Surface-induced droplet fusion in microfluidic devices

Here we demonstrate a new method for droplet fusion based on a surface energy pattern on the walls of a microfluidic device, that does not require active elements nor accurate synchronization of the droplets.     

Pillar-induced droplet merging in microfluidic circuits

A novel method is presented for controllably merging aqueous microdroplets within segmented flow microfluidic devices. Our approach involves exploiting the difference in hydrodynamic resistance of the continuous phase and the surface tension of the discrete phase through the use of passive structures contained within a microfluidic channel. Rows of pillars separated by distances smaller than the representative droplet dimension are installed within the fluidic network and define passive merging elements or chambers. Initial experiments demonstrate that such a merging element can controllably adjust the distance between adjacent droplets. In a typical scenario, a droplet will enter the chamber, slow down and stop. It will wait and then merge with the succeeding droplets until the surface tension is overwhelmed by the hydraulic pressure. We show that such a merging process is independent of the inter-droplet separation but rather dependent on the droplet size. Moreover, the number of droplets that can be merged at any time is also dependent on the mass flow rate and volume ratio between the droplets and the merging chamber. Finally, we note that the merging of droplet interfaces occurs within both compressing and the decompressing regimes.     

Electrowetting-based droplet mixers for microfluidic systems

Mixing of analytes and reagents is a critical step in realizing a lab-on-a-chip. However, mixing of liquids is very difficult in continuous flow microfluidics due to laminar flow conditions. An alternative mixing strategy is presented based on the discretization of liquids into droplets and further manipulation of those droplets by electrowetting. The interfacial tensions of the droplets are controlled with the application of voltage. The droplets act as virtual mixing chambers, and mixing occurs by transporting the droplet across an electrode array. We also present an improved method for visualization of mixing where the top and side views of mixing are simultaneously observed. Microliters of liquid droplets are mixed in less than five seconds, which is an order of magnitude improvement in reported mixing times of droplets. Flow reversibility hinders the process of mixing during linear droplet motion. This mixing process is not physically confined and can be dynamically reconfigured to any location on the chip to improve the throughput of the lab-on-a-chip.     

A switchable digital microfluidic droplet dye-laser

Digital microfluidic devices allow the manipulation of droplets between two parallel electrodes. These electrodes can act as mirrors generating a micro-cavity, which can be exploited for a droplet dye-laser. Three representative laser-dyes with emission wavelengths spanning the whole visible spectrum are chosen to show the applicability of this concept. Sub-microlitre droplets of laser-dye solution are moved in and out of a lasing site on-chip to down-convert the UV-excitation light into blue, green and red laser-pulses.     

Selective droplet coalescence using microfluidic systems

We report a microfluidic approach, which allows selective and controlled 1 : 1, 2 : 1 or 3 : 1 droplet fusion. A surfactant-stabilized droplet with an interfacial surfactant coverage, Γ, of >98% will fuse spontaneously with a second droplet when Γ of the latter droplet is <16%. However, when Γ of the second droplet is ~66%, the two droplets will not fuse, unless they have previously been brought into contact for critical time τ. Therefore, controlling the number of droplets in contact for time τ allows precise control over the number of fused droplets. We have demonstrated efficient (proportion of droplets coalesced p(c) = 1.0, n > 1000) and selective 1 : 1, 2 : 1 or 3 : 1 droplet fusion (proportion of correctly fused droplets p(s) > 0.99, n > 1000). Coalescence in this regime is induced by hydrodynamic flow causing interface separation and is efficient at different Ca numbers and using different dispersed phases, continuous phases and surfactants. However, when Γ of the second droplet is ~96% coalescence is no longer observed. Droplet-based microfluidic systems, in which each droplet functions as an independent microreactor, are proving a promising tool for a wide range of ultrahigh-throughput applications in biology and chemistry. The addition of new reagents to pre-formed droplets is critical to many of these applications and we believe the system described here is a simple and flexible method to do so, as well as a new tool to study interfacial stability phenomena.     

Rapid droplet mixers for digital microfluidic systems

The mixing of analytes and reagents for a biological or chemical lab-on-a-chip is an important, yet difficult, microfluidic operation. As volumes approach the sub-nanoliter regime, the mixing of liquids is hindered by laminar flow conditions. An electrowetting-based linear-array droplet mixer has previously been reported. However, fixed geometric parameters and the presence of flow reversibility have prevented even faster droplet mixing times. In this paper, we study the effects of varying droplet aspect ratios (height:diameter) on linear-array droplet mixers, and propose mixing strategies applicable for both high and low aspect ratio systems. An optimal aspect ratio for four electrode linear-array mixing was found to be 0.4, with a mixing time of 4.6 seconds. Mixing times were further reduced at this ratio to less than three seconds using a two-dimensional array mixer, which eliminates the effects of flow reversibility. For lower aspect ratio (相似文献   

Modeling of droplet traffic in interconnected microfluidic ladder devices

The problem of controlling the droplet motion in multiphase flows on the microscale has gained increasing attention because the droplet-based microfluidic devices provide great potentials for chemical and biological applications. It is critical to understand the relevant physics on droplet hydrodynamics and thus control the generation, motion, splitting, and coalescence of droplets in complex microfluidic networks. Numerical simulations using the volume of fluid algorithm are conducted to investigate the time-dependent dynamics of droplets in gas-liquid multiphase devices. An analytical model based on the electronic-hydraulic analogy is developed to describe the hydrodynamic behavior of the droplets in interconnected microfluidic ladder devices. It is found that the pressure drop caused by the droplets plays a critical role in the droplet synchronization. A fitted formula for pressure drops in the presence of surfactant is achieved by using numerical simulations. Both the numerical and the theoretical results agree well with the corresponding experimental results.     

Capacitance-based droplet position estimator for digital microfluidic devices

Digital microfluidic (DMF) devices manipulate minuscule droplets through basic fluidic operations including droplet transport, mixing and splitting commonly known as the building blocks for complete laboratory analyses on a single device. A DMF device can house various chemical species and confine chemical reactions within the volume of a droplet much like a micro-reactor. The automation of fluidic protocols requires a feedback controller whose sensor is capable of locating droplets independent of liquid composition (or previous knowledge of liquid composition). In this research, we present an estimator that tracks the continuous displacement of a droplet between electrodes of a DMF device. The estimator uses a dimensionless ratio of two electrode capacitances to approximate the position of a droplet, even, in the domain between two adjacent electrodes. This droplet position estimator significantly enhances the control precision of liquid handling in DMF devices compared to that of the techniques reported in the literature. It captures the continuous displacement of a droplet; valuable information for a feedback controller to execute intricate fluidic protocols including droplet positioning between electrodes, droplet velocity and acceleration control. We propose a state estimator for tracking the continuous droplet displacement between two adjacent electrodes. The dimensionless nature of this estimator means that any droplet composition can be sensed. Thus, no calibration for each chemical species within a single DMF device is required. We present theoretical and experimental results that demonstrate the efficacy of the position estimator in approximating the position of the droplet in the interval between two electrodes.     

Sequential microfluidic droplet processing for rapid DNA extraction

This work describes a novel droplet-based microfluidic device, which enables sequential droplet processing for rapid DNA extraction. The microdevice consists of a droplet generation unit, two reagent addition units and three droplet splitting units. The loading/washing/elution steps required for DNA extraction were carried out by sequential microfluidic droplet processing. The movement of superparamagnetic beads, which were used as extraction supports, was controlled with magnetic field. The microdevice could generate about 100 droplets per min, and it took about 1 min for each droplet to perform the whole extraction process. The extraction efficiency was measured to be 46% for λ-DNA, and the extracted DNA could be used in subsequent genetic analysis such as PCR, demonstrating the potential of the device for fast DNA extraction.     

Alternating droplet generation and controlled dynamic droplet fusion in microfluidic device for CdS nanoparticle synthesis

A multifunctional and high-efficiency microfluidic device for droplet generation and fusion is presented. Through unique design of the micro-channels, the device is able to alternately generate droplets, generating droplet ratios ranging from 1 ratio 5 to 5 ratio 1, and fuse droplets, enabling precise chemical reactions in several picoliters on a single chip. The controlled fusion is managed by passive control based on the channel geometry and liquid phase flow. The synthesis of CdS nanoparticles utilizing each fused droplet as a microreactor for rapid and efficient mixing of reagents is demonstrated in this paper. Following alternating droplet generation, the channel geometry allows the exclusive fusion of alternate droplets with concomitant rapid mixing and produces supersaturated solution of Cd2+ and S2- ions to form CdS nanoparticles in each fused droplet. The spectroscopic properties of the CdS nanoparticles produced by this method are compared with CdS prepared by bulk mixing.     

Raman spectroscopic monitoring of droplet polymerization in a microfluidic device

Microfluidic methodologies are becoming increasingly important for rapid formulation and screening of materials, and development of analytical tools for multiple sample screening is a critical step in achieving a combinatorial 'lab on a chip' approach. This work demonstrates the application of Raman spectroscopy for analysis of monomer composition and degree of conversion of methacrylate-based droplets in a microfluidic device. Droplet formation was conducted by flow focusing on the devices, and a gradient of component composition was created by varying the flow rates of the droplet-phase fluids into the microchannels. Raman data were collected using a fiber optic probe from a stationary array of the droplets/particles on the device, followed by partial least squares (PLS) calibration of the first derivative (1600 cm(-1) to 1550 cm(-1)) allowing successful measurement of monomer composition with a standard error of calibration (SEC) of +/-1.95% by volume. Following photopolymerization, the percentage of double bond conversion of the individual particles was calculated from the depletion of the normalized intensity of the C[double bond, length as m-dash]C stretching vibration at 1605 cm(-1). Raman data allowed accurate measurement of the decrease in double bond conversion as a function of increasing crosslinker concentration. The results from the research demonstrate that Raman spectroscopy is an effective, on-chip analytical tool for screening polymeric materials on the micrometre scale.     

Effects of viscosity on droplet formation and mixing in microfluidic channels

This paper characterizes the conditions required to form nanoliter-sized droplets (plugs) of viscous aqueous reagents in flows of immiscible carrier fluid within microfluidic channels. For both non-viscous (viscosity of 2.0 mPa s) and viscous (viscosity of 18 mPa s) aqueous solutions, plugs formed reliably in a flow of water-immiscible carrier fluid for Capillary number less than 0.01, although plugs were able to form at higher Capillary numbers at lower ratios of the aqueous phase flow rate to the flow rate of the carrier fluid (in all the experiments performed, the Reynolds number was less than 1). The paper also shows that combining viscous and non-viscous reagents can enhance mixing in droplets moving through straight microchannels by providing a nearly ideal initial distribution of reagents within each droplet. The study should facilitate the use of this droplet-based microfluidic platform for investigation of protein crystallization, kinetics, and assays.     

On-demand microfluidic droplet manipulation using hydrophobic ferrofluid as a continuous-phase

Multiple essential microdroplet operation units, including splitting, dispensing, oil-phase exchange, trapping, release and demulsification, were successfully implemented by combining hydrophobic ferrofluid with microfluidic chips.     

Engineering superlyophobic surfaces as the microfluidic platform for droplet manipulation

We propose robust engineering superlyophobic surfaces (SLS) as a universal microfluidic platform for droplet manipulation enabling electric actuation, featured with characteristics of highly nonwetting, low adhesion, and low friction for various liquids including water and oil. To functionalize SLS with embedded electrodes, two configurations with continuous and discrete topologies have been designed and compared. The discrete configuration is found to be superior upon comparison of their fabrication, microstructures and nonwetting performances. We also present new formulation of SLS pressure stability for linear, square and hexagonal pattern layouts, and propose a criterion for three wetting states (the Cassie-Baxter, partial Cassie-Baxter and Wenzel states) by introducing two dimensionless parameters, which are supported by our experimental data. Droplet manipulation experiments including deformation and transport on electrode-embedded SLS were performed, showing that present SLS reduce adhesion and flow resistance of oil droplets respectively by 98% and 73% compared with a smooth hydrophobic surface, and the excellent hydrodynamic performances are applicable for a wide range of droplet velocity. Simulation of an oil droplet electrically actuated on SLS predicts the significantly increased droplet motion for a low solid fraction and a relatively large droplet size.     

Picoliter droplet microfluidic immunosorbent platform for point-of-care diagnostics of tetanus

We have developed a sensitive, specific, rapid and low cost picoliter microsphere-based platform for bioanalyte detection and quantification. In this method, a biological sample, biosensing microspheres, and fluorescently labeled detection (secondary) antibodies are co-encapsulated to capture the analyte (here: human anti-tetanus immunoglobulin G) on the surface of the microsphere in microfluidic pL-sized droplets. The absorption of the analyte and detecting antibodies on the microsphere concentrate the fluorescent signal in correlation with analyte concentration. Using our platform and commercially available antibodies, we were able to quantify anti-tetanus antibodies in human serum. In comparison to standard bulk immunosorbent assays, the microfluidic droplet platform presented here reduces the reagent volume by four orders of magnitude, while fast reagent mixing reduces the detection time from hours to minutes. We consider this platform to be a major leap forward in the miniaturization of immunosorbent assays and to provide a rapid and low cost tool for global point-of-care.

The dynamic effects of surfactants on droplet formation in coaxial microfluidic devices

Droplet emulsification in microfluidic devices involves the constant formation of fresh interfaces between two immiscible fluids. When the multiphase system contains surfactant, dynamic mass transfer of the surfactant onto the interface results in a dynamic interfacial tension different from the static interfacial tension measured in an equilibrium state. In this work, we have systematically investigated the effects of surfactant concentration and type on the dynamic interfacial tension of two different liquid-liquid two phase systems [N-hexane/water-sodium dodecyl sulfate (SDS) and N-hexane/water-cetyltrimethylammonium bromide (CTAB)] rapidly producing relatively small droplets in coaxial microfluidic devices. Dynamic interfacial tension experiments using the pendent drop method and a tensiometer were conducted, and a semiempirical equation was developed to put into context the effects of surfactants and the experimental conditions on droplet formation and dynamic interfacial tension in dynamic microchannel flows. The results presented in this work provide a more in-depth understanding of the dynamic effects of surfactants on droplet formation and the precise controllable preparation of monodispersed droplets in microfluidic devices.     

Actuation of adaptive liquid microlens droplet in microfluidic devices: A review

A significant growth of research on adaptive liquid lens is achieved over the past decades, and the field is still attracting increasing attentions, focusing on the transition from the current stage to the commercialized stage. The challenges faced are not limited to fabrication, material, small tuning range in focal lengths, additional control systems, limitations in special actuation methods and so on. In addition, the use of external driving parts or systems induce extra problem on bulky appearance, high cost, low reliability etc. Therefore, adaptive liquid lens will be an interesting research focus in both microfluidics and optofluidics science. This review attempts to summarize and focus on the droplet profile deformation under different driving mechanisms in tunable liquid microlens as well as the application in cameras, cell phone and so on. The driving techniques are generally categorized in terms of mechanisms and driving sources.     

Picoliter cell lysate assays in microfluidic droplet compartments for directed enzyme evolution

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Highlights? Microfluidic droplet compartments miniaturize cell lysate screening assays ? Picoliter single-cell lysate assays comparable in sensitivity to macroscale ? Directed evolution results in improved clones validating this experimental set-up ? Precision of droplet sorting enables enrichment of clones with slight improvements     

On-demand preparation of quantum dot-encoded microparticles using a droplet microfluidic system

Optical barcoding technology based on quantum dot (QD)-encoded microparticles has attracted increasing attention in high-throughput multiplexed biological assays, which is realized by embedding different-sized QDs into polymeric matrixes at precisely controlled ratios. Considering the advantage of droplet-based microfluidics, producing monodisperse particles with precise control over the size, shape and composition, we present a proof-of-concept approach for on-demand preparation of QD-encoded microparticles based on this versatile new strategy. Combining a flow-focusing microchannel with a double T-junction in a microfluidic chip, biocompatible QD-doped microparticles were constructed by shearing sodium alginate solution into microdroplets and on-chip gelating these droplets into a hydrogel matrix to encapsulate CdSe/ZnS QDs. Size-controllable QD-doped hydrogel microparticles were produced under the optimum flow conditions, and their fluorescent properties were investigated. A novel multiplex optical encoding strategy was realized by loading different sized QDs into a single droplet (and thus a hydrogel microparticle) with different concentrations, which was triggered by tuning the flow rates of the sodium alginate solutions entrapped with different-colored QDs. A series of QD-encoded microparticles were controllably, and continuously, produced in a single step with the present approach. Their application in a model immunoassay demonstrated the potential practicability of QD-encoded hydrogel microparticles in multiplexed biomolecular detection. This simple and robust strategy should be further improved and practically used in making barcode microparticles with various polymer matrixes.