Droplet-based chemistry on a programmable micro-chip

Electric manipulation on deformation of ionic ferrofluid sessile droplets

Active electric-driven droplet manipulation in digital microfluidics constitutes a promising domain owing to the unique and programmable wettability inherent in sessile ionic droplets. The coupling between the electric field and flow field enables precise control over wetting characteristics and droplet morphology. This study delves into the deformation phenomena of ionic sessile ferrofluid droplets in ambient air induced by uniform electric fields. Under the assumption of a pinned mode throughout the process, the deformation is characterized by variations in droplet height and contact angle in response to the applied electric field intensity. A numerical model is formulated to simulate the deformation dynamics of ferrofluid droplets, employing the phase field method for tracking droplet deformation. The fidelity of the numerical outcomes is assessed through the validation process, involving a comparison of droplet geometric deformations with corresponding experimental results. The impact of the electric field on the deformation of dielectric droplets is modulated by parameters such as electric field strength and droplet size. Through meticulously designed experiments, the substantial influence of both field strength and droplet size is empirically verified, elucidating the behavior of ionic sessile droplets. Considering the interplay of electric force, viscous force, and interfacial tension, the heightened field intensity is observed to effectively reduce the contact angle, augment droplet height, and intensify internal droplet flow. Under varying electric field conditions, droplets assume diverse shapes, presenting a versatile approach for microfluidic operations. The outcomes of this research hold significant guiding implications for microfluidic manipulation, droplet handling, and sensing applications.

Programmable Control of Nanoliter Droplet Arrays using Membrane Displacement Traps

A unique droplet microfluidic technology enabling programmable deterministic control over complex droplet operations is presented. The platform provides software control over user-defined combinations of droplet generation, capture, ejection, sorting, splitting, and merging sequences to enable the design of flexible assays employing nanoliter-scale fluid volumes. The system integrates a computer vision system with an array of membrane displacement traps capable of performing selected unit operations with automated feedback control. Sequences of individual droplet handling steps are defined through a robust Python-based scripting language. Bidirectional flow control within the microfluidic chips is provided using an H-bridge channel topology, allowing droplets to be transported to arbitrary trap locations within the array for increased operational flexibility. By enabling automated software control over all droplet operations, the system significantly expands the potential of droplet microfluidics for diverse biological and biochemical applications by combining the functionality of robotic liquid handling with the advantages of droplet-based fluid manipulation.

Magnetic Beads inside Droplets for Agitation and Splitting Manipulation by Utilizing a Magnetically Actuated Platform

We successfully developed a platform for the magnetic manipulation of droplets containing magnetic beads and examined the washing behaviors of the droplets, including droplet transportation, magnetic bead agitation inside droplets, and separation from parent droplets. Magnetic field gradients were produced with two layers of 6 × 1 planar coils fabricated by using printed circuit board technology. We performed theoretical analyses to understand the characteristics of the coils and successfully predicted the magnetic field and thermal temperature of a single coil. We then investigated experimentally the agitation and splitting kinetics of the magnetic beads inside droplets and experimentally observed the washing performance in different neck-shaped gaps. The performance of the washing process was evaluated by measuring both the particle loss ratio and the optical density. The findings of this work will be used to design a magnetic-actuated droplet platform, which will separate magnetic beads from their parent droplets and enhance washing performance. We hope that this study will provide digital microfluidics for application in point-of-care testing. The developed microchip will be of great benefit for genetic analysis and infectious disease detection in the future.

Rapid Customization and Manipulation Mechanism of Micro-Droplet Chip for 3D Cell Culture

A full PDMS micro-droplet chip for 3D cell culture was prepared by using SLA light-curing 3D printing technology. This technology can quickly customize various chips required for experiments, saving time and capital costs for experiments. Moreover, an injection molding method was used to prepare the full PDMS chip, and the convex mold was prepared by light-curing 3D printing technology. Compared with the traditional preparation process of micro-droplet chips, the use of 3D printing technology to prepare micro-droplet chips can save manufacturing and time costs. The different ratios of PDMS substrate and cover sheet and the material for making the convex mold can improve the bonding strength and power of the micro-droplet chip. Use the prepared micro-droplet chip to carry out micro-droplet forming and manipulation experiments. Aimed to the performance of the full PDMS micro-droplet chip in biological culture was verified by using a solution such as chondrocyte suspension, and the control of the micro-droplet was achieved by controlling the flow rate of the dispersed phase and continuous phase. Experimental verification shows that the designed chip can meet the requirements of experiments, and it can be observed that the micro-droplets of sodium alginate and the calcium chloride solution are cross-linked into microspheres with three-dimensional (3D) structures. These microspheres are fixed on a biological scaffold made of calcium silicate and polyvinyl alcohol. Subsequently, the state of the cells after different time cultures was observed, and it was observed that the chondrocytes grew well in the microsphere droplets. The proposed method has fine control over the microenvironment and accurate droplet size manipulation provided by fluid flow compared to existing studies.

Characterization of a Droplet Containing the Clustered Magnetic Beads Manipulation by Magnetically Actuated Chips

A magnetically actuated chip was successfully developed in this study to perform the purpose of transportation for a droplet containing clustered magnetic beads. The magnetic field gradient is generated by the chip of the two-layer 4 × 4 array micro-coils, which was commercially fabricated by printing circuit board (PCB) technology. A numerical model was first established to investigate the magnetic field and thermal field for such a micro-coil. Consequently, the numerical simulations were in reasonable agreement with the experimental results. Moreover, a theoretical analysis was derived to predict the dynamic behaviors of the droplets. This analysis will offer the optimal operation for such a magnetically actuated chip. This study aims to successfully implement the concept of "digital microfluidics" in "point-of-care testing" (POCT). In the future, the micro-coil chip will be of substantial benefit to genetic analysis and infectious disease detection.

On‐Chip Chemical Synthesis Using One‐Step 3D Printed Polyperfluoropolyether

Three‐dimensional (3D) printing has already shown its high relevance for the fabrication of microfluidic devices in terms of precision manufacturing cycles and a wider range of materials. 3D‐printable transparent fluoropolymers are highly sought after due to their high chemical and thermal resistance. Here, we present a simple one‐step fabrication process via stereolithography of perfluoropolyether dimethacrylate. We demonstrate successfully printed microfluidic mixers with 800 µm circular channels for chemistry‐on‐chip applications. The printed chips show chemical, mechanical, and thermal resistance up to 200 °C, as well as high optical transparency. Aqueous and organic reactions are presented to demonstrate the wide potential of perfluoropolyether dimethacrylate for chemical synthesis.

Optical Dielectrophoretic (DEP) Manipulation of Oil-Immersed Aqueous Droplets on a Plasmonic-Enhanced Photoconductive Surface

We present a plasmonic-enhanced dielectrophoretic (DEP) phenomenon to improve optical DEP performance of a floating electrode optoelectronic tweezers (FEOET) device, where aqueous droplets can be effectively manipulated on a light-patterned photoconductive surface immersed in an oil medium. To offer device simplicity and cost-effectiveness, recent studies have utilized a polymer-based photoconductive material such as titanium oxide phthalocyanine (TiOPc). However, the TiOPc has much poorer photoconductivity than that of semiconductors like amorphous silicon (a-Si), significantly limiting optical DEP applications. The study herein focuses on the FEOET device for which optical DEP performance can be greatly enhanced by utilizing plasmonic nanoparticles as light scattering elements to improve light absorption of the low-quality TiOPc. Numerical simulation studies of both plasmonic light scattering and electric field enhancement were conducted to verify wide-angle scattering light rays and an approximately twofold increase in electric field gradient with the presence of nanoparticles. Similarly, a spectrophotometric study conducted on the absorption spectrum of the TiOPc has shown light absorption improvement (nearly twofold) of the TiOPc layer. Additionally, droplet dynamics study experimentally demonstrated a light-actuated droplet speed of 1.90 mm/s, a more than 11-fold improvement due to plasmonic light scattering. This plasmonic-enhanced FEOET technology can considerably improve optical DEP capability even with poor-quality photoconductive materials, thus providing low-cost, easy-fabrication solutions for various droplet-based microfluidic applications.

Continuous Droplet-Actuating Platforms via an Electric Field Gradient: Electrowetting and Liquid Dielectrophoresis

This work develops a technology for actuating droplets of any size without the requirement for high voltages or active control systems, which are typically found in competitive systems. The droplet actuation relies on two microelectrodes separated by a variable gap distance to generate an electrostatic gradient. The physical mechanism for the droplet motion is a combination of liquid dielectrophoresis and electrowetting. Investigating the system behavior as a function of the driving frequency identified the relative contribution of these two mechanisms and the optimum operating conditions. A fixed signal frequency of 0.5 kHz actuated various liquids and contaminants. Droplet actuation was demonstrated on several platforms, including linear, radial-symmetric, and bilateral-symmetric droplet motion. The electrode designs are scalable and can be fabricated on a flexible and optically transparent substrate: these key advancements will enable consumer applications that were previously inaccessible. A self-cleaning platform was also tested under laboratory conditions and on the road. This technology has significant potential in microfluidics and self-cleaning platforms, for example, in the automotive sector to clean body parts, camera covers, and sensors.

Dynamically controlled dielectrophoresis using resonant tuning

Electrically polarizable micro- and nanoparticles and droplets can be trapped using the gradient electric field of electrodes. But the spatial profile of the resultant dielectrophoretic force is fixed once the electrode structure is defined. To change the force profile, entire complex lab-on-a-chip systems must be re-fabricated with modified electrode structures. To overcome this problem, we propose an approach for the dynamic control of the spatial profile of the dielectrophoretic force by interfacing the trap electrodes with a resistor and an inductor to form a resonant resistor-inductor-capacitor (RLC) circuit. Using a dielectrophoretically trapped water droplet suspended in silicone oil, we show that the resonator amplitude, detuning, and linewidth can be continuously varied by changing the supply voltage, supply frequency, and the circuit resistance to obtain the desired trap depth, range, and stiffness. We show that by proper tuning of the resonator, the trap range can be extended without increasing the supply voltage, thus preventing sensitive samples from exposure to high electric fields at the stable trapping position. Such unprecedented dynamic control of dielectrophoretic forces opens avenues for the tunable active manipulation of sensitive biological and biochemical specimen in droplet microfluidic devices used for single-cell and biochemical reaction analysis.

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.

Low-Temperature Solution Crystallization of Nanostructured Oxides and Thin Films

As an introduction to this themed issue, a critically selected overview of recent progress on the topic of solution methods for the low-temperature crystallization of nanoscale oxide materials is presented. It is focused on the low-temperature solution processing of oxide nanostructures and thin films. Benefits derived from these methods span from minimizing the environmental impact to reducing the fabrication costs. In addition, this topic is regarded as a key objective in the area because it offers a unique opportunity for the use of these materials in areas like flexible electronics, energy conversion and storage, environmental sciences, catalysis, or biomedicine.

Microfluidic Study of the Electrocoalescence of Aqueous Droplets in Crude Oil

In electrocoalescence, an electric field is applied to a dispersion of conducting water droplets in a poorly conducting oil to force the droplets to merge in the direction of the field. Electrocoalescence is used in petroleum refining to separate water from crude oil and in droplet-based microfluidics to combine droplets of water in oil and to break emulsions. Using a microfluidic design to generate a two-dimensional (2D) emulsion, we demonstrate that electrocoalescence in an opaque crude oil can be visualized with optical microscopy and studied on an individual droplet basis in a chamber whose height is small enough to make the dispersions two dimensional and transparent. From reconstructions of images of the 2D electrocoalescence, the electrostatic forces driving the droplet merging are calculated in a numerically exact manner and used to predict observed coalescence events. Hence, the direct simulation of the electrocoalescence-driven breakdown of 2D emulsions in microfluidic devices can be envisioned.

A floating top-electrode electrowetting-on-dielectric system

Herein, we describe a novel device configuration for a double-plate electrowetting-on-dielectric system with a floating top-electrode. As a conventional double-plate EWOD device requires a grounded electrode on the top-plate, it will cause additional fabrication complicity and cost during the encapsulation process. In this work, we found that by carefully designing the electrode arrangement and configuring the driving electronic circuit, the droplet driving force can be maintained with a floating electrode on the top-plate. This can provide the possibilities to integrate additional electrical or electrochemical sensing functions on the top-plate. We use both finite element analysis and the fabricated system to validate the theory, and the results indicate that floating top-electrode EWOD systems are highly reliable and reproducible once the design considerations are fully met.

A novel device configuration for an electrowetting-on-dielectric system with a floating top-electrode, which provides possibilities to enable a true lab-on-a-chip.

A Liquid-Metal-Based Dielectrophoretic Microdroplet Generator

This paper proposes a novel microdroplet generator based on the dielectrophoretic (DEP) force. Unlike the conventional continuous microfluidic droplet generator, this droplet generator is more like “invisible electric scissors”. It can cut the droplet off from the fluid matrix and modify droplets’ length precisely by controlling the electrodes’ length and position. These electrodes are made of liquid metal by injection. By applying a certain voltage on the liquid-metal electrodes, the electrodes generate an uneven electric field inside the main microfluidic channel. Then, the uneven electric field generates DEP force inside the fluid. The DEP force shears off part from the main matrix, in order to generate droplets. To reveal the mechanism, numerical simulations were performed to analyze the DEP force. A detailed experimental parametric study was also performed. Unlike the traditional droplet generators, the main separating force of this work is DEP force only, which can produce one droplet at a time in a more precise way.

Integration of Horizontal and Vertical Microfluidic Modules for Core-Shell Droplet Generation and Chemical Application

This paper presents a method for utilizing three-dimensional microfluidic channels fully to realize multiple functions in a single device. The final device structure was achieved by combining three independent modules that consisted of horizontal and vertical channels. The device allowed for the one-step generation of water-in-oil-in-water droplets without the need for partial treatment of the polydimethylsiloxane channel surface using separate modules for generating water-in-oil droplets on the horizontal plane and oil-in-water droplets on the vertical plane. The second vertically structured module provided an efficient flow for the generation of highly wettable liquid droplets, and tuning of the first horizontally structured module enabled different modes of inner-core encapsulation within the oil shell. The successful integration of the vertical and horizontal channels for core-shell droplet generation and the chemical synthesis of a metal complex within the droplets were evaluated. The proposed approach of integrating independent modules will expand and enhance the functions of microfluidic platforms.

Microfluidic on-demand droplet generation, storage, retrieval, and merging for single-cell pairing

A multifunctional microfluidic platform combining on-demand aqueous-phase droplet generation, multi-droplet storage, and controlled merging of droplets selected from a storage library in a single integrated microfluidic device is described. A unique aspect of the technology is a microfluidic trap design comprising a droplet trap chamber and lateral bypass channels integrated with a microvalve that supports the capture and merger of multiple droplets over a wide range of individual droplet sizes. A storage unit comprising an array of microfluidic traps operates in a first-in first-out manner, allowing droplets stored within the library to be analyzed before sequentially delivering selected droplets to a downstream merging zone, while shunting other droplets to waste. Performance of the microfluidic trap is investigated for variations in bypass/chamber hydrodynamic resistance ratio, micro-chamber geometry, trapped droplet volume, and overall flow rate. The integrated microfluidic platform is then utilized to demonstrate the operational steps necessary for cell-based assays requiring the isolation of defined cell populations with single cell resolution, including encapsulation of individual cells within an aqueous-phase droplet carrier, screening or incubation of the immobilized cell-encapsulated droplets, and generation of controlled combinations of individual cells through the sequential droplet merging process. Beyond its utility for cell analysis, the presented platform represents a versatile approach to robust droplet generation, storage, and merging for use in a wide range of droplet-based microfluidics applications.

Active or Passive On-Demand Droplet Merging in a Microfluidic Valve-Based Trap

A microfluidic valve-based trap enabling controlled capture, release, and temporary immobilization of droplets together with on-demand merging of selected droplets is presented in this paper. The microfluidic trap technology can merge droplets passively or in active manner via a pneumatically actuated membrane. A microchip is developed with two functional units of droplet generator and merging mechanism to implement the passive or active merging performance of the microfluidic valve-based trap using a low and high surfactant concentrated continuous oil-phase.

Microfluidics in Malignant Glioma Research and Precision Medicine

Glioblastoma multiforme (GBM) is an aggressive form of brain cancer that has no effective treatments and a prognosis of only 12-15 months. Microfluidic technologies deliver microscale control of fluids and cells, and have aided cancer therapy as point-of-care devices for the diagnosis of breast and prostate cancers. However, a few microfluidic devices are developed to study malignant glioma. The ability of these platforms to accurately replicate the complex microenvironmental and extracellular conditions prevailing in the brain and facilitate the measurement of biological phenomena with high resolution and in a high-throughput manner could prove useful for studying glioma progression. These attributes, coupled with their relatively simple fabrication process, make them attractive for use as point-of-care diagnostic devices for detection and treatment of GBM. Here, the current issues that plague GBM research and treatment, as well as the current state of the art in glioma detection and therapy, are reviewed. Finally, opportunities are identified for implementing microfluidic technologies into research and diagnostics to facilitate the rapid detection and better therapeutic targeting of GBM.

Coalescence Processes of Droplets and Liquid Marbles

The coalescence process of droplets and, more recently, of liquid marbles, has become one of the most essential manipulation schemes in digital microfluidics. This process is indispensable for realising microfluidic functions such as mixing and reactions at microscale. This paper reviews previous studies on droplet coalescence, paying particular attention to the coalescence of liquid marbles. Four coalescence systems have been reviewed, namely, the coalescence of two droplets freely suspended in a fluid; the coalescence of two sessile droplets on a solid substrate; the coalescence of a falling droplet and a sessile droplet on a solid substrate; and liquid marble coalescence. The review is presented according to the dynamic behaviors, physical mechanisms and experimental parameters of the coalescence process. It also provides a systematic overview of how the coalescence process of droplets and liquid marbles could be induced and manipulated using external energy. In addition, the practical applications of liquid marble coalescence as a novel microreactor are highlighted. Finally, future perspectives on the investigation of the coalescence process of liquid marbles are proposed. This review aims to facilitate better understanding of the coalescence of droplets and of liquid marbles as well as to shed new insight on future studies.

Generation of droplet arrays with rational number spacing patterns driven by a periodic energy landscape

The generation of droplets at low Reynolds numbers is driven by nonlinear dynamics that give rise to complex patterns concerning both the droplet-to-droplet spacing and the individual droplet sizes. Here we demonstrate an experimental system in which a time-varying energy landscape provides a periodic magnetic force that generates an array of droplets from an immiscible mixture of ferrofluid and silicone oil. The resulting droplet patterns are periodic, owing to the nature of the magnetic force, yet the droplet spacing and size can vary greatly by tuning a single bias pressure applied on the ferrofluid phase; for a given cycle period of the magnetic force, droplets can be generated either at integer multiples (1, 2, ...), or at rational fractions (3/2, 5/3, 5/2, ...) of this period with mono- or multidisperse droplet sizes. We develop a discrete-time dynamical systems model not only to reproduce the phenotypes of the observed patterns but also to provide a framework for understanding systems driven by such periodic energy landscapes.

Model-Based Classification for Digital PCR: Your Umbrella for Rain

Standard data analysis pipelines for digital PCR estimate the concentration of a target nucleic acid by digitizing the end-point fluorescence of the parallel micro-PCR reactions, using an automated hard threshold. While it is known that misclassification has a major impact on the concentration estimate and substantially reduces accuracy, the uncertainty of this classification is typically ignored. We introduce a model-based clustering method to estimate the probability that the target is present (absent) in a partition conditional on its observed fluorescence and the distributional shape in no-template control samples. This methodology acknowledges the inherent uncertainty of the classification and provides a natural measure of precision, both at individual partition level and at the level of the global concentration. We illustrate our method on genetically modified organism, inhibition, dynamic range, and mutation detection experiments. We show that our method provides concentration estimates of similar accuracy or better than the current standard, along with a more realistic measure of precision. The individual partition probabilities and diagnostic density plots further allow for some quality control. An R implementation of our method, called Umbrella, is available, providing a more objective and automated data analysis procedure for absolute dPCR quantification.

A Noncontact Picolitor Droplet Handling by Photothermal Control of Interfacial Flow

We present a noncontact handling of droplets in a microfluidic platform by the Marangoni convection, interfacial tension driven flow, generated by a light-induced local temperature gradient in the surrounding liquid of the droplet. Droplets flowing in a microchannel experience a force due to the interfacial tension gradient when approaching the heated area. This method provides noncontact, selective and flexible manipulation for droplets flowing in microchannel network. In this study, an O/W emulsion system with oleic acid for the dispersed phase and a buffer solution for the continuous one was used. Trajectory control and trapping for droplets with 5 - 65 pL in volume was achieved by patterned laser irradiation. Also, we quantitatively evaluated the driving force exerted on droplets by measuring the fluidic temperature distribution around the droplet. From the balance of the drag force and the photo-induced Marangoni force, the driving force was determined using the measured temperature gradient of the droplet. From the results, the applicability of noncontact droplet manipulation using the photothermal Marangoni effect by continuous-phase heating has been demonstrated.

Reconfigurable liquid-core/liquid-cladding optical waveguides with dielectrophoresis-driven virtual microchannels on an electromicrofluidic platform

An electrically reconfigurable liquid-core/liquid-cladding (L(2)) optical waveguide with core liquid γ-butyrolactone (GBL, ncore = 1.4341, εcore = 39) and silicone oil (ncladding = 1.401, εcladding = 2.5) as cladding liquid is accomplished using dielectrophoresis (DEP) that attracts and deforms the core liquid with the greater permittivity to occupy the region of strong electric field provided by Teflon-coated ITO electrodes between parallel glass plates. Instead of continuously flowing core and cladding liquids along a physical microchannel, the DEP-formed L(2) optical waveguide guides light in a stationary virtual microchannel that requires liquids of limited volume without constant supply and creates stable liquid/liquid interfaces for efficient light guidance in a simply fabricated microfluidic device. We designed and examined (1) stationary and (2) moving L(2) optical waveguides on the parallel-plate electromicrofluidic platform. In the stationary L-shaped waveguide, light was guided in a GBL virtual microchannel core for a total of 27.85 mm via a 90° bend (radius 5 mm) before exiting from the light outlet of cross-sectional area 100 μm × 100 μm. For the stationary spiral waveguide, light was guided in a GBL core containing Rhodamine 6G (R6G, 1 mM) and through a series of 90° bends with decreasing radii from 5 mm to 2.5 mm. With the stationary straight waveguide, the propagation loss was measured to be 2.09 dB cm(-1) in GBL with R6G (0.01 mM). The moving L-shaped waveguide was implemented on a versatile electromicrofluidic platform on which electrowetting and DEP were employed to generate a precise GBL droplet and form a waveguide core. On sequentially applying appropriate voltage to one of three parallel L-shaped driving electrodes, the GBL waveguide core was shifted; the guided light was switched at a speed of up to 0.929 mm s(-1) (switching period 70 ms, switching rate 14.3 Hz) when an adequate electric signal (173.1 VRMS, 100 kHz) was applied.

Innovative Tools and Technology for Analysis of Single Cells and Cell-Cell Interaction

Heterogeneity in single-cell responses and intercellular interactions results from complex regulation of cell-intrinsic and environmental factors. Single-cell analysis allows not only detection of individual cellular characteristics but also correlation of genetic content with phenotypic traits in the same cell. Technological advances in micro- and nanofabrication have benefited single-cell analysis by allowing precise control of the localized microenvironment, cell manipulation, and sensitive detection capabilities. Additionally, microscale techniques permit rapid, high-throughput, multiparametric screening that has become essential for -omics research. This review highlights innovative applications of microscale platforms in genetic, proteomic, and metabolic detection in single cells; cell sorting strategies; and heterotypic cell-cell interaction. We discuss key design aspects of single-cell localization and isolation in microfluidic systems, dynamic and endpoint analyses, and approaches that integrate highly multiplexed detection of various intracellular species.

Online monitoring and analysis for autonomous continuous flow self-optimizing reactor systems

In this review the recent progress in the field of self-optimizing reactor systems for continuous flow chemistry is presented.

Moving droplets between closed and open microfluidic systems

In electric-field-mediated droplet microfluidics, there are two distinct architectures - closed systems using parallel-plate electrodes and open systems using coplanar electrodes fabricated on an open substrate. An architecture combining both closed and open systems on a chip would facilitate many of the chemical and biological processes now envisioned for the laboratory on a chip. To accomplish such an integration requires a means to move droplets back and forth between the two. This paper presents an investigation of the requirements for such manipulation of both water and oil droplets. The required wetting conditions for a droplet to cross the open/closed boundary is revealed by a force balance analysis and predictions of this model are compared to experimental results. Water droplets can be moved between closed and open systems by electrowetting actuation; droplet detachment from the upper plate is facilitated by the use of beveled edge. The force model predicts that driving an oil droplet from a closed to an open structure requires an oleophobic surface. This prediction has been tested and confirmed using <100> silicon wafers made oleophobic by re-entrant microstructures etched into the surface.

LCAT pump optimization for an integrated microfluidic droplet generator

We demonstrate an on-chip integrated droplet generator enabled by lateral cavity acoustic transducer (LCAT) oil and water microfluidic pumps. Both oil-in-water (O/W) and water-in-oil (W/O) droplet generation are demonstrated. The LCAT pumps are energized by piezoelectric acoustic energy to induce rectified microstreaming for pumping liquid. In this work, the analysis of geometric optimization of LCAT pumps was performed. The LCAT droplet generator was characterized in terms of size and frequency of generated droplets. For the W/O droplet generation, the controllable range of droplet diameter was 50-420 μm; while for the O/W droplet generation, the controllable range of droplet diameter is 60-150 μm. The minimum voltage for stable droplet generation can be as low as 4 V_pp. The first LCAT pump for pumping oil is also demonstrated by lipophilic treatment of the microfluidic channel. The integrated LCAT droplet generator offers a valveless, portable, low-cost, and low-power platform for generating microfluidic droplets. The LCAT droplet generator can be a key enabling microfluidic component towards the realization of a portable diagnostic/screening platform.

Selective fusion of anchored droplets via changes in surfactant concentration

We present a robust method to fuse in parallel an array of anchored droplets in a microchannel. Pairs of anchored droplets are fused by the removal of surfactant from the droplet interface by reducing the surfactant content in the flowing external oil phase. By controlling the flow of multiple oil inlets, the selective fusion of rows of droplets in a larger array is demonstrated. The technique is compatible with cells as shown with a trypan blue exclusion vitality assay. The method is easy to implement, requires no active components and is applicable to oil/water combinations where the surfactant is soluble in the external phase.

Industrial lab-on-a-chip: design, applications and scale-up for drug discovery and delivery

Microfluidics is an emerging and promising interdisciplinary technology which offers powerful platforms for precise production of novel functional materials (e.g., emulsion droplets, microcapsules, and nanoparticles as drug delivery vehicles- and drug molecules) as well as high-throughput analyses (e.g., bioassays, detection, and diagnostics). In particular, multiphase microfluidics is a rapidly growing technology and has beneficial applications in various fields including biomedicals, chemicals, and foods. In this review, we first describe the fundamentals and latest developments in multiphase microfluidics for producing biocompatible materials that are precisely controlled in size, shape, internal morphology and composition. We next describe some microfluidic applications that synthesize drug molecules, handle biological substances and biological units, and imitate biological organs. We also highlight and discuss design, applications and scale up of droplet- and flow-based microfluidic devices used for drug discovery and delivery.

Characterization of the geometry of negative dielectrophoresis traps for particle immobilization in digital microfluidic platforms

This paper studies the effect of dielectrophoresis on particle manipulation and immobilization in digital microfluidic (DMF) devices. The dimensions of negative dielectrophoresis (nDEP) traps in the form of circular and square shapes are characterized using numerical and experimental approaches. These efforts will result in defining lifting and trapping zones, the ratio of which is shown to remain constant for trap sizes larger than 40 μm. As a result, a limiting constant K based on the ratio of the particle diameter to the trap size is introduced to identify the status of particle trapping prior to running numerical models or experiments. The results show that K must be less than 0.63 for trapping the particles on the nDEP traps. This study will also result in optimizing the trap size for single particle immobilization which is important for cell printing and growth applications.

Phase-field-based lattice Boltzmann finite-difference model for simulating thermocapillary flows

A phase-field-based hybrid model that combines the lattice Boltzmann method with the finite difference method is proposed for simulating immiscible thermocapillary flows with variable fluid-property ratios. Using a phase field methodology, an interfacial force formula is analytically derived to model the interfacial tension force and the Marangoni stress. We present an improved lattice Boltzmann equation (LBE) method to capture the interface between different phases and solve the pressure and velocity fields, which can recover the correct Cahn-Hilliard equation (CHE) and Navier-Stokes equations. The LBE method allows not only use of variable mobility in the CHE, but also simulation of multiphase flows with high density ratio because a stable discretization scheme is used for calculating the derivative terms in forcing terms. An additional convection-diffusion equation is solved by the finite difference method for spatial discretization and the Runge-Kutta method for time marching to obtain the temperature field, which is coupled to the interfacial tension through an equation of state. The model is first validated against analytical solutions for the thermocapillary driven convection in two superimposed fluids at negligibly small Reynolds and Marangoni numbers. It is then used to simulate thermocapillary migration of a three-dimensional deformable droplet and bubble at various Marangoni numbers and density ratios, and satisfactory agreement is obtained between numerical results and theoretical predictions.

Microfluidic chemical analysis systems

The field of microfluidics has exploded in the past decade, particularly in the area of chemical and biochemical analysis systems. Borrowing technology from the solid-state electronics industry and the production of microprocessor chips, researchers working with glass, silicon, and polymer substrates have fabricated macroscale laboratory components in miniaturized formats. These devices pump nanoliter volumes of liquid through micrometer-scale channels and perform complex chemical reactions and separations. The detection of reaction products is typically done fluorescently with off-chip optical components, and the analysis time from start to finish can be significantly shorter than that of conventional techniques. In this review we describe these microfluidic analysis systems, from the original continuous flow systems relying on electroosmotic pumping for liquid motion to the large diversity of microarray chips currently in use to the newer droplet-based devices and segmented flow systems. Although not currently widespread, microfluidic systems have the potential to become ubiquitous.

Temperature-induced droplet coalescence in microchannels

This paper reports a technique for temperature-induced merging of droplets in a microchannel. The multiphase system consists of water droplet and oil as the dispersed phase and the carrying continuous phase. A resistive heater provides heating in a rectangular merging chamber. The temperature of the chamber is controlled by the voltage applied to the heater. The merging process of two neighboring droplets was investigated with different applied voltage, flow rate ratio between water and oil and total flowrate. Merging is found to be effective at high flow rate ratio, high temperature, and low total flowrate. The presented technique could be used for merging and mixing in droplet-based lab-on-a-chip platforms.

Applications of electrowetting-based digital microfluidics in clinical diagnostics

Digital microfluidics based on electrowetting is a type of microfluidic platform in which liquids are processed as individual unit-sized droplets that are dispensed from a source, merged together, split apart or transported between locations on demand. These devices are implemented using arrays of surface electrodes to control the shape and position of droplets through the electrowetting effect. A major thrust of digital microfluidics research has been the development of integrated lab-on-a-chip devices to perform clinical in vitro diagnostic assays. A variety of preparatory and analytical processes have been implemented and feasibility has been demonstrated for test types ranging from clinical chemistries to immunoassays, nucleic acid tests and cell-based assays. In this article, the current state and future potential of digital microfluidics for clinical diagnostic testing is reviewed and evaluated.

Micro-droplet detection and characterization using thermal responses

We suggest a novel method to detect droplets and determine the protein content of droplets in microfluidic system using the 3ω method, which is a powerful tool to easily detect thermal response changes with a simple device. By measuring the thermal response of droplets and a carrying flow in real time, water droplets in an oleic acid carrying flow can be detected, and the concentration of bovine serum albumin in droplets can be estimated. This method is expected to increase the practicality and power of droplet-based microfluidic systems.

Fast on-demand droplet fusion using transient cavitation bubbles

A method for on-demand droplet fusion in a microfluidic channel is presented using the flow created from a single explosively expanding cavitation bubble. We test the technique for water-in-oil droplets, which are produced using a T-junction design in a microfluidic chip. The cavitation bubble is created with a pulsed laser beam focused into one droplet. High-speed photography of the dynamics reveals that the droplet fusion can be induced within a few tens of microseconds and is caused by the rapid thinning of the continuous phase film separating the droplets. The cavitation bubble collapses and re-condenses into the droplet. Droplet fusion is demonstrated for static and moving droplets, and for droplets of equal and unequal sizes. Furthermore, we reveal the diffusion dominated mixing flow and the transport of a single encapsulated cell into a fused droplet. This laser-based droplet fusion technique may find applications in micro-droplet based chemical synthesis and bioassays.

Droplet microfluidics for the study of artificial cells

In this review, we describe recent advances in droplet-based microfluidics technology that can be applied in studies of artificial cells. Artificial cells are simplified models of living cells and provide valuable model platforms designed to reveal the functions of biological systems. The study of artificial cells is promoted by microfluidics technologies, which provide control over tiny volumes of solutions during quantitative chemical experiments and other manipulations. Here, we focus on current and future trends in droplet microfluidics and their applications in studies of artificial cells.

Droplets formation and merging in two-phase flow microfluidics

Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.

Novel combination of hydrophilic/hydrophobic surface for large wettability difference and its application to liquid manipulation

This paper reports a novel combination of hydrophilic/hydrophobic materials for the evolution of liquid manipulation. Droplet generation based on a hydrophilic/hydrophobic mechanism is a promising method for highly accurate liquid manipulations. Although several droplet manipulation devices utilizing hydrophilic/hydrophobic patterns have been reported, it has been difficult to split fluid into droplets solely through hydrophilic/hydrophobic patterns in a microchannel. In this study, a material combination for fabricating hydrophilic/hydrophobic patterns was investigated and their wettability difference was enhanced for droplet generation. To improve hydrophilicity, we attempted to increase the surface area of silicon oxide through pulsed plasma chemical vapor deposition (PPCVD). To improve hydrophobicity, the damage to the hydrophobic patterns in the fabrication process was reduced. We successfully enhanced the difference in contact angles from 54.3° to 86.6° by combining the developed hydrophilic material and hydrophobic material. The developed material combination could successfully split fluid into a quantitative droplet of 14.1 nL in a microfluidic chip. Because the developed hydrophilic/hydrophobic combination enables the formation of a droplet with desirable shape in microchannels, the developed hydrophilic/hydrophobic combination is a promising component for lab-on-a-chip applications.

Single-sided continuous optoelectrowetting (SCOEW) for droplet manipulation with light patterns

Electrowetting-on-dielectric (EWOD) promises to be an important lab-on-a-chip approach for effectively manipulating droplets with electric field-controlled surface tension. Droplets manipulated in electrowetting-based devices are typically sandwiched between two parallel plates and actuated by digital electrodes. The size of pixilated electrodes limits the minimum droplet size that can be manipulated. Here, we report on a single-sided continuous optoelectrowetting (SCOEW) mechanism that enables light-patterned electrowetting modulation for continuous droplet manipulation on an open, featureless, and photoconductive surface. SCOEW overcomes the size limitation of physical pixilated electrodes by utilizing dynamic and reconfigurable optical patterns and enables the continuous transport, splitting, merging, and mixing of droplets with volumes ranging from 50 microL to 250 pL, over 5-orders of magnitude. This single-sided open configuration provides a flexible interface for integration with other microfluidic components, such as sample reservoirs through simple tubing. Light-triggered, parallel, and volume-tunable droplet injection with volume variation less than 1% has been demonstrated with SCOEW. The unique lateral field-driven optoelectrowetting mechanism also enables extremely low light intensity actuation, and droplet manipulation can be achieved by directly positioning the SCOEW chip on a LCD screen used in a laptop or portable cellular phone.