Role of the channel geometry on the bubble pinch-off in flow-focusing devices

Comprehensive Review on Bubbles: Synthesis, Modification, Characterization and Biomedical Applications

Accurate detection, treatment, and imaging of diseases are important for effective treatment outcomes in patients. In this regard, bubbles have gained much attention, due to their versatility. Bubbles usually 1 nm to 10 μm in size can be produced and loaded with a variety of lipids, polymers, proteins, and therapeutic and imaging agents. This review details the different production and loading methods for bubbles, for imaging and treatment of diseases/conditions such as cancer, tumor angiogenesis, thrombosis, and inflammation. Bubbles can also be used for perfusion measurements, important for diagnostic and therapeutic decision making in cardiac disease. The different factors important in the stability of bubbles and the different techniques for characterizing their physical and chemical properties are explained, for developing bubbles with advanced therapeutic and imaging features. Hence, the review provides important insights for researchers studying bubbles for biomedical applications.

Ultrasound reforms droplets

Size-controlled monodisperse droplets are indispensable in food, cosmetics, and healthcare industries. Although emulsion formation from bulk phases is well-explored, a robust in situ method to continuously reform existing emulsions is unavailable. Remarkably, we introduce a continuous flow acousto-microfluidics technique which enables simultaneous trapping-coalescence-splitting of droplets to reform an existing polydisperse emulsion into size-controlled droplets with improved monodispersity. In contrast to conventional approaches, our platform enables controlling droplet characteristics in situ by regulating acoustic power without altering hydrodynamical parameters thereby improving response time and facilitates continuous nozzle-less clogging-free droplet generation from a liquid plug in a chamber instead of from a liquid stream at a narrow junction. The technique can process polydisperse droplets produced not only due to fluid-source fluctuations or unstable jetting regime but also externally by non-microfluidic or inexpensive setups. Our theoretical scaling suggests that the sum of capillary (Ca) and acousto-capillary (Caa) numbers ∼ (1), and predicts the generated droplet size, both agreeing well with the experimental findings. We identify acousto-visco-capillary number, Caav = (Ca Caa)1/2, which governs the generated droplet size. We also explore and characterize acoustic streaming- and coalescence-based mixing of samples inside the trapped plug. Distinctively, our platform is amenable to continuous mixing of inhomogeneous droplets, offering monodisperse mixed-sample droplets, and holds the potential to match current throughput standards through suitable design modifications.

Universal Correlation for Droplet Fragmentation in a Microfluidic T-Junction

Despite extensive research on droplet dynamics at microfluidic T-junction, for different droplet lengths and Capillary numbers, there remains limited understanding of their dynamics at different viscosity ratio. In this study, we adopt a modeling framework in a three-dimensional (3D) configuration to numerically investigate the droplet dynamics as it passes through a symmetric T-junction with varying Capillary numbers, droplet lengths, and viscosity ratios. We present a 3D regime map for the first time to demarcate the droplet breakup and no breakup regimes. Herein, we propose a simple surface equation accounting for the critical Capillary number for breakup, in terms of viscosity ratio and dimensionless droplet length. The proposed universal relationship aligns well with experimental and computational findings from the existing literature. Furthermore, we reveal a new droplet breakup characteristic at high viscosity ratio and high Capillary number where the droplet spreads almost twice its initial value before splitting. Overall, this research provides comprehensive understanding of droplet dynamics at the T-junction and has significant implications for several related applications, including the large-scale synthesis of microdroplets using microchannel networks.

Development and future of droplet microfluidics

Over the past two decades, advances in droplet-based microfluidics have facilitated new approaches to process and analyze samples with unprecedented levels of precision and throughput. A wide variety of applications has been inspired across multiple disciplines ranging from materials science to biology. Understanding the dynamics of droplets enables optimization of microfluidic operations and design of new techniques tailored to emerging demands. In this review, we discuss the underlying physics behind high-throughput generation and manipulation of droplets. We also summarize the applications in droplet-derived materials and droplet-based lab-on-a-chip biotechnology. In addition, we offer perspectives on future directions to realize wider use of droplet microfluidics in industrial production and biomedical analyses.

Foam Generation Through a Single Pore with Rectangular Cross-Section: Hysteretic Behavior and Geometric Limitation of the Volume Fraction of Bubbles

We study foam production and destabilization through a flow-focusing geometry, namely a single pore of rectangular cross-section, by coinjecting gas and liquid at constant pressure, Pg, and constant flow rate, Qw. We observe that bubble production results from a Rayleigh-Plateau destabilization of the internal gas thread that occurs at the pore neck when its width becomes comparable to the height of the rectangular-section channel. Using a simple model and numerical approach, we (i) predict the shape of the gas jet and its stability range as a function of flow parameters and device geometry, which we successfully compare with our experimental results, and (ii) demonstrate the existence of a critical local pressure drop at the pore neck that determines whether or not a stable gas flow can form. We thus show that bubble foam generation exhibits hysteretic behavior due to hydrodynamic feedback and demonstrate that there is a maximum bubble volume fraction that the generated foam cannot exceed, the value of which is fixed by the geometry. Our results suggest that the foam collapse observed in porous media when the fractional gas flow becomes too large may result from hydrodynamic feedback inhibiting foam generation and not necessarily from coalescence between bubbles, as is usually claimed.

Design of Improved Flow-Focusing Microchannel with Constricted Continuous Phase Inlet and Study of Fluid Flow Characteristics

This paper proposed an improved flow-focusing microchannel with a constricted continuous phase inlet to increase microbubble generation frequency and reduce microbubbles' diameter. The design variables were obtained by Latin hypercube sampling, and the radial basis function (RBF) surrogate model was used to establish the relationship between the objective function (microbubble diameter and generation frequency) and the design variables. Moreover, the optimized design of the nondominated sorting genetic algorithm II (NSGA-II) algorithm was carried out. Finally, the optimization results were verified by numerical simulations and compared with those of traditional microchannels. The results showed that dripping and squeezing regimes existed in the two microchannels. The constricted continuous phase inlet enhanced the flow-focusing effect of the improved microchannel. The diameter of microbubbles obtained from the improved microchannel was reduced from 2.8141 to 1.6949 μm, and the generation frequency was increased from 64.077 to 175.438 kHz at the same capillary numbers (Ca) compared with the traditional microchannel. According to the fitted linear function, it is known that the slope of decreasing microbubble diameter with increasing Ca number and the slope of increasing generation frequency with increasing Ca number are greater in the improved microchannel compared with those in the traditional microchannel.

Research on Micro-Quantitative Detection Technology of Simulated Waterbody COD Based on the Ozone Chemiluminescence Method

Chemical oxygen demand (COD), reflecting the degree of waterbody contaminated by reduction substances, is an important parameter for water quality monitoring. The existing measurement method of waterbody COD takes time and is a complex system, which cannot meet the real-time monitoring requirements of river pollution indicators. We developed the vortex t-structure microfluidic detection chip with the help of microfluidic technology and designed the COD detection system with a high integration degree based on the principle of ozone chemiluminescence, and we have also carried out research on a waterbody COD quantitative detection test. The test results show that the detection chip can generate quantitative and controllable ozone-based bubbles; it also shows the advantages of a simple system and short test time without environmental pollution, which provides some technical support for the online real-time monitoring of river water quality.

Number concentration dependence of ultrasonic disruption ratio of diameter-sorted microcapsules

The use of ultrasound to destroy microcapsules in microbubble-assisted drug delivery systems (DDS) is of great interest. In the present study, the disruption ratios of capsule clusters were measured by observing and experimentally analyzing microcapsules with polymer shells undergoing disruption by ultrasound. The microcapsules were dispersed in a planar microchamber filled with a gelatin gel and sonicated using 1 MHz focused ultrasound. Different capsule populations were obtained using a filtration technique to modify and control the capsule sizes. The disruption ratio as a function of the concentration of capsules was obtained through image processing of the recorded photomicrographs. We found that the disruption ratio for each population exponentially decreases as the particle number concentration (PNC) increases. The maximum disruption ratio of the diameter-sorted capsules was larger than that of polydispersed capsules. Particularly, for resonant capsule populations, the ratio was more than twice that of polydispersed capsules. Furthermore, the maximum disruption ratio occurred at higher concentrations as the mean particle diameter of the capsule cluster decreased.

Biomedical nanobubbles and opportunities for microfluidics

The use of bulk nanobubbles in biomedicine is increasing in recent years, which is attributable to the array of therapeutic and diagnostic tools promised by developing bulk nanobubble technologies. From cancer drug delivery and ultrasound contrast enhancement to malaria detection and the diagnosis of acute donor tissue rejection, the potential applications of bulk nanobubbles are broad and diverse. Developing these technologies to the point of clinical use may significantly impact the quality of patient care. This review compiles and summarizes a representative collection of the current applications, fabrication techniques, and characterization methods of bulk nanobubbles in biomedicine. Current state-of-the-art generation methods are not designed to create nanobubbles of high concentration and low polydispersity, both characteristics of which are important for several bulk nanobubble applications. To date, microfluidics has not been widely considered as a tool for generating nanobubbles, even though the small-scale precision and real-time control offered by microfluidics may overcome the challenges mentioned above. We suggest possible uses of microfluidics for improving the quality of bulk nanobubble populations and propose ways of leveraging existing microfluidic technologies, such as organ-on-a-chip platforms, to expand the experimental toolbox of researchers working to develop biomedical nanobubbles.

The use of bulk nanobubbles in biomedicine is increasing in recent years. This translates into new opportunities for microfluidics, which may enable the generation of higher quality nanobubbles that lead to advances in diagnostics and therapeutics.

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.

Scaleable production of microbubbles using an ultrasound-modulated microfluidic device

Surfactant-coated gas microbubbles are widely used as contrast agents in ultrasound imaging and increasingly in therapeutic applications. The response of microbubbles to ultrasound can be strongly influenced by their size and coating properties, and hence the production method. Ultrasonic emulsification (sonication) is the most commonly employed method and can generate high concentrations of microbubbles rapidly, but with a broad size distribution, and there is a risk of contamination and/or degradation of sensitive components. Microfluidic devices provide excellent control over microbubble size, but are often challenging or costly to manufacture, offer low production rates (<10⁶s^-1), and are prone to clogging. In this study, a hybrid sonication-microfluidic or "sonofluidic" device was developed. Bubbles of ∼180 μm diameter were produced rapidly in a T-junction and subsequently exposed to ultrasound (71-73 kHz) within a microchannel, generating microbubbles (mean diameter: 1-2 μm) at a rate of >10⁸s^-1 using a single device. Microbubbles were prepared using either the sonofluidic device or conventional sonication, and their size, concentration, and stability were comparable. The mean diameter, concentration, and stability were found to be comparable between techniques, but the microbubbles produced by the sonofluidic device were all <5 μm in diameter and thus did not require any post-production fractionation.

Acoustic frequency combs using gas bubble cluster oscillations in liquids: a proof of concept

We propose a new approach to the generation of acoustic frequency combs (AFC)-signals with spectra containing equidistant coherent peaks. AFCs are essential for a number of sensing and measurement applications, where the established technology of optical frequency combs suffers from fundamental physical limitations. Our proof-of-principle experiments demonstrate that nonlinear oscillations of a gas bubble cluster in water insonated by a low-pressure single-frequency ultrasound wave produce signals with spectra consisting of equally spaced peaks originating from the interaction of the driving ultrasound wave with the response of the bubble cluster at its natural frequency. The so-generated AFC posses essential characteristics of optical frequency combs and thus, similar to their optical counterparts, can be used to measure various physical, chemical and biological quantities.

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.

Lattice Boltzmann simulations capture the multiscale physics of soft flowing crystals

The study of the underlying physics of soft flowing materials depends heavily on numerical simulations, due to the complex structure of the governing equations reflecting the competition of concurrent mechanisms acting at widely disparate scales in space and time. A full-scale computational modelling remains a formidable challenge since it amounts to simultaneously handling six or more spatial decades in space and twice as many in time. Coarse-grained methods often provide a viable strategy to significantly mitigate this issue, through the implementation of mesoscale supramolecular forces designed to capture the essential physics at a fraction of the computational cost of a full-detail description. Here, we review some recent advances in the design of a lattice Boltzmann mesoscale approach for soft flowing materials, inclusive of near-contact interactions (NCIs) between dynamic interfaces, as they occur in high packing-fraction soft flowing crystals. The method proves capable of capturing several aspects of the rheology of soft flowing crystals, namely, (i) a 3/2 power-law dependence of the dispersed phase flow rate on the applied pressure gradient, (ii) the structural transition between an ex-two and ex-one (bamboo) configurations with the associated drop of the flow rate, (iii) the onset of interfacial waves once NCI is sufficiently intense. This article is part of the theme issue 'Fluid dynamics, soft matter and complex systems: recent results and new methods'.

Ultrasound-responsive lipid microbubbles for drug delivery: A review of preparation techniques to optimise formulation size, stability and drug loading

Lipid-shelled microbubbles have received extensive interest to enhance ultrasound-responsive drug delivery outcomes due to their high biocompatibility. While therapeutic effectiveness of microbubbles is well established, there remain limitations in sample homogeneity, stability profile and drug loading properties which restrict these formulations from seeing widespread use in the clinical setting. In this review, we evaluate and discuss the most encouraging leads in lipid microbubble design and optimisation. We examine current applications in drug delivery for the systems and subsequently detail shell compositions and preparation strategies that improve monodispersity while retaining ultrasound responsiveness. We review how excipients and storage techniques help maximise stability and introduce different characterisation and drug loading techniques and evaluate their impact on formulation performance. The review concludes with current quality control measures in place to ensure lipid microbubbles can be reproducibly used in drug delivery.

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Breakup Dynamics of Semi-dilute Polymer Solutions in a Microfluidic Flow-focusing Device

Droplet microfluidics involving non-Newtonian fluids is of great importance in both fundamental mechanisms and practical applications. In the present study, breakup dynamics in droplet generation of semi-dilute polymer solutions in a microfluidic flow-focusing device were experimentally investigated. We found that the filament thinning experiences a transition from a flow-driven to a capillary-driven regime, analogous to that of purely elastic fluids, while the highly elevated viscosity and complex network structures in the semi-dilute polymer solutions induce the breakup stages with a smaller power-law exponent and extensional relaxation time. It is elucidated that the elevated viscosity of the semi-dilute solution decelerates filament thinning in the flow-driven regime and the incomplete stretch of polymer molecules results in the smaller extensional relaxation time in the capillary-driven regime. These results extend the understanding of breakup dynamics in droplet generation of non-Newtonian fluids and provide guidance for microfluidic synthesis applications involving dense polymeric fluids.

Three Decades of Ultrasound Contrast Agents: A Review of the Past, Present and Future Improvements

Initial reports from the 1960s describing the observations of ultrasound contrast enhancement by tiny gaseous bubbles during echocardiographic examinations prompted the development of the first ultrasound contrast agent in the 1980s. Current commercial contrast agents for echography, such as Definity, Optison, Sonazoid and SonoVue, have proven to be successful in a variety of on- and off-label clinical indications. Whereas contrast-specific technology has seen dramatic progress after the introduction of the first approved agents in the 1990s, successful clinical translation of new developments has been limited during the same period, while understanding of microbubble physical, chemical and biologic behavior has improved substantially. It is expected that for a successful development of future opportunities, such as ultrasound molecular imaging and therapeutic applications using microbubbles, new creative developments in microbubble engineering and production dedicated to further optimizing microbubble performance are required, and that they cannot rely on bubble technology developed more than 3 decades ago.

Diffusion-Dominated Pinch-Off of Ultralow Surface Tension Fluids

We study the breakup of a liquid thread inside another liquid at different surface tensions. In general, the pinch-off of a liquid thread is governed by the dynamics of fluid flow. However, when the interfacial tension is ultralow (2-3 orders lower than normal liquids), we find that the pinch-off dynamics can be governed by bulk diffusion. By studying the velocity and the profile of the pinch-off, we explain why the diffusion-dominated pinch-off takes over the conventional breakup at ultralow surface tensions.

Self-Similar Relaxation of Confined Microfluidic Droplets

We report an experimental study concerning the capillary relaxation of a confined liquid droplet in a microscopic channel with a rectangular cross section. The confinement leads to a droplet that is extended along the direction normal to the cross section. These droplets, found in numerous microfluidic applications, are pinched into a peanutlike shape thanks to a localized, reversible deformation of the channel. Once the channel deformation is released, the droplet relaxes back to a pluglike shape. During this relaxation, the liquid contained in the central pocket drains towards the extremities of the droplet. Modeling such viscocapillary droplet relaxation requires considering the problem as 3D due to confinement. This 3D consideration yields a scaling model incorporating dominant dissipation within the droplet menisci. As such, the self-similar droplet dynamics is fully captured.

Restoring universality to the pinch-off of a bubble

The pinch-off of a bubble is an example of the formation of a singularity, exhibiting a characteristic separation of length and time scales. Because of this scale separation, one expects universal dynamics that collapse into self-similar behavior determined by the relative importance of viscous, inertial, and capillary forces. Surprisingly, however, the pinch-off of a bubble in a large tank of viscous liquid is known to be nonuniversal. Here, we show that the pinch-off dynamics of a bubble confined in a capillary tube undergo a sequence of two distinct self-similar regimes, even though the entire evolution is controlled by a balance between viscous and capillary forces. We demonstrate that the early-time self-similar regime restores universality to bubble pinch-off by erasing the system's memory of the initial conditions. Our findings have important implications for bubble/drop generation in microfluidic devices, with applications in inkjet printing, medical imaging, and synthesis of particulate materials.

Bubble-bubble pinch-off in symmetric and asymmetric microfluidic expansion channels for ordered foam generation

By incorporating the techniques of geometrically mediated splitting and bubble-bubble breakup, the present work offers a novel microfluidic foam generation system via production of segregated, mono- or bidisperse bubbles at capacities exceeding 10 000 bubbles per second. Bubble-bubble pinch-off is precise at high capillary numbers (Ca > 0.065), generating monodisperse or bidisperse daughter bubbles for a symmetric or an asymmetric expansion respectively. Bi- or tridisperse foam is produced as pinch-off perfectly alternates such that the system contains twice the number of fragmented bubbles as intact bubbles. A relationship between the upstream bubble extension and the capillary number demarcates the different regimes of pinch-off defined with respect to frequency and precision: non-splitting, irregular, polydisperse, and monodisperse (or bidisperse for an asymmetric expansion). For tridisperse foam generation via a fixed asymmetric expansion geometry, the wall bubble confinement can be tuned to adjust the pinch-off accuracy in order to access a spectrum of fragmented bubble size ratios. The simplicity in operating and characterizing our system will enable studies on dynamic bubble interactions and ordered, wet foam applications.

Reliable Manipulation of Gas Bubble Size on Superaerophilic Cones in Aqueous Media

Gas bubbles in aqueous media are ubiquitous in a broad range of applications. In most cases, the size of the bubbles must be manipulated precisely. However, it is very difficult to control the size of gas bubbles. The size of gas bubbles is affected by many factors both during and after the generation process. Thus, precise manipulation of gas bubble size still remains a great challenge. The ratchet and conical hairs of the Chinese brush enable it to realize a significant capacity for holding ink and transferring them onto paper continuously and controllably. Inspired by this, a superhydrophobic/superaerophilic cone interface is developed to manipulate gas bubble size in aqueous media. When the resultant force between the Laplace force and the axial component of the buoyancy force approaches zero, the gas bubble is held steadily by the superhydrophobic/superaerophilic copper cones in a unique position (balance position). A new kind of pressure sensor is also designed based on this principle.

Universal Equations for the Coalescence Probability and Long-Term Size Stability of Phospholipid-Coated Monodisperse Microbubbles Formed by Flow Focusing

Resonantly driven monodisperse phospholipid-coated microbubbles are expected to substantially increase the sensitivity and efficiency in contrast-enhanced ultrasound imaging and therapy. They can be produced in a microfluidic flow-focusing device, but questions remain as to the role of the device geometry, the liquid and gas flow, and the phospholipid formulation on bubble stability. Here, we develop a model based on simple continuum mechanics equations that reveals the scaling of the coalescence probability with the key physical parameters. It is used to characterize short-term coalescence behavior and long-term size stability as a function of flow-focusing geometry, bulk viscosity, lipid cosolvent mass fraction, lipid concentration, lipopolymer molecular weight, and lipopolymer molar fraction. All collected data collapse on two master curves given by universal equations for the coalescence probability and the long-term size stability. This work is therefore a route to a more fundamental understanding of the physicochemical monolayer properties of microfluidically formed bubbles and their coalescence behavior in a flow-focusing device.

Light generated bubble for microparticle propulsion

Light activated motion of micron-sized particles with effective forces in the range of micro-Newtons is hereby proposed and demonstrated. Our investigation shows that this exceptional amount of force results from accumulation of light-generated heat by a micron-sized particle that translates into motion due to a phase transition in the nearby water. High-speed imagery indicates the role of bubble expansion and later collapse in this event. Comparing observations with known models reveals a dynamic behavior controlled by polytropic trapped vapor and the inertia of the surrounding liquid. The potential of the proposed approach is demonstrated by realization of disordered optical media with binary light-activated switching from opacity to high transparency.

Parallelization of microfluidic flow-focusing devices

Microfluidic flow-focusing devices offer excellent control over fluid flow, enabling formation of drops with a narrow size distribution. However, the throughput of microfluidic flow-focusing devices is limited and scale-up through operation of multiple drop makers in parallel often compromises the robustness of their operation. We demonstrate that parallelization is facilitated if the outer phase is injected from the direction opposite to that of the inner phase, because the fluid injection flow rate, where the drop formation transitions from the squeezing into the dripping regime, is shifted towards higher values.

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.

Pinch-off of microfluidic droplets with oscillatory velocity of inner phase flow

When one liquid is introduced into another immiscible one, it ultimately fragments due to hydrodynamic instability. In contrast to neck pinch-off without external actuation, the viscous two-fluid system subjected to an oscillatory flow demonstrates higher efficiency in breaking fluid threads. However, the underlying dynamics of this process is less well understood. Here we show that the neck-thinning rate is accelerated by the amplitude of oscillation. By simply evaluating the momentum transfer from external actuation, we derive a dimensionless pre-factor to quantify the accelerated pinch-off. Our data ascribes the acceleration to the non-negligible inner fluid inertia, which neutralizes the inner phase viscous stress that retards the pinch-off. Moreover, we characterize an equivalent neck-thinning behavior between an actuated system and its unactuated counterpart with decreased viscosity ratio. Finally, we demonstrate that oscillation is capable of modulating satellite droplet formation by shifting the pinch-off location. Our study would be useful for manipulating fluids at microscale by external forcing.