phase separation in the SFD process. Through dynamic analysis on refrigerant atomization, the changes in speed and diameter of refrigerant droplet are obtained. The analysis results are helpful to further improve the design and operation of ice generator. In addition, the work also experimentally investigated the effects of some predominate factors on determining overall performance of ice generator, comprehensively considering ice generation and desalination. The experimental results could be significant for the SFD project by using cold energy of LNG.

3D-Printed High-Density Droplet Array Chip for Miniaturized Protein Crystallization Screening under Vapor Diffusion Mode.

Here we describe the combination of three-dimensional (3D) printed chip and automated microfluidic droplet-based screening techniques for achieving massively parallel, nanoliter-scale protein crystallization screening under vapor diffusion mode. We fabricated high-density microwell array chips for sitting-drop vapor diffusion crystallization utilizing the advantage of the 3D-printing technique in producing high-aspect-ratio chips. To overcome the obstacle of 3D-printed microchips in performing long-term reactions caused by their porousness and gas permeability properties in chip body, we developed a two-step postprocessing method, including paraffin filling and parylene coating, to achieve high sealability and stability. We also developed a simple method especially suitable for controlling the vapor diffusion speed of nanoliter-scale droplets by changing the layer thickness of covering oil. With the above methods, 84 tests of nanoliter-scale protein crystallization under vapor diffusion mode were successfully achieved in the 7 × 12 droplet array chip with a protein consumption of 10 nL for each test, which is 20-100 times lower than that in the conventional large-volume screening system. Such a nanoliter-scale vapor diffusion system was applied to two model proteins with commercial precipitants and displayed advantages over that under microbatch mode. It identified more crystallization conditions, especially for the protein samples with lower concentrations.

Analysis on the Leidenfrost effect of cryoprotectant microdroplets on the liquid nitrogen surface

The Leidenfrost effects of cryoprotectant (CPA) micro droplets on the liquid nitrogen surface are investigated with combination of the infrared photography technology and the theoretical model of non-isothermal crystallization kinetics. The results show that the phase change temperature of water droplet is the highest one (about 273 K), and both the degree of ice crystallization (about 3×10−2) and the maximum speed of droplets (100–130 mm/s) are also the largest ones, respectively. But the Leidenfrost time of water droplet is the shortest one. As the concentration of CPA rising, the phase change temperature obviously shifts to the lower temperature, i.e. 250 K for 20% glycerol solution and 225 K for 50% glycerol solution, and the degree of ice crystallization significantly deceases, i.e. 1.4×10−2 for 20% glycerol solution and 7×10−7 for 50% glycerol solution, which makes the speed of CPA droplets slow down 90–120 mm/s for 20% glycerol solution and 85–105 mm/s for 50% glycerol solution, but prolong the Leidenfrost time with increasing concentration of CPA. In the case of Vs55, both the phase change temperature (about 158 K) and the degree of ice crystallization (about 2.8×10−16) are the lowest ones, respectively. The maximum speed of Vs55 droplets is about 75–95 mm/s, and the Leidenfrost time of it is the longest one (about 2–3 times more than water case). Generally, larger droplets can make the Leidengfrost time longer, and there is a trend that larger droplets let them run faster for high concentration cases, i.e. 50% glycerol solution and Vs55. However, there is no such tendency for both pure water and 20% glycerol solution. These results can be expected to guide the optimal design for vitrification of biomaterials sealed with micro droplets in the future work.

Focal Plane Shift Imaging for the Analysis of Multi-Droplet Jumping

High speed images of coalescence induced three-droplet jumping on a nanostructured superhydrophobic carbon nanotube (CNT) surface are presented (θaapp ≈ 173º). When two or more droplets coalesce on a nanostructured superhydrophobic surface, the resulting droplet can jump away from the surface due to the release of excess surface energy. To more easily study the jumping phenomena, we have developed focal plane shift imaging (FPSI) to determine both jumping speed and direction. Figure 1(a) shows a schematic of the FPSI concept. A high speed camera was attached to an upright optical microscope, and samples were horizontally mounted on a cold stage. Initial conditions were obtained by moving the focal plane to be coincident with the middle of the droplets prior to coalescence (Figure 1b). Then the focal plane was shifted above the droplets by a known distance (Figure 1c), followed by measurement of the time taken for the jumping droplet to pass through the shifted focal plane (Figure 1d). By analyzing the initial and final conditions of the departing droplet for multiple three-droplet coalescence events, the three-droplet jumping droplet speed was determined (Figure 2b). Experimentally measured jumping speeds determined by the FPSI imaging technique show good agreement with three-droplet inertial capillary scaling (Figure 2b, dotted line, equation inset). The FPSI visualization technique provides a novel imaging platform for the study of complex multi-droplet jumping-droplet phenomena.

Splash threshold of a droplet impacting a moving substrate

The effect of ambient pressure on the corona splash resulting from a droplet impacting on a smooth dry moving substrate is investigated experimentally. As the ambient pressure is reduced the upstream-enhanced corona splash is initially suppressed to a prompt splash, and on further reduction of the ambient pressure prompt splash transitions to deposition. The splash threshold pressure is measured under different conditions including droplet diameter, droplet speed, substrate speed, and liquid viscosity. The splash and suppression mechanisms are explained based on aerodynamics. A model for the splash threshold pressure is presented that incorporates the substrate velocity into an existing model for droplet impingement on a stationary substrate. This new model approximately collapses the results from experiments with a multitude of variables onto a single curve.

Multi Pulse Train Modeling of Piezo-Drop-on-Demand Inkjet Print-Head Response

Resonant oscillations set up internal fluid waves within a piezo-drop-on-demand (DoD) print-head channel as a result of actuation drive pulses. Such waves will persist for some time after droplet ejection from the nozzle, and the residual wave amplitude can interfere (constructively or destructively) with all succeeding actuation drive pulses, potentially altering the speed and volume of successive droplets. As uncontrolled interference would worsen printing quality, residual waves are usually reduced by a combination of print-head design and waveform optimization for better performance at continuous (steady state) printing frequencies. However, the residual waves following any changes of printing frequency can influence “first” drops and short bursts of drops. Exact analytic expressions are provided here for the N-pulse burst DoD print-head response function with fixed printing frequency. This article explains the purpose and application of the model predictions to published piezo-driven DoD data. An examination of the effect of fluid properties, the identification of unexpected jetting behavior and some issues with manufacturing prototype quality, tests of assumptions made in the simple model and extensions to the prediction of print-head performance using realistic complex waveforms are also discussed. An earlier shorter article, mainly introducing the multi pulse train modeling approach and some applications within Xaar, was first presented at NIP31/DF2015 [S. D. Hoath, A simple model for DoD inkjet frequency response, Proc. IS&Ts 31st Int'l. Conf. on Digital Printing Technologies (IS&T, Springfield, VA 2015), 8–12]. © 2016 Society for Imaging Science and Technology.

Ice Accretion on Helicopter Fuselage Considering Rotor-Wake Effects

To accurately predict the ice shape on the fuselage under rotor-wake effects, discrete modeling of individual tip vortices is essential to determine the local aerodynamic interactions between the vortex and the fuselage. To this end, the actuator surface model is developed and applied to the present study. Then, water-droplet-trajectory prediction, thermal analysis, and ice growth module are seamlessly integrated. This study aims to perform detailed analysis of the rotor downwash effects on ice that accretes on a helicopter fuselage and to determine the variation with respect to the forward flight speed. As a result of comprehensive numerical computations, the following conclusions are reached. First, the rotor inflow transforms the droplet trajectories, the location of ice accretion and the amount of ice are changed. Actually, 16.5% less ice is accumulated on the overall fuselage in the rotor–fuselage interaction case compared to the isolated fuselage case. Second, a clear-cut distinction of the icing locations on the fuselage is observed with various advance ratios. In hovering conditions, massive ice is accumulated on the tail-boom and fuselage nose region, where the tip vortices collide with fuselage. Due to the large collision area with high speed of droplets, 15.2 and 9.1% more ice is accumulated in hovering case than that of forward flight case with the advance ratio 0.075 and 0.15, respectively. However, as the forward speed grows from 0.15 to 0.2, the total mass of ice exponentially increases. In case of the advance ratio 0.2, 36% more ice is accumulated than that of the hovering case.

Robust bidirectional continuous electrowetting based on metal–semiconductor (M–S) diodes

We demonstrate bidirectional continuous electrowetting by embedding metal–semiconductor diodes in the electrowetting substrate. Unlike conventional electrowetting on dielectric, bidirectional continuous electrowetting uses a single electrode pair to actuate a droplet through long distances. As long as the voltage potential is maintained between two end electrodes, the droplet moves toward the electrode with the higher potential. However, previously reported material systems had limited success in repeated actuation. In this work, diodes based on Schottky barriers were fabricated by forming metal–semiconductor junctions between titanium and high-resistivity n-type silicon. The performance enhancements were evaluated using current–voltage measurements of interface pairs. When the titanium is coated with gold to limit electrochemical reactions, the Schottky diodes achieved superior performance compared to electrochemical diodes previously studied. Droplet speed range from 8 to 240 mm/s is reported. Under repeated actuation, the speed of the droplet showed no degradation for up to 2000 cycles (experiment duration).

Экспериментальное исследование закономерностей испарения капель воды, движущихся последовательно в области высокотемпературных продуктов сгорания

Introduction. The paper presents the results of experimental investigation of cross impact of water droplets in a flow while moving and evaporation in a medium of high-temperature gases, which cor¬respond in temperature to the typical combustion products due to a fire. We considered the sequential schemes of droplet movement and the schemes when the trajectories of droplet movement diverged relative to each other. Materials and methods. The area full of high-temperature (about 1000 K) combustion products was created using the model of fire source (hollow cylinder of height h = 1 m and diameter d = 0.15 m). Droplets moved in the opposite direction of the gas medium. In experiments we applied the tools of high-speed video recording (up to 106 frames per second) and also the optical diagnostic methods of two-phase gas flows (Particle Image Velocimetry, Particle Tracking Velocimetry, Stereoscopic Particle Image Velocimetry, Shadow Photography). Results. In experiments the decrease in typical radius R of water droplets and change in distance Ld between neighboring droplets was the main defining characteristics. From the experiments we deter-mined the velocity variation of droplets moving sequentially, distance, at which the subsequent droplet catches the next one in a flow, and there is a coalescence. The influence of initial distance between neighboring droplets in a flow on their approximation while movement in a channel with high-¬temperature gases at various values of initial speed of droplet was investigated. Also, we determined the impact scale of the vapor trace forming from evaporation of droplets moving ahead on the eva¬pora¬tion intensity of the next droplets. It was revealed that heating and evaporation of the next droplets in a vapor-gas trace of preceding ones are less intense. The influence of initial water temperature on the approach and the coalescence of droplets moving subsequently in a flow of high-temperature gases were determined. It was illustrated that the previous heat-up of water accelerates the droplet coales¬cence in a flow under conditions of relatively short initial distances between droplets. Also, the extreme distances between droplets were defined, in pro-viding which there is a coalescence of droplets when they pass the area of high-temperature combustion products. Conclusions. We illustrated the satisfactory correlation of experimental results with the data of nu¬me¬-rical simulation carried out previously. The theoretical hypothesis about the significant influence of water droplets moving ahead (through high-temperature gases) on the conditions of evaporation and movement of subsequent ones was proved experimentally.

New Drop Fluidics Enabled by Magnetic-Field-Mediated Elastocapillary Transduction.

This research introduces a new drop fluidics that uses a deformable and stretchable elastomeric film as the platform instead of the commonly used rigid supports. Such a soft film impregnated with magnetic particles can be modulated with an external electromagnetic field that produces a vast array of topographical landscapes with varying surface curvature, which, in conjunction with capillarity, can direct and control the motion of water droplets efficiently and accurately. When a thin layer of oil is present on this film that is deformed locally, a centrosymmetric wedge is formed. A water droplet placed on this oil-laden film becomes asymmetrically deformed, thus producing a gradient of Laplace pressure within the droplet and setting it in motion. A simple theory is presented that accounts for the droplet speed in terms of such geometric variables as the volume of the droplet and the thickness of the oil film covering the soft elastomeric film as well as material variables such as the viscosity of the oil and the interfacial tension of the oil-water interfaces. Following the verification of the theoretical result using well-controlled model systems, we demonstrate how the electromagnetically controlled elastocapillary force can be used to manipulate the motion of single and/or multiple droplets on the surface of the elastomeric film and how elementary operations such as drop fusion and thermally addressed chemical transformation can be carried out in aqueous droplets. It is expected that the resulting drop fluidics would be suitable for the digital control of drop motion by simply switching on and off the electromagnetic fields applied at different positions underneath the elastomeric film in a Boolean sequence. We anticipate that this method of directing and manipulating water droplets is poised for application in various biochemical reaction engineering situations, an example of which is the polymerase chain reaction (PCR).

Droplet Velocity in an Electrowetting on Dielectric Digital Microfluidic Device.

In many electrowetting on dielectric (EWOD) based microfluidics devices, droplet actuation speed is a crucial performance-controlling parameter. Our present study aims to characterize and study droplet speed in a typical EWOD device. First, a practical droplet speed measurement method has been methodically demonstrated and some related velocity terms have been introduced. Next, influence of electrode shape on droplet speed has been studied and a new design to enhance droplet speed has been proposed and experimentally demonstrated. Instead of using square shaped electrodes, rectangular electrodes with smaller widths are used to actuate droplets. Additionally, different schemes of activating electrodes are studied and compared for the same applied voltage. The experiments show that a particular scheme of activating the array of rectangular electrodes enhances the droplet speed up to 100% in comparison to the droplet speed in a conventional device with square shaped electrodes.

Quantumlike statistics of deterministic wave-particle interactions in a circular cavity.

A deterministic low-dimensional iterated map is proposed here to describe the interaction between a bouncing droplet and Faraday waves confined to a circular cavity. Its solutions are investigated theoretically and numerically. The horizontal trajectory of the droplet can be chaotic: it then corresponds to a random walk of average step size equal to half the Faraday wavelength. An analogy is made between the diffusion coefficient of this random walk and the action per unit mass ℏ/m of a quantum particle. The statistics of droplet position and speed are shaped by the cavity eigenmodes, in remarkable agreement with the solution of Schrödinger equation for a quantum particle in a similar potential well.

Investigation and in situ removal of spatter generated during laser ablation of aluminium composites

Spatter generated during laser irradiation of an aluminium alloy nanocomposite (AlMg5 reinforced with Al2O3 nanoparticles) was monitored by high speed imaging. Droplets trajectory and speed were assessed by computerized image analysis. The effects of laser peak power and laser focusing on the plume expansion and expulsed droplet speeds were studied in air or under argon flow. It was found that the velocity of visible droplets expulsed laterally or at the end of the plume emission from the metal surface was not dependent on the plasma plume speed. The neighbouring area of irradiation sites was studied by optical and scanning electron microscopy. Droplets deposited on the surface were classified according to their size and counted using a digital image processing software. It was observed that the number of droplets on surface was 1.5–3 times higher when the laser beam was focused in depth as compared to focused beams, even though the populations average diameter were comparable. Three methods were selected for removing droplets in situ, during plume expansion: an argon gas jet crossing the plasma plume, a fused silica plate collector transparent to the laser wavelength placed parallel to the irradiated surface and a mask placed onto the aluminium composite surface. The argon gas jet was efficient only for low power irradiation conditions, the fused silica plate failed in all tested conditions and the mask was successful for all irradiation regimes.

Investigation on droplet momentum in VPPA-GMAW hybrid welding of aluminum alloys

Variable polarity plasma arc-gas metal arc welding (VPPA-GMAW) is a superior technology for welding thick plates of high-strength aluminum alloys. It integrates the advantages of energy focusing and high penetration depth in VPPA welding, and those of high welding efficiency and wide range of technological parameters in GMAW process. In this work, we investigated the droplet momentum in paraxial VPPA-GMAW hybrid welding of 7A52 aluminum alloys, and the technological parameters of welding process was also optimized. The images of droplet transfer were captured by high-speed camera, while the droplet speeds and sizes were statistically analyzed by t tests of independent samples. The results showed that the speeds of droplet arriving at the weld pool were significantly between GMAW and VPPA-GMAW processes, and the droplet speed increases with increasing plasma currents within a certain range. Meanwhile, the droplet momentum in VPPA-GMAW process is larger than that in conventional GMAW process. We also found that as the droplet momentum increased, the depression of weld pool grew more obvious and greatly facilitated the deep-penetration welding. In VPPA-GMAW process, it became more and more easier for the droplet to fall off the wire when the electromagnetic force gradually increased during pulse period. Droplet movement through the arc zone was further accelerated since the central pressure of arc column increased during base period. This research can provide some theoretical support for thick plate welding of high-strength aluminum alloys and help for deeper understanding of the hybrid arc coupling mechanism.

Study the Properties of Water Mist Droplet by Using FDS

Fire suppression systems are an extremely important part of every building that ensures the safety of the occupants and limits damage due to fire. Full scale model is set up for the purpose of predicting the geometry of fire spread and water mist particles distribution and movement. The results are based on full scale model simulation for several properties of water mist droplet. The water mist droplet speed, mist flux, droplet size and droplet distribution are discussed. It can be concluded that The water mist droplet movement, particle size and droplet distribution play a vital role to suppress fire.The present study is carried out using Fire dynamic simulator (FDS) to perform the model of room fire scenario require to investigate and compute particle distribution, droplet velocity, mist flux and droplet size.