Flow-focusing Droplet Generators Research Articles

A coupled volume-of-fluid and level-set method (VOSET) for capturing interface of two-phase flows in arbitrary polygon grid

This article presents an extension of coupled volume-of-fluid and level-set method (VOSET) for simulating free surfaces flows in arbitrary 2D polygon meshes. A series of techniques are introduced for geometric calculations in convex polygons. Newton iteration is adopted for the interface reconstruction in polygons, and incremental remapping approach is employed for the propagation of the volume fractions. The interface tracking test results suggested a second order accuracy in mixed and hexagonal grids. For the validation purpose, a Rayleigh-Taylor instability problem, a liquid column collapse problem and a single bubble rising problem were numerically studied, and the obtained results show excellent agreements with experimental data and benchmark solutions in literatures. Finally the proposed VOSET method was applied in simulating the working process of a flow-focusing microfluidic droplet generator.

Robust Microfabrication of Highly Parallelized Three-Dimensional Microfluidics on Silicon

We present a new, robust three dimensional microfabrication method for highly parallel microfluidics, to improve the throughput of on-chip material synthesis by allowing parallel and simultaneous operation of many replicate devices on a single chip. Recently, parallelized microfluidic chips fabricated in Silicon and glass have been developed to increase the throughput of microfluidic materials synthesis to an industrially relevant scale. These parallelized microfluidic chips require large arrays (>10,000) of Through Silicon Vias (TSVs) to deliver fluid from delivery channels to the parallelized devices. Ideally, these TSVs should have a small footprint to allow a high density of features to be packed into a single chip, have channels on both sides of the wafer, and at the same time minimize debris generation and wafer warping to enable permanent bonding of the device to glass. Because of these requirements and challenges, previous approaches cannot be easily applied to produce three dimensional microfluidic chips with a large array of TSVs. To address these issues, in this paper we report a fabrication strategy for the robust fabrication of three-dimensional Silicon microfluidic chips consisting of a dense array of TSVs, designed specifically for highly parallelized microfluidics. In particular, we have developed a two-layer TSV design that allows small diameter vias (d < 20 µm) without sacrificing the mechanical stability of the chip and a patterned SiO2 etch-stop layer to replace the use of carrier wafers in Deep Reactive Ion Etching (DRIE). Our microfabrication strategy allows >50,000 (d = 15 µm) TSVs to be fabricated on a single 4” wafer, using only conventional semiconductor fabrication equipment, with 100% yield (M = 16 chips) compared to 30% using previous approaches. We demonstrated the utility of these fabrication strategies by developing a chip that incorporates 20,160 flow focusing droplet generators onto a single 4” Silicon wafer, representing a 100% increase in the total number of droplet generators than previously reported. To demonstrate the utility of this chip for generating pharmaceutical microparticle formulations, we generated 5–9 µm polycaprolactone particles with a CV < 5% at a rate as high as 60 g/hr (>1 trillion particles/hour).

Rapid and Inexpensive Fabrication of Multi-Depth Microfluidic Device using High-Resolution LCD Stereolithographic 3D Printing

With the dramatic increment of complexity, more microfluidic devices require 3D structures, such as multi-depth and -layer channels. The traditional multi-step photolithography is time-consuming and labor-intensive and also requires precise alignment during the fabrication of microfluidic devices. Here, we present an inexpensive, single-step, and rapid fabrication method for multi-depth microfluidic devices using a high-resolution liquid crystal display (LCD) stereolithographic (SLA) three-dimensional (3D) printing system. With the pixel size down to 47.25 μm, the feature resolutions in the horizontal and vertical directions are 150 μm and 50 μm, respectively. The multi-depth molds were successfully printed at the same time and the multi-depth features were transferred properly to the polydimethylsiloxane (PDMS) having multi-depth channels via soft lithography. A flow-focusing droplet generator with a multi-depth channel was fabricated using the presented 3D printing method. Experimental results show that the multi-depth channel could manipulate the morphology and size of droplets, which is desired for many engineering applications. Taken together, LCD SLA 3D printing is an excellent alternative method to the multi-step photolithography for the fabrication of multi-depth microfluidic devices. Taking the advantages of its controllability, cost-effectiveness, and acceptable resolution, LCD SLA 3D printing can have a great potential to fabricate 3D microfluidic devices.

Effect of device geometry on droplet size in co-axial flow-focusing microfluidic droplet generation devices

The Co-axial Flow Focusing (CFF) technique is an established microfluidic method for the generation of micro-droplets. It consists of a nozzle co-axially aligned in a (circular cross-section) channel with a downstream orifice. The continuous-phase (c-phase) fluid in the orifice is used to shear off droplets of the dispersed-phase (d-phase) fluid ejected from the nozzle. Circular cross-section CFF devices are preferred due to their simpler fluid hydrodynamics, but not studied extensively as compared to rectangular channels. Understanding the effect of various parameters such as the channel design and fluid properties in CFF requires the development of reliable computational models. In this paper, the effects of multiple local geometries on droplet formation in a microfluidic CFF device were investigated numerically with experimental validations. The geometries included the d-phase nozzle inner diameter, nozzle tip distance from the c-phase channel orifice, and the inner diameter and length of the flow-focusing orifice. Our model is able to successfully predict the effects of these geometrical parameters on the droplet size for a wide range of c-phase flow rates spanning both the dripping and the jetting flow regimes, with an error range of 3–6%. Based on the simulation results, the droplet size is strongly dependent on the c-phase flow rate and the sizes of both the dispensing nozzle and the flow-focusing orifice. The effects of nozzle-to-orifice distance and the length of the orifice on the droplet size were not as pronounced in the range of the simulated c-phase flow rates. Our model can be used for the design and fabrication of CFF devices which yield predetermined droplet sizes with high precision.

Performance tuning of microfluidic flow-focusing droplet generators.

The required step in all droplet-based devices is droplet formation. A droplet generator must deliver an application-specific performance that includes a prescribed droplet size and generation frequency while producing monodisperse droplets. The desired performance is usually reached through several cost- and time-inefficient design iterations. To address this, we take advantage of a low-cost rapid prototyping method and provide a framework that enables researchers to make informed decisions on how to change geometric parameters and flow conditions to tune the performance of a microfluidic flow-focusing droplet generator. We present the primary and secondary parameters necessary for fine-tuning droplet formation over a wide range of capillary numbers and flow rate ratios. Once the key parameters are identified, we demonstrate the effect of geometric parameters and flow conditions on droplet size, generation rate, polydispersity, and generation regime. Using this framework, a wide range of droplet diameters (i.e., 30-400 μm) and generation rates (i.e., 0.5-800 Hz) was achieved.

Formation and manipulation of polyacrylamide spheroids doped with magnetic nanoparticles in microfluidic chip

Nowadays isolation and sorting of biological objects plays an important role in life science and medicine. Magnetic sorting has big opportunities because of its advantages such as sufficient selectivity and bioinerticity. In this work we studied formation and handling of polyacrylamide microparticles doped with magnetic nanoparticles using droplet microfluidics. Using a flow focusing droplet generator magnetic nanoparticles were encapsulated to polyacrylamide droplets which were polymerized inside the device. It was shown that these magnetic droplets could be efficiently sorted by magnetic field in a microfluidic device.

Analysis of LIF and Mie signals from single micrometric droplets for instantaneous droplet sizing in sprays.

Planar droplet sizing (PDS) is a technique relying on the assumption that laser-induced fluorescence (LIF) and Mie scattering optical signals from spherical droplets depend on their volume and surface area, respectively. In this article, we verify the validity of this assumption by experimentally analyzing the light intensity of the LIF and Mie optical signals from micrometric droplets as a function of their diameter. The size of the droplets is controlled using a new flow-focusing monodisperse droplet generator capable of producing droplets of the desired size in the range of 21 µm to 60 µm. Ethanol droplets doped with eosin dye and excited at 532 nm are considered in this study, and the individual droplets were imaged simultaneously at microscopic and macroscopic scale. The effects of laser power, dye concentration, and temperature variation are systematically studied as a function of LIF/Mie ratio in the whole range of droplet sizes. Finally, a calibration curve at tracer concentration of 0.5 vol% is deduced and used to extract the droplet Sauter mean diameter (SMD) from instantaneous images of a transient ethanol spray. This droplet size mapping is done using structured laser illumination planar imaging (SLIPI), in order to suppress the artifacts induced by multiple light scattering.

Simple, low-cost fabrication of acrylic based droplet microfluidics and its use to generate DNA-coated particles

Hydrogel microparticles were copolymerized with surface-immobilized DNA. Particles derived from a microfluidic device and particles derived from mechanical homogenization were compared. The hypothesis was tested that a controlled droplet generation mechanism would produce more homogeneous particles. Surprisingly, the DNA content of both particle types was similarly inhomogeneous. To make this test possible, a simple, low cost, and rapid method was developed to fabricate a microfluidic chip for droplet generation and in-line polymerization. This method used a low-cost laser cutter ($400) and direct heat bonding (no adhesives or intermediate layers). The flow focusing droplet generator produced droplets and hydrogel particles 10–200 μm in diameter.

SIZE-CONTROLLED DROPLET GENERATION IN A MICROFLUIDIC DEVICE FOR RARE DNA AMPLIFICATION BY OPTIMIZING ITS EFFECTIVE PARAMETERS

Versatility and portability of microfluidic devices play a dominant role in their widespread use by researchers. Droplet-based microfluidic devices have been extensively used due to their precise control over sample volume, and ease of manipulating and addressing each droplet on demand. Droplet-based polymerase chain reaction (PCR) devices are particularly desirable in single DNA amplification. If the droplets are small enough to contain only one DNA molecule, single molecule amplification becomes possible, which can be advantageous in several cases such as early cancer detection. In this work, flow-focusing microfluidic droplet generation’s parameters are numerically investigated and optimized for generating the smallest droplet possible, while considering fabrication limits. Taguchi design of experiment method is used to study the effects of key parameters in droplet generation. By exploiting this approach, a droplet with a radius of 111[Formula: see text]nm is generated using a 3[Formula: see text][Formula: see text]m orifice. Since the governing physics of the droplet generation process is not totally understood yet, by means of analysis of variance (ANOVA) analysis, a generalized linear model (GLM) is proposed to predict the droplet radius, given the values of eight major parameters affecting the droplet size. The proposed model shows a correlation of 95.3% and 64.95% for droplets of radius greater than and lower than 5[Formula: see text][Formula: see text]m, respectively. Finally, the source of this variation of behavior in different size scales is identified.

Fabrication of Glass Microchannel via Glass Imprinting using a Vitreous Carbon Stamp for Flow Focusing Droplet Generator.

This study reports a cost-effective method of replicating glass microfluidic chips using a vitreous carbon (VC) stamp. A glass replica with the required microfluidic microstructures was synthesized without etching. The replication method uses a VC stamp fabricated by combining thermal replication using a furan-based, thermally-curable polymer with carbonization. To test the feasibility of this method, a flow focusing droplet generator with flow-focusing and channel widths of 50 µm and 100 µm, respectively, was successfully fabricated in a soda-lime glass substrate. Deviation between the geometries of the initial shape and the vitreous carbon mold occurred because of shrinkage during the carbonization process, however this effect could be predicted and compensated for. Finally, the monodispersity of the droplets generated by the fabricated microfluidic device was evaluated.

Rapid Fabrication of Multilayer Microfluidic Devices Using the Liquid Crystal Display-Based Stereolithography 3D Printing System

We present a generalized process to characterize a 3D printer for fabrication of microfluidic devices. With this process, researchers are able to determine the capability of SLA printers for a specific resin. We employed a liquid crystal display (LCD)-based SLA 3D printer to demonstrate the feasibility of the process and applied optimized parameters for fabricating multilayer 3D microfluidic devices. It has been found that the LCD-based SLA 3D printer can support fabrication of microfluidic devices with the features down to 400 μm for in-plane features and 800 μm for vertical and interconnection features. The optimized curing time of the 100-μm-thick layer is 5.5, 6.5, and 7.5 s for yellow, light green, and dark green resins, respectively. The 3D printed flow-focusing droplet generator worked properly and could generate droplets with sizes between 50 and 185 mm2. Taken together, the presented strategy can be used to quantitatively analyze and understand the capabilities of SLA 3D printing systems, which greatly facilitate optimization of device design and fabrication processes.

Flow regime mapping of aqueous two-phase system droplets in flow-focusing geometries

The recent advent of aqueous two-phase system (ATPS) has shown great potential to rapidly generate microscale aqueous droplets without tedious post-processing. ATPS provides a more biologically friendly and straightforward method to manufacture aqueous droplets compared with conventional oil-water systems and as such, it has been employed in many biomedical applications. Although the cost-effective manufacturing of aqueous droplets has been feasible by direct generation in ATPS, an understanding of the underlying physics of droplet formation in ATPS is still in its infancy. In this paper, we investigated the hydrodynamic behavior and mechanisms of all-aqueous droplet formation in two flow-focusing droplet generators. This study specifically tests whether ATPS in different geometries generates aqueous droplets with different sizes and properties. To achieve these goals, two incompatible polymers, namely polyethylene glycol (PEG) and dextran (DEX), were mixed in water to make ATPS. The influences of inlet pressures and flow-focusing configurations on droplet size, droplet generation frequency, and thread breakup length were quantified. In addition, flow regime mapping for the two different droplet generators at 30° and 90° junction angles was obtained. The results show that droplet size is very susceptible to the junction angle. On the other hand, inlet pressures of the PEG and DEX flows readily control five main flow regimes including PEG Back Flow, DEX Back Flow, Dripping Flow, Jetting Flow and Stratified Flow. It is observed that generated droplets in the Jetting Flow regime are larger in the case of 30°, whereas larger droplets are obtained in the Dripping Flow regime at 90° configuration. The frequency of droplet generation increases and decreases by increasing PPEG and PDEX, respectively. Finally, we characterized flow regimes by the Capillary number (Ca) of the PEG flow.

Geometry-confined bifurcation at low flow rate in flow-focusing droplet generator

Geometry-confined bifurcation at low flow rate in flow-focusing droplet generator

Three-dimensional parallelization of microfluidic droplet generators for a litre per hour volume production of single emulsions.

This paper looks at the design, fabrication and characterization of stackable microfluidic emulsion generators, with coefficients of variation as low as ~6% and with production rates as high as ~1 L h(-1). This work reports the highest throughput reported in the literature for a microfluidic device with simultaneous operation of liquid-liquid droplet generators. The device was achieved by stacking several layers of 128 flow-focusing droplet generators, organized in a circular array. These layers are interconnected via through-holes and fed with designated fractal distribution networks. The proposed layers were milled on poly(methylmethacrylate) (PMMA) sheets and the stack was thermo-compression bonded to create a three-dimensional device with a high density of generators and an integrated hydraulic manifold. The effect of stacking multiple layers was studied and the results show that fabrication accuracy has a greater impact on the dispersity of the emulsion than the addition of more layers to the stack. Particle crystallization of drugs was also demonstrated as a possible application of this technology in industry.

Microfluidic assembly of multistage porous silicon-lipid vesicles for controlled drug release.

A reliable microfluidic platform for the generation of stable and monodisperse multistage drug delivery systems is reported. A glass-capillary flow-focusing droplet generation device was used to encapsulate thermally hydrocarbonized porous silicon (PSi) microparticles into the aqueous cores of double emulsion drops, yielding the formation of a multistage PSi-lipid vesicle. This composite system enables a large loading capacity for hydrophobic drugs.

Sensitive, High Throughput Detection of Proteins in Individual, Surfactant-Stabilized Picoliter Droplets Using Nanoelectrospray Ionization Mass Spectrometry

Droplet-based fluidics is emerging as a powerful platform for single cell analysis, directed evolution of enzymes, and high throughput screening studies. Due to the small amounts of compound compartmentalized in each droplet, detection has been primarily by fluorescence. To extend the range of experiments that can be carried out in droplets, we have developed the use of electrospray ionization mass spectrometry (ESI-MS) to measure femtomole quantities of proteins in individual pico- to nanoliter droplets. Surfactant-stabilized droplets containing analyte were produced in a flow-focusing droplet generation microfluidic device using fluorocarbon oil as the continuous phase. The droplets were collected off-chip for storage and reinjected into microfluidic devices prior to spraying the emulsion into an ESI mass spectrometer. Crucially, high quality mass spectra of individual droplets were obtained from emulsions containing a mixture of droplets at >150 per minute, opening up new routes to high throughput screening studies.

Stable, biocompatible lipid vesicle generation by solvent extraction-based droplet microfluidics.

In this paper, we present a microfluidic platform for the continuous generation of stable, monodisperse lipid vesicles 20-110 μm in diameter. Our approach utilizes a microfluidic flow-focusing droplet generation design to control the vesicle size by altering the system's fluid flow rates to generate vesicles with narrow size distribution. Double emulsions are first produced in consecutive flow-focusing channel geometries and lipid membranes are then formed through a controlled solvent extraction process. Since no strong solvents are used in the process, our method allows for the safe encapsulation and manipulation of an assortment of biological entities, including cells, proteins, and nucleic acids. The vesicles generated by this method are stable and have a shelf life of at least 3 months. Here, we demonstrate the cell-free in vitro synthesis of proteins within lipid vesicles as an initial step towards the development of an artificial cell.

Integrated parallel microfluidic device for simultaneous preparation of multiplex optical-encoded microbeads with distinct quantum dot barcodes

In this paper we describe a versatile microfluidic strategy for simultaneous preparation of quantum dot-encoded microbeads with controllable barcodes. This method involves the generation of multiple dispersed solutions carrying stepwise concentration gradients of quantum dots by using a pyramidal microfluidic network, the formation of corresponding droplets in parallel flow-focusing droplet generators, and on-chip solidification of these droplets into microbeads. Using a designed microfluidic device, we simultaneously generated five kinds of fluorescence-encoded alginate hydrogel microbeads with a five-level stepwise concentration gradient of monochromatic quantum dot or five controllable ratios of two different-sized quantum dots. This method has the following features: (1) the barcode adapter compartment substitutes the respective preparation of precursor solutions containing the correct concentrations and ratios of different quantum dots for corresponding barcodes. (2) Identical microbeads with distinct quantum dot barcodes can be simultaneously generated in homogeneous conditions. (3) The microfluidic device can be upgraded by increasing or decreasing the inlets and outlets of the pyramidal network and the number of the parallel flow-focusing droplet generators to meet the requirements for the particular barcodes. We expect that this microfluidic route will facilitate the construction and mass-production of robust and reproducible barcode materials.

Microfluidic consecutive flow-focusing droplet generators.

This article describes emulsification in a microfluidic double droplet generator (DDR) comprising two consecutive flow-focusing devices with locally modified surface chemistry. We generated W/O/W, O/O/W and O/W/O double emulsions with precisely controlled sizes and morphology of droplets. Secondly, by combining two mechanisms of droplet formation (the flow-focusing mechanism and the break up of liquid threads at T-junction) we produced multiple populations of droplets with varying size and/or composition. These droplets were used as the structural units for the formation of complex dynamic lattices.