Electric Control of Droplets in Microfluidic Devices

Sorting droplets into many outlets

Droplet microfluidics is a commercially successful technology, widely used in single cell sequencing and droplet PCR. Combining droplet making with droplet sorting has also been demonstrated, but so far found limited use, partly due to difficulties in scaling manufacture with injection molded plastics. We introduce a droplet sorting system with several new elements, including: 1) an electrode design combining metallic and ionic liquid parts, 2) a modular, multi-sorting fluidic design with features for keeping inter-droplet distances constant, 3) using timing parameters calculated from fluorescence or scatter signal triggers to precisely actuate dozens of sorting electrodes, 4) droplet collection techniques, including ability to collect a single droplet, and 5) a new emulsion breaking method to collect aqueous samples for downstream analysis. We use these technologies to build a fluorescence based cell sorter that can sort with high (>90%) purity. We also show that these microfluidic designs can be translated into injection molded thermoplastic, suitable for industrial production. Finally, we tally the advantages and limitations of these devices.

Tunable hydrodynamic focusing with dual-neodymium magnet-based microfluidic separation device

Microfluidic separation technologies are the focus of various biological applications, such as disease diagnostics, single-cell analysis, and therapeutics. Different methods and devices were proposed in the micro-separation field, focusing on minimizing the chemical deformation and physical damage to the particles throughout the separation process; however, it is still a challenge. This paper proposes a hydrodynamic focusing-based microfluidic separation device equipped with a dual-neodymium magnet for positive magnetophoretic microparticles and cell separation. Hydrodynamic focusing is used to help to sort the particles and minimize the damage to the microparticles through the proposed different inlet flow rates between the two focusing channels. The dual magnets help to separate the particles in two stages. The system's novelty is integrating the hydrodynamic focusing with the dual magnetics system, where the hydrodynamic focusing is with variable inlet flow rates. The performance of the proposed microfluidic particle separator is numerically assessed under various operating parameters, including the concentration of the particle in the injected solution and flow rate ratios of high to the low focusing flows on the efficiency of the separation. Following the proposed separation method, it was possible to separate the 16 and 10 [Formula: see text] microparticles with the first-round efficiency of 21% with a quality of 92%, respectively. The developed particle separation system can significantly broaden its applications in a variety of biomedical research studies.

Simultaneous Droplet Generation with In-Series Droplet T-Junctions Induced by Gravity-Induced Flow

Droplet microfluidics offers a wide range of applications, including high-throughput drug screening and single-cell DNA amplification. However, these platforms are often limited to single-input conditions that prevent them from analyzing multiple input parameters (e.g., combined cellular treatments) in a single experiment. Droplet multiplexing will result in higher overall throughput, lowering cost of fabrication, and cutting down the hands-on time in number of applications such as single-cell analysis. Additionally, while lab-on-a-chip fabrication costs have decreased in recent years, the syringe pumps required for generating droplets of uniform shape and size remain cost-prohibitive for researchers interested in utilizing droplet microfluidics. This work investigates the potential of simultaneously generating droplets from a series of three in-line T-junctions utilizing gravity-driven flow to produce consistent, well-defined droplets. Implementing reservoirs with equal heights produced inconsistent flow rates that increased as a function of the distance between the aqueous inlets and the oil inlet. Optimizing the three reservoir heights identified that taller reservoirs were needed for aqueous inlets closer to the oil inlet. Studying the relationship between the ratio of oil-to-water flow rates (Φ) found that increasing Φ resulted in smaller droplets and an enhanced droplet generation rate. An ANOVA was performed on droplet diameter to confirm no significant difference in droplet size from the three different aqueous inlets. The work described here offers an alternative approach to multiplexed droplet microfluidic devices allowing for the high-throughput interrogation of three sample conditions in a single device. It also has provided an alternative method to induce droplet formation that does not require multiple syringe pumps.

Surfactant and dilatational viscosity effects on the deformation of liquid droplets in an electric field

HYPOTHESIS

A conducting droplet suspended in an insulating continuous phase, e.g. an aqueous electrolyte in an oil, is deformed by an applied electric field to nonspherical equilibrium shapes, and can even break-up under strong fields. Many technologies use electro-deformation to manipulate fluid dispersions, with surfactants present on the droplet interfaces forming stabilizing monolayers. While surfactants lower the interface tension which facilitates electro-deformation, the monolayer elasticity resists deformation. High molecular weight surfactants, with large dilatational viscosities, can potentially retard the deformation dynamics.

NUMERICS

A boundary integral method simulates the dynamic interfacial deformation of a perfectly conducting droplet in a dielectric in a uniform field. The interface contains an insoluble monolayer which is a Newtonian fluid with constant dilatational viscosity obeying a Langmuir state equation. A range of initial surfactant surface concentrations are studied, with elasticity proportional to concentration.

FINDINGS

Equilibrium drop deformations, unaffected by surface viscosity, are strongly resisted by elasticity at high surface concentrations, and field strengths necessary for break-up increase with elasticity. Dilatational viscosity scales with the ratio, κ^∗, the surface viscosity (divided by the droplet radius) to the bulk viscosity, and can extend the deformation time. Extended times are described by a time rescaling proportional to κ^∗.

Dual dean entrainment with volume ratio modulation for efficient droplet co-encapsulation: extreme single-cell indexing

The future of single cell diversity screens involves ever-larger sample sizes, dictating the need for higher throughput methods with low analytical noise to accurately describe the nature of the cellular system. Current approaches are limited by the Poisson statistic, requiring dilute cell suspensions and associated losses in throughput. In this contribution, we apply Dean entrainment to both cell and bead inputs, defining different volume packets to effect efficient co-encapsulation. Volume ratio scaling was explored to identify optimal conditions. This enabled the co-encapsulation of single cells with reporter beads at rates of ∼1 million cells per hour, while increasing assay signal-to-noise with cell multiplet rates of ∼2.5% and capturing ∼70% of cells. The method, called Pirouette coupling, extends our capacity to investigate biological systems.

Microfluidics-based generation of cell encapsulated microbeads in the presence of electric fields and spatio-temporal viability studies

Microfluidics based techniques for generation of cell-laden microbeads are emerging as an attractive route to 3D cell encapsulation due to the precise control provided by microfluidics. However, existing microfluidics based cell encapsulation methods are restricted to 2D planar devices and use of passive methods for droplet generation. In this work, we report the development of a 3D glass-PDMS (polydimethylsiloxane) hybrid device for complete on-chip generation of cell-laden alginate beads in the presence of electric fields. The 3D hybrid device allows application of electric fields for active control of droplet (sodium alginate) size without the need for electrode patterning or liquid electrodes. Chemical gelation is achieved through on-chip coalescence of sodium alginate droplets and calcium chloride plugs, generated using coflow and T-junction geometries respectively. Using this approach, we successfully encapsulate E. coli cells (with viability ∼90 %) into alginate microbeads and perform comprehensive spatio-temporal growth and viability studies. The active control of droplet size coupled with complete on-chip gelation demonstrated here is a promising technology for cell encapsulation with applications such as cell therapy, organ repair, biocatalysis, and microbial fuel cells.

Fabricating an Electrospray Ionization Chip Based on Induced Polarization and Liquid Splitting

The coupling of the microfluidic chip to mass spectrometry (MS) has attracted considerable attention in the area of chemical and biological analysis. The most commonly used ionization technique in the chip–MS system is electrospray ionization (ESI). Traditional chip-based ESI devices mainly employ direct electrical contact between the electrode and the spray solvent. In this study, a microchip ESI source based on a novel polarization-splitting approach was developed. Specifically, the droplet in the microchannel is first polarized by the electric field and then split into two sub-droplets. In this process, the charge generated by polarization is retained in the liquid, resulting in the generation of two charged droplets with opposite polarities. Finally, when these charged droplets reach the emitter, the electrospray process is initiated and both positive and negative ions are formed from the same solution. Preliminary experimental results indicate that the coupling of this polarization-splitting ESI (PS-ESI) chip with a mass spectrometer enables conventional ESI-MS analysis of various analytes.

An integrated high-throughput microfluidic circulatory fluorescence-activated cell sorting system (μ-CFACS) for the enrichment of rare cells

There is an increasing need for the enrichment of rare cells in the clinical environments of precision medicine, personalized medicine, and regenerative medicine. With the possibility of becoming the next-generation cell sorters, microfluidic fluorescence-activated cell sorting (μ-FACS) devices have been developed to avoid cross-contamination, minimize device footprint, and eliminate bio-aerosols. However, due to highly precise flow control, the achievable throughput of the μ-FACS system is generally lower than the throughput of conventional FACS devices. Here, we report a fully integrated high-throughput microfluidic circulatory fluorescence-activated cell sorting (μ-CFACS) system for the enrichment of clinical rare cells. A microfluidic sorting cartridge has been developed for enriching samples through a sequential sorting process, which was further realized by the integration of both fast amplified piezoelectrically actuated on-chip valves and compact pneumatic cylinders actuated on-chip valves. At an equivalent throughput of ∼8000 events per second (eps), the purity of rare fluorescent microparticles has been significantly increased from ∼0.01% to ∼27.97%. An enrichment of ∼9400-fold from 0.009% to 81.86% has also been demonstrated for isolating fluorescently labelled MCF-7 breast cancer cells from Jurkat cells at an equivalent sorting throughput of ∼6400 eps. With the advantages of high throughput and contamination-free design, the proposed integrated μ-CFACS system provides a new option for the enrichment of clinical rare cells.

Synchronized Reagent Delivery in Double Emulsions for Triggering Chemical Reactions and Gene Expression

Microfluidic methods for the formation of single and double emulsion (DE) droplets allow for the encapsulation and isolation of reactants inside nanoliter compartments. Such methods have greatly enhanced the toolbox for high-throughput screening for cell or enzyme engineering and drug discovery. However, remaining challenges in the supply of reagents into these enclosed compartments limit the applicability of droplet microfluidics. Here, a strategy is introduced for on-demand delivery of reactants in DEs. Lipid vesicles are used as reactant carriers, which are co-encapsulated in double emulsions and release their cargo upon addition of an external trigger, here the anionic surfactant sodium dodecyl sulfate (SDS). The reagent present inside the lipid vesicles stays isolated from the remaining content of the DE vessel until SDS enters the DE lumen and solubilizes the vesicles' lipid bilayer. The versatility of the method is demonstrated with two critical applications chosen as representative assays for high-throughput screening: the induction of gene expression in bacteria and the initiation of an enzymatic reaction. This method not only allows for the release of the lipid vesicle content inside DEs to be synchronized for all DEs but also for the release to be triggered at any desired time.

Active Flow Control and Dynamic Analysis in Droplet Microfluidics

Droplet-based microfluidics has emerged as an important subfield within the microfluidic and general analytical communities. Indeed, several unique applications such as digital assay readout and single-cell sequencing now have commercial systems based on droplet microfluidics. Yet there remains room for this research area to grow. To date, most analytical readouts are optical in nature, relatively few studies have integrated sample preparation, and passive means for droplet formation and manipulation have dominated the field. Analytical scientists continue to expand capabilities by developing droplet-compatible method adaptations, for example, by interfacing to mass spectrometers or automating droplet sampling for temporally resolved analysis. In this review, we highlight recently developed fluidic control techniques and unique integrations of analytical methodology with droplet microfluidics-focusing on automation and the connections to analog/digital domains-and we conclude by offering a perspective on current challenges and future applications.

Microfluidics for Drug Development: From Synthesis to Evaluation

Drug development is a long process whose main content includes drug synthesis, drug delivery, and drug evaluation. Compared with conventional drug development procedures, microfluidics has emerged as a revolutionary technology in that it offers a miniaturized and highly controllable environment for bio(chemical) reactions to take place. It is also compatible with analytical strategies to implement integrated and high-throughput screening and evaluations. In this review, we provide a comprehensive summary of the entire microfluidics-based drug development system, from drug synthesis to drug evaluation. The challenges in the current status and the prospects for future development are also discussed. We believe that this review will promote communications throughout diversified scientific and engineering communities that will continue contributing to this burgeoning field.

Prediction of the spread of Corona-virus carrying droplets in a bus - A computational based artificial intelligence approach

Public transport has been identified as high risk as the corona-virus carrying droplets generated by the infected passengers could be distributed to other passengers. Therefore, predicting the patterns of droplet spreading in public transport environment is of primary importance. This paper puts forward a novel computational and artificial intelligence (AI) framework for fast prediction of the spread of droplets produced by a sneezing passenger in a bus. The formation of droplets of salvia is numerically modelled using a volume of fluid methodology applied to the mouth and lips of an infected person during the sneezing process. This is followed by a large eddy simulation of the resultant two phase flow in the vicinity of the person while the effects of droplet evaporation and ventilation in the bus are considered. The results are subsequently fed to an AI tool that employs deep learning to predict the distribution of droplets in the entire volume of the bus. This combined framework is two orders of magnitude faster than the pure computational approach. It is shown that the droplets with diameters less than 250 micrometers are most responsible for the transmission of the virus, as they can travel the entire length of the bus.

Computer simulation of submicron fluid flows in microfluidic chips and their applications in food analysis

In recent years, countries around the world have maintained a zero-tolerance attitude toward safety problems in the food industry. In order to ensure human health, a fast, sensitive, and high-throughput analysis of food contaminants is necessary to ensure safe food products on the market. Microfluidics, as a high-efficiency and sensitive detection technology, has many advantages in the detection of food contaminants, including foodborne pathogens, pesticides, heavy metal ions, toxic substances, and so forth, especially in conjunction with a variety of submicron fluid driving methods, making food detection and analysis more efficient and accurate. This review introduces the principle of submicron fluid driving modes and discusses the driving simulation of submicron fluid in microfluidic chips. In addition, the latest developments in the application of simulation in food analysis from 2006 to 2020 are discussed, and the computer simulation of submicron fluid flow in microfluidic chips and its application and development trend in food analysis are also highlighted. The review indicates that microfluidic technology, using numerical simulation as an auxiliary tool, combined with traditional methods has greatly improved the detection and analysis of food products. In addition, microfluidics combined with a variety of control methods embodies the ability of specific, multifunctional, and sensitive detection and analysis of food products. The development of high-sensitivity, high-throughput, portable, integrated microfluidic chips will enable the technology to be applied in practice.

Multidimensional Digital Bioassay Platform Based on an Air-Sealed Femtoliter Reactor Array Device

Single-molecule experiments have been helping us to get deeper inside biological phenomena by illuminating how individual molecules actually work. Digital bioassay, in which analyte molecules are individually confined in small compartments to be analyzed, is an emerging technology in single-molecule biology and applies to various biological entities (e.g., cells and virus particles). However, digital bioassay is not compatible with multiconditional and multiparametric assays, hindering in-depth understanding of analytes. This is because current digital bioassay lacks a repeatable solution-exchange system that keeps analytes inside compartments. To address this challenge, we developed a digital bioassay platform with easy solution exchanges, called multidimensional (MD) digital bioassay. We immobilized single analytes in arrayed femtoliter (10^-15 L) reactors and sealed them with airflow. The solution in each reactor was stable and showed no cross-talk via solution leakage for more than 2 h, and over 30 rounds of perfect solution exchanges were successfully performed. With multiconditional assays based on our system, we could quantitatively determine inhibitor sensitivities of single influenza A virus particles and single alkaline phosphatase (ALP) molecules, which has never been achieved with conventional digital bioassays. Further, we demonstrated that ALPs from two origins can be precisely distinguished by a single-molecule multiparametric assay with our system, which was also difficult with conventional digital bioassays. Thus, MD digital bioassay is a versatile platform to gain in-depth insight into biological entities in unprecedented resolution.

Public-Health-Driven Microfluidic Technologies: From Separation to Detection

Separation and detection are ubiquitous in our daily life and they are two of the most important steps toward practical biomedical diagnostics and industrial applications. A deep understanding of working principles and examples of separation and detection enables a plethora of applications from blood test and air/water quality monitoring to food safety and biosecurity; none of which are irrelevant to public health. Microfluidics can separate and detect various particles/aerosols as well as cells/viruses in a cost-effective and easy-to-operate manner. There are a number of papers reviewing microfluidic separation and detection, but to the best of our knowledge, the two topics are normally reviewed separately. In fact, these two themes are closely related with each other from the perspectives of public health: understanding separation or sorting technique will lead to the development of new detection methods, thereby providing new paths to guide the separation routes. Therefore, the purpose of this review paper is two-fold: reporting the latest developments in the application of microfluidics for separation and outlining the emerging research in microfluidic detection. The dominating microfluidics-based passive separation methods and detection methods are discussed, along with the future perspectives and challenges being discussed. Our work inspires novel development of separation and detection methods for the benefits of public health.

Critical conditions for organic thread cutting under electric fields

Conditions for triggering the cutting of organic samples under an AC electric field are investigated in a microchannel to explore the strategy for organic sample manipulation. Based on the nature of triggering and developing instability at liquid interfaces, in combination with an equivalent electric circuit model, a novel electric capillary number method is proposed as a comprehensive critical condition for the cutting. We uncover the physics behind cutting and non-cutting of an organic thread for different electric frequencies, electric properties of fluid, and width of the organic thread. The critical time required and the critical cutting position are studied to offer guidelines for accurate cutting. Higher electric frequency and higher permittivity of the aqueous phase surrounding the organic phase can reduce the voltage required for cutting. In summary, the newly defined electric capillary number is proved to be a comprehensive criterion for determining the cutting phenomena, which is capable of considering the interfacial tension, the electric permittivity and the electric field strength applied. The results offer applicable references for achieving efficient and accurate cutting of organic samples in practical applications.

Recent advances in droplet microfluidics for enzyme and cell factory engineering

Enzymes and cell factories play essential roles in industrial biotechnology for the production of chemicals and fuels. The properties of natural enzymes and cells often cannot meet the requirements of different industrial processes in terms of cost-effectiveness and high durability. To rapidly improve their properties and performances, laboratory evolution equipped with high-throughput screening methods and facilities is commonly used to tailor the desired properties of enzymes and cell factories, addressing the challenges of achieving high titer and the yield of the target products at high/low temperatures or extreme pH, in unnatural environments or in the presence of unconventional media. Droplet microfluidic screening (DMFS) systems have demonstrated great potential for exploring vast genetic diversity in a high-throughput manner (>10⁶/h) for laboratory evolution and have been increasingly used in recent years, contributing to the identification of extraordinary mutants. This review highlights the recent advances in concepts and methods of DMFS for library screening, including the key factors in droplet generation and manipulation, signal sources for sensitive detection and sorting, and a comprehensive summary of success stories of DMFS implementation for engineering enzymes and cell factories during the past decade.

Directed self-propulsion of droplets on surfaces absent of gradients for cargo transport

HYPOTHESIS

Manipulating droplet transportation without inputting work is desired and important in microfluidic systems. Although the creation of wettability gradient on surfaces has been employed to achieve this goal, the transport distance is very limited, hindering its applications in long-term operations.

EXPERIMENTS

Here, we show that programming long-ranged transport of droplets on surfaces can be achieved by the addition of trisiloxane surfactants and the creation of deep grooves. The former provides Marangoni stress to actuate the droplet motion and also reduces the inherent contact line pinning. The latter acts as a railing to guide the motion of surfactant-laden droplets to follow various layouts with geometric features of roads.

FINDINGS

It is found that the droplets with microliters can move over 20 cm. This work-free method is applicable to a variety of substrate materials and liquids. By using self-running shuttles, a convenient platform for liquid cargos transport is developed and demonstrated. Moreover, the coalescence of cargos carried by different shuttles is accomplished in a three-branch layout, revealing new droplet microreactors.

Microwave Heating Induced On-Demand Droplet Generation in Microfluidic Systems

In this note, we report a simple, new method for droplet generation in microfluidic systems using integrated microwave heating. This method enables droplet generation on-demand by using microwave heating to induce Laplace pressure change at the interface of the two fluids. The distance between the interface and junction and microwave excitation power have been found to influence droplet generation. Although this method is limited in generating droplets with a high rate, the fact that it can be integrated with microwave sensing that can be used as the feedback to tune the supply flow of materials presents unique advantages for applications that require dynamic tuning of material properties in droplets.

High-Performance Unidirectional Manipulation of Microdroplets by Horizontal Vibration on Femtosecond Laser-Induced Slant Microwall Arrays

The high-performance unidirectional manipulation of microdroplets is crucial for many vital applications including water collection and bioanalysis. Among several actuation methods (e.g., electric, magnetic, light, and thermal actuation), mechanical vibration is pollution-free and biocompatible. However, it suffers from limited droplet movement mode, small volume range (V_Max /V_Min  < 3), and low transport velocity (≤11.5 mm s^-1 ) because the droplet motion is a sliding state caused by the vertical vibration on the asymmetric hydrophobic microstructures. Here, an alternative strategy is proposed-horizontal vibration for multimode (rolling, bouncing/reverse bouncing, converging/diffusing, climbing, 90^o turning, and sequential transport), large-volume-range (V_Max /V_Min  ≈ 100), and high-speed (≈22.86 mm s^-1 ) unidirectional microdroplet manipulation, which is ascribed to the rolling state on superhydrophobic slant microwall arrays (SMWAs) fabricated by the one-step femtosecond laser oblique ablation. The unidirectional transport mechanism relies on the variance of viscous resistance induced by the difference of contact area between the microdroplet and the slant microwalls. Furthermore, a circular, curved, and "L"-shaped SMWA is designed and fabricated for droplet motion with particular paths. Finally, sequential transport of large-volume-range droplets and chemical mixing microreaction of water-based droplets are demonstrated on the SMWA, which demonstrates the great potential in the field of microdroplet manipulation.

How electrospray potentials can disrupt droplet microfluidics and how to prevent this

A pressure-resistant microfluidic glass chip that integrates a packed-bed HPLC column, a droplet generator and a monolithic electrospray emitter is presented. This approach enables a seamless coupling of chip-HPLC and droplet microfluidics with ESI-MS detection. For the electrical contacting of the emitter, an electrode was integrated into the channel, which reaches up to the emitter tip. The incidental finding that under certain circumstances, the electrospray potential can strongly disturb the droplet microfluidics by electrowetting, was investigated in detail. Strategies to avoid this are evaluated and include electrical shielding and/or chip layouts, where the droplet generator is positioned at a long distance from the emitter.

Amplified piezoelectrically actuated on-chip flow switching for a rapid and stable microfluidic fluorescence activated cell sorter

With the potential to avoid cross-contamination, eliminate bio-aerosols, and minimize device footprints, microfluidic fluorescence-activated cell sorting (μ-FACS) devices could become the platform for the next generation cell sorter. Here, we report an on-chip flow switching based μ-FACS mechanism with piezoelectric actuation as a fast and robust sorting solution. A microfluidic chip with bifurcate configuration and displacement amplified piezoelectric microvalves has been developed to build the μ-FACS system. Rare fluorescent microparticles of different sizes have been significantly enriched from a purity of ∼0.5% to more than 90%. An enrichment of 150-fold from ∼0.6% to ∼91% has also been confirmed for fluorescently labeled MCF-7 breast cancer cells from Jurkat cells, while viability after sorting was maintained. Taking advantage of its simple structure, low cost, fast response, and reliable flow regulation, the proposed μ-FACS system delivers a new option that can meet the requirements of sorting performance, target selectivity, device lifetime, and cost-effectiveness of implementation.

Enhanced single-cell encapsulation in microfluidic devices: From droplet generation to single-cell analysis

Single-cell analysis to investigate cellular heterogeneity and cell-to-cell interactions is a crucial compartment to answer key questions in important biological mechanisms. Droplet-based microfluidics appears to be the ideal platform for such a purpose because the compartmentalization of single cells into microdroplets offers unique advantages of enhancing assay sensitivity, protecting cells against external stresses, allowing versatile and precise manipulations over tested samples, and providing a stable microenvironment for long-term cell proliferation and observation. The present Review aims to give a preliminary guidance for researchers from different backgrounds to explore the field of single-cell encapsulation and analysis. A comprehensive and introductory overview of the droplet formation mechanism, fabrication methods of microchips, and a myriad of passive and active encapsulation techniques to enhance single-cell encapsulation efficiency were presented. Meanwhile, common methods for single-cell analysis, especially for long-term cell proliferation, differentiation, and observation inside microcapsules, are briefly introduced. Finally, the major challenges faced in the field are illustrated, and potential prospects for future work are discussed.

Method for Passive Droplet Sorting after Photo-Tagging

We present a method to photo-tag individual microfluidic droplets for latter selection by passive sorting. The use of a specific surfactant leads to the interfacial tension to be very sensitive to droplet pH. The photoexcitation of droplets containing a photoacid, pyranine, leads to a decrease in droplet pH. The concurrent increase in droplet interfacial tension enables the passive selection of irradiated droplets. The technique is used to select individual droplets within a droplet array as illuminated droplets remain in the wells while other droplets are eluted by the flow of the external oil. This method was used to select droplets in an array containing cells at a specific stage of apoptosis. The technique is also adaptable to continuous-flow sorting. By passing confined droplets over a microfabricated trench positioned diagonally in relation to the direction of flow, photo-tagged droplets were directed toward a different chip exit based on their lateral movement. The technique can be performed on a conventional fluorescence microscope and uncouples the observation and selection of droplets, thus enabling the selection on a large variety of signals, or based on qualitative user-defined features.

Formation and manipulation of ferrofluid droplets with magnetic fields in a microdevice: a numerical parametric study

We present a numerical model that describes the microfluidic generation and manipulation of ferrofluid droplets under an external magnetic field. We developed a numerical Computational Fluid Dynamics (CFD) analysis for predicting and optimizing continuous flow generation and processing of ferrofluid droplets with and without the presence of a permanent magnet. More specifically, we explore the dynamics of oil-based ferrofluid droplets within an aqueous continuous phase under an external inhomogeneous magnetic field. The developed model determines the effect of the magnetic field on the droplet generation, which is carried out in a flow-focusing geometry, and its sorting in T-junction channels. Three-channel depths (25 μm, 30 μm, and 40 μm) were investigated to study droplet deformation under magnetic forces. Among the three, the 30 μm channel depth showed the most consistent droplet production for the studied range of flow rates. Ferrofluids with different loadings of magnetic nanoparticles were used to observe the behavior for different ratios of magnetic and hydrodynamic forces. Our results show that the effect of these factors on droplet size and generation rate can be tuned and optimized to produce consistent droplet generation and sorting. This approach involves fully coupled magnetic-fluid mechanics models and can predict critical details of the process including droplet size, shape, trajectory, dispensing rate, and the perturbation of the fluid co-flow for different flow rates. The model enables better understanding of the physical phenomena involved in continuous droplet processing and allows efficient parametric analysis and optimization.

Experimental Evidence of Enhanced Adsorption Dynamics at Liquid-Liquid Interfaces under an Electric Field

In this work, we investigate the adsorption of surface-active compounds at the water-oil interface subjected to an electric field. A fluid system comprising a pendent water drop surrounded by an asphaltene-rich organic phase is exposed to a DC uniform electric field. Two subfractions of asphaltenes having contrasting affinities to the water-oil interface are used as surface-active compounds. The microscopic changes in the drop shape, as a result of asphaltene adsorption, are captured and the drop profiles are analyzed using our in-house code for axisymmetric drop shape analysis (ADSA) under an electric field. The estimates of dynamic interfacial tension under different strengths of the field (E₀) and concentrations of the asphaltene subfractions (C) are used to calculate adsorption dynamics and surface excess. The experimental observations and careful analyses of the data suggest that the externally applied electric field significantly stimulates the mass-transfer rate at the liquid-liquid interface. The enhancement in mass transport at the water-oil interface can be attributed to the axisymmetric electrohydrodynamic fluid flows generated on either side of the interface. The boost in mass transport is evident from the growing decay in equilibrium interfacial tension (γ_eq) and increased surface excess (Γ_eq) upon increasing strength of the applied electric field. The mass-transfer intensification does not increase monotonously with the electric field strength above an optimum E₀, which is in agreement with the previous theoretical studies in the literature. However, these first explicit experimental measurements of adsorption at an interface under an electric field suggest that the optimum E₀ is determined by characteristics of the surface-active molecules.

Electrohydrodynamics of droplets and jets in multiphase microsystems

Electrohydrodynamics is among the most promising techniques for manipulating liquids in microsystems. The electric stress actuates, generates, and coalesces droplets of small sizes; it also accelerates, focuses, and controls the motion of fine jets. In this review, the current understanding of dynamic regimes of electrically driven drops and jets in multiphase microsystems is summarized. The experimental description and underlying mechanism of force interplay and instabilities are discussed. Conditions for controlled transitions among different regimes are also provided. Emerging new phenomena either due to special interfacial properties or geometric confinement are emphasized, and simple scaling arguments proposed in the literature are introduced. The review provides useful perspectives for investigations involving electrically driven droplets and jets.

Droplet size prediction in a microfluidic flow focusing device using an adaptive network based fuzzy inference system

Microfluidics has wide applications in different technologies such as biomedical engineering, chemistry engineering, and medicine. Generating droplets with desired size for special applications needs costly and time-consuming iterations due to the nonlinear behavior of multiphase flow in a microfluidic device and the effect of several parameters on it. Hence, designing a flexible way to predict the droplet size is necessary. In this paper, we use the Adaptive Neural Fuzzy Inference System (ANFIS), by mixing the artificial neural network (ANN) and fuzzy inference system (FIS), to study the parameters which have effects on droplet size. The four main dimensionless parameters, i.e. the Capillary number, the Reynolds number, the flow ratio and the viscosity ratio are regarded as the inputs and the droplet diameter as the output of the ANFIS. Using dimensionless groups cause to extract more comprehensive results and avoiding more experimental tests. With the ANFIS, droplet sizes could be predicted with the coefficient of determination of 0.92.

Rotational manipulation of a microscopic object inside a microfluidic channel

Observations and analyses of a microscopic object are essential processes in various fields such as chemical engineering and life science. Microfluidic techniques with various functions and extensions have often been used for such purposes to investigate the mechanical properties of microscopic objects such as biological cells. One of such extensions proposed in this context is a real-time visual feedback manipulation system, which is composed of a high-speed camera and a piezoelectric actuator with a single-line microfluidic channel. Although the on-chip manipulation system enables us to control the 1 degree-of-freedom position of a target object by the real-time pressure control, it has suffered from unintended changes in the object orientation, which is out of control in the previous system. In this study, we propose and demonstrate a novel shear-flow-based mechanism for the control of the orientation of a target object in addition to the position control in a microchannel to overcome the problem of the unintended rotation. We designed a tributary channel using a three-dimensional hydrodynamic simulation with boundary conditions appropriate for the particle manipulation to apply shear stress to the target particle placed at the junction and succeeded in rotating the particle at an angular velocity of 0.2 rad/s even under the position control in the experiment. The proposed mechanism would be applied to feedback controls of a target object in a microchannel to be in a desired orientation and at a desired position, which could be a universally useful function for various microfluidic platforms.

Droplet Generation in a Flow-Focusing Microfluidic Device with External Mechanical Vibration

The demand for highly controllable droplet generation methods is very urgent in the medical, materials, and food industries. The droplet generation in a flow-focusing microfluidic device with external mechanical vibration, as a controllable droplet generation method, is experimentally studied. The effects of vibration frequency and acceleration amplitude on the droplet generation are characterized. The linear correlation between the droplet generation frequency and the external vibration frequency and the critical vibration amplitude corresponding to the imposing vibration frequency are observed. The droplet generation frequency with external mechanical vibration is affected by the natural generation frequency, vibration frequency, and vibration amplitude. The droplet generation frequency in a certain microfluidic device with external vibration is able to vary from the natural generation frequency to the imposed vibration frequency at different vibration conditions. The evolution of dispersed phase thread with vibration is remarkably different with the process without vibration. Distinct stages of expansion, shrinkage, and collapse are observed in the droplet formation with vibration, and the occurrence number of expansion–shrinkage process is relevant with the linear correlation coefficient.

Droplet microfluidics: fundamentals and its advanced applications

Droplet-based microfluidic systems have been shown to be compatible with many chemical and biological reagents and capable of performing a variety of operations that can be rendered programmable and reconfigurable. This platform has dimensional scaling benefits that have enabled controlled and rapid mixing of fluids in the droplet reactors, resulting in decreased reaction times. This, coupled with the precise generation and repeatability of droplet operations, has made the droplet-based microfluidic system a potent high throughput platform for biomedical research and applications. In addition to being used as micro-reactors ranging from the nano- to femtoliter (10-15 liters) range; droplet-based systems have also been used to directly synthesize particles and encapsulate many biological entities for biomedicine and biotechnology applications. For this, in the following article we will focus on the various droplet operations, as well as the numerous applications of the system and its future in many advanced scientific fields. Due to advantages of droplet-based systems, this technology has the potential to offer solutions to today's biomedical engineering challenges for advanced diagnostics and therapeutics.

Droplet and Particle Generation on Centrifugal Microfluidic Platforms: A Review

The use of multiphase flows in microfluidics to carry dispersed phase material (droplets, particles, bubbles, or fibers) has many applications. In this review paper, we focus on such flows on centrifugal microfluidic platforms and present different methods of dispersed phase material generation. These methods are classified into three specific categories, i.e., step emulsification, crossflow, and dispenser nozzle. Previous works on these topics are discussed and related parameters and specifications, including the size, material, production rate, and rotational speed are explicitly mentioned. In addition, the associated theories and important dimensionless numbers are presented. Finally, we discuss the commercialization of these devices and show a comparison to unveil the pros and cons of the different methods so that researchers can select the centrifugal droplet/particle generation method which better suits their needs.

Wall Effects on Hydrodynamic Drag and the Corresponding Accuracy of Charge Measurement in Droplet Contact Charge Electrophoresis

Droplet size dependent wall effects on hydrodynamic drag and the corresponding droplet contact charge estimations were experimentally and theoretically investigated. The consistent reduction in the dimensionless droplet contact charges proportional to droplet size was reported and explained by the parallel and approaching wall effects on the drag coefficient. Extrapolation of the size dependent droplet charge data showed that the droplet charge follows the perfect conductor theory when the droplet radius approaches zero. The proposed model was applied to the drag calculation to estimate and compare dimensionless charges before and after consideration of the wall effects. The droplet free fall test concluded that the droplets in the current experimental setup follow Stokes' law. The theoretical velocity profile of the droplet approaching the wall perpendicularly is proposed considering the approaching wall effect on hydrodynamic drag and verified by comparison with the experiment. The droplet size dependent velocity profile shape change was also explained by this approaching wall effect. The shape of the asymmetric velocity profile along the direction of droplet movement was explained by the effect of the image charge through direct numerical calculation of the electric force. The direct calculation of the electric force also showed that the electric correction at the center of the cuvette is negligible; thus, it is sufficient to consider only hydrodynamic correction for accurate charge measurement in this experimental system. The present study will contribute to the accurate measurement of the droplet charges under contact charge electrophoresis. It also provides the basis for precise control of droplet movement in lab-on-a-chip devices.

A novel micro-injection droplet microfluidic system for studying locomotive behavior responses to Cu2+ induced neurotoxin in individual C.elegans

Analysis of C.elegans by droplet microfluidics has been widely used in study of locomotive behavior responses to neurotoxicity due to the capacity of high-throughput manipulating single cells. However, it has been difficult to manipulate droplets flexibly and actively on account of the limitation of the dimension of individual C. elegans droplets. In this study, a novel MiDMS (Micro-injection Droplet Microfluidic System) was proposed, which consisted of three parts: single C. elegans droplet generator, droplets drug micro-injection channels and drug-incubation observation array. Individual C.elegans droplets were produced initially by regulating the flow rates between oil and water phase as well as the concentration of C.elegans in suspension. Then, the drug solution was precisely injected into each C.elegans droplet, which by electricity induced surface tension of droplet changing. In addition, the effect of neurotoxic Cu²⁺ on locomotive behavior of C. elegans was evaluated at single cell resolution. The results showed that the neurotoxicity induced behavioral disorder of the C. elegans was more obvious with the increase of Cu²⁺ concentration or treatment time, and these dose-effect and time-effect relationship in MiDMS were similar as in petri dish. This study will provide a powerful platform for the study of the response of C. elegans to quantitative drug at single cell resolution.

Droplet and Microchamber-Based Digital Loop-Mediated Isothermal Amplification (dLAMP)

Digital loop-mediated isothermal amplification (dLAMP) refers to compartmentalizing nucleic acids and LAMP reagents into a large number of individual partitions, such as microchambers and droplets. This compartmentalization enables dLAMP to be an excellent platform to quantify the absolute number of the target nucleic acids. Owing to its low requirement for instrumentation complexity, high specificity, and strong tolerance to inhibitors in the nucleic acid samples, dLAMP has been recognized as a simple and accurate technique to quantify pathogenic nucleic acid. Herein, the general process of dLAMP techniques is summarized, the current dLAMP techniques are categorized, and a comprehensive discussion on different types of dLAMP techniques is presented. Also, the challenges of the current dLAMP are illustrated together with the possible strategies to address these challenges. In the end, the future directions of the dLAMP developments, including multitarget detection, multisample detection, and processing nucleic acid extraction are outlined. With recently significant advances in dLAMP, this technology has the potential to see more widespread use beyond the laboratory in the future.

Rich topologies of monolayer ices via unconventional electrowetting

Accurate manipulation of a substance on the nanoscale and ultimately down to the level of a single atom or molecule is an ongoing subject of frontier research. Herein, we show that topologies of water monolayers on substrates, in the complete wetting condition, can be manipulated into rich forms of ordered structures via electrowetting. Notably, two new topologies of monolayer ices were identified from our molecular dynamics simulations: one stable below room temperature and the other one having the ability to be stable at room temperature. Moreover, the wettability of the substrate can be tuned from superhydrophobic to superhydrophilic by uniformly changing the charge of each atomic site of the dipole or quadrupole distributed in an orderly manner on the model substrate. At a certain threshold value of the atomic charge, water droplets on the substrate can spread out spontaneously, achieving a complete electrowetting. Importantly, unlike the conventional electrowetting, which involves application of a uniform external electric field, we proposed non-conventional electrowetting, for the first time, by invoking the electric field of dipoles and quadrupoles embedded in the substrate. Moreover, different topologies of water monolayers can be achieved by using the non-conventional electrowetting. A major advantage of the non-conventional electrowetting is that the contact-angle saturation, a long-standing and known limitation in the field of electrowetting, can be overcome by tuning uniformly the lattice atomic charge at the surface, thereby offering a new way to mitigate the contact-angle saturation for various electrowetting applications.

Pulsatile Flow in Microfluidic Systems

This review describes the current knowledge and applications of pulsatile flow in microfluidic systems. Elements of fluid dynamics at low Reynolds number are first described in the context of pulsatile flow. Then the practical applications in microfluidic processes are presented: the methods to generate a pulsatile flow, the generation of emulsion droplets through harmonic flow rate perturbation, the applications in mixing and particle separation, and the benefits of pulsatile flow for clog mitigation. The second part of the review is devoted to pulsatile flow in biological applications. Pulsatile flows can be used for mimicking physiological systems, to alter or enhance cell cultures, and for bioassay automation. Pulsatile flows offer unique advantages over a steady flow, especially in microfluidic systems, but also require some new physical insights and more rigorous investigation to fully benefit future applications.

Size-Based Sorting of Emulsion Droplets in Microfluidic Channels Patterned with Laser-Ablated Guiding Tracks

We demonstrate an autonomous, high-throughput mechanism for sorting of emulsion droplets with different sizes concurrently flowing in a microfluidic Hele-Shaw channel. The aqueous droplets of varying radii suspended in olive oil are separated into different streamlines across the channel upon interaction with a shallow (depth ∼ 700 nm) inclined guiding track ablated into the polydimethylsiloxane-coated surface of the channel with focused femtosecond laser pulses. Specifically, the observed differences in the droplet trajectories along the guiding track arise due to the different scaling of the confinement force attracting the droplets into the track, fluid drag, and wall friction, with the droplet radius. In addition, the distance traveled by the droplets along the track also depends on the track width, with wider tracks providing more stable droplet guiding for any given droplet size. We systematically study the influence of the droplet size and velocity on the trajectory of the droplets in the channel and analyze the sensitivity of size-based droplet sorting for varying flow conditions. The droplet guiding and sorting experiments are complemented by modeling of the droplet motion in the channel flow using computational fluid dynamics simulations and a previously developed model of droplet guiding. Finally, we demonstrate a complete separation of droplets produced by fusion of two independent droplet streams at the inlet of the Hele-Shaw channel from unfused daughter droplets. The presented droplet sorting technique can find applications in the development of analytical and preparative microfluidic protocols.

Simultaneous electric production and sizing of emulsion droplets in microfluidics

Microscale emulsions are widely used in fundamental and applied sciences. To expand their utilization, various methods have been developed for manipulating and measuring the physical properties of fabricated emulsions inside microchannels. Herein, we present an electric emulsification platform that can produce emulsions and simultaneously detect their physical properties (size and production speed). The characterization of the emulsion properties during the fabrication process will broaden the application fields for microscale emulsions because it can avoid time-consuming post image processing and simplify the emulsification platform. To accomplish this, a "bottleneck" channel is implanted between two reservoirs of immiscible fluids (continuous and dispersion phases). This channel can not only confine one fluid within the other when the electric field is on, resulting in emulsification via electrohydrodynamically induced Rayleigh instability, but also act as a resistive pulse sensor (RPS). The fluctuation of the liquid/liquid interface during emulsification induces the fluctuation of the electric resistance in the bottleneck channel, as the two fluid phases have different electrical conductivities. With this simple but dual-functional channel, the emulsion size (radius of 5-10 μm) and production speed (7-12 Hz) can be controlled by adjusting the electric field and the channel-neck geometry. Additionally, the properties can be measured using the RPS; the data obtained through the RPS exhibit high correlations with the validated data obtained using a high-speed camera and microscopy (>95%). The proposed buffer-less electric emulsification with the embedded RPS is a simple and cost-effective emulsion production method that allows real-time emulsion characterization with a limited sample volume.

A micrometer head integrated microfluidic device for facile droplet size control and automatic measurement of a droplet size

A novel microfluidic droplet generator is proposed, which can control the droplet size through turning an integrated micrometer head with ease, and the size of the produced micro-droplet can be automatically and real-time monitored by an open-sourced software and off-the-shelf hardware.