Axisymmetric Flow Focusing Research Articles

Microfluidic droplet generation based on non-embedded co-flow-focusing using 3D printed nozzle

Most commercial microfluidic droplet generators rely on the planar flow-focusing configuration implemented in polymer or glass chips. The planar geometry, however, suffers from many limitations and drawbacks, such as the need of specific coatings or the use of dedicated surfactants, depending on the fluids in play. On the contrary, and thanks to their axisymmetric geometry, glass capillary-based droplet generators are a priori not fluid-dependent. Nevertheless, they have never reached the market because their assembly requires fastidious and not scalable fabrication techniques. Here we present a new device, called Raydrop, based on the alignment of two capillaries immersed in a pressurized chamber containing the continuous phase. The dispersed phase exits one of the capillaries through a 3D-printed nozzle placed in front of the extraction capillary for collecting the droplets. This non-embedded implementation of an axisymmetric flow-focusing is referred to non-embedded co-flow-focusing configuration. Experimental results demonstrate the universality of the device in terms of the variety of fluids that can be emulsified, as well as the range of droplet radii that can be obtained, without neither the need of surfactant nor coating. Additionally, numerical computations of the Navier-Stokes equations based on the quasi-steadiness assumption allow to provide an explanation to the underlying mechanism behind the drop formation and the mechanism of the dripping to jetting transition. Excellent predictions were also obtained for the droplet radius, as well as for the dripping-jetting transition, when varying the geometrical and fluid parameters, showing the ability of this configuration to enventually enhance the dripping regime. The monodispersity ensured by the dripping regime, the robustness of the fabrication technique, the optimization capabilities from the numerical modelling and the universality of the configuration confer to the Raydrop technology a very high potential in the race towards high-throughput droplet generation processes.

Manipulation of jet breakup length and droplet size in axisymmetric flow focusing upon actuation

External sinusoidal actuation is employed in the axisymmetric flow focusing (AFF) for generating uniform droplets in the jetting mode. The perturbations propagating along the meniscus surface can modulate the rupture of the liquid jet. Experiments indicate that the jet breakup length and the resultant droplet size can be precisely controlled in the synchronized regime, which are further confirmed by the scaling law. The finding of this study can help for better understanding of the underlying physics of actuation-aided AFF, and this active droplet generation method with fine robustness, high productivity, and nice process control would be advantageous for various potential applications.

Droplet-based microfluidic method for robust preparation of gold nanoparticles in axisymmetric flow focusing device

A novel microfluidic mixing strategy was developed and used to prepare polyvinylpyrrolidone (PVP) capped gold nanoparticles (AuNPs). In this process, 1 mM tetrachloroauric acid (HAuCl4) stream containing 1% (w/v) PVP was injected through the inner capillary tube and mixed with 20 mM L-ascorbic acid solution delivered co-currently through the outer coaxial capillary. The reaction mixture was hydrodynamically flow focused by the environmentally friendly oil Miglyol 840 delivered from the opposite side of the outer capillary, which resulted in the generation of reaction droplets in a tapered collection tube. The reactants were rapidly mixed within droplets by internal circulating flows induced by hydrodynamic interactions of fluids inside the droplets with the carrier oil. The size of the prepared AuNPs was measured by both dynamic light scattering and transmission electron microscopy and was found to decrease with decreasing the droplet size and increasing the difference in velocity between the two reactant streams, which improved mixing efficiency within droplets. The smallest nanoparticles were obtained when the outlet section of the injection tube was positioned at the entry section of the collection tube due to the highest shear at the liquid interface. The carrier oil formed a hydrophobic barrier between the droplets and the reactor walls preventing deposition of the synthesised particles. As a result, the size of the AuNPs was smaller than in the co-flow mixer operated with two continuous reactant streams.

Numerical study on droplet generation in axisymmetric flow focusing upon actuation

In the pure axisymmetric flow focusing (AFF), it is difficult to generate uniform droplets due to the random breakup of bulk flows. Therefore, applying external perturbations is a desirable approach to control the monodispersity of the droplets and makes it possible to produce uniform droplets at relatively high flow rates. In the present study, the effects of different external perturbations (waveform shape, frequency f and amplitude A) on the droplet generation are numerically investigated. When the focused phase is singly actuated, the size of the uniform droplets can be obtained and manipulated by adjusting f and A. In particular, the jet breakup has the same frequency as the external perturbation in the generation of uniform droplets. There exists a cutoff frequency beyond which the perturbation cannot control the jet breakup, even with large A. This is found to be associated with the critical condition for the onset of the Rayleigh-Plateau instability. In addition, the reservoir effect of the cone in the AFF effectively reduces the influence of the perturbation on the liquid supply to the liquid jet, accounting for the presence of jetting at low f and large A. Moreover, we apply the perturbations either singly to the focusing phase or simultaneously to the focused and focusing phases and assess their effects on the jet breakup. Finally, the square waveform perturbations acting on the droplet generation are discussed. The present work provides a guidance of the actuation-aided AFF for practical applications of on-demand droplet generation.

Global stability of axisymmetric flow focusing

In this paper, we analyse numerically the stability of the steady jetting regime of gaseous flow focusing. The base flows are calculated by solving the full Navier–Stokes equations and boundary conditions for a wide range of liquid viscosities and gas speeds. The axisymmetric modes characterizing the asymptotic stability of those flows are obtained from the linearized Navier–Stokes equations and boundary conditions. We determine the flow rates leading to marginally stable states, and compare them with the experimental values at the jetting-to-dripping transition. The theoretical predictions satisfactorily agree with the experimental results for large gas speeds. However, they do not capture the trend of the jetting-to-dripping transition curve for small gas velocities, and considerably underestimate the minimum flow rate in this case. To explain this discrepancy, the Navier–Stokes equations are integrated over time after introducing a small perturbation in the tapering liquid meniscus. There is a transient growth of the perturbation before the asymptotic exponential regime is reached, which leads to dripping. Our work shows that flow focusing stability can be explained in terms of the combination of asymptotic global stability and the system short-term response for large and small gas velocities, respectively.

Droplet formation in a flow focusing configuration: Effects of viscoelasticity

We numerically investigate the effects of bulk fluid viscoelasticity on droplet formation and dynamics in an axisymmetric flow focusing configuration. Viscoelasticity is modeled using the finitely extensible nonlinear elastic-Chilcott-Rallison (FENE-CR) model. Extensive simulations are performed to examine droplet formation and breakup dynamics for a wide range of parameters including flow rate ratio, Weissenberg number, polymeric viscosity ratio, and extensibility parameter. It is found that these parameters have a significant influence on the droplet size and size distribution (dispersity). Three different regimes are observed in the sequence of squeezing, dripping, and jetting modes as the flow rate ratio is increased. It is also found that the viscoelasticity has a similar effect as decreasing flow rate ratio and acts to delay transition from squeezing to dripping and from dripping to jetting regimes. The strain-rate hardening occurs at a critical Weissenberg number resulting in an abrupt increase in droplet size and this effect is more pronounced as the polymeric viscosity ratio is increased.

Monosized dripping mode of axisymmetric flow focusing.

We identify and analyze the perfectly regular dripping mode of flow focusing. This mode occurs within narrow intervals of injected flow rates and applied pressure drops and leads to homogeneous-size droplets with diameters similar to or smaller than that of the discharge orifice. The balance between the local acceleration of the fluid particle and the applied pressure drop yields the scaling law for the droplet diameter. This scaling law is validated experimentally with excellent accord.

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.

“Off-the-shelf” 3-D microfluidic nozzle

We present the construction and operation of a microfluidic nozzle created using several standard fluidic parts available commercially. By elegantly combining several pieces from a standard assembly, a capillary and a few other standard parts, we were able to develop a novel device. Using this device, precise axisymmetric flow focusing of particles was achieved and observed at the exit of the nozzle and within a connected microfluidic device several centimetres away. Sheath and core flow rates were varied to show influence and control over the width of the focused particles.

Automated Reactor For Extraction And Manipulation Of Sub-Megabase Fragments Of Genomic DNA With Flow-Focusing

Long DNA fragments (0.1-1 Mb) are required in many polymer physics studies, especially implementing single-molecule approaches. Previously we presented a membrane reactor designed to produce samples of sub-megabase DNA fragments. This reactor is capable of extracting and purifying high quality genomic DNA and additionally perform various reactions such as restriction enzyme digestion, intercalation with fluorescent dyes, and labeling with sequence-specific tags. This 125 μl volume reactor performs preparations significantly faster than routine procedures and is completely automated.To extend the ability of the reactor to work with smaller bacterial loads (106 cells vs. 108 cells), we recently introduced an axisymmetric flow focusing mode. In this mode, flow fields created within the chamber focus the bacterial cells to a small area in the center of the membrane. This arrangement helps to limit the interaction of deformable DNA coils with membrane nanopores which leads to decreased sample losses. It also enables elution of the sample in a smaller volume and at 5-10-fold higher concentration. Implementation of flow splitting during the elution process helps to further increase the DNA concentration by an order of magnitude.This work focuses on experimental and numerical characterization of the flow fields employed in the reactor. Experimental study was performed with 160 kb DNA and 240 kDa proteins. Numerically, the semidilute DNA solution on the membrane is modeled with deGennes' reptation model. The models are used to estimate flow fields necessary to carry out reactions and purification of genomic DNA on the membrane without shear degradation.As an illustration of reactor performance, we demostrate successful extraction of genomic DNA from E. coli cells, its purification, specific digestion with NotI restriction enzyme, and intercalation with POPO-1. The largest eluted DNA fragment was nearly 1 Mb-long.

Axisymmetric flow focusing of particles in a single microchannel

Axisymmetric flow focusing of particles in a single microchannel has been proposed on the basis of the observation that a particle migrates toward the tube axis when it lags behind the fluid flow, which is supported by demonstrating that more than 90% of the particles are tightly focused within three times the particle diameter when negative electrophoretic mobility is imposed on the particles.

1P309 3次元マイクロ流路を用いた微生物の均一径ハイドロゲルカプセル化(バイオエンジニアリング、計測,口頭発表,第45回日本生物物理学会年会)

1P309 3次元マイクロ流路を用いた微生物の均一径ハイドロゲルカプセル化(バイオエンジニアリング、計測,口頭発表,第45回日本生物物理学会年会)