Droplet Formation In Microchannel Research Articles

Numerical simulation of droplet formation in a microfluidic T-junction using a dynamic contact angle model

Due to the dominance of interfacial forces, microreactors are commonly applied to multiphase processes where control over the dispersed phase volume is critical. For the design of such microfluidic devices, it is therefore important to accurately predict droplet breakup and the resulting two-phase flow pattern using computational tools. In this work, we show that integrating a dynamic contact angle model into a volume-of-fluid (VOF) solver significantly increases the prediction accuracy of droplet formation in a microfluidic T-junction compared to earlier studies with a constant contact angle model. Furthermore, it was found that the droplet formation is more sensitive to the chosen value of the advancing contact angle, while the receding contact angle showed only a minor influence. The present findings confirm that using a dynamic contact angle model with a VOF approach to simulate droplet formation in microchannels will provide more accurate predictions.

Toward computationally effective modeling and simulation of droplet formation in microchannel junctions

Droplet-based microfluidics is a widely used technology in various chemical and biological applications. Droplet formation is an essential part of droplet-based microfluidics. This work applied and summarized a combination of techniques for speeding up computational fluid dynamics (CFD) simulations based on the Volume of Fluid method for the droplet generation process. To demonstrate the efficiency of the applied techniques, numerical modeling and simulation of droplet formation in a cross-shaped square microchannel were carried out. Water–butanol and water–toluene two-phase systems were considered under various flow rate conditions. Flow visualization experiments were conducted to validate the numerical results. In addition, a parametric analysis on the influence of viscosity, interfacial tension, and three-phase contact angle on the droplet formation was carried out to demonstrate the performance of the numerical setup. The simulated fluid flow dynamics showed good agreement with the experimental data. The CFD model was also verified by simulating the droplet formation in a T-shaped rectangular microchannel. The presented setup shows promising potential for development of chips for droplet generation by testing virtual prototypes. Due to the possibility to model the droplet formation process at an acceptable computational cost, time savings in the research and design cycles can be significant.

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.

Droplet formation in microchannels at low values of the capillary and the reynolds numbers

This paper describes the fabrication of PDMS microchannels of various configurations including T-junction, cross junction and diamond shape that accomplished using Xurography technique. The experimental characterization of droplets formation within the fabricated microchannels was performed by introducing a continuous phase and dispersed phase. The flow regimes of squeezing and dripping were observed in the microchannels. The Capillary number affects the patterns of flow regimes and the droplet frequencies in the microchannels. The droplet frequency was dependent on the volumetric flow rate of the continuous phase. The droplet frequency increases as the continuous phase flow rate increases. The smaller size of the droplets is formed at the larger Capillary number

Numerical study of Janus droplet formation in microchannels by a lattice Boltzmann method

A ternary lattice Boltzmann method based on the color-gradient model was successfully established to numerically investigate the Janus droplet formation in a Y-junction microfluidic device. We first validated the model by comparing the simulation results of contact relationship of mono-dispersed droplets with the theoretical solutions, and then studied Janus droplet formation and the breakup dynamics in Y-junction numerically. The results showed that the Janus droplet size obeys a scaling law during the formation. The dynamic behavior of dispersed thread, including the thread tip and minimum width of thread, revealed that the breakup of the thread is a self-thinning process. The evolution of the minimum width of dispersed thread is dominated by evolution time in a power law relationship. The deep investigation of each dispersed thread indicated that the dynamic behavior of each thread is identical before the final pinch-off in the formation process.

The dynamic mass transfer of surfactants upon droplet formation in coaxial microfluidic devices

In this work, the dynamic adsorption of surfactants is used to evaluate the mass transfer coefficient during droplet formation. As surfactants exist in only one phase and at the interface, the resistance of mass transfer inside and outside the droplet can be decoupled. By exchanging the continuous phase and dispersed phase (N-hexane/water-Tween 80), the mass transfer coefficients inside and outside the droplet were evaluated. A semi-empirical equation with modified parameters was developed to incorporate the effects of convection and the experimental conditions on the mass transfer coefficient during droplet formation in micro-channels. The results presented in this work provide a more in-depth understanding of the dynamic mass transfer of surfactants inside and outside the droplet during the formation of a droplet in microfluidic devices.

CFD Simulation of Droplet Formation in Microchannels by a Modified Level Set Method

A level set method embedded in a Computational Fluidic Dynamics (CFD) simulation is a useful approach in the study of microfluidic two-phase flow. In this work, we established a new governing equation for the level set function to overcome the “mass loss” problem. The feasibility of the modified method was proved by the simulation of a droplet formation in a co-tube microchannel. The modified method was then used to simulate the droplet flow in a focusing shape microchannel. The simulation results showed that the droplet size would decrease with an increase in the continuous phase flow rate. In the early stage of the droplet formation process, there was strong inner circulation flow that enhanced the mass transfer between the two phases and generated a concentration notch at the head of the droplet. In the later stage of the droplet formation process, the droplet moving velocity became higher, and there was no longer inner circulation flow in the droplet. The mass transfer speed slowed, and there was an enrichment of the material at the head of the droplet. The total mass transfer ratio in the droplet formation process reached 50%. The simulation results were consistent with both our previous experiment results and the results reported in the literature, which further proved the feasibility of the modified method.

LBM simulation of droplet formation in micro-channels

This work presented a potential solution to flexibly manipulate the droplet formation in micro-channels by simple modifications on the ordinary T-junction, that is, using Venturi-shaped micro-channels to effectively influence the size of droplet and the frequency of droplet generation. To better understand the flow patterns of droplet formation in these micro-channels, Lattice Boltzmann method was developed to model the multiphase flow phenomena. The simulations were successfully validated by the experimental data, and then adopted to guide the design of the appropriate channel geometry. The results illustrated vividly that the smaller droplets can be high-efficiently generated under the operation of low phase ratio (defined as the flow rate ratio of continuous phase to dispersed phase) in contrast to the much bigger droplets in ordinary T-junction geometry under the same conditions. The mechanism can be understood by the predicted velocity field at the initial stage of droplet formation, which generated the dominant shearing force to cut off the dispersed phase to form droplets. This work indicated that the multiphase flows in micro-devices could be very sensitive to the tiny difference in terms of channel geometry. The theoretical approach helped to disclose the mechanism and then to make further improvement on the micro-channel design.

Lattice Boltzmann simulation of droplet formation in microchannels under an electric field

The intermolecular potential model of the lattice Boltzmann method (LBM) is used to simulate the effect of an imposed electric field on droplet formation process from mixing of immiscible fluid streams in a 2D microchannel with non-wetting walls. The electrodes are placed near the inlet of the main channel where droplet breakup occurs. Simulation results show that droplet size decreases and formation frequency increases as the intensity of the electric field is increased. This is because the gap between the emerging droplet and main channel wall is decreased under the action of the electric field during the droplet formation process, which induces the emerging droplet to block the continuous phase more effectively. The obstruction increases the pressure difference across the droplet interface, leading to stronger squeezing effects on the neck of the droplet. For a fixed velocity ratio of the continuous phase to the disperse phase, there exists a critical value of electric field intensity for which the droplet begins to touch the surfaces of the electrodes. At this critical value of electric field intensity, the droplet size suddenly decreases and droplet formation frequency suddenly increases drastically. At high electric field intensity when the electric force dominates the flow field, deformation of droplet is relatively independent of the velocity of the continuous phase. Simulation results show that an electric field can be used to control droplet size and droplet formation frequency effectively.

Numerical simulation of droplet formation in a micro-channel using the lattice Boltzmann method

This study investigates droplet formation in a micro-channel using the lattice Boltzmann (LB) method. A cross-junction micro-channel and two immiscible, water and oil phase fluids, were used to form the micro-droplets. Droplets are formed by the hydrodynamic instability on the interface between two immiscible fluids when two immiscible fluids are imported simultaneously in a cross-junction micro-channel. The Shan & Chen model, which is a lattice Boltzmann model of two-phase flows, is used to treat the interaction between immiscible fluids. The detailed process of the droplet formation in the cross-junction micro-channel was illustrated. The results of the droplet formation by the LBM predicted well the experimental data by PIV (particle image velocimetry). The effect of the surface tension and the flow rate of water phase fluid on the droplet length and the interval between droplets was also investigated. As the surface tension increased, the droplet length and the interval between droplets were increased. On the other hand, when we increased the flow rate of the water phase fluid under the condition of the fixed oil-phase fluid flow rate, the droplet size was increased while the interval between droplets was decreased.

Thermally mediated droplet formation in microchannels

Precise dispensing of microdroplets is an important process for droplet-based microfluidics. The droplet formation by shear force between two immiscible fluids depends on their flow rates, the viscosities, and the interfacial tension. In this letter, the authors report the use of integrated microheater and temperature sensor for controlling the droplet formation process. The technique exploits the dependency on temperature of viscosities and interfacial tension. Using a relatively low heating temperature ranging from 25to70°C, the droplet diameter can be adjusted to over two times of its original value. The relatively low temperature range makes sure that this concept is applicable for droplets containing biological samples.

Droplet formation in microchannels under static conditions

Droplets generated by oil and water mixtures in microfluidic devices have become popular for generating nanovolume vessels for biochemical assays. The authors report a microfluidic technology that can create nanovolume droplets in a microfluidic channel under static (nonflow) conditions. The technique exploits interfacial surface tension and geometry of the channels to generate aqueous droplets of predetermined size at fixed locations. This results in a method of generating small drops in a microfluidic system at programed locations in a manner that is not strongly dependent on flow rate or viscosity.

Lattice Boltzmann Simulations of Droplet Formation in a T-Shaped Microchannel

We investigated the formation of a droplet from a single pore in a glass chip, which is a model system for droplet formation in membrane emulsification. Droplet formation was simulated with the lattice Boltzmann method, a method suitable for modeling on the mesoscale. We validated the lattice Boltzmann code with several benchmarks such as the flow profile in a rectangular channel, droplet deformation between two shearing plates, and a sessile drop on a plate with different wetting conditions. In all cases, the modeling results were in good agreement with the benchmark. A comparison of experimental droplet formation in a microchannel glass chip showed good quantitative agreement with the modeling results. With this code, droplet formation simulations with various interfacial tensions and various flow rates were performed. All resulting droplet sizes could be correlated quantitatively with the capillary number and the fluxes in the system.

The effect of the hydrophobicity of microchannels and components in water and oil phases on droplet formation in microchannel water-in-oil emulsification

The effects of the hydrophobicity of silicon microchannels and components in the oil and water phases in the production of monodispersed water-in-oil (W/O) emulsions by microchannel emulsification was investigated. Several types of silane coupler reagents were applied to make the microchannel hydrophobic. The hydrophobicity of the surface was evaluated by contact angle measurements. Hexane, decane and oleic acid were used as the oil phase, and pure water and sucrose solution were used as the water phase. Tetraglycerol polyricinoleate, sodium bis(2-ethylhexyl) sulfosuccinate and sorbitan fatty acid esters were used as surfactants. The contact angle of a water droplet on the modified silicon microchannel plate in an oil phase was found to be dependent on the composition of the oil phase, the surfactant type and the silane coupler reagent used for surface modification. When the contact angle was greater than 120°, monodispersed water droplets were formed and the geometric standard deviation was below 1.06. The average droplet size varied from 17 to 23 μm, and was affected by the characteristics of the surfactant and the viscosities of the water and oil phases.