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.     

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.     

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.     

Formation of mono-dispersed droplets with electric periodic dripping regime in electrohydrodynamic (EHD) atomization

The formation of controllable size and dripping frequency in electrohydrodynamic (EHD) atomization with electric periodic dripping regime are of much interest and importance because of significant and wide applications, such as micro-encapsulation and ink-printing. In the present study, the experimental and theoretical works were carried out to explore droplet formation in periodic dripping regime in presence of an electric field. The dimensionless electric charge carried by each droplet produced is smaller than the 50% of critical value of the Rayleigh limit, where charge-to-mass ratio of droplets was obtained through the deflection distance in the presence of an electric field. The droplet in electric periodic dripping regime usually undergoes oscillating deformation, and finally forms a spherical droplet below the tip no more than ten times out diameter of tube. The droplet size tens of microns to one hundreds of microns decreases with an increase in applied potential. In the electric dripping mode, droplets size is independent of flow rate and affected by flow rate due to adsorption of surface active species in micro-dripping. The simplified model to predict droplets size was derived from the balance of electric, surface tension and gravity forces. The droplets size calculated in good agreement with the experiments. Meanwhile, the dripping frequency of droplets with rang of a few to several hundred hertz obtained from timeresolved images is highly dependent of liquid flow rate and electric potential. The largest dripping frequency was predicted and in reasonable agreement with the experimental results. In electric periodic dripping regime drop-on-demand droplets in size and dripping frequency further our understanding on the formation of identical droplets and are beneficial to many practical applications.     

Emulsification using tubular metallic membranes

Tubular metallic membranes with pore diameters of 5 and 10 μm have been used in a cross-flow unit to prepare monodisperse oil-in-water emulsions (O/W) with span values as low as 0.67, significantly lower than for emulsions prepared with a rotor–stator homogenizer. The influence of typical operating parameters (continuous phase flow rate and transmembrane pressure) on droplet size distribution was studied. The smallest droplets were obtained at low transmembrane pressures and high continuous phase flow rates. The droplet production with tubular metallic membranes was higher than with other types of tubular membranes, such as SPG or ceramic.Experimental results were compared with those obtained in a stirred tank unit operating under similar conditions and using flat metallic membranes with the same pore diameter. Droplet size prediction models based on simple force balances were applied to compare theoretical and experimental droplet diameters. The droplet formation regime (dripping, jetting) was also studied for both types of membranes.     

Formation of O/W emulsions by static mixers for pharmaceutical applications

Oil-in-water (O/W) emulsions produced by static mixers in the laminar flow regime are characterized for their oil drop size spectra. The emulsions are used in the first process step for the production of microspheres for pharmaceutical applications by the emulsion extraction method. However, emulsion generation by static mixers in the laminar flow regime is rarely discussed in the scientific literature. Here we deduce a non-dimensional correlation for predicting the Sauter mean oil drop size as a function of the static mixer operation parameters and the liquid properties. First, the material properties of the organic and water phases are characterized. Second, the oil drop size spectra of the emulsions are measured by laser diffraction. Dimensional analysis is used to describe the relationship between the process parameters of the static mixer and the Sauter mean oil droplet size. Emulsion production experiments using SMX static mixers with two different diameters are carried out with the mixing of the two liquids taking place in the laminar flow regime. We provide results covering a wide range of all process parameters, which were identified influencing the droplet size of the emulsion. The correlation achieved is related to the non-dimensional drop-size based Ohnesorge number of the emulsification process and allows for the prediction of the mean oil droplet size with good accuracy, which is an essential information about the emulsion properties relevant for the pharmaceutical application.     

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.     

Intensification of production of O/W emulsions using oscillatory woven metal micro-screen (WMMS)

Using high porosity woven metal micro-screen (WMMS), a novel design oscillatory emulsifier had been used for intensification of production of relatively narrow size distribution oil in water (O/W) emulsions. The average droplet size increased with increasing the dispersed phase flow and decreased with increasing both the oscillation frequency and amplitude. The emulsion polydispersity decreased with increasing both oscillation intensities as well as dispersed phase flux. Although the change in droplet size with oscillation was reasonably predicted using a simple torque balance model based on Stokes oscillatory flow, both the flow patterns and the surface phenomena are more complex, and the final droplet size is affected by interactions between different operating and physical parameters.     

Jet breakup and droplet formation in near-critical regime of carbon dioxide-dichloromethane system

The jet breakup and droplet formation mechanism of a liquid in the near-critical conditions of a solvent-antisolvent system is examined with high-speed visualization experiments and simulated using a front tracking/finite volume method. The size of droplets formed under varying system pressure at various jet breakup regimes is measured with a Global Sizing Velocimetry, using the shadow sizing method. A stainless steel nozzle with 0.25 mm I.D and 1.6 mm O.D was used in this study. Experiments were performed at fixed temperature of 35 °C and system pressure in the range from 61 to 76 bar in the near-critical regime of the DCM-CO₂. At the near mixture critical regime for DCM-CO₂ mixture, the miscibility between the two fluid phases increases and the interfacial tension diminishes. This phase behavior has important applications in particle formation using gas antisolvent (GAS) and supercritical antisolvent (SAS) processes. The jet breakup and droplet formation in the near-critical regime is strongly dependent on the changes in interface tension and velocity of the liquid phase. An understanding of the droplet formation and jet breakup behavior of DCM-CO₂ in this regime is useful in experimental design for particle fabrication using SAS method.     

Relationship between droplet size and fluid flow characteristics in miniemulsion polymerization of methyl methacrylate

Static mixer (SM) can be applied for emulsification, but the fundamental understanding of the nature of fluid flow and mixing in static mixers, is however poor. Droplet size is a very important parameter in miniemulsion systems and affects strongly the mechanism of particle formation in polymerization reactions. In this study, static mixer was used as homogenization device for emulsification of methyl methacrylate (MMA). Re number (Re) was obtained for SM inserted tube in different flow rates. It was demonstrated the nature of fluid flow was turbulent under our experimental conditions. The relationship between droplet size—the most important variable in our study—and Weber number (We) was investigated. The results showed that the ratio of the droplet size to the pipe diameter was fit as an exponential function with an order of −0.35. The polymerization of created droplets under certain We values by SM showed that it is possible to obtain a reasonable 1 : 1 copy of droplets to the particles. All these, indicate that using relationship between We and droplet size allow one to obtain acceptable condition of droplet nucleation in miniemulsion polymerization. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

局部几何构型对聚焦流微通道内液滴生成特性的影响

在微流控技术中,微通道结构的优化设计是一种被动实现液滴精确调控的有效方法。为探究分散相入口、通道下游孔口以及二者共存模式下的通道结构变化对液滴生成特性的影响,采用VOF / CSF耦合level set的界面捕捉法对聚焦流微通道内的液滴生成开展了数值模拟研究。结果表明,当孔口为单一变量时,液滴生成周期和直径随孔口宽度呈近线性增大,且颈部宽度收缩率随孔口宽度的增大而不断减小。孔口的收缩有助于强化连续相Y方向的挤压和X方向的黏性剪切作用。当孔口宽度较小,聚焦作用较强时,液滴生成周期和直径整体上对分散相入口竖直和水平边锥形角的变化并不敏感;此时,孔口对连续相的聚焦效应主要影响液滴的生成特性。当孔口和分散相入口水平边锥形角θ₂同步变化时,二者可协同影响液滴的生成。孔口宽度的增大削弱了孔口的聚焦作用,液滴挤压破裂时间在单个周期中的占比逐渐增大。此外,当孔口宽度较大时,液滴生成开始对θ₂敏感,其周期和直径随θ₂增大而增大,且液滴可从滴流向射流模式转变。     

CFD analysis of microchannel emulsification: Droplet generation process and size effect of asymmetric straight flow-through microchannels

Asymmetric straight flow-through microchannel (MC) arrays are high-performance MC emulsification devices for stable mass production of uniform droplets. This paper presents computational fluid dynamics (CFD) simulation and analysis of the generation of soybean oil-in-water emulsion droplets via asymmetric straight flow-through MCs, each consisting of a microslot and a narrow MC. We also used CFD to investigate the effects of the channel size and the flow of the dispersed phase on MC emulsification using asymmetric straight flow-through MCs with a characteristic channel size of 5–400 μm. The overall shape of an oil–water interface and the time scale during droplet generation via a control asymmetric straight flow-through MC were appropriately simulated. Better insight was obtained on the flow profile of the two phases and the internal pressure balance of the dispersed phase during droplet generation. Comparison of the CFD and experiment results also provided insight into dynamic interfacial tension during droplet generation. Successful droplet generation was observed below a critical dispersed-phase velocity. In this case, the resultant droplet size was proportional to the channel size and was not sensitive to the dispersed-phase velocity applied. The maximum droplet generation rate per channel was inversely proportional to the channel size, unless the buoyancy force did not promote droplet detachment. The maximum droplet productivity per unit area of an asymmetric straight flow-through MC array was estimated to be constant, regardless of channel size.     

Influence of Flow Conditions in High‐Pressure Orifices on Droplet Disruption of Oil‐in‐Water Emulsions

Flow conditions in and behind high‐pressure orifices are described by a characteristic correlation between discharge coefficient and Reynolds number. The use of a pressure vessel and variations in viscosity allowed for non‐pulsating flow conditions from laminar to turbulent flow. Emulsions were homogenized under each condition. A considerable difference was observed in the final droplet size distribution depending on laminar, transitional, and turbulent flow. When the flow was pulsating as found when applying a plungers pump, transition of the flow from laminar to turbulence was more difficult to detect. Emulsions homogenized under these conditions indicated broader droplet size distributions. The Sauter mean diameter, however, was not affected by the pulsating flow.     

基于T形共流聚焦法的液滴生成技术

针对传统流动共聚焦法两相流注入结构复杂,外部封装体积大的问题,本文开展了毛细管中微液滴生成的相关技术研究,借助商品化T形管,提出了一种基于T形共流聚焦结构的液滴生成方法。该方法不但简化了两相流的注入结构,较好地解决了封装难的问题,而且便于对液滴生成的相关参数开展进一步研究。对于液滴生成的相关参数,本文深入研究了液滴生成中液滴尺寸、流速比和生成频率之间的关系;通过正交试验深入研究了流速比、出口锥角角度和出口管径对液滴均一性的影响,其影响的主次顺序为：锥角角度＞流速比＞出口管径,在最优参数锥角角度为4°,流速比190∶1,出口管径72μm的条件下,所生成液滴平均体积为8.3nL,生成频率0.7Hz,均一性0.011。     

Droplet formation of H2SO4/alkane system in a T‐junction microchannel: Gravity effect

A special system of concentrated sulfuric acid (H₂SO₄) and n‐hexane was used to study the droplet formation in a glass T‐junction microchannel with H₂SO₄ as the continuous phase. The effects of capillary number, flow ratio, and viscosity ratio on the droplet formation were investigated. The effect of gravity was explored by changing the flow direction in the microchannel. Results showed that the formation of transition flow pattern from squeezing to dripping is much easier for this special system compared with common aqueous/organic systems. This phenomenon is due to the considerably higher viscosity of H₂SO₄ than that of common aqueous phase and the higher density difference of the system compared with those of common systems. In addition to capillary number and flow ratio, gravity evidently affects the formation of droplets and flow patterns. The droplet size is smaller than that during the horizontal flow when the flow direction is consistent with gravity. By contrast, flow direction contrary to that of gravity results in larger droplet size than that at horizontal flow. This phenomenon provides guidance on the operation of these special systems in microchannels. Finally, mathematical models of droplet size at different flow patterns have been established, and these models can predict droplet size very well. This study could be helpful to extend the application of microreactors to new working systems. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4564–4573, 2016     

A descriptive force‐balance model for droplet formation at microfluidic Y‐junctions

In a previous article, we studied the basics of emulsification in microfluidic Y‐junctions, however, without considering the effect of viscosity of the disperse phase. As it is known from investigations on many different microstructures that viscosity and viscosity ratio are governing parameters for droplet size, we here investigate whether this is also the case for microfluidic Y‐junctions and do so for a wide range of process conditions. The investigated Y‐junctions have a width of 19.9 or 12.8 μm and a depth of 5.0 μm, and the formed monodisperse droplets (CV < 1%) are between 3 and 20 μm. We varied the disperse‐phase viscosity using different oils (1–105 mPa s), and continuous‐phase viscosity using glycerol–water and ethanol–water mixtures (1.0–6.2 mPa s), which corresponds to disperse‐to‐continuous‐phase viscosity ratios from 0.4 to 105.0. Through the variation of the liquids, also a range in interfacial tensions (12–55 mN m^?1) is assessed. The disperse‐phase flow rate is varied from 0.039 to 18.0 μL h^?1, the continuous‐phase flow rate from 1.39 μL h^?1 to 0.41 mL h^?1, and this corresponds to flow rate ratios from 1.1 × 10^?3 to 0.14, which is once again based on wide range of conditions. For all these conditions, in which droplets are formed in the dripping and jetting regime, the droplet size could be described with a model based on the existing force‐balance model, but now extended to incorporate the cross‐sectional area of the droplet and the resistance with the wall. Surprisingly enough, it was found that the droplet size is not influenced by the disperse‐phase viscosity, or the viscosity ratio, but it is dominated by the resistance with the wall and the continuous‐phase properties. Because of this, emulsification with Y‐junctions is intrinsically simpler than any other shear‐based method as droplet size is only determined by the continuous phase. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

流动聚焦微通道内牛顿微液滴在幂律剪切致稀流体中的生成研究

采用开源CFD软件OpenFOAM中的interFoam求解器对流动聚焦微通道内微液滴的形成过程进行了数值模拟。通过与文献中的实验数据进行对比,验证了VOF模型和幂律非牛顿流体模型的准确性。并以此为基础模拟了幂律剪切致稀流体中牛顿微液滴的形成过程,研究了幂律流体的幂律指数n和稠度系数K对微液滴生成的影响。研究表明,在滴状和挤压状流型中,离散线颈部宽度与周期内剩余时间呈幂律关系;离散线长度在坍塌阶段呈现线性缓慢增长,在夹断阶段呈现近似指数迅速增长的趋势。随着n和K的增大,液滴的尺寸逐渐减小,而生成频率则逐渐增大,且n的变化比K的变化对其产生的影响更明显。     

Droplet splitting in multi-furcating microchannel: A three-dimensional numerical simulation study

This work investigates the splitting of a droplet in a multi-furcating microfluidic channel for a two-phase system employing 3D simulation. The simulations were performed using an explicit volume of fluid (VOF) method and have been validated using experimental data taken from the literature. The width ratio of the branch channel to the main channel is set to 0.25 for five branches of the multi-furcating microchannel, as it is the width ratio at which multiple splitting takes place. Simulations have been carried out at different oil velocities (V_o) ranging from 0.12 to 0.22 m/s and at different water velocities (V_w) ranging from 0.002 to 0.10 m/s. Oil fraction data in the main channel has been recorded and compared with the homogenous model. The average difference between the homogeneous model and the 3D simulations is 22.68%. Analysis of dimensionless droplet length in ±0°, ± 40°, and 90° branch channels has been done. α (length of the droplet in branch channel/width of the main channel) increases up to a flow rate ratio of 0.38, and then decreases, whereas β (length of the droplet in the main channel/width of the main channel) increases with an increase in flow rate ratio. A flow pattern map has been developed to identify the various droplet breakup regimes at the junction. Frequency (counts per unit time) of droplet generation increases with capillary number for all the branch channels except for the 0° branch channel, where the regime is that the droplet passes through three branch channels. The volume distribution ratio (λ) decreases at first, then increases with an increase in capillary number for 0°:90° and 40°:90° angle branch channels for the regime where the droplet passes through five branch channels. For the regime where the droplet passes through three branch channels, the trend is likely linear with λ = 0.3 ± 0.04. The dimensionless mother droplet length increases with an increase in capillary number for V_o = 0.13 and 0.16 m/s, but for V_o = 0.19 and 0.22 m/s, the dimensionless mother droplet length becomes constant after capillary number = 0.26 and 0.30 respectively. The droplet breakup time (t) for regime (a), where the droplet passes through three branch channels, is 0.002 s; for regime (b), where the droplet passes through five branch channels, it is 0.001 s; and for regime (c), where multi-furcation and coalescence of the droplet occurs, it is 0.0005 s. Multiple splitting is a topic covered in this paper that can be applied to upcoming microfluidic platform-based devices.