Concurrent droplet charging and sorting by electrostatic actuation

This paper presents a droplet-based microfluidic device for concurrent droplet charging and sorting by electrostatic actuation. Water-in-oil droplets can be charged on generation by synchronized electrostatic actuation. Then, simultaneously, the precharged droplets can be electrostatically steered into any designated laminar streamline, thus they can be sorted into one of multiple sorting channels one by one in a controlled fashion. In this paper, we studied the size dependence of the water droplets under various relative flow rates of water and oil. We demonstrated the concurrent charging and sorting of up to 600 droplets∕s by synchronized electrostatic actuation. Finally, we investigated optimized voltages for stable droplet charging and sorting. This is an essential enabling technology for fast, robust, and multiplexed sorting of microdroplets, and for the droplet-based microfluidic systems.     

Microfluidic generation of droplet interface bilayer networks incorporating real-time size sorting in linear and non-linear configurations

In this study, a novel droplet based microfluidic method for the generation of different sized droplet interface bilayers is reported. A microfluidic platform was designed, which allows the generation and packing of picoliter lipid coated water droplets. Droplets were generated by hydrodynamic focusing coupled with selective transport along grooves according to their size. A trapping structure at the end of the groove and a fine control of the flow pressures allowed for the droplets to be successfully trapped and aligned on demand. This technology facilitates the fine control of droplet size production as well as the generation of extended networks from a variety of lipids including 1,2-diphytanoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine in linear and non-linear configurations, which is vital to the application of Droplet Interface Bilayers to biological network construction on-chip.     

A novel actuation method of transporting droplets by using electrical charging of droplet in a dielectric fluid

We evaluate the feasibility of manipulating droplets in two dimensions by exploiting Coulombic forces acting on conductive droplets immersed in a dielectric fluid. When a droplet suspended in an immiscible fluid is located near an electrode under a dc voltage, the droplet can be charged by direct contact, by charge transfer along an electrically conducting path, or by both mechanisms. This process is called electrical charging of droplet (ECOD). This charged droplet may then be transported rapidly by exploiting Coulombic forces. We experimentally demonstrate electrical actuation of a charged droplet by applying voltage sequences. A charged droplet is two dimensionally actuated by following the direction of the electrical field signal. The droplet does not contact the surface of the microfluidic chip when it moves. This characteristic is very advantageous because treatments of the substrate surfaces of microfluidic chip become simpler. In order to test the feasibility of using ECOD in a droplet-based microreactor, electrocoalescence of two oppositely charged droplets is also studied. When two droplets approach each other due to Coulombic attraction, a liquid bridge is formed between them. We postulate that if the applied electric field is weaker than a certain critical level, the two droplets coalesce instantaneously when the charges are exchanged and redistributed through this liquid bridge.     

基于SOA的科技文献随需应变知识服务

分析了科技文献信息价值实现困境,提出只有应用随需应变理念才能一方面满足知识经济背景下日益多样的用户需求,另一方面促进科技文献的开发利用,实现其自身价值。同时,应用SOA系统架构建立随需应变微服务组配机制,利用其独立自治、松散耦合、潜在复用的特点支持随需应变知识服务的实现。并构建了基于SOA的科技文献随需应变知识服务模型。     

补贴政策下考虑充电设施建设水平的充电定价与策略研究

针对不同补贴政策下运营商的决策与不同充电设施建设水平下政策实施的策略问题,文章构建基准模式、投资补贴模式、运营补贴模式和复合补贴模式等四种政策情境,分析比较不同政策情境下运营商定价及补贴策略,结果表明：①充电价格随着运营补贴的强度提升而下降,电动汽车的市场渗透率提升,充电运营商收益增加;②政策性因素、市场性因素、成本性因素和时间性因素对价格、需求和收益具有直接和交叉影响。③当充电设施建设水平小于0.5时,运营补贴更能刺激新能源汽车的扩散。当充电设施建设水平大于0.5时,可适时退出补贴政策。     

Microfluidic sorting of microtissues

Increasingly, invitro culture of adherent cell types utilizes three-dimensional (3D) scaffolds or aggregate culture strategies to mimic tissue-like, microenvironmental conditions. In parallel, new flow cytometry-based technologies are emerging to accurately analyze the composition and function of these microtissues (i.e., large particles) in a non-invasive and high-throughput way. Lacking, however, is an accessible platform that can be used to effectively sort or purify large particles based on analysis parameters. Here we describe a microfluidic-based, electromechanical approach to sort large particles. Specifically, sheath-less asymmetric curving channels were employed to separate and hydrodynamically focus particles to be analyzed and subsequently sorted. This design was developed and characterized based on wall shear stress, tortuosity of the flow path, vorticity of the fluid in the channel, sorting efficiency and enrichment ratio. The large particle sorting device was capable of purifying fluorescently labelled embryoid bodies (EBs) from unlabelled EBs with an efficiency of 87.3% ± 13.5%, and enrichment ratio of 12.2 ± 8.4 (n = 8), while preserving cell viability, differentiation potential, and long-term function.     

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.     

Robotic automation of droplet microfluidics

Droplet microfluidics enables powerful analytic capabilities but often requires workflows involving macro- and microfluidic processing steps that are cumbersome to perform manually. Here, we demonstrate the automation of droplet microfluidics with commercial fluid-handling robotics. The workflows incorporate common microfluidic devices including droplet generators, mergers, and sorters and utilize the robot''s native capabilities for thermal control, incubation, and plate scanning. The ability to automate microfluidic devices using commercial fluid handling will speed up the integration of these methods into biological workflows.     

无线自组网按需平面距离矢量路由协议研究

文章介绍了AdHoc网络中按需生成路由方式的典型协议——AODV路由协议,它是应用于无线网状网络(也称做无线mesh网络)中进行路由选择的路由协议,它能够实现单播和多播路由。本文从路由建立、路由维护方面介绍了该路由的组网过程。     

Emerging open-channel droplet arrays for biosensing

Open-channel droplet arrays have attracted much attention in the fields of biochemical analysis,biofluid monitoring,biomarker recognition and cell interactions,as they have advantages with regard to miniaturization,parallelization,high-throughput,simplicity and accessibility.Such droplet arrays not only improve the sensitivity and accuracy of a biosensor,but also do not require sophisticated equipment or tedious processes,showing great potential in next-generation miniaturized sensing platforms....     

Label-free density difference amplification-based cell sorting

The selective cell separation is a critical step in fundamental life sciences, translational medicine, biotechnology, and energy harvesting. Conventional cell separation methods are fluorescent activated cell sorting and magnetic-activated cell sorting based on fluorescent probes and magnetic particles on cell surfaces. Label-free cell separation methods such as Raman-activated cell sorting, electro-physiologically activated cell sorting, dielectric-activated cell sorting, or inertial microfluidic cell sorting are, however, limited when separating cells of the same kind or cells with similar sizes and dielectric properties, as well as similar electrophysiological phenotypes. Here we report a label-free density difference amplification-based cell sorting (dDACS) without using any external optical, magnetic, electrical forces, or fluidic activations. The conceptual microfluidic design consists of an inlet, hydraulic jump cavity, and multiple outlets. Incoming particles experience gravity, buoyancy, and drag forces in the separation chamber. The height and distance that each particle can reach in the chamber are different and depend on its density, thus allowing for the separation of particles into multiple outlets. The separation behavior of the particles, based on the ratio of the channel heights of the inlet and chamber and Reynolds number has been systematically studied. Numerical simulation reveals that the difference between the heights of only lighter particles with densities close to that of water increases with increasing the ratio of the channel heights, while decreasing Reynolds number can amplify the difference in the heights between the particles considered irrespective of their densities.Separating specific cells from heterogeneous or homogeneous mixtures has been considered as a key step in a wide variety of applications ranging from biomedicine to energy harvesting. For example, the separation and sorting of rare circulating tumor cells (CTCs) from whole blood has gained significant importance in the potential diagnosis and treatment of metastatic cancers.^1,2 Similarly, malaria detection relies on the collection of infected red blood cells (RBCs) from whole blood.^3,4 In addition, the selective separation of lipid-rich microalgae from homogeneous mixtures of microalgae is a promising technique in biomass conversion.⁵To date, conventional cell separation can be done by labelling cells with biomolecules to induce differences in physical properties. For instance, in a fluorescence-activated cell sorter (FACS), cells to be separated are labelled with antibodies or aptamers with fluorescent molecules, and then sorted by applying an electrical potential.^6,7 Similarly, magnetic-activated cell sorter (MACS) uses magnetic.^8,9 Alternatively, label-free cell separation methods have exploited inherent differences in the physical properties (e.g., size and dielectric properties) of different kinds of cells. For example, acoustophoresis forces particles larger than a desired size to move into the center of a fluidic channel by using ultrasonic standing waves.^10–12 Inertial microfluidics takes advantage of curved fluidic channels in order to amplify the size differences between particles.^13,14 Mass-dependent separation of particles based on gravity and hydrodynamic flow was also reported.¹⁵ Particles with different dielectric properties can also be sorted by dielectrophoresis which induces the movement of polarizable particles.^16–18The disadvantage of these methods, however, is that they require external forces and labels that may cause unexpected damage to biological cells.^19–21 More importantly, most methods are limited in separating cells of the same kind or cells with similar sizes and dielectric properties.Here we designed a novel, label-free density difference amplification-based cell sorting (dDACS) that allows the separation of particles with the same size and charge by exploiting subtle differences in density without the use of external forces. Figure 1(a) illustrates the proposed microfluidic model and its underlying mechanism. The conceptual microfluidic system consists of an inlet, a separation chamber (hydraulic jump cavity), and multiple outlets. Particles entering through the inlet experience gravity (F_G), buoyancy (F_B), and drag (F_D) forces in the separation chamber. The net force acting on the particles can be described as F = F_G + F_B + F_D.(1)As particles enter the separation chamber (i.e., hydraulic jump cavity), F_D acting on the particles changes its direction along the streamline. The particles experience additional forces in the y direction due to large tangential angle (Fig. 1(b)). For lighter particles, whose densities are close to that of the surrounding water, F_D becomes comparable to F_G (i.e., in the y direction), while the net force for heavier particles is less affected by this additional contribution of F_D due to a large F_G. As a result, the height (H) and distance (D) that each particle can travel are different depending on its density. The difference in the maximum height (ΔH_max) between two particles with different density (ρ_p1 and ρ_p2) can be further approximated as ΔHmax∝(vyp0)2(vyf−vyp0), (ρp1−ρp2) ,(2)where v_yp0 and v_yf represent the velocity of particle and fluid along the y direction at the entrance of hydraulic jump cavity, respectively.Open in a separate windowFIG. 1.Schematic illustration of label-free density difference amplification-based cell sorting (dDACS), which exploits differences in the densities (ρ₁ > ρ₂) of particles with similar diameters (d) and charge. (a) The conceptual microfluidic design consists of an inlet, a separation chamber (hydraulic jump cavity), and multiple outlets. Incoming particles experience gravity (F_G), buoyancy (F_B), and drag (F_D) forces in the separation chamber, and depending on their densities, the height (H) and distance (D) that each particle is able to reach will be different, allowing the particles to be separated into multiple outlets. (b) Possible microfluidic channel configurations for density-based separation: Uniform channel height (left), gradual channel expansion (middle), and hydraulic jump cavity with sudden channel expansion (right). The height difference between particles with different densities can be amplified by the sudden channel expansion compared to the other two cases due to the relatively large tangential angle, θ of F_D. (|θ₁|≪ |θ₂|) (see Fig. S1 in the supplementary material²²).In comparison with the other two cases (Fig. 1(b) uniform channel height and gradual channel expansion), the height difference between the particles with different densities can be amplified by the sudden channel expansion in the hydraulic jump cavity due to relatively large tangential angle (see supplementary material²²). Therefore, the particles can be separated through the multiple outlets, depending on their height and distance.In order to analyze the separation behavior of particles in the chamber according to differences in their densities, H and D are systematically investigated. The numerical simulations are performed using a commercial CFD software (CFX 14.0; ANSYS 14.0; ANSYS, Inc.). Particles with the same density may have different trajectories in the separation chamber depending on their inlet positions (Fig. 2(a)). Prior to this investigation, the maximum height (H_max) and distance (D_max) for each particle are compared by examining H and D of 100 identical particles at different inlet positions since the inlet position of particles could be controlled.²⁰ Fig. 2(b) shows H_max and D_max of particles with respect to density at a fixed Reynolds number (Re = 0.1). Note that Reynolds number is defined as Re = ρ_fv_fD_h/μ, where ρ_f, v_f, D_h, μ are density of fluid, velocity of the fluid, hydraulic diameter of a channel, and dynamic viscosity of the fluid, respectively. The hydraulic diameter in the Reynolds number is determined with the inlet channel. Particle densities in the range of 1.1 to 2.0 g/cm³ are chosen with the increase of 0.1 g/cm³. These values are quite reasonable in that the densities of many microorganisms such as microalgae are typically within this range and their densities can be varied by 0.2 g/m³ depending on their cellular context.²³ The lighter particles travel with a higher H_max, and longer D_max. With the separation chamber, the height difference between particles with densities of 1.1 and 1.2 g/cm³ can be amplified by about 10 times as compared to that in a channel without the chamber, judging from the position where the 1.1 g/cm³ particle reaches its H_max.Open in a separate windowFIG. 2.Microfluidic particle separation with respect to Reynolds number (Re). (a) Trajectories in the separation chamber of a hundred particles with the same density starting from inlet positions chosen arbitrarily in order to investigate the effect of the inlet positions on the maxima of the height (H_max) and distance (D_max) prior to further simulation. (b) Representative trajectories of particles having different densities from 1.1 to 2.0 g/cm³. (c) The maximum height (H_max) of each particle with respect to Re. (d) Representative maximum distance (D_max) of each particle at Re = 0.1. (Left) Streamline of fluid and representative trajectories of particles with densities of 1.1 and 2.0 g/cm³ in the separation chamber at Re = 0.1 (right).In Fig. 2(c), the values for H_max of particles with respect to Reynolds number (Re) are presented. Since in our study, the maximum height (H_max) and distance (D_max) for each particle were compared by examining H and D of 100 identical particles that are randomly distributed in the channel (throughout all figures), there is little variation in H_max and D_max between each simulation. However, the standard deviation between each simulation is quite small and can be negligible. The H_max values particles at Re = 0.5 with densities of 1.1 g/cm³ and 1.2 g/cm³ are 2.21 × 10³μm and 2.17 × 10³μm, respectively. The difference between H_max of different particles, ΔH_max, increases with decreasing Re. For example, ΔH_max between particles with densities of 1.1 and 2.0 g/cm³ becomes 0.26 × 10³μm at Re = 1.0, but increases to 1.38 × 10³μm as Re decreases to 0.1. As Re increases (velocity of fluid increases), the relative velocity in the y direction between the fluid and the particle increases resulting in increasing of F_D in the y direction since the velocity of particle in the y direction is very small at the entrance of the separation chamber. Thus, contribution of F_D becomes comparable to the net force in the y direction. As a result, most of the particles even in the case of heavier ones travel quite similarly with the streamline, and ΔH_max subsequently decreases. On the other hand, as Re decreases, the contribution of F_G becomes dominant due to the decrease of F_D in the y direction. Consequently, the particles start to cross downwards streamlines as the density of the particles increases and H_max gradually decreases. In addition, irrespective of their densities, ΔH_max of the particles increases with decreasing Re.Fig. 2(d) shows D_max with respect to the density of the particles (left). Different densities of particles show different trajectories due to the relative contribution of F_D to the net force in the y direction depending on the particle density (right). At Re = 0.1, D_max of particles with densities of 1.1 cm³ and 1.2 g/cm³ are 2.91 × 10⁴μm and 1.43 × 10⁴μm, respectively. As the density of a particle increases, its D_max dramatically decreases. The difference in D_max between particles with densities of 1.1 and 1.2 g/cm³ is 1.48 × 10⁴μm, and 0.0037 × 10⁴μm for particles with densities of 1.9 and 2.0 g/cm³. The effect of F_D is stronger compared to that of F_G on lighter particles. Thus, lighter particles travel quite similarly with the streamline and finally have a large D_max. On the other hand, heavier particles where effect of F_G is stronger compared to that of F_D cross downwards streamlines and finally have a small D_max.Next, in order to investigate the separation behavior of particles with respect to the geometry of the microfluidic device, the effect of the ratio of the height of the separation chamber (h_c) to the inlet (h_i) on H_max is investigated as shown in Fig. ​Fig.3.3. Interestingly, H_max of particles with density of 1.1 g/cm³ increases from 1.93 × 10³μm to 6.48 × 10³μm while that of particles with density of 1.9 g/cm³ slightly changes from 0.70 × 10³μm to 0.73 × 10³μm as h_c/h_i increases from 5 to 20.Open in a separate windowFIG. 3.Microfluidic particle separation with respect to the ratio of the height of the inlet (h_i) to the separation chamber (h_c).This result can be attributed to two effects: (1) the change in the streamline and (2) the relative contribution of drag force to the net force depending on the density. With increasing h_c/h_i, dramatic increase in H_max for lighter particles is because the streamline for the lighter ones experiences more vertical displacement in the separation chamber and the contribution of F_D to the net force acting on the lighter one is more significant (see Fig. S2 in the supplementary material²²).Based on this approach, we propose a microfluidic device for the selective separation of the lightest particle. Fig. 4(a) shows one unit (with three outlets) of the proposed microfluidic device that can be connected in series. The ratio of channel heights (h_c/h_i) is set to 20, and the particle densities are in the range of 1.1 ∼ 1.5 g/m³. Fig. 4(b) shows the representative separation behavior of the particles. A portion of the lightest particles (1.1 g/cm³) is selectively separated into the upper and middle outlets, while remaining light particles together with four other heavier particles with densities in the range of 1.2 to 1.5 g/cm³ leave through the lowest outlet. With a single operation of this unit, 40% of the lightest particles are recovered. In addition, the yield increases with increasing number of cycles (Fig. 4(c)).Open in a separate windowFIG. 4.(a) One unit of the proposed microfluidic device for the selective separation of the lightest particle based on the simulation results. Particles are separated into two outlets based on differences in both the height and distance travelled stemming from differences in density. (b) Representative separation behavior of particles observed in the device. (c) The yield of the lightest particle (1.1 g/cm³) with the proposed microfluidic device according to the number of cycles (i.e., this unit is assumed to be connected in series).In summary, we have demonstrated a label-free microfluidic system for the separation of particles according to subtle differences in their densities without external forces. Our microfluidic design consists simply of an inlet, a separation chamber, and multiple outlets. When entering the separation chamber, the particles experience an additional drag force in the y direction, amplifying the difference in both the height and the distance that the particles with different densities can travel within the chamber. At a fixed Reynolds number, with increasing particle density, H_max decreases monotonously, and D_max decreases dramatically. On the other hand, as Reynolds number increases, the difference between the heights of particles with different densities is attenuated. In addition, the simulation reveals that increasing the ratio of the channel heights increases the difference between the heights of particles only when their densities are close to that of the surrounding water. Based on this approach, a microfluidic device for the separation of the lightest particles has been proposed. We expect that our density-based separation design can be beneficial to the selective separation of specific microorganisms such as lipid-rich microalgae for energy harvesting application.     

Shape-tunable wax microparticle synthesis via microfluidics and droplet impact

Spherical and non-spherical wax microparticles are generated by employing a facile two-step droplet microfluidic process which consists of the formation of molten wax microdroplets in a flow-focusing microchannel and their subsequent off-chip crystallization and deformation via microdroplet impingement on an immiscible liquid interface. Key parameters on the formation of molten wax microdroplets in a microfluidic channel are the viscosity of the molten wax and the interfacial tension between the dispersed and continuous fluids. A cursory phase diagram of wax morphology transition is depicted depending on the Capillary number and the Stefan number during the impact process. A combination of numerical simulation and analytical modeling is carried out to understand the physics underlying the deformation and crystallization process of the molten wax. The deformation of wax microdroplets is dominated by the viscous and thermal effects rather than the gravitational and buoyancy effects. Non-isothermal crystallization kinetics of the wax illustrates the time dependent thermal effects on the droplet deformation and crystallization. The work presented here will benefit those interested in the design and production criteria of soft non-spherical particles (i.e., alginate gels, wax, and polymer particles) with the aid of time and temperature mediated solidification and off-chip crosslinking.     

Controlling droplet incubation using close-packed plug flow

Controlling droplet incubation is critical for droplet-based microfluidic applications; however, current techniques are either of limited precision or place strict limits on the incubation times that can be achieved. Here, we present a simple technique to control incubation time by exploiting close-packed plug flow. In contrast to other techniques, this technique is applicable to very short and very long incubation times.     

ANSYS在二维静电场分析中的应用研究

利用有限元分析软件ANSYS对形状不规则的二维静电场电场进行求解,其研究结果可以用于分析求解电容器的电场分布,电力电容器分为串联电容器和并联电容器,它们都能改善电力系统的电压质量和提高输电线路的输电能力,是电力系统的重要设备。     

On the theory of electrostatic generators

A summary is given of the general quantitative theory of electrostatic generators in the final, simplified form, as progressively developed by the author in a number of previous papers. The method is illustrated by detailed application to (1) a four carrier, bisymmetric replenisher, (2) a six carrier, trisymmetric replenisher, (3) a four carrier, four symmetric, electrostatic alternator, and (4) a four carrier, bisymmetric, constant potential electrostatic generator. Various artifices for simplifying and shortening the labor of mathematical solution are given.     

Versatile on-demand droplet generation for controlled encapsulation

We present a droplet-based microfluidic system for performing bioassays requiring controlled analyte encapsulation by employing highly flexible on-demand droplet generation. On-demand droplet generation and encapsulation are achieved pneumatically using a microdispensing pump connected to a constant pressure source. The system generates single droplets to the collection route only when the pump is actuated with a designated pressure level and produces two-phase parallel flow to the waste route during the stand-by state. We analyzed the effect of actuation pressure on the stability and size of droplets and optimized conditions for generation of stable droplets over a wide pressure range. By increasing the duration of pump actuation, we could either trigger a short train of identical size droplets or generate a single larger droplet. We also investigated the methodology to control droplet contents by fine-tuning flow rates or implementing a resistance bridge between the pump and main channels. We demonstrated the integrated chip for on-demand mixing between two aqueous phases in droplets and on-demand encapsulation of Escherichia coli cells. Our unique on-demand feature for selective encapsulation is particularly appropriate for bioassays with extremely dilute samples, such as pathogens in a clinical sample, since it can significantly reduce the number of empty droplets that impede droplet collection and subsequent data analysis.     

Real-time detection, control, and sorting of microfluidic droplets

We report the design and implementation of capacitive detection and control of microfluidic droplets in microfluidic devices. Integrated microfluidic chip(s) with detection∕control circuit enables us to monitor in situ the individual volume of droplets, ranging from nanoliter to picoliter, velocity and even composition, with an operation frequency of several kilohertz. Through electronic feedback, we are able to easily count, sort, and direct the microfluidic droplets. Potential applications of this approach can be employed in the areas of biomicrofluidic processing, microchemical reactions as well as digital microfluidics.