Dynamic formation and scaling law of hollow droplet with gas/oil/water system in dual‐coaxial microfluidic devices

Based on the one‐step microfluidic method of producing hollow droplet with thin film, this article studies the effect of water and oil flow rate, gas pressure, and viscosity of aqueous phase on the dynamic formation and size of hollow droplet by analyzing large amounts of data acquired automatically. The results show that the filling stage of hollow droplet is similar to that of microbubble formation, while the necking stage is similar to that of droplet formation process. Furthermore, based on the data and mathematical model describing droplet formation mechanism, a filling stage model including Capillary number of continuous phase is developed. Considering the dynamic interface breakup and displacement of droplet in necking stage, a necking stage model is developed. The results show that the model results considering filling and necking stage fit well with the experimental data, and the relative error is less than 5%. Finally, the same model with parameters is used to predict the size of hollow droplet with other systems and devices, and the model is proved to be relative precise in our experimental conditions. The results presented in this work provide a more in‐depth understanding of the dynamic formation and scaling law of hollow droplet with G/L/L systems in microfluidic devices. © 2017 American Institute of Chemical Engineers AIChE J, 64: 730–739, 2018     

Gain scheduling PID control for directed self-assembly of colloidal particles in microfluidic devices

Directed self-assembly provides a promising route to fabricate small-scale structures for various engineering applications. The use of the external fields of variable strength and frequency allows for active controls to be implemented. For assembling specific structural features, precise control over a local particle density is often needed, which can be achieved using feedback control. However, the strong nonlinear behavior of directed self-assembly complicates such control. In this work, a gain-scheduling feedback control strategy for directed self-assembly of colloidal particles is presented. The process gain is described by an empirical steady-state model, which is used for scheduling a proportional-integral feedback controller. The automated controller is implemented experimentally in a microfluidic device for directed self-assembly of colloidal particles. The gain-scheduled controller shows a significantly improved dynamic performance compared with a conventional proportional-integral controller over a broad range of operating parameters.     

Enhancement in the limit of detection of lab-on-chip microfluidic devices using functional nanomaterials

Technological developments in recent years have witnessed a paradigm shift towards lab-on-chip devices for various diagnostic applications. Lab-on-chip technology integrates several functions typically performed in a large-scale analytical laboratory on a small-scale platform. These devices are more than the miniaturized versions of conventional analytical and diagnostic techniques. The advances in fabrication techniques, material sciences, surface modification strategies, and their integration with microfluidics and chemical and biological-based detection mechanisms have enormously enhanced the capabilities of these devices. The minuscule sample and reagent requirements, capillary-driven pump-free flows, faster transport phenomena, and ease of integration with various signal readout mechanisms make these platforms apt for use in resource-limited settings, especially in developing and underdeveloped parts of the world. The microfluidic lab-on-a-chip technology offers a promising approach to developing cost-effective and sustainable point-of-care testing applications. Numerous merits of this technology have attracted the attention of researchers to develop low-cost and rapid diagnostic platforms in human healthcare, veterinary medicine, food quality testing, and environmental monitoring. However, one of the major challenges associated with these devices is their limited sensitivity or the limit of detection. The use of functional nanomaterials in lab-on-chip microfluidic devices can improve the limit of detection by enhancing the signal-to-noise ratio, increasing the capture efficiency, and providing capabilities for devising novel detection schemes. This review presents an overview of state-of-the-art techniques for integrating functional nanomaterials with microfluidic devices and discusses the potential applications of these devices in various fields.     

Effects of fluid–fluid interfacial elasticity on droplet formation in microfluidic devices

Biomolecules adsorb at fluid–fluid interfaces and can form a cohesive interfacial network imparting distinctive local interfacial mechanics. This modification of interfacial behavior is empirically known to significantly affect the stability and flow behavior of foams and emulsions. Droplet formation in the presence of interfacial networks is investigated in a flow‐focusing microfluidic device using designed peptide surfactants, which allow decoupled control of interfacial rheology and interfacial tension. The influence of interfacial elasticity on droplet breakup, satellite droplet formation and droplet size are reported. The presence of high interfacial elasticity strongly affects the mechanism of droplet breakup by delaying neck thinning and altering interfacial shape at the point of droplet detachment, resulting in the suppression of satellite droplet formation and a decrease in droplet size. We report a correlation between dimensionless droplet size and a new dimensionless grouping which combines flow‐rate ratio with the ratio of interfacial tension and interfacial elasticity. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

A comparison of different geometrical elements to model fluid wicking in paper-based microfluidic devices

Recently, microfluidic paper-based analytical devices (μPADs) have outstripped polymeric microfluidic devices in the ease of fabrication and simplicity. Surface tension-based fluid motion in the paper's porous structure has made the paper a suitable substrate for multiple biological assays by directing fluid into multiple assay zones. The widespread assumption in most works for modeling wicking in a paper is that the paper is a combination of capillaries with the same diameter equal to the effective pore diameter. Although assuming paper as a bundle of capillaries gives a good insight into pressure force that drives the fluid inside the paper, there are some difficulties using the effective pore radius. The effective pore radius is totally different from the average geometrical pore radius which makes it impossible to predict wicking in μPADs based on geometrical parameters. In this article, we introduce different analytical and numerical models to investigate the possibility of determining the permeability of the paper, based on geometrical parameters rather than effective parameters. The lattice Boltzmann method is used for numerical simulations. The permeability of each of the proposed models was compared with the experimental permeability. Results indicated that assuming paper as a combination of capillaries and annuluses leads to accurate results that totally depend on average geometrical values rather than effective values. This paves the way for prediction of the fluid wicking only by considering average geometrical pore and fiber diameters.     

三维石墨毡阳极自呼吸微流体燃料电池性能数值模拟

针对采用三维石墨毡可渗透阳极的空气自呼吸无膜微流体燃料电池,建立了三维等温稳态数学模型,对电池中燃料及电解液的流动和传输、电极过程动力学及电荷传递过程进行了模拟,计算获得了具有石墨毡阳极的微流体燃料电池内的传质和燃料渗透特性,研究了石墨毡厚度及反应物（燃料和电解液）流量对电池性能的影响。结果表明：当入口流量为333 μl·min^-1时,采用石墨毡可渗透阳极相比碳纸和碳布可渗透阳极,极限电流密度和极限功率密度分别提升12%和50%;电池电压为0.8 V时燃料渗透引起的寄生电流密度仅占电流密度的0.86%。电池性能随着石墨毡电极厚度增加而升高,但增幅逐渐减小;反应物流量增大,电池的性能先增加后逐渐趋于稳定。     

Development of microfluidic devices for biomedical and clinical application

This review focuses on the development and use of microfluidic devices within a clinical setting. The underlying theoretical background of microfluidics is briefly elucidated. The materials and techniques used to fabricate the devices and their applicability to the clinical environment are described. The current research in this area is appraised and projections for future applications are discussed. Copyright © 2010 Society of Chemical Industry     

CFD数值模拟技术在液滴微流控多相流特性研究的应用进展

高集成化的微流控系统具有界面面积大、传递距离短、混合速度快等优势,已被广泛应用于许多科学领域。然而,微通道内多相流间的相互作用及动力学行为受多方面的影响,仅依靠试验观测技术和理论预测方法难以全面了解多相流传质传热过程、获取流场特性参数、揭示多相流相互作用规律。当下CFD数值模拟技术的快速发展为预测和分析微流控通道内的多相流问题提供了更为直观、有效、准确的帮助。本文对数值模拟技术在液滴微流控多相流特性研究的应用进展进行全面综述,涵盖液滴微流控装置结构及演变、液滴微流控模拟方法及优化以及微通道内多相流作用过程及原理。     

Breakup dynamics of slender bubbles in non‐newtonian fluids in microfluidic flow‐focusing devices

This study aims to investigate the breakup of slender bubbles in non‐Newtonian fluids in microfluidic flow‐focusing devices using a high‐speed camera and a microparticle image velocimetry (micro‐PIV) system. Experiments were conducted in 400‐ and 600‐μm square microchannels. The variation of the minimum width of gaseous thread with the remaining time before pinch‐off could be scaled as a power‐law relationship with an exponent less than 1/3, obtained for the pinch‐off of bubbles in Newtonian fluids. The velocity field and spatial viscosity distribution in the liquid phase around the gaseous thread were determined by micro‐PIV to understand the bubble breakup mechanism. A scaling law was proposed to describe the size of bubbles generated in these non‐Newtonian fluids at microscale. The results revealed that the rheological properties of the continuous phase affect significantly the bubble breakup in such microdevices. © 2012 American Institute of Chemical Engineers AIChE J,, 2012     

Observations and analysis of electrokinetically driven particle trapping in planar microelectrode arrays

In situ observations of submicron fluorescent tracers suspended in high ionic strength media sealed in a confined geometry are combined with 3‐D simulations in order to provide a better understanding of the synergism between dielectrophoresis and electrothermal flows that cause rapid particle transport and trapping on the surface of planar quadrupolar microelectrodes. The influence of electrode design on the microfluidic patterns and observed particle collection is examined by employing two different types of microelectrodes in the experiments. The potential use of quadrupolar microelectrodes as means for achieving accelerated sampling and signal amplification in future surface based biosensor devices is illustrated with an experiment involving stable capture of antigen‐coated polystyrene particles on the surface of an antibody‐functionalized microelectrode array.     

电场作用下基面液滴蒸发与内部流动数值模拟

为探究电场强化基面液滴蒸发的原理,本文采用有限元方法,对外加电场作用下的固体基面上液滴的蒸发过程进行了数值模拟,对比了不同电导率液滴的蒸发过程,分析了电场、液滴蒸发速率和内部流动的影响及其成因,以及液滴在电场作用下的内部流动与液滴传热传质的关系,结果表明,电场力的作用能够显著强化液滴内部的流动,对液滴的传热传质具有促进作用。此外,本文分析了温度对电场下基面液滴蒸发及内部流动的影响,发现温度对电场、液滴内部流动及蒸发的强化作用也有着较为明显的影响：对于电导率较低的纯水液滴,当电场强度低于和高于临界值6kV/cm时,温度对电场强化液滴内部流动和蒸发的影响有所不同;对于电导率较高的盐酸液滴,温度对电场强化液滴内部流动和蒸发的影响随电场强度升高均较大。本文为发展高效静电喷雾冷却技术提供了研究基础。     

Droplets formation and merging in two-phase flow microfluidics

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

Drop mass transfer in a microfluidic chip compared to a centrifugal contactor

A model system was developed for enabling a multiscale understanding of centrifugal‐contactor liquid–liquid extraction. The system consisted of Nd(III) + xylenol orange in the aqueous phase buffered to pH = 5.5 by KHP, and dodecane + thenoyltrifluroroacetone (HTTA) + tributyphosphate (TBP) in the organic phase. Diffusion constants were measured for neodymium in both the organic and aqueous phases, and the Nd(III) partition coefficients were measured at various HTTA and TBP concentrations. A microfluidic channel was used as a high‐shear model environment to observe mass transfer on a droplet scale with xylenol orange as the aqueous‐phase metal indicator; mass‐transfer rates were measured quantitatively in both diffusion and reaction limited regimes on the droplet scale. The microfluidic results were comparable to observations made for the same system in a laboratory scale liquid–liquid centrifugal contactor, indicating that single drop microfluidic experiments can provide information on mass transfer in complicated flows and geometries. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3071–3078, 2014     

Mechanism of bubble formation in step-emulsification devices

Step-emulsification device is an emerging technology for bubble generation, which is convenient for numbering-up. However, the mechanism of bubble formation in such devices remains unclear. In this article, the mechanism of bubble formation in step-emulsification devices is investigated by using a high-speed camera. First, the evolution of gas–liquid interface with time during bubble formation is observed. Second, the variation of characteristic parameters of bubbles, such as bubble's volume V_B and generation frequency f, with gas and liquid flow rates is described under various liquid viscosities and step widths. Meanwhile, the coupling law of interface evolution with characteristic parameters of bubbles is explored. Finally, the basic guideline for generation of monodispersed bubbles in step-emulsification is put forward: low liquid viscosity. Connecting the interface evolution with the characteristic parameters, predictive models for bubble's volume, and generation frequency in step-emulsification are proposed, respectively.     

液滴微流控的集成化放大方法研究进展

相比于传统乳化方法,液滴微流控技术可以在微通道内可控制备单分散液滴模板用于合成各种功能微球,被广泛应用于生物、医疗、制药、环境等领域。由于单个液滴制备微流控单元的产量低,液滴微流控的集成化放大成为了液滴微流控技术面向工业应用的技术难点。本文综述了近年来液滴微流控集成化放大方法的研究进展,重点介绍了不同类型液滴制备微流控单元集成化放大的研究进展,包括基于剪切力形成液滴、基于界面张力形成液滴和基于被动分裂形成液滴的液滴制备微流控单元的集成化放大方法。     

Enhancing single molecule imaging in optofluidics and microfluidics

Microfluidics and optofluidics have revolutionized high-throughput analysis and chemical synthesis over the past decade. Single molecule imaging has witnessed similar growth, due to its capacity to reveal heterogeneities at high spatial and temporal resolutions. However, both resolution types are dependent on the signal to noise ratio (SNR) of the image. In this paper, we review how the SNR can be enhanced in optofluidics and microfluidics. Starting with optofluidics, we outline integrated photonic structures that increase the signal emitted by single chromophores and minimize the excitation volume. Turning then to microfluidics, we review the compatible functionalization strategies that reduce noise stemming from non-specific interactions and architectures that minimize bleaching and blinking.     

钒微流体燃料电池性能的数值模拟

对一种新型钒微流体燃料电池进行了理论分析并建立了三维数值模型。该模型包含了层流、物质传输与电化学反应等电池内部的物理和化学过程。计算得到的极化曲线与实验数据吻合较好,说明模型是可靠的。通过多场耦合求解,数值模拟了体积流速、燃料纯度等对电池性能的影响。研究结果表明：增大体积流速可以提高电池的功率,但燃料利用率会大幅降低;燃料纯度对燃料电池的电压有较大影响;燃料利用率低是制约微流体燃料电池发展的主要因素之一。通过改进原有Y形流道设计,设计了一种双Y形流道微流体电池,仿真结果显示其可以较大地改善燃料的利用率。     

微混合器内流体混合的研究进展

Microreaction technology is one of the most innovative and rapid developing fields in chemical engineering, synthesis and process technology. Many expectations toward enhanced product selectivity, yield and purity, improved safety, and access to new products and processes are directed to the microreaction technology. Microfluidic mixer is the most important component in microfluidic devices. Based on various principles, active and passive micromixers have been designed and investigated. This review is focused on the recent developments in microfluidic mixers. An overview of the flow phenomena and mixing characteristics in active and passive micromixers is presented, including the types of physical phenomena and their utilization in micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, T-micromixer is highlighted as an example to illustrate the effect of design and operating parameters on mixing efficiency and fuid flow inside microfluidic mixers.