T-junction Microchannels Research Articles

Experimental Studies of Droplet Formation Process and Length for Liquid–Liquid Two-Phase Flows in a Microchannel

In this study, changes in the droplet formation mechanism and the law of droplet length in a two-phase liquid–liquid system in 400 × 400 μm standard T-junction microchannels were experimentally studied using a high-speed camera. The study investigated the effects of various dispersed phase viscosities, various continuous phase viscosities, and two-phase flow parameters on droplet length. Two basic flow patterns were observed: slug flow dominated by the squeezing mechanism, and droplet flow dominated by the shear mechanism. The dispersed phase viscosity had almost no effect on droplet length. However, the droplet length decreased with increasing continuous phase viscosity, increasing volume flow rate in the continuous phase, and the continuous-phase capillary number Cac. Droplet length also increased with increasing volume flow rate in the dispersed phase and with the volume flow rate ratio. Based on the droplet formation mechanism, a scaling law governing slug and droplet length was proposed and achieved a good fit with experimental data.

Water Droplets Translocation and Fission in a 3D Bi-Planar Multifurcated T-Junction Microchannels

Droplet fission has gained notable interest in drug delivery applications due to its ability to perform parallel operations in single device. Hitherto, droplet flow behavior in a 3D constriction was scarcely investigated. This study aims to investigate droplets fission inside a 3D bi-planar multifurcated microfluidic device. The flow behavior and droplet size distribution were studied in trifurcated microchannels using distilled water as dispersed phase (1 mPa·s) and olive oil (68 mPa·s) as continuous phase. Various sizes of subordinate daughter droplets were manipulated passively through the modulation of flowrate ratio (Q) (0.15 < Q < 3.33). Overall, we found droplet size coefficient of variations (CV%) ranging from 0.72% to 69%. Highly monodispersed droplets were formed at the upstream T-junction (CV% < 2%) while the droplet fission process was unstable at higher flowrate ratio (Q > 0.4) as they travel downstream (1.5% < CV% < 69%) to splitting junctions. Complex responses to the non-monotonic behavior of mean droplet size was found at the downstream boundaries, which arose from the deformations under nonuniform flow condition. CFD was used as a tool to study the preliminary maximum velocity (Umax) profile for the symmetrical (0.01334 m/s < Umax < 0.0153 m/s) and asymmetrical branched channels (0.0223 m/s< Umax < 0.00438 m/s), thus complementing the experimental model studies.

New insights into generation of highly controllable monodisperse high-throughput microdroplets in a T-junction microchannel with step structure

With the development of droplet microfluidic technology, there are some problems in the current generation of droplets using microchannels, such as high cost, and difficulty in controlling the size of droplets and the generation frequency. This paper applies numerical simulation method to study T-junction microchannels. Studies have shown that the pressure difference in the T-junction microchannel and the viscous shear force of the continuous phase act as a driving force in the droplet formation process. The effective droplet diameter decreases as the Ca increases, and the generation frequency increases as Ca increases. Based on the level-set method, three T-junction microchannels with step structure at different positions are designed to study the problem of generating monodisperse high-throughput microdroplets. This paper achieves flexible control of droplet generation using three different designs. In the three designs, when the step position is in the middle, the generated droplet diameter is the smallest and the frequency is the fastest. Therefore, studies have shown that the step structure in the middle is the optimal solution for generating monodisperse high-throughput microdroplets.

Slug Formation Analysis of Liquid–Liquid Two-Phase Flow in T-Junction Microchannels

Slug flow is a common flow pattern in the liquid–liquid two-phase flow in microchannels. It is an ideal pattern for mass transfer enhancement. Many factors influence the slug formation such as the channel geometries (channel widths, channel depth), flow rates of the two phase, and physical properties. In this paper, in order to investigate the liquid–liquid two-phase slug formation in a T-junction microchannel quantitatively, the volume of fluid (VOF) method is adopted to simulate the whole slug formation process. With the validated model, the effects of the disperse phase channel width, channel depth, and two-phase flow rate ratio on slug formation frequency and slug size (slug volume and slug length) are analyzed with dimensionless parameters. Dimensionless parameters include the disperse-to-continuous phase channel width ratio, height-to-width ratio, and two-phase flow rate ratio. Results show that both the channel geometry and two-phase flow rate ratio have a significant influence on slug formation. Compared with the conventional slug formation stages, a new stage called the lag stage emerges when the disperse phase channel width decreases to half of the continuous phase channel width. When the channel depth decreases to one third of the continuous phase channel width, the flow patterns become unstable and vary with the two-phase flow rate ratio. Moreover, empirical correlations are proposed to predict the slug formation frequency. The correlation between slug formation frequency and slug volume is quantified.

Prediction of Microdroplet Breakup Regime in Asymmetric T-Junction Microchannels.

Splitting droplets is becoming a major functional component in increasing number of droplet microfluidic applications, and there is an increasing interest in splitting droplets into two daughter droplets with different volumes. However, designing an asymmetric droplet splitter and predicting how a droplet splits in such designs is not trivial. In this study, numerical simulations were conducted to study droplet breakup in asymmetric T-junctions of square cross-sections having different pressure gradient ratios (i.e. T-junctions with outlet branches of different lengths). The goal of the simulation is to identify the conditions where a parent droplet breaks or does not break into two smaller droplets of different sizes (so called critical condition) and to identify the important fluid and microchannel parameters in this process. Four modes of droplet breakup (primary-, transition-, bubble-, and non-breakups) are identified and an empirical correlation is introduced that can predict the breakup/non-breakup of the droplet based on the parent droplet size and the capillary number. The simulation results are then compared with experimental data to verify its accuracy and the effect of fluids properties on the proposed correlation are studied. Two major asymmetric breakup mechanisms are determined, namely "breakup with permanent obstruction" and "unstable breakup". The numerical results show that the splitting ratio for the asymmetric breakup mechanisms depends on flow conditions and dwell time of the droplet at the junction prior to splitting. Finally, the results from two-dimensional and three-dimensional simulations were compared. It is shown that two-dimensional simulation may not accurately predict the breakup behavior for asymmetric droplet breakup and viscosity ration has a greater effect on the prediction critical condition.

Modeling of the pressure fluctuations induced by the process of droplet formation in a T-junction microdroplet generator

To quantitatively study the pressure fluctuations induced by the process of droplet formation in a T-junction microdroplet generator, the mathematical model which can accurately predict the amplitude and the frequency of the pressure fluctuations is demonstrated. Different geometries of the T-junction microchannels are designed for the experiments of droplet formation, and the time-varying pressure difference between the upstream and downstream of each droplet is measured during the droplet production progress. From both theoretical and experimental study, it can be observed the pressure difference is a periodic function of time for the whole process of droplet generation. In particular, the frequency of the pressure fluctuations coincides with the production rate of droplets, and the amplitude of the pressure fluctuations varies with the geometrical parameters of the T-junctions. Additionally, good agreements are shown between the theoretical calculations and the experimental measurements of the magnitude of the pressure fluctuations. Therefore, our mathematical model is validated experimentally, and the magnitude of the pressure fluctuations can be drastically reduced by optimization of the geometrical parameters of the T-junction.

Quantitative study of the production rate of droplets in a T-junction microdroplet generator

In a T-junction microdroplet generator, a mathematical model which can quantify the production rate of droplets is demonstrated. The experiments of droplet formation are performed for different geometries of the T-junction microchannels, and good agreements are shown between the predicted and the measured values of the droplet production rates. From both theoretical and experimental study, the production rate of droplets varies nonlinearly with the flow-rate ratio of the two phases during droplet formation, and by fixing the flow-rate ratio, the production rate of droplets is approximately a linear function of the flow rate of the fluids. In particular, the coefficients of the linear relation are only determined by the geometrical parameters of the T-junction microchannel. As a result, our model can be validated experimentally, and especially for a specific geometry of the T-junction, the production rate of droplets can be precisely predicted and controlled based on the flow rate of the fluids.

Lattice Boltzmann simulation of drop formation in T-junction microchannel

In this study, we numerically studied the drop formation in the ordinary and modified T-junction microchannels. The lattice Boltzmann method based on the pesudo-potential method is utilized for the multiphase flow simulation. The results are verified by several well-known benchmarks, such as the Laplace test, coalescence of two static drops, contact angle, and the drop formation in the T-junction microchannels. Present results depict a good agreement with the previous published numerical and experimental studies. A detailed investigation of the effect of various parameters including Capillary number, the flow rate ratio, the width ratio, and the contact angle on the length of drop, as well for the distance between drops in the ordinary and the modified T-junction microchannels are performed and the results are presented in details. The results reveal that by simple modifications on the ordinary T-junction, the smaller drops and the lower distance between them can be generated in the modified T-junction microchannels in the comparison of the ordinary T-junction geometry under the same conditions. The results reveal that the size of drops and the distance between them are very sensitive to the geometry of the micro-channel. This study shows that the Pseudo-Potential lattice Boltzmann method is an effective way to simulate the generation of drops in microchannels.

Comparative analysis of gas–liquid flow in T-junction microchannels with different inlet orientations

In this study, a comparative analysis of two-phase flow in T-junction microchannels with different inlet orientations was carried out. Based on computational fluid dynamics and the volume-of-fluid model, bubble size, bubble velocities, and pressure distributions were analyzed. The numerical algorithm was validated with the experimental observations from former literatures; the results show that when the capillary number, Ca, is low, the length of gas and liquid slug in the symmetric T-junction is higher than that in the cross-flow T-junction. The effect of different forces acting on the slugs during the process of bubble formation was studied by investigating the velocity gradient and pressure distribution in the mixing zone. The result shows that the shear stress in cross-flow T-junction is over two times of that in T-junction with two symmetric inlets under the same operating conditions, which indicates that the size of bubble and liquid slug depends on the shear stress at low Ca number.

Lattice Boltzmann Simulation of Droplet Formation in Non-Newtonian Fluids

AbstractNewtonian and non-Newtonian dispersed phase droplet formation in non-Newtonian continuous phase in T-junction and cross junction microchannels are investigated by the immiscible lattice BGK model. The effects of the non-Newtonian fluid power-law exponent, viscosity and interfacial tension on the generation of the droplet are studied. The final droplet size, droplet generation frequency, and detachment point of the droplet change with the power-law exponent. The results reveal that it is necessary to take into account the non-Newtonian rheology instead of simple Newtonian fluid assumption in numerical simulations. The present analysis also demonstrates that the lattice Boltzmann method is of potential to investigate the non-Newtonian droplet generation in multiphase flow.

Effects of finite distance between a pair of opposite transversal dimensions in microchannel configurations: DSMC analysis in transitional regime

The influence of transversal walls on a stationary near-isothermal gas flow in three dimensional microchannel configurations is studied using the Direct Simulation Monte Carlo (DSMC) method in the late slip regime and the transitional regime of the gas flow, for some selected values of the rarefaction parameter. The study was performed considering a straight, a 90 degree bend and a T-junction microchannels, with a rectangular cross-section. Implicit boundary conditions were utilized for applying a given pressure at the inlet and outlet boundaries. Some of the obtained results concern two dimensional configurations by confining the third dimension between two specularly reflecting walls, so that the flow in the third direction is homogeneous. A comparison of the DSMC produced results with those given by the finite volume based SIMPLE-TS was performed. The hard sphere model was used. The No Time Counter (NTC) scheme was used for computing the particle collisions. A DSMC based software was developed to study the gas flow in microchannel configurations, and a part of the results were obtained using an OpenMP parallel version of the software.

Alternating and merged droplets in a double T-junction microchannel

In this work, we report experimental and numerical studies of alternating and merged droplets in a double T-junction microchannel. The microchannel device is fabricated using PDMS substrate and experiments are performed with mineral oil with surfactant as the continuous phase and aqueous glycerol as the discrete phase. Based on the flow rate fraction ϕ and Capillary number Ca, four different flow regimes are identified: merging, stable alternating droplets, alternating droplets with transition and laminar. A numerical model that employs volume-of-fluid formulations is used to predict the alternating droplet generation process. In the stable alternating droplet regime, the effect of the discretephase flow rate ratio α on the droplet diameter ratio β is experimentally studied and compared with that predicted from the simulations. It is observed that the droplet diameter ratio β increases linearly with increase in the flow rate ratio α and a good match between experiments and simulations is observed. The diameters of droplets at different Capillary numbers Ca generated using single and double T-junction microchannels are compared and it is observed that, at low Ca, the double T-junction generates larger droplets as compared to single T-junction. In merged droplet regime, the effect of the continuous phase flow rate Qc and discrete phase viscosity μd on diameter dm and interdistance between the droplets λ of the merged droplets are studied. It is observed that the merged droplet diameter dm is reduced and interdistance between the droplets λ increases with increase in the continuous phase flow rate Qc. As the viscosity of the discrete phase μd increases, the diameter dm and interdistance between the droplets λ of the merged droplets decreases.

Mass transfer characteristics of bubbly flow in microchannels

The mass transfer characteristics of bubbly flows in co-flowing and T-junction microchannels are described in this study. A CO2N2 gas mixture and a monoethanolamine–ethylene glycol solution were used as the gas and liquid phases, respectively, to generate gas/liquid bubbly microflows. The bubble dimensions changed obviously during the bubble-forming and bubble-flowing stages. The overall mass transfer coefficients (KL) at these stages were determined by developing an online measurement method through which time-dependent changes in bubble volume were analyzed. Investigation of the effects of phase flow rate and concentration of the two phases on KL showed that KL was in the range of 1×10−4–4.9×10−3m/s. In addition, empirical correlations were established to predict KL for different mass transfer stages.

Y- and T-junction microfluidic devices: effect of fluids and interface properties and operating conditions

Water-in-oil emulsions were produced in microchannels with Y- and T-junction geometries by individual droplet generation. For each microchannel configuration, the effect of the fluids and interface properties as well as of the process conditions was evaluated. The size of the droplets depended mainly on the relative velocity between continuous and dispersed phases and the relative fluid viscosity between phases. Those variables were related to the shear stress between the phases, which caused the droplet detachment. In addition, the interfacial forces played a minor role in Y-junction, and they had no effect in the droplets formation in T-junction microchannels. In Y-junction, a large variation in the droplet size was observed, depending on the system composition and the operating conditions. At low relative velocity and fluid viscosity, no droplets were generated. In contrast, the process in T-junction resulted in a lower variation of droplets size and the droplets were formed even at less favorable conditions. Such results indicate that the knowledge of the mechanism of droplets generation in each microchannel geometry makes it possible to choose the appropriate configuration according to the type of fluid, and the operating conditions can be adjusted to obtain the desired final emulsion.

Experimental characterization of gas–liquid–liquid flows in T-junction microchannels

Micro-reaction/reactor technology has been used to intensify several chemical processes involving gas–liquid and liquid–liquid reactions. However, potential of intensifying gas–liquid–liquid reactions still remains unexplored. In the present work, we report experimental investigations of gas–liquid–liquid three-phase flows in microchannels. High-speed imaging experiments were performed to identify and to map flow regimes; to understand the effect of flow rates of continuous (kerosene) and dispersed phases (air and water) on the flow regimes, formation mechanisms of air bubbles/slugs and water drops/slugs and also to measure the slug lengths. In particular, the effects of different gas and liquid distributors and addition of surfactant were investigated to understand controlled generation of bubbles and drops and formation of different flow regimes in microchannels. The results presented in this manuscript will be important to device microreactor systems for catalytic reactions involving gas–liquid–liquid flows.

New regime of droplet generation in a T-shape microfluidic junction

We present an experimental study of a new regime of monodisperse micro-droplet generation that we named the balloon regime. A dispersion of oil in water in a T-junction microfluidic system was studied. Several microfluidic devices having different cross-sections of the continuous and the dispersed phases micro-channels were tested. This new regime appears only for low- dispersed phase velocity. The micro-droplet size is mainly related to the geometry of the T-junction micro-channels especially its width and depth, and independent of the continuous and dispersed phases velocities. In our experiments, the velocities of the continuous and the dispersed phases $$\overline v_{\rm c}$$ and $$\overline v_{\rm d}$$ respectively, have been varied in a wide range: $$\overline v_{\rm c}$$ from 0.5 to 500 mm/s, and $$\overline v_{\rm d}$$ from 0.01 to 30 mm/s. We show that the continuous phase only controls the micro-droplet density, while the dispersed phase linearly changes the frequency of the micro-droplet generation. Another particularity of the present regime, which differentiates it from all other known regimes, is that the micro-droplet retains its circular shape throughout its formation at the T junction, and undergoes no deformation due to the drag forces. We propose a mechanism to explain the formation of micro-droplets in this new regime.

Mass transfer characteristic in the formation stage of gas–liquid segmented flow in microchannel

In this paper, the mass transfer characteristic in the formation stage of gas–liquid segmented flow in microchannel was investigated. Modified T-junction microchannels with differing contact angles were applied as the microdevices. The length of formed gas slug was determined firstly. An in situ method was developed to measure the mass transfer characteristic. The results showed that the initial length of gas slug with mass transfer was much smaller than that without mass transfer. The slug formation time was in the range of 0.2–0.4s and the mass transfer during the formation stage could contribute around 30–40% of the total transferred solute. The overall mass transfer coefficient during the slug formation stage was in the range of 1.4–5.5×10−4 m/s. The comparison between the experimental results and the calculated data by diffusion model indicated that the recirculation flow in the gas slug significantly intensified the mass transfer with the enhancement factor in the range of 5–15. A correlation equation was developed to predict the enhancement factor, considering both the influences of the surface renewal and the moving speed of the gas phase. The calculated data was in good coincidence with the experimental results.

Droplet generation in micro-sieve dispersion device

Microfluidic devices with micro-sieve plate as the dispersion medium have been widely used for the mass production of emulsions. While unfortunately, few studies have so far been made for the droplet generation rules in those devices. In this work, the droplet generation processes in micro-sieve dispersion devices are investigated with specially designed micro-sieve pore arrays. The effects of channel structure, pore arrangement, and feeding method of dispersed phase on the average size and distribution of droplets are studied carefully. It is found the dimensionless average droplet diameters (dav/de) in micro-sieve dispersion devices can be represented by a linear relation with Ca−1/4 of continuous phase, the same as the scaling law in T-junction microchannels. The flow distribution among pores and the steric hindrance between droplets affect the diameter distribution of generated droplet very much. Monodispersed droplets with polydispersity index less than 5% can be made at Ca number larger than 0.01 and phase ratio (QD/QC) less than 1/6 in the present investigation.

Staged alkylation in microchannels

The liquid–liquid distribution and droplet dynamics in branching T-junction microchannels were studied experimentally. The droplet flow patterns in the main channel are divided into: short-slug flow, long-slug flow and parallel flow. According to the distribution characteristics of droplets in branching T-junction microchannels, it is divided into break up at branching microchannels C1-C4 and break up at branching microchannels C1-C5. The effects of flow rate of both phases and crossing flow on the two-phase distribution uniformity in branching microchannels C1-C4 and C1-C5 are described. Finally, the two-phase distribution in branching microchannels C1-C4 and C1-C5 is analyzed and predicted, and it has achieved a good prediction effect on liquid–liquid two phases distribution in branching microchannels.