Global stability of the focusing effect of fluid jet flows

Study on Two-Phase Mixing Inside Flow Focusing/Blurring Nozzle by Gray Distribution Analysis Method

The application prospect of a flow focusing/blurring nozzle is broad but research on gas-liquid flow inside the nozzle is not comprehensive. The gas-liquid mixing inside the nozzle is difficult to study by visualization experiment, so this paper proposes to study the gas-liquid flow or mixing inside the nozzle by the gray scale level distribution of the experimental images. The results show that the increase of air flow rate is beneficial to two-phase mixing inside the nozzle, while the influence of water flow rate, tube hole distance (the distance between inner tube and nozzle outlet) and orifice diameter increase is opposite. The influence of air inertia force on two-phase mixing is weaker than the water inertia force under different parameters, the effect of the air inertia force on two-phase mixing is similar to tube hole distance under a small flow rate, the effect of the orifice diameter on two-phase mixing is relatively weak. In addition, the analysis of the gas-liquid flow field in the mixing zone shows that the gas-liquid flow in the nozzle is stable in the flow focusing mode. In the flow blurring mode, the gas-liquid flow inside the nozzle has radial stability but axial pulsation. In the transition mode, the gas-liquid flow inside the nozzle is unstable, but the gas-liquid flow is close to the flow blurring mode.

Alternative Geometric Arrangements of the Nozzle Outlet Orifice for Liquid Micro-Jet Focusing in Gas Dynamic Virtual Nozzles

Liquid micro-jets are crucial for sample delivery of protein crystals and other macromolecular samples in serial femtosecond crystallography. When combined with MHz repetition rate sources, such as the European X-ray free-electron laser (EuXFEL) facility, it is important that the diffraction patterns are collected before the samples are damaged. This requires extremely thin and very fast jets. In this paper we first explore numerically the influence of different nozzle orifice designs on jet parameters and finally compare our simulations with the experimental data obtained for one particular design. A gas dynamic virtual nozzle (GDVN) model, based on a mixture formulation of Newtonian, compressible, two-phase flow, is numerically solved with the finite volume method and volume of fluid approach to deal with the moving boundary between the gas and liquid phases. The goal is to maximize the jet velocity and its length while minimizing the jet thickness. The design studies incorporate differently shaped nozzle orifices, including an elongated orifice with a constant diameter and an orifice with a diverging angle. These are extensions of the nozzle geometry we investigated in our previous studies. Based on these simulations it is concluded that the extension of the constant diameter channel makes a negligible contribution to the jet’s length and its velocity. A change in the angle of the nozzle outlet orifice, however, has a significant effect on jet parameters. We find these kinds of simulation extremely useful for testing and optimizing novel nozzle designs.

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.

3D printing of gas-dynamic virtual nozzles and optical characterization of high-speed microjets

Gas dynamic virtual nozzles (GDVNs) produce microscopic flow-focused liquid jets and droplets and play an important role at X-ray free-electron laser (XFEL) facilities where they are used to steer a stream of hydrated biomolecules into an X-ray focus during diffraction measurements. Highly stable and reproducible microjet and microdroplets are desired, as are flexible fabrication methods that enable integrated mixing microfluidics, droplet triggering mechanisms, laser illumination, and other customized features. In this study, we develop the use of high-resolution 3D nano-printing for the production of monolithic, asymmetric GDVN designs that are difficult to fabricate by other means. We also develop a dual-pulsed nanosecond image acquisition and analysis platform for the characterization of GDVN performance, including jet speed, length, diameter, and directionality, among others. We show that printed GDVNs can form microjets with very high degree of reproducibility, down to sub-micron diameters, and with water jet speeds beyond 170 m/s.

Rapid sample delivery for megahertz serial crystallography at X-ray FELs

Liquid microjets are a common means of delivering protein crystals to the focus of X-ray free-electron lasers (FELs) for serial femtosecond crystallography measurements. The high X-ray intensity in the focus initiates an explosion of the microjet and sample. With the advent of X-ray FELs with megahertz rates, the typical velocities of these jets must be increased significantly in order to replenish the damaged material in time for the subsequent measurement with the next X-ray pulse. This work reports the results of a megahertz serial diffraction experiment at the FLASH FEL facility using 4.3 nm radiation. The operation of gas-dynamic nozzles that produce liquid microjets with velocities greater than 80 m s-1 was demonstrated. Furthermore, this article provides optical images of X-ray-induced explosions together with Bragg diffraction from protein microcrystals exposed to trains of X-ray pulses repeating at rates of up to 4.5 MHz. The results indicate the feasibility for megahertz serial crystallography measurements with hard X-rays and give guidance for the design of such experiments.

Passive and active droplet generation with microfluidics: a review

Precise and effective control of droplet generation is critical for applications of droplet microfluidics ranging from materials synthesis to lab-on-a-chip systems. Methods for droplet generation can be either passive or active, where the former generates droplets without external actuation, and the latter makes use of additional energy input in promoting interfacial instabilities for droplet generation. A unified physical understanding of both passive and active droplet generation is beneficial for effectively developing new techniques meeting various demands arising from applications. Our review of passive approaches focuses on the characteristics and mechanisms of breakup modes of droplet generation occurring in microfluidic cross-flow, co-flow, flow-focusing, and step emulsification configurations. The review of active approaches covers the state-of-the-art techniques employing either external forces from electrical, magnetic and centrifugal fields or methods of modifying intrinsic properties of flows or fluids such as velocity, viscosity, interfacial tension, channel wettability, and fluid density, with a focus on their implementations and actuation mechanisms. Also included in this review is the contrast among different approaches of either passive or active nature.

Generation of micro-sized PDMS particles by a flow focusing technique for biomicrofluidics applications

Polydimethylsiloxane (PDMS), due to its remarkable properties, is one of the most widely used polymers in many industrial and medical applications. In this work, a technique based on a flow focusing technique is used to produce PDMS spherical particles with sizes of a few microns. PDMS precursor is injected through a hypodermic needle to form a film/reservoir over the needle's outer surface. This film flows towards the needle tip until a liquid ligament is steadily ejected thanks to the action of a coflowing viscous liquid stream. The outcome is a capillary jet which breaks up into PDMS precursor droplets due to the growth of capillary waves producing a micrometer emulsion. The PDMS liquid droplets in the solution are thermally cured into solid microparticles. The size distribution of the particles is analyzed before and after curing, showing an acceptable degree of monodispersity. The PDMS liquid droplets suffer shrinkage while curing. These microparticles can be used in very varied technological fields, such as biomedicine, biotechnology, pharmacy, and industrial engineering.

Ceramic micro-injection molded nozzles for serial femtosecond crystallography sample delivery

Serial femtosecond crystallography (SFX) using X-ray Free-Electron Lasers (XFELs) allows for room temperature protein structure determination without evidence of conventional radiation damage. In this method, a liquid suspension of protein microcrystals can be delivered to the X-ray beam in vacuum as a micro-jet, which replenishes the crystals at a rate that exceeds the current XFEL pulse repetition rate. Gas dynamic virtual nozzles produce the required micrometer-sized streams by the focusing action of a coaxial sheath gas and have been shown to be effective for SFX experiments. Here, we describe the design and characterization of such nozzles assembled from ceramic micro-injection molded outer gas-focusing capillaries. Trends of the emitted jet diameter and jet length as a function of supplied liquid and gas flow rates are measured by a fast imaging system. The observed trends are explained by derived relationships considering choked gas flow and liquid flow conservation. Finally, the performance of these nozzles in a SFX experiment is presented, including an analysis of the observed background.

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.

Microfluidic liquid jet system with compatibility for atmospheric and high-vacuum conditions

We present microfluidic chip based devices that produce liquid jets with micrometer diameters while operating at very low flow rates. The chip production is based on established soft-lithographical techniques employing a three-layer design protocol. This allows the exact, controlled and reproducible design of critical parts such as nozzles and the production of nozzle arrays. The microfluidic chips reproducibly generate liquid jets exiting at perfect right angles with diameters between 20 μm and 2 μm, and under special circumstances, even down to 0.9 μm. Jet diameter, jet length, and the domain of the jetting/dripping instability can be predicted and controlled based on the theory for liquid jets in the plate-orifice configuration described by Gañán-Calvo et al. Additionally, conditions under which the device produces highly reproducible monodisperse droplets at exact and predictable rates can be achieved. The devices operate under atmospheric and under vacuum conditions making them highly relevant for a wide range of applications, for example, for free-electron lasers. Further, the straightforward integration of additional features such as a jet-in-jet is demonstrated. This device design has the potential to integrate more features based on established microfluidic components and may become a standard device for small liquid jet production.

Building functional materials for health care and pharmacy from microfluidic principles and Flow Focusing

In this review, we aim at establishing a relationship between the fundamentals of the microfluidics technologies used in the Pharmacy field, and the achievements accomplished by those technologies. We describe the main methods for manufacturing micrometer drops, bubbles, and capsules, as well as the corresponding underlying physical mechanisms. In this regard, the review is intended to show non-specialist readers the dynamical processes which determine the success of microfluidics techniques. Flow focusing (FF) is a droplet-based method widely used to produce different types of fluid entities on a continuous basis by applying an extensional co-flow. We take this technique as an example to illustrate how microfluidics technologies for drug delivery are progressing from a deep understanding of the physics of fluids involved. Specifically, we describe the limitations of FF, and review novel methods which enhance its stability and robustness. In the last part of this paper, we review some of the accomplishments of microfluidics when it comes to drug manufacturing and delivery. Special attention is paid to the production of the microencapsulated form because this fluidic structure gathers the main functionalities sought for in Pharmacy. We also show how FF has been adapted to satisfy an ample variety of pharmaceutical requirements to date.

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

A microfluidic technique is proposed to produce microbubbles. A gaseous stream is injected through a T junction into a channel transporting a liquid current. The gas adheres to a hydrophobic strip printed on the channel surface. When the gas and liquid flow rates are set appropriately, a gaseous rivulet flows over that strip. The rivulet breaks up downstream due to a capillary pearling instability, which leads to a monodisperse collection of microbubbles that can be much smaller than the channel size. The physics of the process is theoretically investigated, using both full numerical simulation of the Navier-Stokes equations and a linear stability analysis of an infinite gaseous rivulet driven by a coflowing liquid stream. This stability analysis allows one to determine a necessary condition to get this effect in a T junction device. It also provides reasonably good predictions for the size of the produced microbubbles as obtained from numerical experiments.