T-junction Microchannel Research Articles

Taylor droplet breakup in T-type microchannels: A detailed flow analysis

The study of droplet break-up in microchannels can be used to improve designs for both biological or chemical microreactors and microfluidic devices e.g., heat exchangers and microchips. In this study, 3D computer simulations were carried out to investigate the detailed behavior of droplet breakup in a T-junction microchannel using the Volume of Fluid (VOF) and Level Set (LS) coupling methods (S-CLSVOF) implemented in OpenFoam. The evolution of droplet morphology and the rupture regimens were analyzed considering pressure results, velocity neck thickness, and tunnel width. The results show four rupture regimes for a range of capillary numbers and dimensionless initial droplet length of 0.005≤Ca≤0.055 and 1≤l0/w≤5. The stages of each rupture were also analyzed and compared with experimental and numerical results from literature. A detailed description of each step in the droplet breakup is also given, including analyses of the influence that the local pressure field and Laplace pressure have on the rupture processes. The 3D results reveal that the Laplace pressure decreases in the main channel due to continuous flux at the top and bottom of the 3D channel. This in turn, leads to a slight change in the rear curvature of the bubble. Laplace pressure is an important parameter that can be used to predict the opening tunnel point, large changes in curvature, and whether the droplet will breakup or not. Just before breakup, the 3D simulations captured a backflow near the neck, a region, characterized by pressure drops, and thus smaller breakup times.

Control of plug flow dynamics in microfluidic T-junction using pulsations of dispersed phase flow rate

Active control methods are commonly employed in droplet-based microfluidics. While passive methods of droplet generation face limitations, active methods, including flow rate pulsations provide an avenue for precise control. We investigate the influence of sinusoidal pulsations of dispersed phase flow rate on the plug flow dynamics in a T-junction microchannel. The study covers a range of fluid sets, including gas-liquid and liquid-liquid systems. In the absence of pulsations, distinct regimes of plug formation were identified, and plug length, velocity, and natural frequency of formation were determined. Introducing flow rate pulsations at frequencies proportional to the natural ones f = fpulse/fplug, we observed distinct plug length distributions, including double- or triple-mode, drop-on-demand mode, and multi-mode distribution. The influence of pulsations on plug length was particularly notable at low dimensionless frequencies (f < 1), especially for fluid combinations with higher viscosity ratios. The model was proposed to explain plug length distribution patterns under pulsatile conditions. Micro-PIV measurements provided insights into the velocity fields within plugs, revealing the stretching of streamlines without altering plug length. Our findings contribute to the understanding of active control methods in droplet-based microfluidics and suggest potential applications for precise control over microfluidic processes.

Modelling the passive breakup of a surfactant-contaminated droplet in a T-junction microchannel

A lattice Boltzmann method is used to explore the effect of surfactants on the unequal volume breakup of a droplet in a T-junction microchannel, and the asymmetry due to fabrication defects in real-life microchannels is modelled as the pressure difference between the two branch outlets ( $\Delta {P^\ast }$ ). We first study the effect of the surfactants on the droplet dynamics at different dimensionless initial droplet lengths ( $l_0^\ast $ ) and capillary numbers (Ca) under symmetric boundary conditions ( $\Delta {P^\ast } = 0$ ). The results indicate that the presence of surfactants promotes droplet deformation and breakup at small and moderate $l_0^\ast $ values, while the surfactant effect is weakened at large $l_0^\ast $ values. When the branch channels are completely blocked by the droplet, a linear relationship is observed between the dimensionless droplet length ( $l_d^\ast $ ) and dimensionless time ( ${t^\ast }$ ), and two formulas are proposed for predicting the evolution of $l_d^\ast $ with ${t^\ast }$ for the two systems. We then investigate the effect of the surfactants on the droplet breakup at different values of $\Delta {P^\ast }$ and bulk surfactant concentrations ( ${\psi _b}$ ) under asymmetric boundary conditions ( $\Delta {P^\ast } \ne 0$ ). It is observed that, as $\Delta {P^\ast }$ increases, the volume ratio of the generated droplets ( ${V_1}/{V_2}$ ) decreases to 0 in both systems, while the rate of decrease is higher in the clean system, i.e. the presence of surfactants could cause a decreased pressure difference between the droplet tips. As ${\psi _b}$ increases, ${V_1}/{V_2}$ first increases rapidly, then remains almost constant and finally decreases slightly. We thus establish a phase diagram that describes the ${V_1}/{V_2}$ variation with $\Delta {P^\ast }$ and ${\psi _b}$ .

Microdroplet formation of water and alumina nanofluid in a T-junction microchannel

A scarcity of studies about nanofluids’ utilization in droplet formation inside microdevices currently hovers in the literature although potential applications of nanoparticles in a microfluidic environment are foreseen. For this purpose, experimentally assessing both nanofluid and microdroplet characteristics is fundamental. This work reports a series of experimental tests on the microdroplet formation of distilled water (DIW) and DIW-based aluminum oxide (Al2O3) nanofluid in a microfluidic T-junction. While water and nanofluid are used as the dispersed phase, mineral oil is used as the continuous phase. Microdroplet formation in the squeezing, transitional, and dripping regimes is characterized and scaling laws for the non-dimensional droplet volumes are presented. The effects of flow rate, capillary number, microchannel aspect ratio, and nanoparticle concentration are investigated. The addition of Al2O3 nanoparticles to the water is observed to have a major impact in the transitional regime (up to 40% increase), whereas in the dripping regime its influence is lower, with less than 10% difference. This was attributed to the nanofluid's enhanced interfacial tension and viscosity compared to the DIW, as well as possible adsorption at the surface.

Effect of fluid properties on intensification of heat transfer process in oil-water droplet flow in a T-shaped microchannel

Droplet-based microfluidics refers to the production and utilization of micrometer-sized droplets. This technology has found widespread applications in drug delivery, biotechnology, disease prevention, chemical analysis, cell research, and functional material synthesis. Cooling of high heat flux electronic systems and hot spots is another application of the droplet flow which is focused by the current work. In this paper, formation and hydrothermal performance of oil droplets in water flow in a T-junction microchannel is studied by a 3D numerical simulation. Effects of different parameters such as surface tension, wettability and heat flux on the droplet size, heat transfer rate and pressure drop are investigated. It is observed that the shear between the discrete and continuous phases results in recirculating zones in both phases which contribute to improved mixing and heat transfer rates. Liquid film thickness adjacent to the heated surface and the droplet velocity are two other factors that positively affect the heat transfer. Decreasing the surface tension is shown to enhance the heat transfer rate. Moreover, an optimum value is found for the contact angle as 135° at which a 23 % heat transfer enhancement is achieved. Results also indicate that droplet size decreases with increase of contact angle while it slightly increases with increase of the surface tension and negligibly varies with the applied heat flux.

Influence of dispersed phase flow-rate pulsations on the liquid–liquid parallel flow in a T-junction microchannel

Active flow control in microfluidic devices to broaden the range of desired conditions is a significant practical concern. In this study, we experimentally investigate liquid–liquid parallel flow under the dispersed phase flow-rate pulsations in a T-junction microchannel. Sinusoidal flow rate disturbances with variations in amplitude and period were applied to dispersed phases of different viscosities. The study involves flow visualization and velocity field measurements using the micro-PIV technique for both free and excited flow conditions. The analysis of flow pattern maps, velocity fields, velocity gradients, and phase-averaged velocity profiles in a T-junction region and far downstream provided insights into the evolution of disturbances, which is different in low- and high-viscosity liquids. In the less viscous liquid, transverse waves amplify longitudinal ones due to relaxation of the liquid–liquid interface, while in the high-viscosity dispersed phase, longitudinal waves are damped significantly, with the transverse wave amplitude remaining almost constant. As a result, we revealed two distinct mechanisms of disturbance wave propagation and the loss of parallel flow stability, which are both dependent on the Ohnesorge number of the dispersed phase. Destabilization of the parallel flow led to the formation of plugs with a narrow length distribution. Single-mode, double-mode, triple-mode, and multi-mode plug formation regimes were identified. The results of the study can potentially be used to extend the range of segmented flow regimes and generate plugs of a desired length.

Bubble formation dynamics in aerospace applications

Under microgravity conditions, simulations of bubble formation in a T-junction microchannel with hybrid wettability surfaces allows for more precise control of multiphase flow in space exploration technologies

Controlling gas–liquid flow and enhancing heat transfer in a T-junction microchannel by wettability-engineered walls

Characteristics of gas–liquid flow and heat transfer in a cross-flow T-junction microchannel with wettability-engineered walls are numerically investigated in this paper. The validated diffuse interface method is adopted for interface capture. First, the effects of wall wettability on bubble formation and transportation are studied. Three flow patterns are observed due to different combinations of the bottom and the top wall contact angles. On this basis, two methods are proposed to enhance the heat transfer. One is to increase the two-phase interfacial contact area by dividing the microchannel into three functional regions, which can promote the heat exchange at the two-phase interface. The other is to increase the velocity fluctuation intensity by alternating the contact angle along the channel, which can enhance mixing between the hot liquid layer adjacent to the wall and the cool liquid core. These two methods are applicative for steady and unsteady problems, respectively. The flow states, velocity vectors, and streamlines are used to analyze the fluid and thermal mixing mechanism. Meanwhile, a quantitative comparison of the wall temperature is made at a given wall heat flux. The obtained results can provide fresh insights into the gas–liquid flow control and the heat transfer enhancement in a microchannel, which are valuable for the design of microreactors and radiators.

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.

CO2 capturing in cross T-junction microchannel using numerical and experimental approach

CO2 capturing in cross T-junction microchannel using numerical and experimental approach

Numerical investigation on formation mechanism and flow law of droplet in T-junction by electric field

The mechanism of constant electric field regulation on droplet formation in a T-junction microchannel is investigated numerically, and the law of discrete phase fluid behavior in different viscosity systems is summarized. This research develops a two-phase-electrohydrodynamic solver in the OpenFOAM® framework. The solver uses a leaky medium model to simulate the microfluidic system and uses the volume of fluid method to trace the phase interface. It is found that the mechanism of droplet generation within the microfluidic system can be divided into squeeze and shear effect, where the shear mechanism will have transition to the squeeze mechanism under the influence of the electric field. Additionally, the effect of the electric field on the discrete phase fluid is significantly influenced by the viscosity of the continuous phase fluid. At low to medium viscosity, droplet length can be affected by wall shear resistance and undergo sudden changes at a certain electric field strength, but the mutation will decrease to disappear with the increase in the system viscosity. At high viscosity, droplets undergo significant agglomeration in the interaction of electric field force and viscous shear. Then, the regulating mechanism of constant electric field on a discrete phase fluid flow pattern in an ultra-high viscosity system is also studied. Finally, a map is proposed that adequately describes the variation of discrete phase fluid behavior with constant electric field strength with different continuous phase fluid viscosities.

Numerical Investigation of Flow Patterns and Plug Hydrodynamics in a 3D T-junction Microchannel

A three-dimensional (3D) numerical simulation of a two-phase flow liquid/liquid is performed in a rectangular microchannel with a T-junction. The volume of fluid (VOF) method was used under ANSYS Fluent to capture the interface between the two phases. The dynamic mesh adaptation technique together with the assumption of symmetry plane helps us to reduce the computational cost. The study focuses on the flow patterns and hydrodynamics of plugs. So, the influence of the flow rate ratio $$q$$ , the capillary number $$Ca$$ , and the viscosity ratio on the liquid film, the plug/droplet shape, and velocity are examined here. Particularly, the plug/droplet lengths predicted by the simulation show good agreement with the experimental and correlation available in the literature. The results revealed six distinct flow patterns by dispersing water in a continuous phase of silicone oil. By decreasing the flow rate ratio as well as the viscosity ratio, the liquid film thickness increases in the corners and side planes. In turn, this greatly impacts the liquid film velocity and the plug velocity. Furthermore, capillary number (based on two-phase flow velocity) is also shown to have a greater impact than viscosity ratio and flow rate ratio on plug shape, with the curvature radii of the tail always larger than the front one.

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.

INFLUENCE OF LOW-FREQUENCY EXTERNAL PULSATIONS OF LIQUID FLOW RATE ON GAS-LIQUID SLUG FLOW IN A RECTANGULAR MICROCHANNEL

This paper presents the results of an experimental study of the influence of the liquid flow rate of external pulsations on the gas-liquid flow characteristics of periodic slug flow in a microchannel. The experimental data were obtained using high-speed visualization of the flow of an ethanol-nitrogen mixture in a T-junction microchannel with a 390 &amp;times; 150 &amp;mu;m rectangular cross section. A novelty of this work was the use of pulsations of the continuous liquid phase on the gas-liquid flow. The form of external pulsations was a sine wave with different frequencies (f &amp;#61; 1 and 2 Hz) and amplitude values (A &amp;#61; 8&amp;#37;, 20&amp;#37;, and 40&amp;#37;). The results obtained showed that using high-amplitude and low-frequency external pulsations destroys periodic slug flow and changes the monodispersity of bubbles and liquid slugs. The statistical characteristics of the flow were obtained based on the frequency and amplitude of the pulsations.

Effects of surface wettability and flow rates on the interface evolution and droplet pinch-off mechanism in the cross-flow microfluidic systems

In this study, the effect of surface wettability on the two-phase immiscible fluids flow and dynamics of droplet pinch-off in a T-junction microchannel has been numerically investigated using the finite element method. A conservative level set method (CLSM) has been adopted to capture the interface topology in the squeezing regime (Cac<10-2) for wide flow rate ratio (1/10⩽Qr⩽10) and contact angle (120°⩽θ⩽180°). This study has revealed that the wettability is a dominant factor in determining the hydrodynamic features of the droplet. Based on the instantaneous phase flow profiles, the droplet formation stages are classified as: initial, filling, squeezing, pinch-off and stable droplet. Wettability effects are insignificant in the filling stage. However, the hydrophobic effects are more vital in the squeezing and pinch-off stages. In general, it is shown that engineering parameters have complex dependence on the dimensionless parameters (Cac,Qr,θ). Capturing the instantaneous interface evolution has revealed that the droplet shape is sensitive to the contact angle. Interface shape profiles transform from convex into concave immediately for hydrophobic conditions (120°⩽θ⩽150°) whereas slowly for the super hydrophobic conditions (150°<θ⩽180°). In contrast to the literature, the pressure in the dispersed phase is not constant, but it is an anti-phase with the pressure in the continuous phase. Maximum pressure in the continuous phase, and neck width of the interface are complex function of the governing conditions (Cac,Qr,θ). Comparison of the filling and pinch-off time based on the pressure and phase profiles has brought new insights that the droplet pinch-off mechanism can be elucidated by installing the pressure sensors even without the flow visualization and phase profiles. The interface curvature adopts a flattened to a more concave shape when the Laplace pressure varies from a smaller to higher value. The interface neck width (2r) shows an increasing trend up to a threshold value and then decreases linearly with the contact angle.

Coalescence dynamics of nanofluid droplets in T-junction microchannel

The effect of nanoparticles on droplet coalescence in microchannel was investigated experimentally. The interaction between acid-functionalized nanoparticles and polymers with complementary terminal functional groups facilitates the adsorption of particles at the oil–water interface. Three flow patterns were observed: squeezing coalescence, decompression coalescence and non-coalescence. The presence of nanoparticles significantly reduces the droplet coalescence percentage, and stable droplets could be obtained when the particles concentration increased to 1%. Furthermore, it was found that the film drainage time of squeezing coalescence was independent of the nanoparticles concentration. However, the film drainage time of decompression coalescence increases with nanoparticles concentration. Taking the influence of nanoparticles on the film drainage time into consideration, a new prediction model was proposed. Coupling the droplet contact time with the film drainage time, the critical capillary number for droplet coalescence was obtained. This work could provide important data and technical support for the application of nanoparticle-stabilized emulsions.

Fragmentation of asymmetric liquid filaments and formation of satellite droplets in a microchannel

This study analyzes the breakup dynamic of fluid filament in a T-junction microchannel, including droplet stretching process, liquid filament retraction process and breakup process. The liquid filaments spontaneously evolve into droplets or break into multiple satellite droplets, which can be considered as the result of the competition between the retraction and elongation mechanisms of the liquid filaments. There is an obvious local interfacial tension difference during the shrinkage of the droplet filament, which promotes the discharge of the internal fluid, and then the filament is pinched, resulting in the formation of satellite droplets. On the basis of symmetrical filament pinch-off, and the fully considering of the influence of viscosity and the crossing flow, the elongation and retraction mechanism of droplet filaments in the microchannel are proposed. The study reveals the breakup of asymmetric liquid filaments in a microchannel and the formation of satellite droplets, which provides references for the manipulation of satellite droplets in a microchannel.

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.

Effect of hybrid wall contact angles on slug flow behavior in a T-junction microchannel: A numerical study

This study investigates the effects of hydrophilic-to-superhydrophobic discontinuous wall contact angles on gas-liquid slug flow in a T-junction microchannel. The two-phase volume-of-fluid model with dynamic contact angle are used by solving wall-adhesion equations, which determine the shape of the fluid’s interface at and close to the wall. The governing equations of the flow field are solved numerically using a finite volume method. The obtained results are first validated against previously reported experimental data. Subsequently, the effects of changes in the wall contact angles on the slug flow behavior are examined resulting in the new insights into the size, shape, and velocity of slug flow as a function of the wall contact angle. Using a hydrophilic-superhydrophobic hybrid surface leads to the formation of an asymmetric bubble and separation of the bubble from the hydrophobic part. It also causes adhesion to the hydrophilic part, a change in slug length, a reduction in pressure drop, and a variation in the heat transfer characteristics and continuous phase structure. It is shown that using a particular pattern significantly increases the thickness of the liquid wall film.