阶梯式T型微通道内液滴、气泡分散规律

采用高速摄像仪对嵌入毛细管的阶梯式T型微通道内液滴和气泡的分散规律进行研究。考察了两相流量、黏度、表面活性剂浓度等因素对分散流型及分散尺寸的影响规律。结果表明，对于液滴分散过程，表面活性剂的浓度和连续相流量决定了分散流型，随二者增大，流型从dripping流向jetting流转变。对于气泡分散过程，实验范围内仅存在squeezing、dripping流型，表面活性剂的加入对气泡分散过程影响可忽略。嵌入毛细管的阶梯式T型微通道内获得的液滴、气泡直径小于微通道直径，根据实验结果基于两相流量和毛细管数分别建立了计算液滴、气泡分散尺寸的半经验模型，模型与实验结果符合良好。     

十字型微通道中乳液流变行为的数值模拟

单分散的高品质乳液在生物医学、化学化工等领域有着广泛的应用,利用十字型微通道所制备的乳液单分散性好、均一度高、球形度好,并且该方法可以大大简化了乳液的制备过程,在高品质乳液的制备中具有重要的应用价值。基于VOF相界面追踪方法,建立了十字型微通道中乳液形成过程的非稳态理论模型,对十字型微通道中乳液的形成过程进行了数值模拟研究,发现了4种典型的流型,即挤压流(squeezing)、滴式流(dripping)、喷式流(jetting)和协流(co-flowing),并对其形成机理和演化过程进行了分析,发现界面张力和黏性力在液滴生成过程中起到主导作用。此外,还研究了通道角度对液滴尺寸和流型的影响,计算表明,在挤压流中液滴的长度会受到显著的影响。但是,通道的角度对流型的影响并不明显。     

微流控双水相贴壁液滴流动强化酶促反应研究

以微流控双水相液滴流技术为基础,开发了一种酶促反应平台,将微流控贴壁液滴流快速传递、高效混合的特点与双水相反应分离耦合过程优化结合。本体系克服了传统宏观双水相体系传质传热慢以及耗时耗能的问题,并建立了贴壁液滴微反应器,产生更大的内环流,进一步增强传质效果。探究了双水相液滴界面的分子限域能力、对酶和产物的选择性分配能力。通过比较贴微通道壁和未贴微通道壁两类液滴微反应器的酶促反应效果,发现贴微通道壁液滴微反应器仅6 min转化率即可达到40%,其反应速率可达未贴微通道壁液滴微反应器9.4倍。本文通过微流控双水相贴壁液滴流实现了酶促反应的强化,为微尺度下的酶催化反应过程强化提供了一种新的思路。     

正弦型微通道内液-液两相流型及流动特性实验研究

采用实验的方法对不混溶的液液两相流体在不同入口结构下的正弦微通道(直通道正弦、波峰正弦和波中正弦)内液滴的流动特性进行了分析。硅油作为离散相,含有0.5% SDS的蒸馏水作为连续相,观测到弹状流、滴状流和射状流。分析了两相流动参数及不同的微通道入口结构对流型和液滴长度的影响。流型受微通道入口结构影响较大,波峰正弦微通道能够生成最大范围的稳定的流型。液滴长度随离散相体积流量和离散相与连续相体积流量之比的增大而增大,随连续相的体积流量和毛细数的增大而降低。微通道入口结构对液滴长度有影响,直通道的正弦微通道内液滴长度最短,更有利于液滴的形成。三种通道生成的液滴中,最大的液滴尺寸是最小的液滴尺寸的1.15~1.39倍,但正弦流动段对液滴速度几乎没有影响。     

微通道液-液两相流动特性实验研究

利用高速摄像机与Canny算法,以硅油为离散相,含0. 5%SDS的蒸馏水为连续相,研究了凹穴型微通道内液-液两相流动特性。结果表明,直通道内观察弹状流、过渡流、滴状流3种流型。随着毛细数的增大,液滴形成机理由挤压机制向剪切机制转变,液滴速度逐渐增大,液滴长度逐渐减小。随着连续相流量的增加,液滴形成时间逐渐减小,且挤压机制生成液滴的时间大于剪切机制。凹穴结构减弱了壁面对液滴的限制,液滴速度降低,T型交汇处压差降低,相同工况下的液滴尺寸大于对冲T型微通道的液滴尺寸。     

T型微通道内液滴尺寸的实验测定与关联

利用高速摄像仪对不同深宽比的T型微通道内液滴尺寸进行了实验研究。分别采用3种不同尺寸(深度×宽度)的微通道:400 μm×400 μm、400 μm×600 μm、400 μm×800 μm。以环己烷为分散油相,含0.3%表面活性剂十二烷基硫酸钠(SDS)的蒸馏水-甘油(质量分数分别为20%、40%、60%)溶液为连续相。考察了弹状流、过渡流和滴状流流型下微通道尺寸、两相流率、物性对液滴尺寸的影响。结果表明:液滴尺寸随微通道深宽比、连续相流率、黏度和毛细数的增加而减小,随分散相流率的增加而增加。用毛细数、两相流量比和通道深宽比对微通道内液滴尺寸进行了关联和预测,预测值与实验结果吻合良好。     

微通道内液滴/气泡破裂动力学分析

微化学工程与技术是现代化学工程学科的前沿领域。微通道内液滴及气泡破裂动力学是决定多相过程并行微通道数目放大的基础与难点。破裂流型转换条件、界面动力学和尺寸调控等三方面是微通道内液滴与气泡破裂动力学的主要研究对象。讨论了对称微通道、非对称微通道、多级微通道、旁路微通道、含有障碍物的微通道内气泡和液滴破裂行为及影响因素,指出了目前微尺度下气泡与液滴破裂行为相关研究工作存在的不足,并对该领域未来的发展进行了展望。     

液滴滴浸微通道入口段的动力学特性分析

采用流体体积(Volume of Fluid, VOF)函数捕捉气液相界面,研究液滴滴浸微通道入口段的运动,通过改变微通道入口段的截面宽度、润湿特性及液滴雷诺数(Re)和韦伯数(We)研究滴浸过程的动力学特性。结果表明,微通道入口段的截面宽度对液滴浸入微通道时的撞击过程影响最明显,随截面宽度减小,液滴撞击通道入口后通过微通道的难度增加,整个过程液滴所受阻力逐渐增大;当微通道截面宽度减至0.2 mm时,壁面润湿性效应凸显,表现为壁面静态接触角越大,液滴滴浸微通道时所受的阻力也越大。表面接触角较大时,为使液体通过微通道入口段,可适当增大液滴的Re,液体在通道内的浸润长度随Re增加成比例增大,当Re增至4000时,通道内开始出现射流现象。We减小,表面张力效应变得明显,通道内的流动阻力变大,液体流过微通道入口段的难度增大。     

重力对微圆管中H2SO4/正己烷体系流动特性影响的数值模拟

采用VOF模型对U型微圆管中H_2SO_4/正己烷两相的流动特性受重力的影响进行了三维数值模拟。比较了U型微圆管中的竖直向上、竖直向下和水平方向的液滴的流动形态。结果表明重力会改变液滴的尺寸和界面形态,竖直向上运动时液滴的尺寸最大。分析了该通道中的压力分布,结果表明竖直向上流动时压降最大,水平次之,竖直向下最小。分析了通道中流体的内部流场,发现重力会改变通道中流体内部的流场分布,但并不会引起流体宏观运动速度的明显改变。     

微通道内非牛顿流体中液滴生成机理研究进展

微流体技术良好的可控性为制备均一可控的微液滴提供了新的途径,而非牛顿流体因广泛的应用而受到关注。综述了近年来剪切变稀和黏弹性两类典型非牛顿流体中液滴生成机理的研究进展。围绕流动聚焦型和T型微通道两种典型微通道构型,介绍了非牛顿流体分别作为分散相和连续相时液滴生成过程的界面演化动力学,并与牛顿流体液滴生成过程进行了对比,分析了剪切变稀特性和弹性对主液滴和卫星液滴生成的影响。展望了非牛顿流体液滴生成过程待解决的关键科学问题,为进一步的模拟和实验研究提供了借鉴和参考。     

Squeezing-to-dripping transition for bubble formation in a microfluidic T-junction

The aim of this paper is to investigate the squeezing-to-dripping transition for bubble formation in a microfluidic T-junction by cross-flowing rupture technique using a high-speed digital camera. Experiments were conducted in a glass microfluidic T-junction with the cross-section of the microchannel of 120 μm wide and 40 μm deep. N₂ bubbles were generated in glycerol–water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Three different regimes were identified for generating different kinds of bubbles: squeezing, dripping and transition regimes. Various forces exerted on the gaseous thread in different regimes were analyzed. Long slug bubbles were formed in the squeezing regime, while dispersed bubbles in the dripping regime. The transition regime formed short slug bubbles. The bubble sizes in various regimes could be correlated with several dimensionless numbers such as the ratio of gas/liquid flow rates and capillary number. The two-step model for droplets (Steegmans et al., 2009) was extended to describe the bubble formation.     

The liquid–liquid flow dynamics and droplet formation in a modified step T-junction microchannel

The droplet generation mechanism in the asymmetrically enhanced step T-junction remains unknown, especially for the transition stage from dripping to jetting regimes. In this work, the droplet generation mechanism was systematically investigated in a modified step T-junction by modulating a large flowrate range and altering different interfacial tensions. We found that under different fluid regimes, both the capillary number and flow rate ratio of continuous and dispersed phase showcase completely different impacts over droplet generation. In dripping regime, the interfacial tension, which was controlled by changing the surfactant concentration, dominated the formation mechanism when the surfactant concentration was found below micelle concentration. In jetting regime, our experimental results showed that the influence of the surfactant concentration on the size of generated droplets was rather negligible while the flow rate ratio of continuous and dispersed phase indeed determined such a parameter. In the dripping-jetting transition stage, an increase of droplet size was observed despite the increase of continuous phase flow. After reaching a peak, the droplet dimension started to decrease with the increase of continuous phase flow as expected. To the best for our knowledge, it is the first study to report generation mechanism in modified step T-junction from dripping to jetting regimes.     

十字交叉微通道内微液滴生成过程的数值模拟

采用VOF模型对十字交叉微通道内微液滴的生成进行三维数值模拟,获得了拉伸挤压、滴状剪切、单分散射流等单分散微液滴的生成机制以及紊乱射流、节状形变流、管状流和滑移流等两相流型,模拟与实验结果相吻合验证了模拟的有效性。液液两相流型主要受两相流速、两相界面张力以及连续相黏度的影响,发现随着连续相的流量增大,微液滴的生成尺寸减小,生成频率增大;而离散相流量的影响则相反。两相表面张力与连续相黏度分别在低连续相Ca数和高连续相Ca数条件下分别起主导作用。在低连续相Ca数(U_d<0.03 m·s^-1)的拉伸挤压和滴状剪切流流型下,微液滴生成尺寸随着表面张力系数的减小而减小,在射流条件下反而增大,微液滴的生成频率变化则相反。在高连续相Ca数(U_d>0.03 m·s^-1)下,微液滴的生成尺寸随着连续相黏度的增大而减小,微液滴的生成频率变化则相反。另外,壁面接触角在拉伸挤压流型下对微液滴生成无太大影响,但在滴状剪切和单分散射流流型下,接触角减小会导致微液滴无法稳定生成。     

Visualization study of emulsion droplet formation in a coflowing microchannel

An experimental visualization study is conducted to investigate the hydrodynamic characteristics of emulsion droplet formation in a coflowing microchannel. Both monodisperse and polydisperse patterns of drop formation are observed, including dripping regime, jetting regime (widening jetting and narrowing jetting). Especially, two dripping-to-jetting transition regimes and wavy regime with no individual droplet produced are captured and analyzed. A corresponding phase diagram is provided to characterize the transitions between different emulsification patterns through the control of flow rate of continuous phase. In addition, the dependence of generated droplet size on the Capillary number of the continuous phase (Ca) and the Weber number of the dispersed phase (We) is presented. It is indicated that, when Ca is below 3, the generated droplet size is sensitive to the viscous force and the drop formation regime is widening jetting and dripping. However, when Ca exceeds 3, the generated droplet size is approximately independent of Ca, and the droplet formation regime is thinning jetting.     

An experimental investigation of gas-liquid two-phase flow in single microchannel contactors

This paper describes two-phase flow pattern and pressure drop characteristics during the absorption of CO₂ into water in three horizontal microchannel contactors which consist of Y-type rectangular microchannels having hydraulic diameters of 667, 400 and , respectively. With the help of a high-speed photography system, flow patterns such as bubbly flow, slug flow (including two sub-regimes, Taylor flow and unstable slug flow), slug-annular flow, churn flow and annular flow were observed in these microchannels. The applicability of the currently available correlations for describing flow pattern transitions in microchannels has been examined. Generally, the predicting performance of these correlations deteriorates as the channel diameter further reduces. Toward solving this discrepancy, an empirical correlation based on the superficial Weber numbers was developed to interpret the transition from Taylor flow to unstable slug flow in three microchannels. Taylor bubble formation process in microchannels was found to be in the squeezing regime at lower superficial liquid velocities (Ca ranging from 0.0019 to 0.029) while the transition to the dripping regime was observed at the highest superficial liquid velocity of 1.0 m/s. Lengths of Taylor bubbles formed in the squeezing regime can be well represented by the scaling relation proposed by Garstecki et al. [Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up. Lab on a Chip, 6, 437-446]. For flow patterns including slug-annular flow, annular flow and churn flow, a simple analysis based on the separated flow model has been performed in order to reveal the observed effect of the superficial liquid velocity on two-phase frictional multiplier in the present microchannels. Then, reasonable correlations for the prediction of two-phase frictional pressure drop under these flow patterns were suggested.     

Experimental and computational study of microfluidic flow‐focusing generation of gelatin methacrylate hydrogel droplets

This work presents the experimental and computational study of droplet generation for hydrogel prepolymer solution in oil using a flow‐focusing device. Effects of different parameters on hydrogel droplet generation and droplet sizes in a flow‐focusing device were investigated experimentally and computationally. First, three dimensional (3D) computational simulations were conducted to describe the physics of droplet formation in each regime and mechanism of three different regimes: squeezing, dripping, and jetting regime of hydrogel were investigated. Subsequently, the effects of viscosity, inertia force, and surface tension force on droplet generation, and droplet size were studied through these experiments. The experiments were carried out using different concentration of gelatin methacrylate (GelMA) hydrogel (5 wt % and 8 wt %) as the dispersed phase and two different continuous phase liquids (light mineral oil and hexadecane) with various concentrations of surfactant (0 wt %, 3 wt %, and 20 wt %). All experimental data was summarized by capillary number of dispersed phases and the continuous phases to characterize the different regimes of droplet generation and to predict the transition of dripping to a jetting regime for GelMA solution in flow‐focusing devices. It is shown that the transition of dripping to a jetting regime for GelMA happens at lower capillary numbers compared to aqueous solutions. Moreover, by increasing the viscous force of continuous phase or decreasing the interfacial force, the size of GelMA droplets was decreased. By controlling these parameters, the droplet sizes can be controlled between 30 μm and 200 μm, which are very suitable for cell encapsulation. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43701.     

Droplets generation under different flow rates in T-junction microchannel with a neck

The 2D top view of the droplet formation in microfluidic T-junction devices with a neck is recorded to estimate the droplet volume under different flow conditions. The channel with a neck at the T-junction, to provide a narrow structure, is proposed but has not been analyzed as the normal T-junction. The droplet generation process is separated into two stages, including the filling stage and the squeezing stage, to develop a droplet size predictive model based on the continuity of both liquids. In a wide range of flow conditions for multiphase microfluidics, this study validates and physically explains the model by analyzing the generation of droplets and coefficients of the model. Results of this study can help to design droplet microfluidic devices, where it is requisite to know the relation between flow conditions and the droplet size.     

Dynamics of Emerging Water Droplet Subjected to Sidewall with Different Wettabilities in a Fuel Cell Cathode Channel

On‐site experiments on low temperature fuel cells revealed that water droplets tend to emerge into the gas channel at the corner. This motivates the present investigation on the effects of the location where water emerges from and the wettability of sidewall on the dynamic behaviour of liquid water in gas channel by numerical simulations that employ the volume‐of‐fluid method. A microchannel with a square cross‐section of 0.25 mm in width and a pore of 0.05 mm in diameter is adopted. The simulation results for different pore locations and wettabilities of sidewall show that the behaviour of water droplet only depends on the wettability of bottom wall when it emerges from the centreline without attaching to the sidewall and gains the highest pressure drop. When the emergence location shifts towards the corner, the water droplet unavoidably attaches to the sidewall and thus the wettability of sidewall is found to have significant effects on its dynamics: hydrophobic sidewall results in droplet detachment and fast removal with the highest pressure drop and the minimal water saturation, whereas hydrophilic sidewall leads to a water film and accumulation with the lowest pressure drop and the minimal water coverage on the bottom wall.     

Robust generation of monodisperse droplets using a microfluidic step emulsification device with triangular nozzle

In this work, the droplet generation process in the microfluidic step emulsification chip with a triangular nozzle (SE-T) was investigated in the combination of visualization experiment and numerical simulation, through a comparison with a rectangular nozzle (SE-R). The flow regimes, including dripping, dripping-jetting transition, and jetting, were observed in the SE-T, among which the dripping is the preferred flow regime to generate monodispersed droplet with corresponding C.V. (coefficient of variation) of the droplet size smaller than 1.9%. Compared with the SE-R, the larger space and expanding structure of the triangular nozzle in the SE-T enhance the wall wetting effects, which induces earlier appearance and accelerates shrinking of the neck. As a result, the SE-T exhibits more robust droplet performance under the dripping regime, which produces the droplets with nearly unchanged size and higher monodispersity, especially little related to the variations of surfactant concentrations and dispersed phase flow rates.     

High-frequency formation of bubble with short length in a capillary embedded step T-junction microdevice

The performances of slug flow gas–liquid reactors are mostly determined by slug length, especially for the high gas–liquid flow rate ratio condition. This work is the first time to report the short bubble generated with high frequency in a capillary embedded step T-junction microdevice. The aspect ratio of bubble could be around 0.5 with a frequency higher than 750 s⁻¹ when the gas–liquid flow rate ratio is even higher than 5. The specific surface area of the generated gas–liquid microdispersion system is larger than 10,400 m²/m³. The short bubble formation process includes two periods, and its formation mechanism is mainly because of the relatively higher pressure drop in the step T-junction, which provides a much higher breakup force for the squeezing flow. Finally, two models are developed to predict the bubble frequency and volume. This work provides a highly promising dispersion technology for the gas–liquid process intensification in microreactors.