Surface-tension-confined droplet microfluidics

This article is a concise overview about the developing microfluidic systems named surface-tension-confined droplet microfluidics(STORMs). Different from traditional complexed droplet microfluidics which generated and confined the droplets by three-dimensional(3D) poly(dimethylsiloxane)-based microchannels, STORM systems provide twodimensional(2D) platforms for control of droplets. STORM devices utilize surface energy, with methods such as surface chemical modification and mechanical processing, to control the movement of fluid droplets. Various STORM devices have been readily prepared, with distinct advantages over conventional droplet microfluidics, which generated and confined the droplets by 3D poly(dimethylsiloxane)-based microchannels, such as significant reduction of energy consumption necessary for device operation, facile or even direct introduction of droplets onto patterned surface without external driving force such as a micropump, thus increased frequency or efficiency of droplets generation of specific STORM device, among others. Thus, STORM devices can be excellent alternatives for majority areas in droplet microfluidics and irreplaceable choices in certain fields by contrast. In this review, fabrication methods or strategies, manipulation methods or mechanisms,and main applications of STORM devices are introduced.     

Acoustically driven planar microfluidics

Surface acoustic waves are used to actuate and process smallest possible amounts of fluids on the planar surface of a piezoelectric chip. Chemical modification of the chip surface is employed to create virtual wells and tubes to confine the liquids. Lithographically modulated wetting properties of the surface define a fluidic network, in analogy to the wiring of an electronic circuit. Acoustic radiation pressure exerted by the surface wave leads to internal streaming in the fluid and eventually to an actuation of small droplets along predetermined trajectories.     

Current-voltage characteristic of electrospray processes in microfluidics

We use a glass-based microfluidic device to study the electric current behavior of an electrospray process in the presence of a coflowing liquid. The current shows strong voltage dependence and weak flow rate dependence, in stark contrast to classical electrospray. By considering that the current is dominated by convection near the apex of the conical meniscus and driven by tangential electric stresses, we quantitatively capture the voltage and flow rate dependence of the current. Our results elucidate the influence of external field strength and open the way to achieve robust electric control of the current and of the drop size in microfluidics.     

"Bottleneck effect" in two-dimensional microfluidics

An anomalously long transient is needed to achieve a steady pressurization of a fluid when forced to flow through micronarrowed channels under constant mechanical driving. This phenomenon, known as the "bottleneck effect" is here revisited from a different perspective, by using confined displacements of interfacial fluids. Compared to standard microfluidics, such effect admits in this case a neat quantitative characterization, which reveals intrinsic material characteristics of flowing monolayers and permits to envisage strategies for their controlled micromanipulation.     

Discrete Boltzmann equation for microfluidics

We propose a discrete Boltzmann model for microfluidics based on the Boltzmann equation with external forces using a single relaxation time collision model. Considering the electrostatic interactions in microfluidics systems, we introduce an equilibrium distribution function that differs from the Maxwell-Boltzmann distribution by an exponential factor to represent the action of an external force field. A statistical mechanical approach is applied to derive the equivalent external acceleration force exerting on the lattice particles based on a mean-field approximation, resulting from the electro-static potential energy and intermolecular potential energy between fluid-fluid and fluid-substrate interactions.     

A Multiphase Godunov Method for Compressible Multifluid and Multiphase Flows

We propose a new model and a solution method for two-phase compressible flows. The model involves six equations obtained from conservation principles applied to each phase, completed by a seventh equation for the evolution of the volume fraction. This equation is necessary to close the overall system. The model is valid for fluid mixtures, as well as for pure fluids. The system of partial differential equations is hyperbolic. Hyperbolicity is obtained because each phase is considered to be compressible. Two difficulties arise for the solution: one of the equations is written in non-conservative form; non-conservative terms exist in the momentum and energy equations. We propose robust and accurate discretisation of these terms. The method solves the same system at each mesh point with the same algorithm. It allows the simulation of interface problems between pure fluids as well as multiphase mixtures. Several test cases where fluids have compressible behavior are shown as well as some other test problems where one of the phases is incompressible. The method provides reliable results, is able to compute strong shock waves, and deals with complex equations of state.     

微流控制备单分散微米级SiO2微球

采用软模板法制备出了聚二甲基硅氧烷微流控装置。利用该装置讨论了正硅酸乙酯和氨水的用量分别对反应体系凝胶化时间的影响,确定了制备SiO2微球的优化反应体系,即二甲基乙酰胺、正硅酸乙酯和氨水的体积比为8∶4∶1,实验所需的反应温度为60 ℃。实验发现:在微流体通道中,分散相的流速越大,粒径越大;连续相流速越大,粒径越小。因此,通过控制微流控装置中分散相和连续相的流速制备了粒径40~220 m的单分散SiO2微球,并对其形貌进行表征。光学显微镜和粒径分析均表明所制备的SiO2微球球形度高,单分散性好。     

Inertial microfluidics with multi-particle collision dynamics

Using the method of multi-particle collision dynamics (MPCD), we investigate inertial focussing in microfluidic channels that gives rise to the Segré-Silberberg effect. At intermediate Reynolds numbers, we model the motion of a spherical colloid in a circular microchannel under pressure-driven flow. We determine the radial distribution function and show how its width and the location of its maximum are strongly influenced by the colloid size and the Reynolds number of the Poiseuille flow. We demonstrate that MPCD is well suited for calculating mean values for the lift force acting on the colloid in the cross-sectional plane and for its mean axial velocity. We introduce a Langevin equation for the cross-sectional motion whose steady state is the Boltzmann distribution that contains the integrated lift force as potential energy. It perfectly coincides with the simulated radial distribution function.     

Preparation of nanoparticles by continuous-flow microfluidics

We review a variety of micro- and nanoparticle formulations produced with microfluidic methods. A diverse variety of approaches to generate microscale and nanoscale particles has been reported. Here we emphasize the use of microfluidics, specifically microfluidic systems that operate in a continuous flow mode, thereby allowing continuous generation of desired particle formulations. The generation of semiconductor quantum dots, metal colloids, emulsions, and liposomes is considered. To emphasize the potential benefits of the continuous-flow microfluidic methodology for nanoparticle generation, preliminary data on the size distribution of liposomes formed using the microfluidic approach is compared to the traditional bulk alcohol injection method.     

Multiphase patterns in self-phase-locked optical parametric oscillators

Multiphase patterns are found in a mean-field model of a singly-resonant optical parametric oscillator that converts a pump field at frequency 3ω into signal and idler fields at frequencies 2ω and ω. A complex Ginzburg-Landau equation without diffusion and with a quadratic phase-sensitive nonlinear term is derived under single-longitudinal and paraxial propagation approximations. Owing to the phase-matched multistep parametric process ω + ω = 2ω, phase locking of the resonated signal field is possible with three distinct phase states. Three-armed rotating spirals, target patterns and light filamentation are found by a numerical analysis of the mean-field equation. Received 19 April 2001 and Received in final form 21 June 2001     

Electrokinetic ‘ratchet’ pumps for microfluidics

The symmetry argument underlying ‘ratchet’ schemes for the motion of molecular motors and for selective transport of particles is shown to yield new means for the pumping of liquids. A practical realization consists in using surfaces bearing polar periodic arrays of electrodes addressed by an ac voltage difference. The resulting surface-induced pumping remains efficient under miniaturization and may find application in microfluidics. Received: 19 October 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002     

Wetting in Multiphase Systems with Complex Geometries

We address the shape and distribution of two-phase systems embedded within a third phase. To motivate this work, we begin by describing transmission electron microscopy observations of the configurations adopted by the solid and vapor phases of lead when these are confined together within a silicon cavity. We then perform analytical calculations of the stability of various possible configurations of two-phase systems confined in a cubic-shaped cavity. The most stable configurations are a function of the volume ratio of the two phases in the cavity, and of a parameter describing the wetting behavior in the three-phase system. The wealth of configurations obtained for embedded solid/fluid or condensed/fluid phases within a solid cavity is presented. Wetting anisotropy on the walls of the cavity, and the faceted or isotropic character of the interface between the two embedded phases, are shown to be physical parameters that determine the number of possible stable configurations.     

微尺度血液透析的多相数值模拟

建立血液流动的剪切稀化模型,并对3-D微管道内血液多相流动进行数值模拟,得到的结果与实验数据比较显示,该模型不仅能预测微管道内速度分布,还可以体现微尺度下血液流动的F-L效应.为了探索无膜透析可行性,利用该非牛顿两相流模型对T型微槽道内血液透析过程进行数值模拟,结果表明,血浆中质量扩散系数较大的组份很容易从血液扩散到透析液中,实现了组份分离.     

微流体技术在电子芯片冷却中的应用研究进展

随着微电子技术向小型化集成化及高频高速方向发展,计算机芯片集成度的提高受到因电子元器件发热而引起的热障所限制,芯片冷却问题成为影响计算机进一步发展的关键因素之一。介绍了电子芯片发展的现状及主要冷却方法的发展,从冷却驱动器件、微通道结构各个角度论述了微流体技术在电子芯片冷却中的重要作用,重点介绍了微槽道冷却和微喷冷却的微流体技术特征,论述了压电泵、电渗、热管等微流体驱动技术在冷却液驱动的应用。     

Particle separation in microfluidics using different modal ultrasonic standing waves

Microfluidic technology has great advantages in the precise manipulation of micro and nano particles, and the separation of micro and nano particles based on ultrasonic standing waves has attracted much attention for its high efficiency and simplicity of structure. This paper proposes a device that uses three modes of ultrasonic standing waves to continuously separate particles with positive acoustic contrast factor in microfluidics. Three modes of acoustic standing waves are used simultaneously in different parts of the microchannel. According to the different acoustic radiation force received by the particles, the particles are finally separated to the pressure node lines on both sides and the center of the microchannel. In this separation method, initial hydrodynamic focusing and satisfying various equilibrium constraints during the separation process are the key. Through numerical simulation, the resonance frequency of the interdigital transducer, the distribution of sound pressure in the liquid, and the relationship between the interdigital electrode voltage and the output sound pressure are obtained. Finally, the entire separation process in the microchannel was simulated, and the separation of the two particles was successfully achieved. This work has laid a certain theoretical foundation for the rapid diagnosis of diseases in practical applications.     

Chameleonlike metashells in microfluidics: A passive approach to adaptive responses

Heat energy can transfer via convection, conduction, and radiation. Based on convection and conduction in microfluidics, people have designed and fabricated many novel devices. However, almost none of them has adaptivity, thus restricting practical applications under different conditions. To solve this problem, here we propose a passive approach to adaptive responses. That is,we consider the thermal convection-conduction process in microfluidic structures where Darcy’s law and Fourier’s law are both valid. By carefully designing two key parameters(i.e., tensorial thermal conductivity and tensorial permeability) of a metashell,we theoretically reveal that its effective properties(i.e., effective thermal conductivity and effective permeability) can adaptively change according to the inside object, thus yielding the chameleonlike metashell. Further, this metashell is passive since it requires no prior knowledge of the inside object. We also report that the chameleonlike behavior can occur for anisotropic inside objects, nonuniform external fields, or even complex shapes. All theoretical analyses agree well with finite-element simulations.The chameleonlike metashell can act as an intelligent metamaterial in microfluidics for its adaptive responses, and it can also benefit other physical fields where convection plays a role, such as mass diffusion.     

Driving and analysis of micro-objects by digital holographic microscope in microfluidics

We propose an optical configuration in which floating particles in a microfluidic chamber can be characterized by an interference microscopy configuration to obtain quantitative phase-contrast maps. The configuration is simply made by two laser beams from the same laser source. One beam provides the optical forces for driving the particle along appropriate paths, but at same time works as the object illumination beam in the holographic microscope. The second beam plays the role of the reference beam, allowing recording of an interference fringe pattern (i.e., the digital hologram) in an out-of-focus image plane. The system and method are illustrated and experimental results are offered for polymeric particles as well as for in vitro cells with the aim to demonstrate the approach.     

纳米流体多相流动的多尺度模拟方法

针对纳米流体多相流动的微观特征,提出一种基于格子Boltzmann的多尺度耦合方法,在速度和悬浮纳米粒子分布变化比较剧烈的区域采用细网格多相模型,在其它区域视纳米流体为均匀混合的单相流体,使用粗网格单相模型.为保证不同尺度区域之间的物理信息(参数)的准确传递,运用质量和动量守恒原理,建立跨区域的边界耦合模型.几个算例表明,该方法既可以反映纳米流体流动的微观特征,又能提高计算效率,与单纯使用多相模型相比,节省大量时间.