T-junction Microchannels Research Articles

Merged and alternating droplets generation in double T-junction microchannels using symmetrically inserted capillaries

Merged and alternating droplets generation in double T-junction microchannels using symmetrically inserted capillaries

Numerical investigation on flow regime transition mechanism and length prediction method of droplet in T-junction by electric field

Although droplet microfluidics has received attention and research in the fields of chemistry and materials in recent years, research on electric field modulation of microchannel droplets is currently fragmented. In this paper, the effect of electric field and flow rate ratio on the behavior of discrete-phase flow in systems with different viscosity ratios was simulated. Subsequently, we delineated the regions of slug flow, drip flow, jet flow, and parallel flow under different parameters according to the simulation results, and investigated the mechanism of electric field manipulating the transformation of the above four flow types by combining the velocity vector diagram with the force situation. Additionally, in order to fill the gap in methods for predicting droplet length in T-junction microchannels under the presence of electric field, we present a scaling law for predicting droplet length that couples the effects of electric field, velocity, and viscosity. And it was verified that the error of the model built based on this method is less than 12 % when the flow ratio is less than 1.

Experimental study and multistage design of multiphase droplets splitting based on T-junction microchannels

The splitting of multiphase droplets in T-junction microchannels is investigated in this work. Based on the single-phase droplet splitting regime and mechanism, the splitting of multiphase droplets is visualized and further analyzed. Focusing on the influence of the interfacial tension between the phases of multiphase droplets on the splitting behavior of multiphase droplets, it is found that the larger the interfacial tension between the two phases, the stronger the hindering effect of droplet fragmentation, and the more difficult the droplet is to break up. Meanwhile, the strengthening effect of baffle structure at the bottom of the T-junction on the multiphase droplet splitting process is evaluated, where the introduction of the baffle greatly reduces the possibility of no breakup. In addition, multistage splitting T-junction microchannel systems are designed and established for the scale-up preparation of multiphase droplets. This study is of great significance for the in-depth understanding of the splitting process of multiphase droplets. The “self-similar” designs of multistage splitting microchannel from droplet splitting phase diagram provide solutions for the large-scale and cross-scale preparation of multiphase droplets in various fields.

Hydrodynamic characteristics of core/shell microdroplets formation: Map of the flow patterns in double-T microchannel

Liquid-liquid–liquid (L/L/L) flows in microchannels involving three-phase interactions are important in various chemical applications that entail liquid–liquid–liquid reactions. This work comprehensively investigates core (silicone oil)/shell (Polycaprolactone) microdroplet formation using a continuous third phase (distilled water) in a double T-junction microchannel with a diameter of 600 µm, employing Volume-of-Fluid (VOF) simulations and high-speed imaging experimental observation. Due to the satisfactory agreement between computational and experimental techniques, the theoretical flow patterns were identified and categorized into three distinct categories at various flow rates of three phases: core/shell dripping, core/shell slug, and core/shell jetting. The shell thickness increases as the ratio of shell to core phase velocity (ushell:ucore) increases. Based on the continuous phase Capillary number (Cac) and core and shell phases Weber number (Wecore,shell), a flow pattern map was generated that segregates the distinct flow patterns into different regions. When the flow of the core and shell phases fails to form a droplet due to the inertia of the continuous phase, a core/shell dripping/slug pattern is observed. The channel walls encompass the droplets that form in the slug pattern (2×10-5≤Cac≤5×10-4, 0<Weshell≤1.5×10-3, and 0<Wecore≤5×10-5) because the continuous phase has less inertia, whereas, in the dripping pattern (5.5×10-5≤Cac≤9×10-4, 7×10-5≤Weshell≤2.5×10-4, and 1×10-5≤Wecore≤7×10-5), the shell phase fluid is enclosed by the continuous phase. The transition from dripping to jetting (Cac≥0.5, 0<Weshell≤0.15, and 1×10-2≤Wecore≤8×10-2) occurs as the flow rate of a continuous fluid or shell fluid increases. A dimensionless correlation for shell thickness estimation with a 12 % tolerance was proposed with a core/shell/continuous phase's physical properties, surface tensions, and microdroplet size, using Buckingham's theory based on 200 CFD simulations. Additionally, the flow rate and viscosity substantially impact the internal fluid velocity and vorticity of core/shell microdroplets, whereas the influence of interfacial tension can be disregarded. This research lays the groundwork for future studies on mass transfer and reaction behaviors in liquid–liquid–liquid three-phase flow in double T-junction microchannels by exploring various hydrodynamic characteristics under different operating conditions for core/shell microdroplet formation.

A strategy for regulation of gas–liquid microflow patterns by changing gas kinetic energy

Controllable regulation of gas–liquid microflow pattern and bubble size is significant to bubble-based microfluidics, but it is hard to be realized with the existing technologies. Accordingly, this work reports a convenient strategy that changes the gas kinetic energy to controllably regulate the microflow patterns from Taylor flow to short bubble flow and bubbly flow. Results exhibit that the desired flow patterns and bubble size can be effectively obtained by varying the gas inlet size to change gas kinetic energy, and a much wider bubbly flow regime which could not be reached normally in conventional T-junction microchannels is easily obtained. Besides, from the bubble’s characteristic width and length, the behaviors of the bubble generation in different T-junctions were compared to reveal the mechanism behind the generation of bubbles. Importantly, as the hydraulic diameter of the gas inlet decreases from 365.0 to 50.0 μm, the dominative factors on the bubble generation transfer mainly from the liquid phase to both the two-phases because the gas–liquid Weber number ratio increases from 6.6 × 10−5–5.4 × 10−3 to 4.2 × 10−2–3.4. Finally, the parameter of the two-phase Weber number ratio is the first time introduced to the unified prediction model for bubble generation frequency.

Asymmetric branch selection and splitting of droplets in T-junction microchannels

The droplet motion in the T-junction is the basis for the design of droplet microfluidic chip. This paper investigates droplet motion in T-junction microchannels based on experiments and simulations to analyze the droplet motion mode and its effect on the downstream flow of the channel. The droplet motion can be divided into three modes, including flowing into the downstream main channel, flowing into the side branch, and splitting, whose transitions depend on the inlet flow ratio and droplet length. The critical droplet length, to determine whether the droplet is complete through the junction, follows a non-linear relationship of capillary number with the coefficient of 0.3–0.5, which is influenced by the liquid viscosity ratio. It was found that the droplets can be divided into two kinds according to its length by l0 = 0.8wm, which have significantly different interfacial deformations and cause various fluctuations of the branch flow even under the same flow conditions.

Effect of channel width on droplet generation inside T-junction microchannel

In a T-junction microchannel, channel geometry plays a major role that affects the physics behind droplet generation. The effect of channel width on droplet size and frequency in a T-junction microchannel is investigated in the present study. The current work is an extension of our previous work, where a model was developed to predict the size of the droplets generated in a T-junction microchannel when both the continuous and dispersed phase channels have equal widths. In the present work, we extended the model to account for the varying width ratio between the dispersed and continuous phase channels. We performed in-house experiments by varying the channel width and viscosity ratios between the fluids to study the size of the droplets generated and to validate the proposed scaling law. We further investigated the effect of channel geometry on the frequency of droplet generation in the T-junction microchannels. The experimental results show that the droplet length increases with an increase in the width of the continuous phase channel. On the other hand, the droplet production frequency decreases with an increase in the width of the continuous phase channel. With variations in the width of the dispersed phase channel, similar behavior in droplet sizes and the frequency of droplet production is observed. The analysis of this study provides new insight into the effect of channel width on the droplet length and frequency. Overall, this research intends to provide a thorough understanding of the design of microchannels based on the geometry and manipulation of droplets with varying widths.

Dripping and jetting generation mode in T-junction microchannels with contractive structures

Droplet generation in T-junction microchannels with contractive structures is investigated under different flow rates and viscosity conditions by numerical simulation to improve the monodispersity and controllability of the microdroplets. Basic flow modes are observed, including dripping, transition, and jetting, under various flow rates and dispersed phase viscosities. The flow condition for the transition mode is modeled as a function of capillary number, flow rate ratio, and viscosity ratio to indicate the conversion of generation modes. Unlike ordinary T-junction microchannels, in this case, the droplet diameter first decreases and then increases with the increasing viscosity ratio in T-junction microchannels with contractive structures. By analyzing the velocity fields, pressure fields, and forces in droplet formation, the dynamic mechanism of the viscosity ratio on the generation mode and droplet size is achieved. The droplet volume in the dripping mode is analyzed to propose a prediction formula that takes into account the influence of the viscosity ratio.

Nanoparticle-enhanced hydrogen separation from CO2 in cylindrical and cubic microchannels: A 3D computational fluid dynamics simulation

In this study, hydrogen is purified from its mixture with CO2 using Monoethanolamine-based nanofluids in a T-junction microchannels at dynamic conditions with various lengths and diameters simulated with the aid of computational fluid dynamics. Navier-Stokes and continuity equations, reaction rate, and two-phase transport models are utilized in COMSOL Multiphysics software for this investigation. The impacts of different parameters on hydrogen purification such as solvent and nanoparticles (NPs) concentrations, and channel geometry are investigated. By decreasing the channel diameter, hydrogen purification increases. Furthermore, an increase in channel length enhances hydrogen purification exponentially. Results reveals that the cubic channel has higher hydrogen purification compared to the cylindrical channel. Besides, hydrogen is purified to 98.83% in 15 mm length. NPs concentration has an optimum value after which further increase in NPs concentration reduces in hydrogen purification. Among different NPs in this research, TiO2 has maximum hydrogen purification efficiency at 0.06 wt% which hydrogen purity can be increased up to 98.62 wt%.

Enhanced droplet formation in a T-junction microchannel using electric field: A lattice Boltzmann study

The electric field-driven droplet formation technique can effectively improve the formation throughput and control the droplet size, which is important for the application of microscale droplets in biopharmaceuticals and chemical analysis. In this paper, the droplet formation characteristics in T-junction microchannels under the action of electric field are investigated by coupling a three-dimensional lattice Boltzmann method (3 D LBM) with the leaky dielectric model, focusing on the effects of electric capillary number, a flow ratio, and a viscosity ratio on the droplet size. It is shown that as the electrical capillary number increases, the non-uniformly distributed electric force stretches the dispersed phase to form a Taylor cone and increases shear force at the interface of the two liquids to overcome the surface tension force. This facilitates the transition from squeezing to dropping and reduces the droplet size. At high flow ratios, increasing the electric capillary number leads to a pinning effect between the dispersed phase and the wall, which intensifies the compression of continuous phase on the neck of dispersed phase, resulting in a significant decrease in the droplet size. As the viscosity ratio increases, the vortex resistance caused by electrical force decreases, and thus, the electric field effect will dominate the droplet formation process.

Effects of nanoparticle surfactants on droplet splitting in a T-junction microchannel

<p indent="0mm">Droplet microfluidics can generate and manipulate discrete droplets through immiscible multiphase flows inside microchannels, which effectively controls the size and generation rate of droplets. Due to its remarkable advantages, droplet microfluidics bears a significant value in an extremely wide range of areas, such as bio(chemical) analysis, particle preparation or petroleum engineering. Droplet splitting is an important form of obtaining monodisperse and small droplets in microfluidic devices, which can obtain smaller droplets to meet the requirements of biochemical reaction and material encapsulation. Nanoparticle surfactants, formed at liquid−liquid interfaces by the interactions between functional groups on nanoparticles and polymers having complementary end-functionality, are recently proposed as an excellent interface stabilizer to cover liquid droplets for applications of substance encapsulation and delivery. Apparently, understanding droplet splitting behavior with consideration of nanoparticle surfactants is of great significance for obtaining smaller and better dispersive functional droplets. In this paper, by using the approach of microfluidic experiment and theoretical analysis, the effects of nanoparticle surfactants on droplet splitting behavior within T-junction microchannels were investigated. Comparisons were conducted among clean droplets and those stabilized by nanoparticles alone, surface-active polymers alone and nanoparticle surfactants, respectively. Besides, the effects of concentrations of both nanoparticles and amino-terminal polymers, which determined nanoparticle surfactants concentration on the fluid-fluid interface, on the droplet splitting behavior have been systematically investigated. We have found that the droplet splitting process can be divided into three stages: Entering stage, squeezing stage and post-splitting stage. The generation of nanoparticle surfactants can effectively reduce the time required for droplet splitting. Besides, the nanoparticle surfactants induced reduction of the total split time is mainly attributed to the decrease of the entering time, while no significant change is presented in the squeezing and post-splitting time. The dynamical behavior of droplets splitting under different reagent conditions can be divided into four categories: Blocking splitting, transition state, non-blocking splitting and non-splitting. The blocking splitting state refers to that the droplets are always in contact with the channel wall during the splitting process. Otherwise, there is always a gap between the droplet and the channel wall, which is called non-blocking splitting. Besides, the transition state is a special state of non-blocking splitting, which is in contact with the upper wall of the channel and has a gap with the bottom of the channel. More importantly, by analyzing the transient decreasing of neck width, we found that the thinning rate of the droplet was suddenly accelerated in the process of neck thinning, i.e., the rapid fracture stage. During neck thinning, the depression is fanned out until it reaches a critical width and becomes funnel-shaped. By geometric analysis of neck shape, a theoretical model of neck width thinning was obtained, which is applicable to different reagent conditions. Besides, the influence mechanism of nanoparticle surfactants on droplet splitting was obtained, i.e., affecting necking rate via decreasing the interfacial tension. Finally, via analyzing the droplet splitting state, a theoretical formulation was established for predicting the transition of blocking splitting and non-blocking splitting, which shows a good agreement with the experimental data. These findings provide a theoretical basis for the preparation of smaller functional droplets by droplet splitting.

Insights into the breaking and dynamic mixing of microemulsion (W/O) in the T-junction microchannel

In this paper we conduct a numerical simulation study of droplet generation and dynamic mixing in T-junction microchannels to investigate the effects of two-phase flow rate, viscosity of the dispersed phase and wall wettability on the mixing quality of droplets. It is found that both droplet size and contact length of droplets with the wall are key factors affecting mixing. The larger the droplet size, the worse the mixing effect, and the larger the contact length of droplets with the wall, the better the mixing effect. At the same time, the size of the droplet also constrains the contact length of droplets with the wall. The larger the size of the droplet, the greater the contact length of droplets with the wall, so that an equilibrium is reached, and when the size of the droplet is large enough, this equilibrium will be broken. As the viscosity of the dispersed phase increases, the internal circulation of the droplets becomes difficult and the droplets break up later, which reduces the mixing efficiency. As the contact angle increases, the inner wall of the channel becomes more hydrophobic and the viscous shear of the continuous phase dominates the mixing, changing the direction of the droplet's internal circulation. The precondition for the continuous phase viscous shear to dominate the mixing is that, there exists the velocity difference between the continuous phase fluid and the droplet, and there will be no good mixing effect otherwise. The droplets with good mixing quality should meet the following conditions: reduce the viscosity of the dispersed phase, prepare droplets with smaller size, make the droplets contact with the wall as much as possible, and let the viscous shear of the wall dominate the mixing.

Studies of the Movement Mechanism of Droplets in Power-Law Fluids in a T-Junction Microchannel by the Lattice Boltzmann Method

The movement mechanisms of Newtonian droplets in power-law fluids in T-junction microchannels were studied with the lattice Boltzmann method. The effects of power-law index <i>n</i> , capillary number <i>C</i>_a, flow ratio <i>Q</i> , viscosity ratio <i>M</i>, and surface wettability <i>θ</i> on the droplet formation size, the formation time and the deformation index (<i>D</i>_I) were investigated in detail. The results show that, first, with the increase of power-law index n from 0.4 to 1.6, the droplet formation size decreases almost linearly, and both the droplet formation time and the deformation index decrease quickly at first and then much more slowly. Second, the influences of the viscosity ratio on the droplet formation size, the formation time and the deformation index are basically the same as those of the power law index. In addition, with the increases of <i>C</i>_a and <i>θ</i> of the main channel, the droplet formation size decreases almost linearly, while the droplet formation time and deformation index decrease rapidly at first and then slowly, and the decreasing rates weaken with the increase of the power-law index. At last, with the increase of flow ratio <i>Q</i> of the continuous phase over the dispersed phase, the droplet formation size increases, and the droplet formation time as well as the deformation index decrease.

Distribution of liquid–liquid two-phase flow in branching T-junction 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.

A comprehensive study of droplet formation in a capillary embedded step T-junction: From squeezing to jetting

The understanding of microdroplet formation characteristics in unconfined T-junction microchannels is relatively weak. Accordingly, an unconfined step T-junction microdevice was specially designed and fabricated to systematically investigate the droplet formation mechanism. A new flow named pseudo-squeezing flow is defined. A liquid–liquid flow pattern map including the pseudo-squeezing, dripping, and jetting flow in the unconfined T-junction is plotted, which shows that a much higher capillary number is needed to realize the flow regime transformation when compared with the confined T-junction. The unconfined T-junction could provide a much wider dripping flow regime, and generate monodispersed smaller droplets. Besides, the droplet diameter decreases with the continuous phase flow rate in the pseudo-squeezing and dripping flow but suddenly increases when the flow regime transfers from dripping to jetting. Importantly, the very short liquid plug can be easily formed in this unconfined T-junction. Finally, because of the same formation mechanism, the droplet size in pseudo-squeezing and dripping flow can be perfectly scaled with a same dimensionless model. This work could be the first comprehensive study of the droplet formation in three flow regimes in the unconfined T-junction.

Impact of flow feedback on bubble generation in T-junction microchannels under pressure-driven condition

Under pressure-driven conditions of gas, the bubble size was experimentally studied in different types of T-junction microchannels having widths of 300 μm and heights of 100 μm. The bubble length increases with the flow rate, and we observed that it also increased with the driving pressure. This was proved through the published formulas for the bubble flow pressure difference using the experimental results. Three types of downstream channels—normal, movable, and deformed walls (of different lengths)—were used to study emerging bubbles, and a nonlinear relationship was observed between them. The channel type directly influenced the bubble size by affecting the resistance of the channel with bubbles. Furthermore, the difference between the resistances of a normal channel and a deformed channel with the same typical dimension was determined using a comparator structure between two parallel channels to evaluate the effect of flow resistance on bubble generation. The adjustment of the flow rate at downstream will give feedback to the bubble generation in the double T-junction channel.

Droplet behavior and its effects on flow characteristics in T-junction microchannels

The single-phase flow and droplet flow are investigated in a rectangular microchannel with a T-junction, through experiments and simulations to improve the understanding of a droplet flow and its effect on overall flow in channels with junctions. Droplet behavior can be divided into three modes: flow into the side branch, a split at the junction, and flow into the downstream channel. In branches of the junction, the flow rate ratio and the pressure difference are affected by droplets with the same flow behavior flowing in the junction. The change in the volumetric flow rate ratio and pressure difference between two channels also depend on droplet size and flow conditions. Furthermore, the length of the droplet affects whether the droplet splits at the junction, and this behavior can be documented by a power law relationship between the capillary number Ca and droplet length.

Characterization of dissimilar liquids mixing in T-junction and offset T-junction microchannels

Micromixing process in microfluidic devices has been broadly employed in bio-, nano-, and environmental technologies using either miscible or immiscible liquids. However, there are limited experimental studies investigating the mixing process of different densities and viscosities liquids in relation to microfluidics. Therefore, the mixing process of propan-2-ol and water, water and sodium chloride solution, propan-2-ol and sodium chloride solution were experimented and reported at 5 ≤ Re ≤ 50 in T-junction and offset T-junction microchannels. For miscible mixing experiments, i.e. propan-2-ol and water, water and sodium chloride solution, both microchannels show mixing index for each Reynolds number is directly proportional to the mixing time. At low Reynolds number, higher molecular diffusion takes place while at low flow rate, the residence time of fluid is high. The mixing performance is relatively good at high Reynolds number of 40 and 50 due to the significant convection which is caused by the effect of stretching and thinning of liquid lamellae. For immiscible propan-2-ol and sodium chloride solution mixing, offset T-junction microchannel offers better mixing performance than T-junction microchannel at both low and high Reynolds number. The chaotic mixing happened at the intersection of the T-junction microchannel due to the direct interaction of two liquids entering the junction at high momentum.

Geometric Effect on Gas–Liquid Bubbly Flow in Capillary-Embedded T-Junction Microchannels

Gas–liquid bubbly flows in T-junction microdevices with different microstructures were investigated in this study. Embedded capillaries were used to modify the microstructures of T-junction microch...

Prediction of droplet sizes in a T-junction microchannel: Effect of dispersed phase inertial forces

The generation of monodispersed droplets in T-junction microchannels has wide range applications in biochemical analysis and material synthesis. While the generation of these monodispersed droplets was previously considered to be a balance between forces acting from continuous phase and interfacial force, it is shown here that the inertial force from the dispersed phase also plays an important role in determining the size of the generated droplets. A theoretical analysis for the size of monodisperse droplets generated in a microfluidic T-junction device is developed, and it is validated with a large set of experimental observations. The theoretical analysis accounts for the inertial forces from the dispersed phase along with the forces from the continuous phase and the interfacial forces to define the non-dimensional numbers that govern the droplet breakup in the T-junction microchannel.