Predictive model for the size of bubbles and droplets created in microfluidic T-junctions

3D printing of monolithic gravity-assisted step-emulsification device for scalable production of high viscosity emulsion droplets

Microfluidic technology widely used in generating monodisperse emulsion droplets often suffers from complexity, scalability, applicability to practical fluids, as well as operation instability due to its susceptibility to flow perturbations, low clearance, and depletion of surfactants. Herein, we present a monolithic 3D-printed step-emulsification device (3D-PSD) for scalable and robust production of high viscosity emulsion droplets up to 208.16 mPa s, which cannot be fully addressed using conventional step-emulsification devices. By utilizing stereo-lithography (SLA), 24 triangular nozzles with a pair of 3D void flow distributors are integrated within the 3D-PSD to ensure uniform flow distribution followed by monodisperse droplet formation. The outlets positioned vertically downward enables gravity-assisted clearing to prevent droplet accumulation and thereby maintain size monodispersity. Deposition of silica nanoparticles (SiNP) within the device was also shown to alter the surface wettability from hydrophobic to hydrophilic, enabling the production of both water-in-oil (W/O) as well as oil-in-water (O/W) emulsion droplets, operated at a maximum production rate of up to 50 mL h-1. The utility of the device is further verified through continuous production of biodegradable polycaprolactone (PCL) microparticles using O/W emulsion as templates. We envision that the 3D-PSD presented in this work marks a significant leap in high-throughput production of high viscosity emulsion droplets as well as the particle analogs.

Self-synchronization of reinjected droplets for high-efficiency droplet pairing and merging

Droplet merging serves as a powerful tool to add reagents to moving droplets for biological and chemical reactions. However, unsynchronized droplet pairing impedes high-efficiency merging. Here, we develop a microfluidic design for the self-synchronization of reinjected droplets. A periodic increase in the hydrodynamic resistance caused by droplet blocking a T-junction enables automatic pairing of droplets. After inducing spacing, the paired droplets merge downstream under an electric field. The blockage-based design can achieve a 100% synchronization efficiency even when the mismatch rate of droplet frequencies reaches 10%. Over 98% of the droplets can still be synchronized at nonuniform droplet sizes and fluctuating reinjection flow rates. Moreover, the droplet pairing ratio can be adjusted flexibly for on-demand sample addition. Using this system, we merge two groups of droplets encapsulating enzyme/substrate, demonstrating its capacity to conduct multi-step reactions. We also combine droplet sorting and merging to coencapsulate single cells and single beads, providing a basis for high-efficiency single-cell sequencing. We expect that this system can be integrated with other droplet manipulation systems for a broad range of chemical and biological applications.

Review of the role of surfactant dynamics in drop microfluidics

Surfactants are employed in microfluidic systems not just for drop stabilisation, but also to study local phenomena in industrial processes. On the scale of a single drop, these include foaming, emulsification and stability of foams and emulsions using statistically significant ensembles of bubbles or drops respectively. In addition, surfactants are often a part of a formulation in microfluidic drop reactors. In all these applications, surfactant dynamics play a crucial role and need to be accounted for. In this review, the effect of surfactant dynamics is considered on the level of standard microfluidic operations: drop formation, movement in channels and coalescence, but also on a more general level, considering the mechanisms controlling surfactant adsorption on time- and length-scales characteristic of microfluidics. Some examples of relevant calculations are provided. The advantages and challenges of the use of microfluidics to measure dynamic interfacial tension at short time-scales are discussed.

Microfluidic Platform with Serpentine Geometry Providing Chaotic Mixing in Induction Time Experiments

We present a droplet microfluidic platform mixing the contents of the droplet chaotically in microfluidic induction time measurements, a promising method for quantifying nucleation kinetics with minute amounts of solute. The nucleation kinetics of aqueous potassium chloride droplets dispersed in mineral oil without surfactants is quantified in the presence and absence of chaotic mixing. We demonstrate the ability of the proposed platform to dictate droplet size, to provide a homogeneous temperature distribution, and to chaotically mix the droplet contents. Chaotic mixing in induction time measurements is facilitated by the motion of droplets through serpentine micromixer bends, while the extent of mixing is controlled by how much droplets move. Different nucleation kinetics are observed in experiments where the droplets are static, mixed, and in motion. We hypothesize that the droplet motion induces formation of a thin-liquid Bretherton film surrounding the droplets. The thin film shields droplets from solid boundaries that are more efficient heteronucleant surfaces compared to liquid-liquid interfaces. We observed that repeated microfluidic induction time measurements, particularly with moving droplets, produce significantly distinct cumulative nucleation probability curves, indicating that the measured nucleation kinetics depend strongly on the details of the experimental procedure, which we discuss in detail. Finally, we compare the microfluidic experiments to well-mixed, milliliter volume, turbidity-based measurements in the context of classic nucleation theory.

Fabrication of Biomaterials and Biostructures Based On Microfluidic Manipulation

Biofabrication technologies are of importance for the construction of organ models and functional tissue replacements. Microfluidic manipulation, a promising biofabrication technique with micro-scale resolution, can not only help to realize the fabrication of specific microsized structures but also build biomimetic microenvironments for biofabricated tissues. Therefore, microfluidic manipulation has attracted attention from researchers in the manipulation of particles and cells, biochemical analysis, tissue engineering, disease diagnostics, and drug discovery. Herein, biofabrication based on microfluidic manipulation technology is reviewed. The application of microfluidic manipulation technology in the manufacturing of biomaterials and biostructures with different dimensions and the control of the microenvironment is summarized. Finally, current challenges are discussed and a prospect of microfluidic manipulation technology is given. The authors hope this review can provide an overview of microfluidic manipulation technologies used in biofabrication and thus steer the current efforts in this field.

Easy-to-Operate Co-flow Step Emulsification Device for Droplet Digital Polymerase Chain Reaction

Digital polymerase chain reaction (PCR) plays important roles in the detection and quantification of nucleic acid targets, while there still remain challenges including high cost, complex operation, and low integration of the instrumental system. Here, in this work, a novel microfluidic chip based on co-flow step emulsification is proposed for droplet digital PCR (ddPCR), which can achieve droplet generation, droplet array self-assembly, PCR amplification, and fluorescence detection on a single device. With the combination of single-layer lithography and punching operation, a step microstructure was constructed and it served as the key element to develop a Laplace pressure gradient at the Rayleigh-Plateau instability interface so as to achieve droplet generation. It is demonstrated that the fabrication of step microstructure is low cost, easy-to-operate, and reliable. In addition, the single droplet volume can be adjusted flexibly due to the co-flow design; thus, the ddPCR chip can get an ultrahigh upper limit of quantification to deal with DNA templates with high concentrations. Furthermore, the volume fraction of the resulting droplets in this ddPCR chip can be up to 72% and it results in closely spaced droplet arrays, makes the best of CCD camera for fluorescence detections, and is beneficial for the minimization of a ddPCR system. The quantitative capability of the ddPCR chip was evaluated by measuring template DNA at concentrations from 20 to 50 000 copies/μL. Owing to the characteristics of low cost, easy operation, excellent quantitative capability, and minimization, the proposed ddPCR chip meets the requirements of DNA molecule quantification and is expected to be applied in the point-of-care testing field.

Droplet formation of biological non-Newtonian fluid in T-junction generators. II. Model for final droplet volume prediction

This work represents the second part of a two-part series on the dynamics of droplet formation in a T-junction generator under the squeezing regime when using solutions of red blood cells as the dispersed phase. Solutions containing red blood cells are non-Newtonian; however, these solutions do not behave in the same way as other non-Newtonian fluids currently described in the literature. Hence, available models do not capture nor predict important features useful for the design of T-junction microfluidic systems, including droplet volume. The formation of a red blood cell-containing droplet consists of three stages: a lag stage, a filling stage, and a necking stage, with the lag stage only observed in narrow dispersed phase channel setups. Unlike other shear-thinning fluids, thread elongation into the main channel at the end of the necking stage is not observed for red blood cell solutions. In this work, a model that predicts the final droplet volume of a red blood cell containing droplets in T-junction generators is presented. The model combines a detailed analysis of the geometrical shape of the droplet during the formation process, with force and Laplace pressure balances to obtain the penetration depth (b_{fill}^{*}) and the critical neck thickness (2r_{pinch}^{*}) of the droplet. The performance of the model was validated by comparing the operational parameters (droplet volume, the spacing between the droplet, and the generation frequency) with the experimental data across a range of the dimensionless parameters (flow rate ratios, continuous phase viscosities, and channel geometries).

Droplet formation of biological non-Newtonian fluid in T-junction generators. I. Experimental investigation

The extension of microfluidics to many bioassay applications requires the ability to work with non-Newtonian fluids. One case in point is the use of microfluidics with blood having different hematocrit levels. This work is the first part of a two-part study and presents the formation dynamics of blood droplets in a T-junction generator under the squeezing regime. In this regime, droplet formation with Newtonian fluids depends on T-junction geometry; however, we found that in the presence of the non-Newtonian fluid such as red blood cells, the formation depends on not only to the channel geometry, but also the flow rate ratio of fluids, and the viscosity of the phases. In addition, we analyzed the impact of the red blood cell concentration on the formation cycle. In this study, we presented the experimental data of the blood droplet evolution through the analysis of videos that are captured by a high-speed camera. During this analysis, we tracked several parameters such as droplet volume, spacing between droplets, droplet generation frequency, flow conditions, and geometrical designs of the T junction. Our analysis revealed that, unlike other non-Newtonian fluids, where the fourth stage exists (stretching stage), the formation cycle consists of only three stages: lag, filling, and necking stages. Because of the detailed analysis of each stage, a mathematical model can be generated to predict the final volume of the blood droplet and can be utilized as a guide in the operation of the microfluidic device for biochemical assay applications; this is the focus of the second part of this study [Phys. Rev. E 105, 025106 (2022)10.1103/PhysRevE.105.025106].

Scaling up the throughput of microfluidic droplet-based materials synthesis: A review of recent progress and outlook

The last two decades have witnessed tremendous progress in the development of microfluidic chips that generate micrometer- and nanometer-scale materials. These chips allow precise control over composition, structure, and particle uniformity not achievable using conventional methods. These microfluidic-generated materials have demonstrated enormous potential for applications in medicine, agriculture, food processing, acoustic, and optical meta-materials, and more. However, because the basis of these chips' performance is their precise control of fluid flows at the micrometer scale, their operation is limited to the inherently low throughputs dictated by the physics of multiphasic flows in micro-channels. This limitation on throughput results in material production rates that are too low for most practical applications. In recent years, however, significant progress has been made to tackle this challenge by designing microchip architectures that incorporate multiple microfluidic devices onto single chips. These devices can be operated in parallel to increase throughput while retaining the benefits of microfluidic particle generation. In this review, we will highlight recent work in this area and share our perspective on the key unsolved challenges and opportunities in this field.

Experimental Studies of Droplet Formation Process and Length for Liquid–Liquid Two-Phase Flows in a Microchannel

In this study, changes in the droplet formation mechanism and the law of droplet length in a two-phase liquid–liquid system in 400 × 400 μm standard T-junction microchannels were experimentally studied using a high-speed camera. The study investigated the effects of various dispersed phase viscosities, various continuous phase viscosities, and two-phase flow parameters on droplet length. Two basic flow patterns were observed: slug flow dominated by the squeezing mechanism, and droplet flow dominated by the shear mechanism. The dispersed phase viscosity had almost no effect on droplet length. However, the droplet length decreased with increasing continuous phase viscosity, increasing volume flow rate in the continuous phase, and the continuous-phase capillary number Cac. Droplet length also increased with increasing volume flow rate in the dispersed phase and with the volume flow rate ratio. Based on the droplet formation mechanism, a scaling law governing slug and droplet length was proposed and achieved a good fit with experimental data.

Three-dimensional visualization and analysis of flowing droplets in microchannels using real-time quantitative phase microscopy

Recent years have witnessed the development of droplet-based microfluidics as a useful and effective tool for high-throughput analysis in biological, chemical and environmental sciences. Despite the flourishing development of droplet manipulation techniques, only a few methods allow for label-free and quantitative inspection of flowing droplets in microchannels in real-time and in three dimensions (3-D). In this work, we propose and demonstrate the application of a real-time quantitative phase microscopy (RT-QPM) technique for 3-D visualization of droplets, and also for full-field and label-free measurement of analyte concentration distribution in the droplets. The phase imaging system consists of a linear-CCD-based holographic microscopy configuration and an optofluidic phase-shifting element, which can be used for retrieving quantitative phase maps of flowing objects in the microchannels with a temporal resolution only limited to the frame rate of the CCD camera. To demonstrate the capabilities of the proposed imaging technique, we have experimentally validated the 3-D image reconstruction of the droplets generated in squeezing and dripping regimes and quantitatively investigated the volumetric and morphological variation of droplets as well as droplet parameters related to the depth direction under different flow conditions. We also demonstrated the feasibility of using this technique, as a refractive index sensor, for in-line quantitative measurement of carbamide analyte concentration within the flowing droplets.

µPump: An open-source pressure pump for precision fluid handling in microfluidics

An open-source precision pressure pump system and control software is presented, primarily designed for the experimental microfluidics community, although others may find additional uses for this precision pressure source. This mechatronic system is coined 'µPump,' and its performance rivals that of commercially available systems, at a fraction of the cost. The pressure accuracy, stability, and resolution are 0.09%, 0.02%, and 0.02% of the full span, respectively. The settling time to reach 2 bar from zero and stabilize is less than 2 s. Material for building a four-channel µPump (approx. $3000 USD) or an eight-channel µPump (approx. $5000 USD) is approximately a quarter, or a third of the cost of buying a high-end commercial system, respectively. The design rationale is presented, together with documented design details and software, so that the system may be replicated or customized to particular applications. µPump can be used for two-phase droplet microfluidics, single-phase microfluidics, gaseous flow microfluidics and any other applications requiring precise fluid handling. µPump provides researchers, students, and startups with a cost-effective solution for precise fluid control.

Application of Microfluidics in the Production and Analysis of Food Foams

Emulsifiers play a key role in the stabilization of foam bubbles. In food foams, biopolymers such as proteins are contributing to long-term stability through several effects such as increasing bulk viscosity and the formation of viscoelastic interfaces. Recent studies have identified promising new stabilizers for (food) foams and emulsions, for instance biological particles derived from water-soluble or water-insoluble proteins, (modified) starch as well as chitin. Microfluidic platforms could provide a valuable tool to study foam formation on the single-bubble level, yielding mechanistic insights into the formation and stabilization (as well as destabilization) of foams stabilized by these new stabilizers. Yet, the recent developments in microfluidic technology have mainly focused on emulsions rather than foams. Microfluidic devices have been up-scaled (to some extent) for large-scale emulsion production, and also designed as investigative tools to monitor interfaces at the (sub)millisecond time scale. In this review, we summarize the current state of the art in droplet microfluidics (and, where available, bubble microfluidics), and provide a perspective on the applications for (food) foams. Microfluidic investigations into foam formation and stability are expected to aid in optimization of stabilizer selection and production conditions for food foams, as well as provide a platform for (large-scale) production of monodisperse foams.

Rapid and Highly Controlled Generation of Monodisperse Multiple Emulsions via a One-Step Hybrid Microfluidic Device

Multiple Emulsions (MEs) contain a drop laden with many micro-droplets. A single-step microfluidic-based synthesis process of MEs is presented to provide a rapid and controlled generation of monodisperse MEs. The design relies on the interaction of three immiscible fluids with each other in subsequent droplet formation steps to generate monodisperse ME constructs. The design is within a microchannel consists of two compartments of cross-junction and T-junction. The high shear stress at the cross-junction creates a stagnation point that splits the first immiscible phase to four jet streams each of which are sprayed to micrometer droplets surrounded by the second phase. The resulted structure is then supported by the third phase at the T-junction to generate and transport MEs. The ME formation within microfluidics is numerically simulated and the effects of several key parameters on properties of MEs are investigated. The dimensionless modeling of ME formation enables to change only one parameter at the time and analyze the sensitivity of the system to each parameter. The results demonstrate the capability of highly controlled and high-throughput MEs formation in a one-step synthesis process. The consecutive MEs are monodisperse in size which open avenues for the generation of controlled MEs for different applications.

Nanoparticle-Coated Microbubbles for Combined Ultrasound Imaging and Drug Delivery

Biomedical microbubbles stabilized by a coating of magnetic or drug-containing nanoparticles show great potential for theranostics applications. Nanoparticle-coated microbubbles can be made to be stable, to be echogenic, and to release the cargo of drug-containing nanoparticles with an ultrasound trigger. This Article reviews the design principles of nanoparticle-coated microbubbles for ultrasound imaging and drug delivery, with a particular focus on the physical chemistry of nanoparticle-coated interfaces; the formation, stability, and dynamics of nanoparticle-coated bubbles; and the conditions for controlled nanoparticle release in ultrasound. The emerging understanding of the modes of nanoparticle expulsion and of the transport of expelled material by microbubble-induced flow is paving the way toward more efficient nanoparticle-mediated drug delivery. This Article highlights the knowledge gap that still remains to be addressed before we can control these phenomena.

A review of bubble break-up

The coalescence and break-up of bubbles are important steps in many industrial processes. To date, most of the literature has been focussed on the coalescence process which has been studied using high speed cinematographic techniques. However, bubble break-up is equally important and requires further research. This review essentially details the break-up process and initially summarizes the different types of bubble deformation processes which lead to break-up. Break-up is considered in high and low turbulent (pseudo-static) conditions and the effect of fluctuations and shear forces on the break-up is reviewed. Different mechanisms of break-up are discussed including shearing-off, coalescence induced pitching and impact pinching following air entrapment. Also, the influence of bubble size, interfacial stability, and surfactant on break-up are reviewed and a summary of recent experimental techniques presented. Finally, the break-up process which occurs in micro-fluidics is summarized.

Rapid Determination of Interfacial Tensions in Nanopores: Experimental Nanofluidics and Theoretical Models

A rapid and accurate determination of interfacial tensions (IFTs) in nanopores is scientifically and practically significant, while most existing experimental measurements are restricted to the micrometer scale and theoretical calculations are relatively limited. In this study, six series of the IFT measurement tests for the binary CO₂-C₁₀, C₁-C₁₀, and N₂-C₁₀ mixtures are conducted at temperatures ( T) of 25.0 and 53.0 °C in a self-manufactured nanofluidic system. Moreover, a nanoscale-extended equation-of-state model considering the effects of the confinement, intermolecular interactions, and disjoining pressure and a semianalytical correlation are proposed to calculate the IFTs of the three mixtures in bulk phase and nanopores. Third, a new Tolman length formulation is developed for the IFT corrections in nanopores. Overall, the calculated IFTs from the two theoretical methods agree well with the measured results for most cases in nanopores. On the other hand, effects of the pore scale, temperature, pressure, and fluid composition on the IFTs of the three mixtures are effectively validated and specifically investigated. One suggestion comes from this work that the two theoretical methods for calculating the IFTs are better to be applied concurrently to minimize errors. Another important future work is to include more pore surface parameter (e.g., wettability) into the theoretical model.

Accounting for corner flow unifies the understanding of droplet formation in microfluidic channels

While shear emulsification is a well understood industrial process, geometrical confinement in microfluidic systems introduces fascinating complexity, so far prohibiting complete understanding of droplet formation. The size of confined droplets is controlled by the ratio between shear and capillary forces when both are of the same order, in a regime known as jetting, while being surprisingly insensitive to this ratio when shear is orders of magnitude smaller than capillary forces, in a regime known as squeezing. Here, we reveal that further reduction of—already negligibly small—shear unexpectedly re-introduces the dependence of droplet size on shear/capillary-force ratio. For the first time we formally account for the flow around forming droplets, to predict and discover experimentally an additional regime—leaking. Our model predicts droplet size and characterizes the transitions from leaking into squeezing and from squeezing into jetting, unifying the description for confined droplet generation, and offering a practical guide for applications.

T-junctions are a tool for droplet generation; they are well-described by models that distinguish for squeezing and jetting regimes for different capillary numbers. By considering the usually neglected corner flow, the authors identify an additional leaking regime for very low capillary numbers.

Advances in the Use of Microfluidics to Study Crystallization Fundamentals

This review compares droplet-based microfluidic systems used to study crystallization fundamentals in chemistry and biology. An original high-throughput droplet-based microfluidic platform is presented. It uses nanoliter droplets, generates a chemical library, and directly solubilizes powder, thus economizing both material and time. It is compatible with all solvents without the need for surfactant. Its flexibility permits phase diagram determination and crystallization studies (screening and optimizing experiments) and makes it easy to use for nonspecialists in microfluidics. Moreover, it allows concentration measurement via ultraviolet spectroscopy and solid characterization via X-ray diffraction analysis.

An opto-acousto-fluidic microscopic system with a high spatiotemporal resolution for microfluidic applications

In this work, we develop a new opto-acouto-fludic microsopic system, which employs a high-speed one-dimensional galvanometer scanner and an ultrafast pulse laser (600 kHz). The new system has achieved a high two-dimensional frame rate of up to 2500 Hz with a lateral resolution of 1.7 μm and an axial resolution of 36 μm at the imaging plane. To demonstrate the improved performance of the new system compared to our previous one, we carried out experiments to image the flowing droplets generated with T-junction and flow focusing configurations. We also successfully imaged dynamic migration of magneto particles subjected to non-uniform magnetic field in the microchannel. The results suggest that our new system has sufficient spatiotemporal resolutions to carry out studies for high throughput microfluidic applications.

Examining the Effect of Flow Rate Ratio on Droplet Generation and Regime Transition in a Microfluidic T-Junction at Constant Capillary Numbers

The focus of this work is to examine the effect of flow rate ratio (quotient of the dispersed phase flow rate over the continuous phase flow rate) on a regime transition from squeezing to dripping at constant capillary numbers. The effect of the flow rate ratio on the volume of droplets generated in a microfluidic T-junction is discussed, and a new scaling law to estimate their volume is proposed. Existing work on a regime transition reported by several researchers focuses on the effect of the capillary number on regime transition, and the results that are presented in this paper advance the current understanding by indicating that the flow rate ratio is another parameter that dictates regime transition. In this paper, the transition between squeezing and dripping regimes is reported at constant capillary numbers, with a transition region identified between squeezing and dripping regimes. Dripping is observed at lower flow rate ratios and squeezing at higher flow rate ratios, with a transition region between the two regimes at flow rate ratios between 1 and 2. This is presented in a flow regime map that is constructed based on the observed mechanism. A scaling model is proposed to characterise droplet volume in terms of flow rate ratio and capillary number. The effect of flow rate ratio on the non-dimensional droplet volume is presented, and lastly, the droplet volume is expressed in terms of a range of parameters, such as the viscosity ratio between the dispersed and the continuous phase, capillary number, and the geometrical characteristics of the channels.

Microfluidic Encapsulation of Demembranated Sperm Nuclei in Xenopus Egg Extracts

The cell-free nature of Xenopus egg extract makes it a uniquely tractable experimental model system. The extract, effectively unconfined cytoplasm, allows the direct and relatively straight-forward addition of purified proteins and other reagents, a characteristic that renders the system amenable to many biochemical and cell biological manipulations. Accessibility to the system also facilitates the direct physical manipulation and probing of biological structures, in turn enabling mechanical properties of intracellular assemblies and organelles, such as the mitotic spindle and nucleus, to be measured. Recently, multiphase microfluidics have been combined with Xenopus egg extracts to encapsulate discrete cytoplasmic volumes. Described here is a protocol detailing the use of multiphase microfluidic devices to encapsulate sperm nuclei within extract droplets of defined size and shape. This protocol can also be applied more generally to encapsulation of microbeads and other particles.