Generating microbubbles in a co-flowing microfluidic device

High-Speed Generation of Microbubbles with Constant Cumulative Production in a Glass Capillary Microfluidic Bubble Generator

This work reports a simple bubble generator for the high-speed generation of microbubbles with constant cumulative production. To achieve this, a gas-liquid co-flowing microfluidic device with a tiny capillary orifice as small as 5 μm is fabricated to produce monodisperse microbubbles. The diameter of the microbubbles can be adjusted precisely by tuning the input gas pressure and flow rate of the continuous liquid phase. The co-flowing structure ensures the uniformity of the generated microbubbles, and the surfactant in the liquid phase prevents coalescence of the collected microbubbles. The diameter coefficient of variation (CV) of the generated microbubbles can reach a minimum of 1.3%. Additionally, the relationship between microbubble diameter and the gas channel orifice is studied using the low Capillary number (Ca) and Weber number (We) of the liquid phase. Moreover, by maintaining a consistent gas input pressure, the CV of the cumulative microbubble volume can reach 3.6% regardless of the flow rate of the liquid phase. This method not only facilitates the generation of microbubbles with morphologic stability under variable flow conditions, but also ensures that the cumulative microbubble production over a certain period of time remains constant, which is important for the volume-dominated application of chromatographic analysis and the component analysis of natural gas.

Microflow chemistry and its electrification for sustainable chemical manufacturing

Sustainability is vital in solving global societal problems. Still, it requires a holistic view by considering renewable energy and carbon sources, recycling waste streams, environmentally friendly resource extraction and handling, and green manufacturing. Flow chemistry at the microscale can enable continuous sustainable manufacturing by opening up new operating windows, precise residence time control, enhanced mixing and transport, improved yield and productivity, and inherent safety. Furthermore, integrating microfluidic systems with alternative energy sources, such as microwaves and plasmas, offers tremendous promise for electrifying and intensifying modular and distributed chemical processing. This review provides an overview of microflow chemistry, electrification, their integration toward sustainable manufacturing, and their application to biomass upgrade (a select number of other processes are also touched upon). Finally, we identify critical areas for future research, such as matching technology to the scale of the application, techno-economic analysis, and life cycle assessment.

Dimensionless Analysis of the Effects of Junction Angle on the Gas-Liquid Two-Phase Flow Transition and the Scaling Law of the Microbubble Generation Characteristics in Y-Junctions

Gas-liquid two-phase flow patterns and gas slug hydrodynamics were experimentally studied in three Y-junctions with different junction angles of 60°, 90° and 120°. Microbubbles were generated in the sodium alginate aqueous solution with the surfactant Tween20. Four main flow patterns were observed, i.e., stratified flow, annular flow, dispersed bubble flow and slug bubble flow. The formation mechanism of the bubble flow was explained by a force analysis, which was based on the dimensionless analysis regarding Capillary number, Weber number and Euler number. The transition criteria of the gas-liquid two-phase flow patterns was set up by these three dimensionless numbers. Additionally, the characteristics of the slug bubble were investigated, which made a scaling criterion for eliminating the influence of the angle factor become possible. A new scaling law (validity range within 2.88 < Re1 < 14.38, 0.0068 < We1 < 0.1723) was proposed to predict the bubble size and it showed a good agreement with the experimental results.

Nonlinear Phenomena in Microfluidics

This review focuses on experimental work on nonlinear phenomena in microfluidics, which for the most part are phenomena for which the velocity of a fluid flowing through a microfluidic channel does not scale proportionately with the pressure drop. Examples include oscillations, flow-switching behaviors, and bifurcations. These phenomena are qualitatively distinct from laminar, diffusion-limited flows that are often associated with microfluidics. We explore the nonlinear behaviors of bubbles or droplets when they travel alone or in trains through a microfluidic network or when they assemble into either one- or two-dimensional crystals. We consider the nonlinearities that can be induced by the geometry of channels, such as their curvature or the bas-relief patterning of their base. By casting posts, barriers, or membranes─situated inside channels─from stimuli-responsive or flexible materials, the shape, size, or configuration of these elements can be altered by flowing fluids, which may enable autonomous flow control. We also highlight some of the nonlinearities that arise from operating devices at intermediate Reynolds numbers or from using non-Newtonian fluids or liquid metals. We include a brief discussion of relevant practical applications, including flow gating, mixing, and particle separations.

Experimental studies on droplet characteristics in a microfluidic flow focusing droplet generator: effect of continuous phase on droplet encapsulation

The efficacy of droplet-based microfluidic assays depends on droplet size, pattern, generation rate, etc. The size of the droplet is affected by numerous variables as flow rate ratio, viscosity ratio, microchannel geometry, surfactants, nature of fluids and other dimensionless numbers. This work reports rigorous analysis and optimization of the behavior of droplets with change in flow rate ratio and viscosity ratio in a flow-focusing device. Droplets were produced for different flow rate ratios maintaining a constant aqueous phase and varying the continuous phase, to have capillary numbers ranging from 0.01 to 0.1. It was observed that the droplet size decreased with the increase in flow rate ratio, and vice versa. It was noted that as the viscosity ratio was increased, the dispersed phase elongated before the complete breakup and long droplets were formed in the microchannel. Smaller droplets were formed for lower viscosity ratios with a combination of higher flow rate ratios. An empirical relation has been developed to predict the droplet length in terms of capillary number and flow rate ratio for different viscosity ratios. In addition, microparticle encapsulation in individual droplets was attempted to realize the effect of flow rate of the continuous phase for various flow rate ratios on encapsulation efficiency.

Microbubbles versus Extracellular Vesicles as Therapeutic Cargo for Targeting Drug Delivery

Extracellular vesicles (EVs) and microbubbles are nanoparticles in drug-delivery systems that are both considered important for clinical translation. Current research has found that both microbubbles and EVs have the potential to be utilized as drug-delivery agents for therapeutic targets in various diseases. In combination with EVs, microbubbles are capable of delivering chemotherapeutic drugs to tumor sites and neighboring sites of damaged tissues. However, there are no standards to evaluate or to compare the benefits of EVs (natural carrier) versus microbubbles (synthetic carrier) as drug carriers. Both drug carriers are being investigated for release patterns and for pharmacokinetics; however, few researchers have focused on their targeted delivery or efficacy. In this Perspective, we compare EVs and microbubbles for a better understanding of their utility in terms of delivering drugs to their site of action and future clinical translation.

Experimental Studies of Microchannel Tapering on Droplet Forming Acceleration in Liquid Paraffin/Ethanol Coaxial Flows

The formations of micro-droplets are strongly influenced by the local geometries where they are generated. In this paper, through experimental research, we focus on the roles of microchannel tapering in the liquid paraffin/ethanol coaxial flows in their flow patterns, flow regimes, and droplet parameters, i.e., their sizes and forming frequencies. For validity, the non-tapering coaxial flows (the convergence angle α=0∘) are investigated, the experimental methods and experimental data are examined and analyzed by contrasting the details with previous works, and consistent results are obtained. We consider a slightly tapering microchannel (the convergence angle α=2.8∘) and by comparison, the experiments show that the tapering has significant effects on the flow patterns, droplet generation frequencies, and droplet sizes. The regimes of squeezing, dripping, jetting, tubing, and threading are differentiated to shrink toward the coordinate origin of the Cac–Wed space. The closer it is to the origin, the less variations will occur. For the adjacent regimes of the origin, i.e., dripping and squeezing, slight changes have occurred in both flow patterns, as well as the droplet characters. In the dripping and squeezing modes, the liquid droplets are generated near the orifice of the inner tube. Their forming positions (geometry) and flow conditions are almost the same. Therefore, the causes of minute changes in such regimes are physically understandable. While in the jetting regimes, the droplets shrink in size and their forming frequencies increase. The droplet sizes and the frequencies are both linearly related to those of the non-tapering cases with the corresponding relations derived. Furthermore, the threading and the tubing patterns almost did not emerged in the non-tapering data, as it seemed easier to form elongated jets, thinning or widening, in the tapered tubes. This can be explained by the stable analysis of the coaxial jets, which indicates that the reductions in the microchannel diameters can suppress the development of the interface disturbances.

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.

Fabrication of Millimeters-Sized Poly(Divinylbenzene) Foam Shells from Controllable Double Emulsion in Microfluidic Device

A geometrically confined dripping was employed to enable precise control over the dimension and structure of millimeters-sized double-emulsion precursors of poly(divinylbenzene) foam shells in a new kind of double Y-shaped compound channels. Due to the 3D axial-symmetric microfluidic device, a more stable and robust flow field was maintained to obtain a continuous and regular emulsification. Various factors were systematically investigated for the precise size control of dripping in confined channel geometry, such as outlet channel size, fluid properties and flow rates. It was seen that phase properties and synergistic effects of main factors played key roles in determining droplet size. Thus, we used the optimized microfluidic approach to fabricate predetermined size foams to satisfy inertial fusion energy experiments, ranging from 4 to 4.6[Formula: see text]mm in diameter with a 50–300[Formula: see text][Formula: see text]m wall thickness and a coefficient of variation [Formula: see text]%. The results presented in this work provided a practical guideline for creating size-desired polymersome from comparable double emulsions.

Design and Scaling Up of Microchemical Systems: A Review

The past two decades have witnessed a rapid development of microreactors. A substantial number of reactions have been tested in microchemical systems, revealing the advantages of controlled residence time, enhanced transport efficiency, high product yield, and inherent safety. This review defines the microchemical system and describes its components and applications as well as the basic structures of micromixers. We focus on mixing, flow dynamics, and mass and heat transfer in microreactors along with three strategies for scaling up microreactors: parallel numbering-up, consecutive numbering-up, and scale-out. We also propose a possible methodology to design microchemical systems. Finally, we provide a summary and future prospects.

Investigation of dynamic surface tension in gas–liquid absorption using a microflow interfacial tensiometer

Dynamic surface tension in gas–liquid absorption is studied using a microflow device.

Ultra-thin liquid film extraction based on a gas–liquid–liquid double emulsion in a microchannel device

A gas–liquid–liquid double emulsion with ultra-thin liquid film is proposed for the mass transfer enhancement of an extreme phase ratio system.

Hydrostatic pressure effect on micro air bubbles deposited on surfaces with a retreating tip

The effect of hydrostatic pressure on 6 μL air bubbles formed on micropillar structured PDMS and silicone surfaces using a 2 mm diameter stainless steel tip retreated at 1 mm/s was investigated. Dimensional analysis of the tip retraction process showed the experiments to be conducted in the condition where fluid inertial forces are comparable in magnitude with surface tension forces, while viscous forces were lower. Larger bubbles could be left behind on the structured PDMS surface. For hydrostatic pressures in excess of 20 mm H2O (196 Pa), the volume of bubble deposited was found to decrease progressively with pressure increase. The differences in width of the deposited bubbles (in contact with the substrate) were significant at any particular pressure but marginal in height. The attainable height before rupture reduced with pressure increase, thereby accounting for the reducing dispensed volume characteristic. On structured PDMS, the gaseous bridge width (in contact with the substrate) was invariant with tip retraction, while on silicone it was initially reducing before becoming invariant in the lead up to rupture. With silicone, hence, reductions in the contact width and height were both responsible for reduced volumes with pressure increase. Increased hydrostatic pressure was also found to restrict the growth in contact width on silicone during the stage when air was injected in through the tip. The ability to effect bubble size in such a simple manner may already be harnessed in nature and suggests possibilities in technological applications.