Droplet generation in a microchannel with a controllable deformable wall

We report the droplet generation behavior of a microfluidic droplet generator with a controllable deformable membrane wall using experiments and analytical model. The confinement at the droplet generation junction is controlled by using external pressure, which acts on the membrane, to generate droplets smaller than junction size (with other parameters fixed) and stable and monodispersed droplets even at higher capillary numbers. A non-dimensional parameter, i.e., controlling parameter K _p, is used to represent the membrane deformation characteristics due to the external pressure. We investigate the effect of the controlled membrane deformation (in terms of K _p), viscosity ratio λ and flow rate ratio r on the droplet size and mobility. A correlation is developed to predict droplet size in the controllable deformable microchannel in terms of the controlling parameter K _p, viscosity ratio λ and flow rate ratio r. Due to the deflection of the membrane wall, we demonstrate that the transition from the stable dripping regime to the unstable jetting regime is delayed to a higher capillary number Ca (as compared to rigid droplet generators), thus pushing the high throughput limit. The droplet generator also enables generation of droplets of sizes smaller than the junction size by adjusting the controlling parameter.     

Generation of droplets with different concentrations using gradient-microfluidic droplet generator

This study successfully uses the micro-mixers and flow-focusing devices, which are integrated into a gradient-microfluidic droplet generator, to generate the different sizes of the droplets with different concentrations simultaneously and applies these microcapsules for drug release. The sizes of these four types of droplet with different concentrations are uniformity with a coefficient of variation less than 5% and can be precisely controlled by adjusting the water phase flow rate and oil phase flow rate. Moreover, Ca-alginate microcapsules with different concentrations of the bovine serum albumin are used for uniform size drug release, and the Ca-alginate microcapsule size is from 60 to 105 μm in diameter. This developed microfluidic chip has the advantages of actively controlling the droplet diameter, simultaneously generating uniform-sized droplets with different concentrations, and having a simple process and a high throughput. This preparation approach for Ca-alginate microcapsules of four different concentrations will provide many potential applications for drug delivery and pharmaceutical area.     

Detection of microdroplet size and speed using capacitive sensors

Detection of the presence, size and speed of microdroplets in microfluidic devices is presented using commercially available capacitive sensors which make the droplet based microfluidic systems scalable and inexpensive. Cross-contamination between the droplets is eliminated by introducing a passivation layer between the sensing electrodes and droplets. A simple T-junction generator is used to generate droplets in microchannels. Coplanar electrodes are used to form a capacitance through the microfluidic channel. The change in capacitance due to the presence of a droplet in the sensing area is detected and used to determine the size and speed of the droplet. The design of a single pair of electrodes is used to detect the presence of a droplet and the interdigital finger design is used to detect the size and speed of the droplet. An analytical model is developed to predict the detection signal and guide the experimental optimization of the sensor geometry. The measured droplet information is displayed through a Labview interface in real-time. The use of capacitance sensors to monitor droplet sorting at a T-junction is also presented. The discussions in this paper can be generalized to any droplet detection application and can serve as a guideline in sensor selection.     

Microdroplet generation in gaseous and liquid environments

As trends in biology, chemistry, medicine and manufacturing have pushed macroscopic processes onto the micro scale, droplet generation has been a key factor in allowing these methods to translate. For both surface-based liquid-in-gas generation and lab-on-a-chip-based liquid-in-liquid generation, the ability to create small monodisperse liquid droplets is critically important in constructing reliable and practical devices. This article reviews liquid microdroplet generation in gaseous and liquid environments, covering the general characteristics of generators and the specific methods and technologies used for generation. Furthermore, this study compiles the issues encountered when operating generators, and the measurements and instrumentation used to characterize generated droplets. Applications of droplet generation in printing, analysis, synthesis and manufacturing are also discussed.     

Input Shaping of the Synchronous Generator for Reduction of Self‐induced Oscillations

This article suggests the application of input shaping for the elimination of electromechanical oscillations arising from changes in the operating point of the synchronous generator. Poorly damped poles are the cause of electromechanical oscillations of the synchronous generator. Various power system stabilizers (PSS) are now used to reduce the electromechanical oscillations. Input shaping is a feedforward control and therefore does not require the measurement of electrical and mechanical quantities of the synchronous generator unlike the PSS. It is possible to apply the input shaping for the excitation system of synchronous generators with or without PSS.     

Valve-based microfluidic droplet micromixer and mercury (II) ion detection

A valve-based microfluidic micromixer was developed for multiply component droplets generation, manipulation and active mixing. By integrating pneumatic valves in microfluidic device, droplets could be individually generated, merged and well mixed automatically. Moreover, droplet volume could be controlled precisely by tuning loading pressure or the flow rate of the oil phase, and certain droplets fusion conditions were also investigated by adjusting the droplet driving times and oil flow rates. In these optimized conditions, fluorescence enhancement of droplets was used to detect Hg (II) ions in droplet by mixing with probe droplets (Rhodamine B quenched by gold nanoparticle). This method would have powerful potential for tiny volume sample assay or real-time chemical reaction study.     

压电发电器建模研究

压电发电器的优化设计必须以模型为基础。提出了一种新的压电发电器模型建立方法,运用该方法分别推导出2种结构的压电发电器的性能参数表达式,实验数据与表达式计算结果比较吻合,证明表达式可以用于压电发电器的性能预测。通过对输出参数表达式进行深入讨论,得出压电发电器的一般特性和相关参数与发电器性能参数之间的关系,可指导压电发电器的优化设计。     

Microchannel geometry design for rapid and uniform reagent distribution

Rapid and uniform reagent distribution is critical to the performance of a high-throughput microfluidic system, and its geometric design of the microchannels dominates the accuracy and distribution uniformity of the daughter droplets. This research’s purpose is to optimize the geometry of the T-junction to achieve a uniform distribution of two daughter droplets from a single liquid droplet. Models of gas–liquid flow were realized in the transient numerical simulations to investigate the geometry-dependent pressure distributions and the flowing velocities inside the droplet during the splitting process that leads to an improved design of the T-junction that can increase the stability of the droplet splitting process. To validate that increasing the stability of the splitting process can help improve the distribution uniformity of the daughter droplets, microfluidic devices were manufactured on poly(methyl methacrylate) substrates with micromilling and thermal bonding for experiments. In the multiple experiments, 2 μl of reagent was loaded into the microfluidic device and a uniform pneumatic pressure was applied to push the droplet into the T-junction for splitting. The experimental results, after statistical analysis, show that the improved T-junction can achieve better distribution uniformity of the daughter droplets with a higher reliability and a less reagent loss during the splitting process.     

Decoupling and stabilization of large-scale power systems by a geometric graph-theoretic method

Cross-couplings are present in all the generators belonging to interconnected power systems. As is well known, the presence of these couplings is one of the major sources of voltage variations and rotor oscillations in the generators belonging to the system. To solve this problem a geometric graph-theoretic approach is proposed by which a state feedback decoupling controller is derived. The proposed controller can also assure a desired location to all the critical eigenvalues of the decoupled closed-loop system, so that the improvement of the dynamic performance of each generator is guaranteed. The capability and usefulness of the proposed technique are illustrated by considering a 14-generator 34-busbar test system.     

Rotating flow within a droplet actuated with AC EWOD

Electrowetting-on-dielectric (EWOD) is now used in numerous microsystems like digital lab-on-chips. This paper deals with a characteristic hydrodynamic flow appearing in droplets actuated by EWOD with AC voltage. In the coplanar electrode configuration, two pairs of vortex flows are observed to form in a droplet centred on the electrode gap. All experiments are performed in oil as ambient phase and flows in the droplet are analysed using fluorescent beads. At the same time, droplet oscillations induced by AC EWOD are also revealed under stroboscopic lighting. These experiments show that vortex location can be controlled by frequency actuation with fair degree of reproducibility.     

基于共现流增强双向金字塔卷积网络的密集液滴识别

基于深度学习的数字聚合酶链式反应（PCR）液滴识别对PCR图像中的目标进行高阶语义建模,能够减少人工参与特征设计和筛选带来的误差,但忽略了目标的低层物理结构和几何外观细节信息,且在特征建模的过程中重复使用了大量冗余信息,对特征的表征能力有待改善。提出一种共现流增强双向金字塔卷积网络（CoF-BiPCN）框架用于PCR液滴识别和统计。为增强金字塔的内部和层间相关性,设计具有时空分支的双向金字塔卷积网络,从正反2个方向对金字塔卷积网络得到的多尺度特征进行聚合,模拟PCR图像中液滴的上下文语义以及不同层级的细节信息,以捕获液滴的物理外观等低层信息。同时,设计切片的共现注意力（SCo-AN）模块,将不同尺度的高低层信息在不同的切片子空间中进行共享聚合,并交叉传递到不同分支的BiPCN中,强化高低层特征信息的交互和依赖关系,进一步增强信息流对PCR图像上液滴的表征,实现低层和高阶信息流的共享与交叉聚合。实验结果表明,CoF-BiPCN具备良好的识别性能,准确率和平均精度均值分别达到84.74%和45.09%,与Cascade RCNN模型相比分别提高4.3和3.12个百分点。     

Driving and sorting of the fluorescent droplets on digital microfluidic platform

In this paper, we present a digital microfluidic droplet sorting platform to achieve automated droplet sorting based on fluorescent detection. We design and fabricate a kind of digital microfluidic chip for manipulating nano-liter-sized liquid droplets, and the chip is integrated with a fluorescence-initiated feedback system for real-time sorting control. The driving and sorting characteristics of fluorescent droplets encapsulating fluorescent-labeled particles are studied on this platform. The droplets dispensed from on-chip reservoir electrode are transported to a fluorescence detection site and sorted according to their fluorescence signals. The fluorescent droplets and non-fluorescent droplets are successfully separated and the number of fluorescent particles inside each droplet is quantified by its fluorescent intensity. We realize droplet sorting at 20 Hz and obtain a linear relationship between the fluorescent particle concentrations and the fluorescence signals. This work is easily adapted for sorting out fluorescent-labeled microparticles, cells and bacteria and thus has the potential of quantifying catalytic or regulatory bio-activities.     

Piezoelectric-actuated drop-on-demand droplet generator control using adaptive wavelet neural network controller

This paper presents the design, fabrication and control of a piezoelectric-type droplet generator which is applicable for on-line dispensing. Adaptive wavelet neural network (AWNN) control is applied to overcome nonlinear hysteresis inherited in the LPM. The adaptive learning rates are derived based on the Lyapunov stability theorem so that the stability of the closed-loop system can be assured. Unlike open-loop dispensing system, the system proposed can potentially generate droplets with high accuracy. Experimental verifications focusing on regulating control are performed firstly to assure the reliability of the proposed control schemes. Real dispensing is then conducted to validate the feasibility of the piezoelectric-actuated drop-on-demand droplet generator. In order to illustrate the effectiveness of the proposed method, experimental results obtained using the AWNN scheme are compared with their counterparts using traditional PID control. The results indicate that the proposed AWNN scheme not only outperforms PID control but also works well in developing the piezoelectric-actuated drop-on-demand dispensing system. The proposed dispensing system provides droplet chains with an averaged mass as small as 31.5 mg while the associated standard deviation is as low as 0.72%.     

Reliable addition of reagents into microfluidic droplets

This article reports a design that reliably adds reagents into droplets by exploiting the physics of fluid flow at a T-junction in the microchannel. An expanded section right after the T-junction enhances merging of a stream with a droplet, eliminates the drawbacks such as extra droplet formation and long mixing time. The expanded section reduces the pressure buildup at the T-junction and minimizes the tendency to form extra droplets; plays the role in creating low Laplace pressure jump across the interface of the droplet forming from the T-junction which reduces the probability of forming extra droplet in the merging process; provides space for droplet coalescence if there is an extra droplet due to droplet break-up before merging. In this design, after merging, the reactants are in axial arrangement inside the droplets which lead to faster mixing. Reliable addition of reagent to the droplets happens for the combination of flow rates in a broad range from 25 to 250 μl/h, for both DI water (Q _DI) and fluorescent (Q _fluo) streams.     

Generation of uniform-size droplets by multistep hydrodynamic droplet division in microfluidic circuits

A microfluidic system is presented to generate multiple daughter droplets from a mother droplet, by the multistep hydrodynamic division of the mother droplet at multiple branch points in a microchannel. A microchannel network designed based on the resistive circuit model enables us to control the distribution ratio of the flow rate, which dominates the division ratios of the mother droplets. We successfully generated up to 15 daughter droplets from a mother droplet with a variation in diameter of less than 2%. In addition, we examined factors affecting the division ratio, including the average fluid velocity, interfacial tension, fluid viscosity, and the distribution ratio of volumetric flow rates at a branch point. Additionally, we actively controlled the volume of the mother droplets and examined its influence on the size of the daughter droplets, demonstrating that the size of the daughter droplets was not significantly influenced by the volume of the mother droplet when the distribution ratio was properly controlled. The presented system for controlling droplet division would be available as an innovative means for preparing monodisperse emulsions from polydisperse emulsions, as well as a technique for making a microfluidic dispenser for digital microfluidics to analyze the droplet compositions.     

基于国产FPGA及DAC的任意波形发生器的设计

任意波形发生器作为测试测量设备的一种重要仪器,在航空航天的测量与控制技术领域中得到了广泛应用;当前市场成熟任意波形发生器产品多为国外产品或者国内厂商基于国外FPGA和DAC研制的产品;为了打破技术垄断,提高国产任意波形发生器的自主技术保障能力,研制基于国产芯片的任意波形发生器愈发重要;随着国产芯片设计技术提升,国产FPGA和DAC的性能显著提高,并得到了广泛应用;PXI总线作为当前仪器领域的主要总线类型之一,可以满足大部分测试仪器的通讯要求;基于国产FPGA和DAC器件,从硬件设计和软件设计两个方面出发,成功研制了一款采样率为100 MSa/s的 PXI总线任意波形发生器模块,实现了43 MHz信号输出,通过实验测试了模块的功能和性能,完全满足模块指标要求,充分证明了国产芯片在工程设计中的性能特性。     

Oscillations in light-triggered logic microfluidic circuit

Control of droplets in microfluidic environments has numerous applications ranging from analysis and sample preparation for biomaterials synthesis (Mann and Ozin Nature 382:313–318, 1996) and medical diagnostics (Pipper et al. Nat Med 13:1259–1263, 2007) to photonics (Schmidt and Hawkins Nat Photonics 5:598–604, 2011). Here we study the oscillations present in a microfluidic circuit capable of sorting curable droplets on demand by triggering the circuit with UV-light. Prior to this paper we showed that a simple circuit can self-sort particles and produce a binary output, sorted or rejected stream of particles, based on the hydrodynamic resistance induced by the particles as they flow through the microfluidic channels. We showed that the cross-linking of droplets can modulate the resistance, and demonstrated particle switching by sorting of otherwise identical droplets of uncured and cured photocurable solution immersed in mineral oil solution. Before arriving at the sorting circuit, droplets made of a photocurable solution were illuminated by a UV-light from a mercury lamp, curing them. By tuning the outlet pressures, the switching threshold could be tuned so that uncured droplets were rejected while cured droplets were switched (Raafat et al. μTAS Proc 1826–1828, 2010; Cartas-Ayala et al. Small 9:375–381, 2013). Here we use this system to study the oscillations in this circuit due to particle–particle interactions in the circuit. The circuit oscillation can be used as a counter with a light ON/OFF switch. The circuit behavior agrees well with theoretical predictions of droplet oscillations. Furthermore, the circuit oscillations can be switched on or off by UV-light illumination. This experiment demonstrates switching of particles based on deformability, illustrates the switching of particles by using light, and the possibility of creating new managing schemes for droplets by combining light control with droplet generation-rate control.     

High throughput micro droplet generator array controlled by two-dimensional dynamic virtual walls

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. The nozzle density of this compact droplet generator can be five to ten times higher than that of commercial inkjet printheads in one-dimensional formats. The volume and initial speed of the generated droplets was 3.6–5.7 pL and 14–15 m/s, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise meniscus control. The analyzed data shows the meniscus undergoes a “push–pull–push” progress which effectively cuts the liquid column short. The refilling time of the innovative micro droplet generator was estimated to be 0.296 μs from the simplified chamber model, and it was one-tenth of the commercial printheads. In addition, the frequency response was estimated to be higher than 20 kHz by observing the meniscus fluctuation condition. Finally, a 3 × 5 heater array was used to generate two droplets simultaneously, which shows that the crosstalk problem can be eliminated by precise time-delayed control. An interlacing operation was also proposed to address the large array control algorithm. To summarize, a 330-dpi monolithic micro droplet generator prototype has been proposed for high speed and large 2D format printing.     

Exploitation of a microfluidic device capable of generating size-tunable droplets for gene delivery

This study presents a new microfluidic chip that generates micro-scale emulsion droplets for gene delivery applications. Compared with conventional methods of droplet formation, the proposed chip can create uniform droplets (size variation <7.1%) and hence enhance the efficiency of the subsequent gene delivery. A new microfluidic chip was developed in this study, which used a new design with a pneumatic membrane chamber integrated into a T-junction microchannel. Traditionally, the size of droplets was controlled by the flow rate ratio of the continuous and disperse phase flows, which can be controlled by syringe pumps. In this study, a pneumatic chamber near the intersection of the T-junction channel was designed to locally change the flow velocity and the shear force. When the upper air chamber was filled with compressed air, the membrane was deflected and then the droplet size could be fine-tuned accordingly. Experimental data showed that using the new design, the higher the air pressure applied to the active tunable membrane, the smaller the droplet size. Finally, droplets were used as carriers for DNA to be transfected into the Cos-7 cells. It was also experimentally found that the size of the emulsion droplets plays an important role on the efficiency of the gene delivery. The preliminary results of this paper have been presented at the 2007 IEEE International Conference of Nano/Molecular Medicine and Engineering (IEEE NANOMED 2007), Macau, China, 6–9 August, 2007.