Theoretical investigation of a technique to produce microbubbles by a microfluidic T junction

Dripping, jetting and tip streaming

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Acoustofluidic control of bubble size in microfluidic flow-focusing configuration

This paper reports a method to control the bubble size generated in a microfluidic flow-focusing configuration. With an ultrasonic transducer, we induce acoustic streaming using a forward moving, oscillating gas-liquid interface. The induced streaming substantially affects the formation process of gas bubbles. The oscillating interface acts as a pump that increases the gas flow rate significantly and forms a larger bubble. This method is applicable to a wide range of gas pressure from 30 to 90 kPa and flow rate from 380 to 2700 μL h(-1). The bubble size can be tuned repeatedly with the response time on the order of seconds. We believe that this method will enhance the capability of a microfluidic bubble generator to produce a tunable bubble size.

Computational investigations of the mixing performance inside liquid slugs generated by a microfluidic T-junction

Droplet-based microfluidics has gained extensive research interest as it overcomes several challenges confronted by conventional single-phase microfluidics. The mixing performance inside droplets/slugs is critical in many applications such as advanced material syntheses and in situ kinetic measurements. In order to understand the effects of operating conditions on the mixing performance inside liquid slugs generated by a microfluidic T-junction, we have adopted the volume of fluid method coupled with the species transport model to study and quantify the mixing efficiencies inside slugs. Our simulation results demonstrate that an efficient mixing process is achieved by the intimate collaboration of the twirling effect and the recirculating flow. Only if the reagents are distributed transversely by the twirling effect, the recirculating flow can bring in convection mechanism thus facilitating mixing. By comparing the mixing performance inside slugs at various operating conditions, we find that slug size plays the key role in influencing the mixing performance as it determines the amount of fluid to be distributed by the twirling effect. For the cases where short slugs are generated, the mixing process is governed by the fast convection mechanism because the twirling effect can distribute the fluid to the flow path of the recirculating flow effectively. For cases with long slugs, the mixing process is dominated by the slow diffusion mechanism since the twirling effect is insufficient to distribute the large amount of fluid. In addition, our results show that increasing the operating velocity has limited effects on improving the mixing performance. This study provides the insight of the mixing process and may benefit the design and operations of droplet-based microfluidics.

Dynamics of an axisymmetric liquid bridge close to the minimum-volume stability limit

We analyze both theoretically and experimentally the dynamical behavior of an isothermal axisymmetric liquid bridge close to the minimum-volume stability limit. First, the nature of this stability limit is investigated experimentally by determining the liquid bridge response to a mass force pulse for volumes just above that limit. In our experiments, the liquid bridge breakup takes place only when the critical volume is surpassed and is never triggered by the mass force pulse. Second, the growth of the small-amplitude perturbation mode initiating the liquid bridge breakage is measured experimentally and calculated from the linearized Navier-Stokes equations. The results of the linear stability analysis allow one to explain why liquid bridges with volumes just above the stability limit are so robust. Finally, the nonlinear process leading to the liquid bridge breakup is described from both experimental data and the solution of the full Navier-Stokes equations. Special attention is paid to the free-surface pinchoff. The results show that the flow becomes universal (independent of both the initial and boundary conditions) sufficiently close to that singularity and suggest that the transition from the inviscid to the viscous regime is about to take place in the final stage of both the experiments and numerical simulations.

Production of microbubbles from axisymmetric flow focusing in the jetting regime for moderate Reynolds numbers

We analyze both experimentally and numerically the formation of microbubbles in the jetting regime reached when a moderately viscous liquid stream focuses a gaseous meniscus inside a converging micronozzle. If the total (stagnation) pressure of the injected gas current is fixed upstream, then there are certain conditions on which a quasisteady gas meniscus forms. The meniscus tip is sharpened by the liquid stream down to the gas molecular scale. On the other side, monodisperse collections of microbubbles can be steadily produced in the jetting regime if the feeding capillary is appropriately located inside the nozzle. In this case, the microbubble size depends on the feeding capillary position. The numerical simulations for an imposed gas flow rate show that a recirculation cell appears in the gaseous meniscus for low enough values of that parameter. The experiments allow one to conclude that the bubble pinch-off comprises two phases: (i) a stretching motion of the precursor jet where the neck radius versus the time before the pinch essentially follows a potential law, and (ii) a final stage where a very thin and slender gaseous thread forms and eventually breaks apart into a number of micron-sized bubbles. Because of the difference between the free surface and core velocities, the gaseous jet breakage differs substantially from that of liquid capillary jets and gives rise to bubbles with diameters much larger than those expected from the Rayleigh-type capillary instability. The dependency of the bubble diameter upon the flow-rate ratio agrees with the scaling law derived by A. M. Gañán-Calvo [Phys. Rev. E 69, 027301 (2004)], although a slight influence of the Reynolds number can be observed in our experiments.

Formation and dynamics of partially wetting droplets in square microchannels

Droplet motion and dynamic wetting transitions are experimentally investigated over a wide range of viscosities and flow rates in square microchannels

Effect of the fluid injection configuration on droplet size in a microfluidic T junction

The effect of confinement on the droplet formation in T junctions was studied for three configurations of fluid injection. The sizes of the main droplets and the satellite droplets were measured in the squeezing and dripping regimes. The evolution of droplet sizes with capillary number in the continuous phase is similar to that in flow-focusing junctions, i.e., the size of the main droplets decreases with an increase of this capillary number, while the size of the satellite droplets increases with an increase of this capillary number. While in the range of flow rates investigated the injection configuration does not exhibit a significant effect on the main droplet sizes, it does have an effect on the size of the satellite droplets. The latter ones are smaller when the neck rupture of the droplet occurs on an angle of the microsystem.