T-junction Microchannel Research Articles

Dynamic behavior and driving force model of droplet formation in a T-junction microchannel

The formation mechanism and breakup process of droplets in a T-junction microchannel are illustrated by analyzing the dynamic behavior and driving forces. The changes in the pressure in the microchannel during the four stages of droplet breakup and the effects of the capillary number are obtained. The driving force model of the droplet breakup is obtained by analyzing the magnitude and position of the forces acting on the emerging droplets. Furthermore, the governing equations of the droplet breakup and predictive equations for the equivalent length and generation frequency of the droplet are obtained. The influences of the capillary numbers of the continuous phase, the viscosity ratio, the flow ratio, and the channel size on the droplet generation parameters are analyzed. To verify the predictive formula of the equivalent length of a droplet, the effects of the capillary number and viscosity ratio on the droplet length and frequency are determined. The theoretical analysis and experimental results indicate that the droplet length is inversely proportional to the capillary number and directly proportional to the flow rate ratio and the viscosity ratio. The influence of the flow rate ratio is greater than that of the viscosity ratio. The wall affects the droplet formation and the shape of the droplets but does not affect the size of the droplets.

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.

Effect of Geometry Configuration on the Merged Droplet Formation in a Double T-Junction

The merged droplets have great practical application value in protein synthesis and crystallization. In this paper, the effect of geometry configuration on the merged droplet formation in a double T-junction microchannel is studied by three-dimensional numerical simulation using the level-set method. There are three important parameters in the geometry configuration of the microchannel, namely angle between two phases (α), height-to-width ratio (Ʌ = H/wc), and intersection width ratio (Γ = wd/wc). In this study we found that a critical value of the two-phase angle is 60°. When the angle is 60°, the effective diameter of the merged droplet is the smallest and the generation frequency is the fastest under the same physical condition. We found that height-to-width ratio and intersection width ratio have a critical value of 1.0. When height-to-width ratio or intersection width ratio is 1.0, the shearing capacity of the continuous relative dispersed phase reaches a maximum, thus the droplet diameter is minimized and the frequency is the fastest. Therefore, reasonable adjustment of these three factors is an effective method to solve the problem of the high-throughput monodisperse merged droplet formation. This work lays a solid theoretical foundation for the merged droplets in practical applications.

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75°. The effect of water to oil flow rate ratio (Qwat/Qoil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Qwat/Qoil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Qwat/Qoil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.

Growth characteristic of microbubble in a T-junction microchannel in microfluidic chip

Microbubble formation in a T-junction microchannel in polydimethylsiloxane (PDMS) microfluidic chip were developed. The experiment based on high-speed microscopic camera system was set up to investigate the effects of liquid flow rate, gas pressure and gas channel width on microbubble growth. Three stages of microbubble growth process were obtained through the experiment firstly. The control variable method was used for investigating the growth characteristics of microbubble, including volume change rate and the ratio of length to width. The present study provided an empirical reference for the growth of microbubble in microchannel, which helped to achieve precise control of microbubble volume.

Quantitative analysis of hydrodynamic effect on transesterification process in T-junction microchannel reactor system

Microfluidic technology offers high interfacial area particularly in the multiphase reaction such as transesterification process to produce biodiesel. Methanol-to-oil molar ratio is one of the important parameters to enhance the biodiesel production. However, very few in literature are focusing on the hydrodynamic effect that is caused by the molar ratio accurately. In this study, stable and highly consistent droplet-slug flow inside a microchannel was generated using a T-junction to investigate the impact of hydrodynamic factor on the reaction. High molar ratio creates high interfacial area and number of droplets, hence resulting in the highest conversion of oil. This finding has revealed that it is possible to obtain 98% oil conversion at room temperature and relatively short reaction time of 80 s. It demonstrates that microchannel reactor has high potential for intensified process in the production of biodiesel.

New insights into the pressure during the merged droplet formation in the squeezing time

In this paper, a three-dimensional numerical simulation of the merged droplet formation (MDF) in the double T-junction microchannel was performed to investigate the effects of the flow rate of the continuous phase, continuous viscosity and the interfacial tension on the pressure by the level-set method. We divide the droplet formation process into three stages, namely filling, blocking and breakup. We define the time to generate a droplet as the droplet generation period. The characteristic length L/W (L is the length of the droplet, and W is the width of the main channel) decreases as the capillary number increases. The pressure (PA) at a special position at the junction of the two phases occurs periodically during the droplet formation process, which directly and precisely reflects the mechanism of the MDF in the microchannel. Therefore, we define the time during the pressure fluctuates periodically as the pressure fluctuation period. New insights into the pressure during the MDF in the squeezing time are reported. The results show that the pressure fluctuation period and pressure drop period are the same as the droplet generation period. When the continuous flow rate and viscosity increase, the peak and valley values of PA and the pressure drop increase, and the time of the pressure cycle decreases. However, when the interfacial tension increases, the peak and theperiod increase and the valley decreases.

Microbubble characteristic in a T-junction microchannel in microfluidic chip

ABSTRACTFormation characteristic of microbubble in a T-junction microchannel in polydimethylsiloxane (PDMS) microfluidic chip is investigated. Microbubble formation experiments are carried out to investigate the effects of liquid flow rate, gas pressure and gas channel width on the detachment volume and formation time of microbubble. The growth process of the bubble is described by the form of force analysis, and the definition of the bubble detachment distance is introduced. The equivalent spherical theoretical model of bubble detachment diameter prediction is established, and the numerical solution of the model is obtained by the fourth-order Runge–Kutta method. The prediction results of theoretical model correlate well with the experimental data.

Numerical study of droplet formation in the T-junction microchannel with wall velocity slip

Effect of hydrophobicity on droplet formation in the T-junction microfluidic devices is numerically studied. A Navier slip model is adopted. Results reveal that, as increasing of slip length, the velocity profile across the channel becomes flattened. With the increase of slip length, the shear stress on dispersed phase exerted by the continuous phase decreases, thus, the deformation of the dispersed phase weakens while the final droplet size increases. The pressure distributions along the central axis of main channel show a smaller pressure drop when the wall velocity slip length increases.

Mass-Transfer Characteristics of CO2 Absorption into Aqueous Solutions of N-Methyldiethanolamine + Diethanolamine in a T-Junction Microchannel

The mass-transfer process of CO2 absorption into aqueous solution of N-methyldiethanolamine (MDEA) + diethanolamine (DEA) in a T-shaped microchannel was studied using a high-speed camera. The effec...

Liquid–Liquid Extraction of Yttrium(III) Using 2-Ethylhexyl Phosphonic Acid Mono-2-ethylhexyl (EHEHPA) in a Microreactor: A Comparative Study

Experiments were carried out in a slug flow microreactor to systematically investigate reaction behavior under variation of flow rate, and a comparative study was made. Y-junction microreactor and T-junction microreactor have been used to extract yttrium(III) using 2-ethylhexyl phosphonic acid mono-2-ethylhexyl (EHEHPA or P507). Results show that the maximum extraction efficiency of 90.4% in both microreactors could be achieved corresponding to the minimum flow rate of 10 and 100 μL/min. The values of specific interfacial area remain unchanged with the increase of flow rate, and the specific interfacial area of the Y-junction serpentine microreactor is much higher than that of the T-junction microreactor. Maximum values of volumetric mass-transfer coefficient (1.642 s–1) in the Y-junction microreactor are found to be several orders of magnitude higher than those of the T-junction microchannel (0.043 s–1) and conventional extractors (0.0197 s–1).

Numerical study of the dynamics of a droplet in a T-junction microchannel using OpenFOAM

The mechanisms of droplet breakup in a Microfluidic T-junction are mainly influenced by the capillary number Ca and the ratio between the length of the droplet and the transverse dimension of the microchannel (lo/w). This work presents numerical simulations of an isolated Taylor droplet in a rectangular T-junction microchannel in the range of 0.0096 to 0.0904 of capillary numbers and viscosity ratio of the dispersed and continuous phases, that is, μd/μc=0.125. The numerical simulations were carried out with OpenFOAM 2.4 by using the S-CLSVOF methodology which was implemented for this study. The snappyHexMesh tool was used for grid refinement near the microchannel wall in order to correctly model the thin fluid layer between the wall and the drop. It was verified that in this range of the capillary numbers, parasitic currents could affect the dynamics of the droplet in the microchannel. The use of S-CLSVOF, the time step control and mesh resolution enabled a better capture of the interface dynamics. A mathematical correlation was obtained in the obstructed breakup regime for the dimensionless thickness of the droplet (δ/w) in relation to Ca, dimensionless time (τ∗=Udt/w) and droplet length (lo/w). Two types of non-obstructed breakup were observed in the simulations, namely, partially obstructed, and with permanent tunnel. The results showed that the mechanisms of a droplet breakup inside a T-junction are in good agreement with literature.

Droplets Tracing in a T-junction Microchannel

Emulsions consist of small liquid droplets immersed in another liquid, typically either a mix of oil in water or water in oil. Emulsions have wide applications in the production of pharmaceutical products, food and cosmetics. The properties and quality of an emulsion typically depend on the size and the distribution of the droplets. Thus, the objective of this study was to investigate in detail the formation and behaviour of droplets in a T-junction microchannel. By setting up a model and applying a laminar two-phase flow in ANSYS© simulation, the particle droplets distribution was observed. The model used the predefined wetted wall boundary condition at the solid walls, with a contact angle of 135°. In this study, the behaviour and flow pattern of the particles along the T-junction microchannel were observed with regard to the effect of the initial particle concentration, the flow rate of the particles, and the initial velocity feed through the inlets of the microchannel. From the results, the effects of the velocity, mixing time and flow rate of the particles on the particle distribution and mixing were studied. It was shown that the optimization process was achieved at a flow rate of 0.025 mL/s, with the mixing process occurring within 1.6 seconds and the velocity feed at the two inlets being VA = 0.02 m/s and VB = 0.04 m/s, where the particles experienced less lift shear and compressive forces near the outlet, which caused the mixing process to become efficient.

Three-dimensional numerical simulation of a droplet generation in a double T-junction microchannel

With MEMS technology developing rapidly, the microfluidic droplet technology has been employed into many biological or chemical experiments. To satisfy the increasingly high demands in various applications of the microfluidic chips, the size of the droplet needs accurate control. We have designed and performed a three-dimensional numerical simulation of a droplet generation in a double T-junction microchannel by a level-set method in COMSOL5.0 for discussing the process of the droplet formation and the influences on the size of the droplet. We have studied out that the flow rate ratio, the continuous phase viscosity, the interfacial tension, and the contact angle influence the size of the droplet. The effective droplet diameter increases when the flow rate ratio and the interfacial tension increase. It decreases when the continuous phase viscosity and the contact angle increase.

Determination of the Liquid/Liquid Mass Transfer Coefficient for Each Phase in Microchannels

The overall mass transfer coefficient of liquid/liquid microflows has been extensively studied, but the liquid/liquid mass transfer coefficient for each phase in the microchannels has never been reported the best of our knowledge. In this work, the overall mass transfer coefficient and mass transfer coefficient for each phase (inside and outside droplet) of liquid–liquid microflows were determined using H2SO4/cyclohexanone oxime/n-hexane as the working system. Concentrated sulfuric acid (H2SO4) was used as the continuous phase, and the n-hexane solution containing a certain concentration of cyclohexanone oxime was used as the dispersed phase. A T-junction microchannel was used to generate liquid/liquid microflows. The influences of initial concentration of cyclohexanone oxime, two phase flow rates, and phase ratio were studied. The results showed that the mass transfer coefficient for each phase increased with the increasing of continuous phase rate. New mass transfer coefficient prediction models for eac...

Slug formation mechanism for air–water system in T-junction microchannel: a numerical investigation

In this paper, numerical investigation of slug formation mechanism of gas–liquid two-phase flow in a T-junction microchannel has been carried out. A two-dimensional (2D) model for the microchannel was developed using ANSYS Academic research CFD 18.2 software package, and volume of fluid method was adopted to solve the model numerically. The results obtained are in good agreement with the experimental results. In this work, slug length, pressure drop and velocity variations inside the slugs were captured at several operating conditions. The effects of contact angle (0°–155°), fluid viscosity, and surface tension on two phase flow interaction parameters along with the effect of both gas and liquid superficial velocities are discussed in detail. For gas–liquid two phase flow velocities were ranging from 0.025 to 0.5 m s−1 and capillary number (Ca) from 6.96 × 10−4 to 1.39 × 10−2. One of the key objectives of this investigation was to study the occurrence and behaviour of liquid film around the slug units. In fact, liquid film formed at low capillary number (Ca) was very thin and observed only for fine meshing near wall of the channel.

Effect of geometrical parameters on slug behaviour and two phase pressure drop in microchannel T-junctions

Geometric parameters of a microchannel such as cross-sectional area, shape and aspect ratio are important factors in determining the two phase flow characteristics. Two phase flow analyses were carried out experimentally for microchannel T-junction geometries at low values of capillary number. Three hydraulic diameters were used, namely 250, 400 and 550 μm, while the aspect ratio was varied between 0.1 to 1. Experiments were performed for the gas volume flow rate ranging from 2 to 30 ml/min while the liquid volume flow rate ranged between 0.1 to 10 ml/min. Slug length was estimated from the processing of high speed images of slug flow and pressure drop was simultaneously recorded. For constant mass flux in rectangular channels, an increase in the aspect ratio leads to an entrapment of continuous phase at the corner. This influences the transport characteristic of such channels. The effects of aspect ratio on slug length and pressure drop are reported. A simplified scaling model based on the geometric parameters of microchannels as well as gas and liquid flux is proposed which agrees reasonably well (within ±15%) with the present experimental data as well as data available in literature. A flow based pressure drop prediction model was developed which agrees well (within ±10%) with the experimentally measured two phase pressure drop. It is believed that a priori information on slug length and pressure drop in a given flow through a microchannel will help to design the flowing system better.

Numerical Study of Surfactant Dynamics during Emulsification in a T-Junction Microchannel.

Microchannel emulsification requires large amounts of surfactant to prevent coalescence and improve emulsions lifetime. However, most numerical studies have considered surfactant-free mixtures as models for droplet formation in microchannels, without taking into account the distribution of surfactant on the droplet surface. In this paper, we investigate the effects of nonuniform surfactant coverage on the microfluidic flow pattern using an extended lattice-Boltzmann model. This numerical study, supported by micro-particle image velocimetry experiments, reveals the likelihood of uneven distribution of surfactant during the droplet formation and the appearance of a stagnant cap. The Marangoni effect affects the droplet breakup by increasing the shear rate. According to our results, surfactant-free and surfactant-rich droplet formation processes are qualitatively different, such that both the capillary number and the Damköhler number should be considered when modeling the droplet generation in microfluidic devices. The limitations of traditional volume and pressure estimation methods for determining the dynamic interfacial tension are also discussed on the basis of the simulation results.

Optimization of Wavy-Channel Micromixer Geometry Using Taguchi Method.

The micro-mixer has been widely used in mixing processes for chemical and pharmaceutical industries. We introduced an improved and easy to manufacture micro-mixer design utilizing the wavy structure micro-channel T-junction which can be easily manufactured using a simple stamping method. Here, we aim to optimize the geometrical parameters, i.e., wavy frequency, wavy amplitude, and width and height of the micro channel by utilizing the robust Taguchi statistical method with regards to the mixing performance (mixing index), pumping power and figure of merit (FoM). The interaction of each design parameter is evaluated. The results indicate that high mixing performance is not always associated with high FoM due to higher pumping power. Higher wavy frequency and amplitude is required for good mixing performance; however, this is not the case for pumping power due to an increase in Darcy friction loss. Finally, the advantages and limitations of the designs and objective functions are discussed in the light of present numerical results.

Numerical Simulation of the Droplet Formation in a T-Junction Microchannel by a Level-Set Method

To satisfy the increasingly high demands in many applications of microfluidics, the size of the droplet needs accurate control. In this paper, a level-set method provides a useful method for studying the physical mechanism and potential mechanism of two-phase flow. A detailed three-dimensional numerical simulation of microfluidics was carried out to systematically study the generation of micro-droplets and the effective diameter of droplets with different control parameters such as the flow rate ratio, the continuous phase viscosity, the interfacial tension, and the contact angle. The effect of altering the pressure at the x coordinate of the main channel during the droplet formation was analysed. As the simulation results show, the above control parameters have a great influence on the formation of droplets and the size of the droplet. The effective droplet diameter increases when the flow rate ratio and the interfacial tension increase. It decreases when the continuous phase viscosity and the contact angle increase.