Microfluidic Y-junction Research Articles

Splitting of double-core solid-in-water-in-oil droplet in a microfluidic Y-junction

Solid particle-encapsulated droplets have significant applications in biochemistry, advanced materials, and inertial confinement fusion (ICF) experiments. However, there is a problem of encapsulating two solid cores in a single droplet during the preparation of single-core droplets, which reduces the utilization efficiency. In this study, an effective microfluidic approach for continuous splitting of solid-in-water-in-oil droplets encapsulating double solid cores is developed. Visualization experiments are conducted to analyze the movements of solid cores and evolution of liquid–liquid interface during the splitting. The results show that the squeezing stage during the splitting process is shortened due to the presence of solid cores. The splitting mechanisms were also revealed by analyzing the interaction forces between the solid cores and aqueous phase. The force analysis of the aqueous phase showed that sum of squeezing and shear force could overcome the interfacial tension, ensuring the successful splitting of the double-core droplets. The force analysis of the solid cores revealed that the motion of the core could be divided into three typical stages: deceleration, hitting and separation. The combined effect of the aqueous phase, channel wall, and interfacial forces ensured the stable separation of the two solid cores. The length distribution of the daughter droplets exhibited excellent monodispersity. The microfluidic method proposed in this work would effectively improve the controlled preparation efficiency of solid-in-water-in-oil droplets.

Numerical Investigation on the Symmetric Breakup of Bubble within a Heated Microfluidic Y-Junction

This study numerically investigated the symmetric breakup of bubble within a heated microfluidic Y-junction. The established three-dimensional model was verified with the results in the literature. Two crucial variables, Reynolds number (Re) and heat flux (q), were considered. Numerical results demonstrated that the bubble breakup was significantly affected by phase change under the heated environment. The “breakup with tunnel” and “breakup with obstruction” modes respectively occurred at the low and high q. The breakup rate in pinch-off stage was much larger than that in squeezing stage. As Re increased, the bubble broke more rapidly, and the critical neck thickness tended to decrease. The bubble annihilated the vortices existing in the divergence region and made the fluid flow more uniform. The heat transfer was enhanced more drastically as Re was decreased or q was increased, where the maximum Nusselt number under two-phase case was 6.53 times larger than single-phase case. The present study not only helps understanding of the physical mechanisms of bubble behaviors and heat transfer within microfluidic Y-junction, but also informs design of microfluidic devices.

Breakup regimes of double emulsion droplets in a microfluidic Y-junction

The droplet breakup technology can effectively increase the generation throughput and adjust the droplets size, which has an important impact on the performance of the double emulsion droplets in medical, chemical, and other applications. This work presents an experimental study on the breakup regimes of double emulsion droplets after their on-chip generation. Five distinct breakup regimes are categorized according to the breakup times and the existence of the coupling effect during breakup process. Evolutions of the neck widths and thinning rates of both inner droplets and outer droplets are provided to discuss the dynamics of different regimes as well as different stages. In particular, the influences of the coupling effect on the interfacial evolution, collapsing mechanism, force analysis, and breakup critical condition are confirmed by comparisons with the results of single emulsion droplets.

The breakup dynamics of bubbles stabilized by nanoparticles in a microfluidic Y-junction

The breakup flow patterns, the neck dynamics and the tip dynamics of bubbles stabilized by adsorptive nanoparticles in microfluidic Y-junction were investigated. Three flow patterns were observed in the Y-junction: non-breakup, breakup with partial obstruction, and breakup with permanent obstruction. Compared with bubbles stabilized only by surfactants (conventional bubbles), bubbles stabilized by nanoparticles (hardening bubbles) are more difficult to breakup. The breakup process with partial obstruction could be divided into three stages: squeezing, transition and pinch-off. Nanoparticles have little effect on the squeezing stage, but could greatly prolong the transition stage while accelerate the pinch-off stage. The difference between hardening and conventional bubbles is weakened, even disappears as the dispersion flow rate increases. Besides, the “rigid” surface of hardening bubbles could cause the decrease of the curvature of daughter bubble tips and the tunnels between the daughter bubbles and the channel wall in comparison with conventional bubbles.

Dynamics of non-Newtonian droplet breakup with partial obstruction in microfluidic Y-junction

Dynamics of non-Newtonian droplet (CMC aqueous solution) breakup with partial obstruction in microfluidic Y-junction was visually investigated. Three breakup stages were found: squeezing, transition and pinch-off. The effects of the concentration c of CMC, continuous capillary number Cac and dispersed phase local capillary number Cad on droplet breakup were studied. In squeezing stage and transition stage, the breakup process could be promoted by increasing Cad or decreasing c. The pinch-off stage relied primarily on the surface tension and was almost independent of inertial force and viscous force. For the shear thinning non-Newtonian droplet, the length of the daughter droplet was always linearly stretched with time until breakup and the maximum gap width increased with Cac, but decreased with the increases of c and Cad.

Hydrodynamics of double emulsion passing through a microfuidic Y-junction

A scheme of passive breakup of generated droplet into two daughter droplets in a microfluidic Y-junction is characterized by the precisely controlling the droplet size distribution. Compared with the T-junction, the microfluidic Y-junction is very convenient for droplet breakup and successfully applied to double emulsion breakup. Therefore, it is of theoretical significance and engineering value for fully understanding the double emulsion breakup in a Y-junction. However, current research mainly focuses on the breakup of single phase droplet in the Y-junction. In addition, due to structural complexity, especially the existence of the inner droplet, more complicated hydrodynamics and interface topologies are involved in the double emulsion breakup in a Y-junction than the scenario of the common single phase droplet. For these reasons, an unsteady model of a double emulsion passing through microfluidic Y-junction is developed based on the volume of fluid method and numerically analyzed to investigate the dynamic behavior of double emulsion passing through a microfluidic Y-junction. The detailed hydrodynamic information about the breakup and non-breakup is presented, together with the quantitative evolutions of driving and resistance force as well as the droplet deformation characteristics, which reveals the hydrodynamics underlying the double emulsion breakup. The results indicate that the three flow regimes are observed when double emulsion passes through a microfluidic Y-junction: obstructed breakup, tunnel breakup and non-breakup; as the capillary number or initial length of the double emulsion decreases, the flow regime transforms from tunnel breakup to non-breakup; the upstream pressure and the Laplace pressure difference between the forefront and rear droplet interfaces, which exhibit a correspondence relationship, are regarded as the main driving force and the resistance to double emulsion breakup through a microfluidic Y-junction; the appearance of tunnels affects the double emulsion deformation, resulting in the slower squeezing speed and elongation speed of outer droplet as well as the slower squeezing speed of inner droplet; the critical threshold between breakup and non-breakup is approximately expressed as a power-law formula <inline-formula><tex-math id="M5">\begin{document}${l^*} = \beta C{a^b}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M5.png"/></alternatives></inline-formula>, while the threshold between tunnel breakup and obstructed breakup is approximately expressed as a linear formula <inline-formula><tex-math id="M6">\begin{document}${l^*} = \alpha $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M6.png"/></alternatives></inline-formula>; comparing with the phase diagram for single phase droplet, the coefficients <inline-formula><tex-math id="M7">\begin{document}$\alpha $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M7.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M8">\begin{document}$\beta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20181877_M8.png"/></alternatives></inline-formula> of the boundary lines between the different regimes in phase diagram for double emulsion are both increased.

Dynamics of partially obstructed breakup of bubbles in microfluidic Y-junctions.

For revealing the dynamics of partially obstructed breakup of bubbles in microfluidic Y-junctions, the combination of dimensionless power-law and geometric model was applied to study the effects of capillary number, bubble length, and channel angle on the bubble rupture process. In the squeezing process, the gas-liquid interface curve follows the parabolic model. For the evolution of the bubble neck during breakup, the increase of the bubble length, the channel angle, and the capillary number leads to the decrease of the focus distance α. The chord m increases with the increase of the capillary number and the decrease of the bubble length, and it would reach the maximum value when the channel angle is 90°. In the fast pinch-off stage during bubble breakup, the bubble's neck curve no longer conforms to the parabolic model so the focus and chord no longer exist. For the evolution of the bubble head during breakup, the value of γ approaches 1 with the increase of the capillary number and the bubble length, and with the close of the channel angle to 90°. It is found that the quadrilateral model can be applied for the partially obstructed rupture of bubbles in the symmetrical microfluidic Y-junction.

Dynamic fluid interface formation in microfluidics: Effect of emulsifier structure and oil viscosity

Abstract Microfluidic devices are known for their accurate control of emulsification, but are less known for their suitability to investigate involved dynamic mechanisms. We previously showed that a microfluidic Y-junction can be used to measure interfacial tension in the millisecond time-scale, at high interface expansion rates, and under convective mass transport. In the present work, we further use this device to elucidate and compare dynamic adsorption behaviour of water- or oil-soluble surfactants, in combination with different alkanes. We found that oil viscosity affects adsorption of the oil-soluble surfactant Span 20 because surfactant transport is influenced by viscosity through the internal velocity. Conversely, adsorption of the water-soluble surfactant Tween 20 was not affected by oil viscosity. When comparing surfactant adsorption rates, it was clear that surfactant structure became more important when more surfactants were present at the interface; Tween 20 adsorption was slower than Span 20 because of steric repulsion at the interface.

Apparent Interfacial Tension Effects in Protein Stabilized Emulsions Prepared with Microstructured Systems.

Proteins are mostly used to stabilize food emulsions; however, production of protein containing emulsions is notoriously difficult to capture in scaling relations due to the complex behavior of proteins in interfaces, in combination with the dynamic nature of the emulsification process. Here, we investigate premix membrane emulsification and use the Ohnesorge number to derive a scaling relation for emulsions prepared with whey protein, bovine serum albumin (BSA), and a standard emulsifier Tween 20, at various concentrations (0.1%, 0.5%, 1.25% and 2%). In the Ohnesorge number, viscous, inertia, and interfacial tension forces are captured, and most of the parameters can be measured with great accuracy, with the exception of the interfacial tension. We used microfluidic Y-junctions to estimate the apparent interfacial tension at throughputs comparable to those in premix emulsification, and found a unifying relation. We next used this relation to plot the Ohnesorge number versus P-ratio defined as the applied pressure over the Laplace pressure of the premix droplet. The measured values all showed a decreasing Ohnesorge number at increasing P-ratio; the differences between regular surfactants and proteins being systematic. The surfactants were more efficient in droplet size reduction, and it is expected that the differences were caused by the complex behavior of proteins in the interface (visco-elastic film formation). The differences between BSA and whey protein were relatively small, and their behavior coincided with that of low Tween concentration (0.1%), which deviated from the behavior at higher concentrations.

Bubbling and foaming assisted clearing of mucin plugs in microfluidic Y-junctions.

Microfluidic Y-junctions were used to study mechanical mechanisms involved in pig gastric mucin (PGM) plug removal from within one of two bifurcation branches with 2-phase air and liquid flow. Water control experiments showed moderate plug removal due to shear from vortex formation in the blockage branch and suggest a PGM yield stress of 35Pa, as determined by computational fluid dynamics. Addition of hexadecyltrimethylammonium bromide (CTAB) surfactant improved clearing effectiveness due to bubbling in 1mm diameter channels and foaming in 500μm diameter channels. Plug removal mechanisms have been identified as vortex shear, bubble scouring, and then foam scouring as air flow rate is increased with constant liquid flow. The onset of bubbling and foaming is attributed to a flow regime transition from slug to slug-annular. Flow rates explored for 1mm channels are typically experienced by bronchioles in generations 8 and 9 of lungs. Results have implications on treatment of cystic fibrosis and other lung diseases.

Convective mass transport dominates surfactant adsorption in a microfluidic Y-junction.

Surfactant adsorption during emulsification can be quantified by measuring the acting interfacial tension using a Y-junction microfluidic device. To obtain insight into the surfactant transport mechanism to the interface, the effect of shear force on the acting interfacial tension was assessed by systematically varying the continuous phase viscosity and velocity. Varying the continuous phase viscosity did not affect the acting interfacial tension, indicating that surfactant adsorption during Y-junction emulsification is not diffusion-limited. The acting interfacial tension was inversely dependent on the continuous phase velocity, which indicates that surfactant adsorption is governed by convective mass transfer resulting from the continuous phase velocity. The acting interfacial tension can be measured in the sub-millisecond time scale and under convective transport conditions using the Y-junction. These conditions are relevant to industrial emulsification and cannot be assessed by conventional tensiometry techniques (e.g., drop tensiometers) where surfactant adsorption is mostly driven by diffusion. We believe, therefore, that this method can be used to understand emulsifier adsorption during industrial emulsification, which can, in turn, be used to rationally design emulsion formulations and processes.

Magnetofluidic control of the breakup of ferrofluid droplets in a microfluidic Y-junction

Breakup of the ferrofluid droplets at the Y-junction divergence under various flow rate ratios.

Dynamics of bubble breakup in a microfluidic Y-bifurcation

A highspeed camera was used to investigate experimentally the bubble breakup and dynamics in a symmetric microfluidic Y-junction divergence. N2 and deionized water with 0.3% surfactant sodium dodecyl sulfate (SDS)-glycerol were adopted as dispersed and continuous phases, respectively. The symmetric breaking with permanent obstruction, symmetric breaking with gaps and non-breaking regimes were observed in experiments. Experimental results showed that: whether bubbles breakup or not depends primarily on both bubble initial lengths and capillary number. And the higher the viscosity of the fluid is, the higher is the capillary number needed to break bubbles. Dynamics of bubbles breakup were investigated under different regimes. The breakup with permanent obstruction included two stages: squeezing stage and rapid breakup stage. In squeezing stage, the breakup process was only controlled by the augmented pressure. The rapid breakup stage was affected by surface tension. The breakup with gaps consisted of three stages: squeezing stage, squeezing-stretching stage and rapid breakup stage. The squeezing stage and the rapid breakup stage were similar to the stage of the breakup with permanent obstruction. The squeezing-stretching was commonly controlled by the viscous force, inertia force and the augment pressure.

Numerical Analysis on Two-phase Flow Characteristics at Convection Microfluidic Y-junctions

Numerical simulation on two-phase flow characteristics at convection microfluidic Y-junctions based on the two-phase flow interface tracking method. It is found that the Y-angle of the microfluidic Y-junctions, capillary number of continuous phase, and the flux of two phases have a great influence on droplet generation time, rate and size. The decreasing of continuous phase capillary number and Y-angle results in the increasing of the droplet volume. At the same time, with the increasing of flux ratio of the dispersed phase and continuous phase, the influence on the droplet volume is becoming smaller. But the flow flux ratio cannot infinite increase or decrease. Because the dispersed phase can only takes the form of liquid column or silk and it unable to generate the droplet when the ratio is greater than 0.5 or no more than 0.05. When the dispersed phase flux is increasing, the corresponding droplet formation rate is almost proportional increasing; the influence of the dispersed phase flow on the length of the droplets evolution is greater than the continuous phase. In addition, in the formation process of droplet, the filling time becomes longer and the necking time becomes shorter with the increasing of the Y-angle. The droplet detachment mechanism is mainly under the effect of normal stress of the continuous phase.

Dynamic Interfacial Tension Measurements with Microfluidic Y-Junctions

Emulsification in microdevices (microfluidic emulsification) involves micrometer-sized droplets and fast interface expansion rates. In addition, droplets are formed in less than milliseconds, and therefore traditional tensiometric techniques cannot be used to quantify the actual interfacial tension. In this paper, monodisperse droplets formed at flat microfluidic Y-junctions were used to quantify the apparent dynamic interfacial tension during (microfluidic) emulsification. Hexadecane droplets were formed in ethanol-water solutions with a range of static interfacial tensions to derive a calibration curve, which was subsequently used to access the dynamic interfacial tension of hexadecane droplets formed in surfactant solutions. For SDS and Synperonic PEF108, various continuous- and disperse-phase (hexadecane) flow rates were studied, and these conditions were linked to interfacial tension effects, which also allowed convective transport of surfactants to be investiagted. On the basis of these findings, various strategies for the formation of emulsion droplets can be followed and are discussed.