Three-electrode analytical and preparative electrochemistry in micro-volume hanging droplets

Three-electrode micro-cells equipped with a conventional reference electrode (SCE) were easily constructed based on micro-volume droplets suspended by capillary forces to the fritted glass of the SCE bridge. Working and counter electrodes were simply inserted through the droplet surface, allowing classical electrochemistry to be readily performed in minute samples.     

电喷雾离子源(ESI)中带电液滴的形成与碎裂模拟

基于电喷雾离子源(ESI)中液流的输运行为,构建了相应的物理模型,并利用Fluent软件对电喷雾离子源中带电液滴的形成与裂变过程进行模拟研究.分别考察了毛细管电压、离子源温度和溶液表面张力3个参数对源内液滴粒径分布的影响.模拟结果表明,较大的毛细管电压、较高的离子源温度和较低的表面张力条件下得到的液滴粒径较小,液滴碎裂效果较好.模拟结果与文献报道及经验公式结果一致.     

Hydrodynamic control of droplet division in bifurcating microchannel and its application to particle synthesis

We describe herein a microfluidic system for active and precise control of droplet division at a bifurcation point in a microchannel. Water-in-oil or oil-in-water droplets, which were initially formed at a T-junction, were introduced into the bifurcation point, and then divided into two daughter droplets. By continuously introducing 'tuning flow' into the downstream of one of the branch channels, and by controlling the flow rates distributed into the two branch channels, the sizes of the daughter droplets could be precisely tuned. The ratio of the volumetric flow rates into the branch channels was estimated by regarding the microchannel network as a resistive circuit. In addition, we performed synthesis of monodispersed polymer particles with controlled sizes utilizing the presented system. The ability to hydrodynamically control the droplet sizes will open new possibilities not only for producing useful emulsions, but also for conducting controlled chemical and biochemical reactions in a confined space.     

Capillary bridges in electric fields

We analyzed the morphology of droplets of conductive liquids placed between two parallel plate electrodes as a function of the two control parameters electrode separation and applied voltage. Both electrodes were covered by thin insulating layers, as in conventional electrowetting experiments. Depending on the values of the control parameters, three different states of the system were found: stationary capillary bridges, stationary separated droplets, and periodic self-excited oscillations between both morphologies, which appear only above a certain threshold voltage. In the two stationary states, the morphology of the liquid is modified by the electric fields due to electrowetting and due to mutual electrostatic attraction, respectively. We determined a complete phase diagram within the two-dimensional phase space given by the control parameters. We discuss a model based on the interfacial and electrostatic contributions to the free energy. Numerical solutions of the model are in quantitative agreement with the phase boundaries found in the experiments. The dynamics in the oscillatory state are governed by electric charge relaxation and by contact angle hysteresis.     

Controlled generation of droplets using an electric field in a flow-focusing paper-based device

Droplet-based microfluidics is a modular platform in high-throughput single-cell and small sample analyses. However, this droplet microfluidic system was widely fabricated using soft lithography or glass capillaries, which is expensive and technically demanding for various applications, limiting use in resource-poor settings. Besides, the variation in droplet size is also restricted due to the limitations on the operating forces that the paper-based platform is able to withstand. Herein, we develop a fully integrated paper-based droplet microfluidic platform for conducting droplet generation and cell encapsulation in independent aqueous droplets dispersed in a carrier oil by incorporating electric fields. Through imposing an electric field, the droplet size would decrease with increasing the electric field and smaller droplets can be produced at high applied voltage. The droplet diameter can be adjusted by the ratio of inner and outer flow velocities as well as the applied electric field. We also demonstrated the proof of concept encapsulation application of our paper device by encapsulating yeast cells under an electric field. Using a simple wax printing method, carbon electrodes can be integrated on the paper. The integrated paper-based microfluidic platform can be fabricated easily and conducted outside of centralized laboratories. This microfluidic system shows great potential in drug and cell investigations by encapsulating cells in resource-limited environments.     

Equilibrium shapes of liquid bridges under gravity: Symmetry breaking and imperfect bifurcations of two-dimensional bridges

The equilibrium shapes of a two-dimensional liquid bridge are constructed via a shooting and continuation numerical solution of the Laplace—Young equation which focuses on the bifurcation points of the solution branches. For the neutrally buoyant case, two new asymmetric shapes bifurcate from the point of maximum excess pressure where circular profiles with reflective symmetry have been shown to be unstable with respect to constant pressure disturbances. This occurs when the profile is exactly at right angles to the support. With the introduction of gravitational effects, this codimension 5 singularity is retained although the critical contact angle decreases slightly below 90° as one increases the difference between the interior and the exterior densities. However, when a slight tilt angle is introduced to the bridge, the maximum pressure singularity degenerates into two turning points (folds) and the solution branches are isolated from each other. This indicates that hysteretic jumps in the surface curvature and excess pressure exist with respect to changes in diameter/length ratio or liquid volume. Our numerical solution also reveals the existence of a maximum bridge volume that can be sustained by a nonneutrally buoyant bridge. No equilibrium shapes, symmetric or asymmetric, exist for bridge volumes beyond this critical value.     

Droplet size effects on film drainage between droplet and substrate

When a droplet approaches a solid surface, the thin liquid film between the droplet and the surface drains until an instability forms and then ruptures. In this study, we utilize microfluidics to investigate the effects of film thickness on the time to film rupture for water droplets in a flowing continuous phase of silicone oil deposited on solid poly(dimethylsiloxane) (PDMS) surfaces. The water droplets ranged in size from millimeters to micrometers, resulting in estimated values of the film thickness at rupture ranging from 600 nm down to 6 nm. The Stefan-Reynolds equation is used to model film drainage beneath both millimeter- and micrometer-scale droplets. For millimeter-scale droplets, the experimental and analytical film rupture times agree well, whereas large differences are observed for micrometer-scale droplets. We speculate that the differences in the micrometer-scale data result from the increases in the local thin film viscosity due to confinement-induced molecular structure changes in the silicone oil. A modified Stefan-Reynolds equation is used to account for the increased thin film viscosity of the micrometer-scale droplet drainage case.     

A feedback control system for high-fidelity digital microfluidics

Digital microfluidics (DMF) is a technique in which discrete droplets are manipulated by applying electrical fields to an array of electrodes. In an ideal DMF system, each application of driving potential would cause a targeted droplet to move onto an energized electrode (i.e., perfect fidelity between driving voltage and actuation); however, in real systems, droplets are sometimes observed to resist movement onto particular electrodes. Here, we implement a sensing and feedback control system in which all droplet movements are monitored, such that when a movement failure is observed, additional driving voltages can be applied until the droplet completes the desired operation. The new system was evaluated for a series of liquids including water, methanol, and cell culture medium containing fetal bovine serum, and feedback control was observed to result in dramatic improvements in droplet actuation fidelity and velocity. The utility of the new system was validated by implementing an enzyme kinetics assay with continuous mixing. The new platform for digital microfluidics is simple and inexpensive and thus should be useful for scientists and engineers who are developing automated analysis platforms.     

Cross-scale electric manipulations of cells and droplets by frequency-modulated dielectrophoresis and electrowetting

Two important electric forces, dielectrophoresis (DEP) and electrowetting-on-dielectric (EWOD), are demonstrated by dielectric-coated electrodes on a single chip to manipulate objects on different scales, which results in a dielectrophoretic concentrator in an EWOD-actuated droplet. By applying appropriate electric signals with different frequencies on identical electrodes, EWOD and DEP can be selectively generated on the proposed chip. At low frequencies, the applied voltage is consumed mostly in the dielectric layer and causes EWOD to pump liquid droplets on the millimetre scale. However, high frequency signals establish electric fields in the liquid and generate DEP forces to actuate cells or particles on the micrometre scale inside the droplet. For better performance of EWOD and DEP, square and strip electrodes are designed, respectively. Mammalian cells (Neuro-2a) and polystyrene beads are successfully actuated by a 2 MHz signal in a droplet by positive DEP and negative DEP, respectively. Droplet splitting is achieved by EWOD with a 1 kHz signal after moving cells or beads to one side of the droplet. Cell concentration, measured by a cell count chamber before and after experiments, increases 1.6 times from 8.6 x 10(5) cells ml(-1) to 1.4 x 10(6) cells ml(-1) with a single cycle of positive DEP attraction. By comparing the cutoff frequency of the voltage drop in the dielectric layer and the cross-over frequency of Re(fCM) of the suspended particles, we can estimate the frequency-modulated behaviors between EWOD, positive DEP, and negative DEP. A proposed weighted Re(fCM) facilitates analysis of the DEP phenomenon on dielectric-coated electrodes.     

Pumpless dispensing of a droplet by breaking up a liquid bridge formed by electric induction

Dispensing uniform pico‐to‐nanoliter droplets has become one of essential components in various application fields from high‐throughput bio‐analysis to printing. In this study, a new method is suggested and demonstrated for dispensing a droplet on the top plate with an inverted geometry by using electric field. The process of dispensing droplets consists of two stages: (i) formation of liquid bridge by moving up the charged fluid mass using the electrostatic force between the charges on the fluid mass and the induced charges on the substrate and (ii) its break‐up by the motion of the top plate. Different from conventional electrohydrodynamic methods, electric induction enables the droplets to be dispensed on various surfaces including non‐conducting substrate. The use of capillarity with an inverted geometry removes the need of external pumps or elaborates control for constant flow feed. The droplet diameter has been characterized as a function of the nozzle‐to‐plate distance and the plate moving velocity. The robustness of the present method is shown in terms of nozzle length and applied voltage. Finally, its practical applicability is confirmed by rendering a 19 by 24 array of highly uniform droplets with only 1.8% size variation without use of any active feedback control.     

Functional bionetworks from nanoliter water droplets

We form networks from aqueous droplets by submerging them in an oil/lipid mixture. When the droplets are joined together, the lipid monolayers surrounding them combine at the interface to form a robust lipid bilayer. Various protein channels and pores can incorporate into the droplet-interface bilayer (DIB), and the application of a potential with electrodes embedded within the droplets allows ionic currents to be driven across the interface and measured. By joining droplets in linear or branched geometries, functional bionetworks can be created. Although the interfaces between neighboring droplets comprise only single lipid bilayers, the structures of the networks are long-lived and robust. Indeed, a single droplet can be "surgically" excised from a network and replaced with a new droplet without rupturing adjacent DIBs. Networks of droplets can be powered with internal "biobatteries" that use ion gradients or the light-driven proton pump bacteriorhodopsin. Besides their interest as coupled protocells, the droplets can be used as devices for ultrastable bilayer recording with greatly reduced electrolyte volume, which will permit their use in rapid screening applications.     

Delayed coalescence behavior of droplets with completely miscible liquids

Because of capillary forces, sessile droplets usually fuse instantaneously after contact. We find however a delay of the droplet fusion by many seconds if the droplets consist of different but completely miscible liquids. After the initial contact, the main bodies of the droplets remain separated, connected only through a shallow conduit with a flow from the low to the high surface tension liquid. Sporadically, this connecting film can thicken with turbulent or pulsating flows. The droplets will finally fuse when the flow has sufficiently reduced the difference in composition and surface tension. We present calculations which explain this delayed droplet fusion with the compensation of the fusion-promoting capillary pressure by a droplet-separating dynamic pressure caused by the flow between the droplets. Droplets with high contact angles fuse instantaneously. In this case, no separation-stabilizing dynamic pressure can build up because the interdroplet flow becomes turbulent.     

High-throughput sorting of nanoliter droplets enabled by a sequentially addressable dielectrophoretic array

Droplet microfluidics has emerged as a powerful tool for a diverse range of biomedical and industrial applications such as single-cell analysis, directed evolution, and metabolic engineering. In these applications, droplet sorting has been effective for isolating small droplets encapsulating molecules, cells, or crystals of interest. Recently, there is an increased interest in extending the applicability of droplet sorting to larger droplets to utilize their size advantage. However, sorting throughputs of large droplets have been limited, hampering their wide adoption. Here, we report our demonstration of high-throughput fluorescence-activated droplet sorting of 1 nL droplets using an upgraded version of the sequentially addressable dielectrophoretic array (SADA), which we reported previously. The SADA is an array of electrodes that are individually and sequentially activated/deactivated according to the speed and position of a droplet passing nearby the array. We upgraded the SADA by increasing the number of driving electrodes constituting the SADA and incorporating a slanted microchannel. By using a ten-electrode SADA with the slanted microchannel, we achieved fluorescence-activated droplet sorting of 1 nL droplets at a record high throughput of 1752 droplets/s, twice as high as the previously reported maximum sorting throughput of 1 nL droplets.     

Voltage control of droplet interface bilayer lipid membrane dimensions

A novel approach to control the area of anchor-free droplet interface bilayer (DIB) lipid membranes is presented. Unsupported DIB lipid membranes are formed at the interface of phospholipid-coated aqueous droplets dispensed in dodecane oil. Using electrodes inserted into the droplets, an external voltage is applied which modulates the effective DIB area. Electrical (capacitance or current) and optical (imaging of DIB lateral length) recordings were simultaneously performed. Alpha-hemolysin (αHL) single channel insertions into the DIB were recorded. Currents across the DIB were measured as a function of voltage and αHL concentration in the droplets. Nonlinear response is observed for current, DIB lateral length and area, and capacitance with respect to voltage. Voltage induced changes in interfacial tension modulated the DIB-oil contact angle and the membrane contact length, which provided control of membrane dimensions. Comparison of these results is made to the electrowetting effect, which is also governed by effect of voltage on the interfacial tension. This approach provides active control of the number of ion channels inserted into the DIB.     

Rupture work of pendular bridges

Capillary bridging can generate substantial forces between solid surfaces. Impacted technologies and sciences include micro- and nanomachining, disk drive interfaces, scanning probe microscopy, biology, and granular mechanics. Existing calculations of the rupture work of capillary bridges do not consider the thermodynamics relating to the evaporation that can occur in the case of volatile liquids. Here, we show that the occurrence of evaporation decreases the rupture work by a factor of about 2. The decrease arises from heat taken from the surroundings that is converted into work. The treatment is based on a thermodynamic control-volume analysis of the pendular bridge geometry. We extend the mathematical formulation of Orr et al., solving the meniscus problem exactly for non-wetting surfaces. The extension provides analytical results for conditions at the rupture point and at a possible inflection point and for the rupture work. A simple equation (eq 32) is shown to fit the rupture work for the two cases over a meniscus curvature range of 3 orders of magnitude. Coefficients for the equation are given in tabular form for different contact angle pairs.     

A microfluidic device for self-synchronised production of droplets

The primary requirement for a mixing operation in droplet-based microfluidic devices is an accurate pairing of droplets of reaction fluids over an extended period of time. In this paper, a novel device for self-synchronous production of droplets has been demonstrated. The device uses a change in impedance across a pair of electrodes introduced due to the passage of a pre-formed droplet to generate a second droplet at a second pair of electrodes. The device was characterised using image analysis. Droplets with a volume of ~23.5 ± 3.1 nl (i.e.~93% of the volume of pre-formed droplets) were produced on applying a voltage of 500 V. The synchronisation efficiency of the device was 83%. As the device enables self-synchronised production of droplets, it has a potential to increase the reliability and robustness of mixing operations in droplet-based microfluidic devices.     

Fat retention at the tongue and the role of saliva: adhesion and spreading of 'protein-poor' versus 'protein-rich' emulsions

Fat perception of food emulsions has been found to relate to in-mouth friction. Previously, we have shown that friction under mouth-like conditions strongly depends on the sensitivity of protein-stabilized emulsion droplets to coalescence. Here, we investigated whether this also implies that oral fat retention depends in a similar manner on the stability of the emulsion droplets against coalescence. We investigate the separate contributions of droplet adhesion and droplet spreading to fat retention at the tongue, as well as the role of saliva. We perform ex vivo (Confocal Raman Spectroscopy; Confocal Scanning Laser Microscopy) experiments using pig's tongue surfaces in combination with human in vivo experiments. These reveal that protein-poor (unstable) emulsions are retained more at the tongue than protein-rich (stable) emulsions. Furthermore, the layer formed by adhering protein-poor droplets is more stable against rinsing. Saliva is found to be very efficient in removing fat and emulsion droplets from the oral surface but its role in fat retention needs further research. We relate our results to the colloidal forces governing droplet adhesion and spreading.     

Deformation and breakup of micro- and nanoparticle stabilized droplets in microfluidic extensional flows

Using a microfluidic flow-focusing device, monodisperse water droplets in oil were generated and their interface populated by either 1 μm or 500 nm amine modified silica particles suspended in the water phase. The deformation and breakup of these Pickering droplets were studied in both pure extensional flow and combined extensional and shear flow at various capillary numbers using a microfluidic hyperbolic contraction. The shear resulted from droplet confinement and increased with droplet size and position along the hyperbolic contraction. Droplet deformation was found to increase with increasing confinement and capillary number. At low confinements and low capillary numbers, the droplet deformation followed the predictions of theory. For fully confined droplets, where the interface was populated by 1 μm silica particles, the droplet deformation increased precipitously and two tails were observed to form at the rear of the droplet. These tails were similar to those seen for surfactant covered droplets. At a critical capillary number, daughter droplets were observed to stream from these tails. Due to the elasticity of the particle-laden interface, these drops did not return to a spherical shape, but were observed to buckle. Although increases in droplet deformation were observed, no tail streaming occurred for the 500 nm silica particle covered droplets over the range of capillary numbers studied.     

Asymmetric electrowetting--moving droplets by a square wave

Here droplet oscillation and continuous pumping are demonstrated by asymmetric electrowetting on an open surface with embedded electrodes powered by a square wave electrical signal without control circuits. The polarity effect of electrowetting on an SU-8 and Teflon coated electrode is investigated, and it is found that the theta-V (contact angle-applied voltage) curve is asymmetric along the V = 0 axis by sessile drop and coplanar electrode experiments. A systematic deviation of measured contact angles from the theoretical ones is observed when the electrode beneath the droplet is negatively biased. In the sessile drop experiment, up to a 10 degrees increment of contact angle is measured on a negatively biased electrode. In addition, a coplanar electrode experiment is designed to examine the contact angles at the same applied potential but opposite polarities on two sides of one droplet at the same time. The design of the coplanar electrodes is then expanded to oscillate and transport droplets on square-wave-powered symmetric (square) and asymmetric (polygon) electrodes to demonstrate manipulation capability on an open surface. The frequency of oscillation and the speed of transportation are determined by the frequency of the applied square wave and the pitch of the electrodes. Droplets with different volumes are tested by square waves of varied frequencies and amplitudes. The 1.0 microl droplet is successfully transported on a device with a loop of 24 electrodes continuously at a speed up to 23.6 mm s(-1) when a 9 Hz square wave is applied.     

Rupture kinetics of liquid bridges during a pulling process: a kinetic density functional theory study

Capillary bridge is a common phenomenon in nature and can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. Especially, it plays a crucial role in the operation of atomic force microscopy (AFM) and influences in the measured force. In the present work, we study the rupture kinetics and transition pathways of liquid bridges connecting an AFM tip and a flat substrate during a process of pulling the tip off. Depending on thermodynamic conditions and the tip velocity, two regimes corresponding to different transition pathways are identified. In the single-bridge regime, the initial equilibrium bridge persists as a single one during the pulling process until the liquid bridge breaks. While, in the multibridge regime the stretched liquid bridge transforms into an intermediate state with a collection of slender liquid bridges, which then break gradually during the pulling process. Moreover, the critical rupture distance at which the bridges break changes with the tip velocity and thermodynamic conditions, and its maximum value occurs near the boundary between the single-bridge regime and the multibridge regime, where the longest range capillary force is produced. In this work, the effects of tip velocity, tip size, tip-fluid interaction, and humidity on rupture kinetics and transition pathways are also systematically studied.