微型硅基内肋管内的流动与换热特性

目前内肋管强化换热研究大多基于铜、铝或者不锈钢等常规材质,其加工精度和加工尺寸均受到限制。本文采用MEMS工艺首次在硅片上制作成带有方形内肋的平行硅基微通道,并对其内部流动和换热特性进行了实验研究,结果表明：在通道深度和宽度相同的情况下,肋高和肋间距对工质在通道内的流动阻力和换热特性有显著的影响;微通道内加肋以后,通道内的流动阻力和换热性能有不同程度的增加,且提高程度与肋高和肋间距有关;通过合理的设计肋高和肋间距,可以起到明显的强化换热作用。结果还显示,进口段效应对硅基微通道内的流动和换热影响较为显著。     

Modeling of velocity distribution among microchannels with triangle manifolds

A model of velocity distribution among microchannels with triangle manifolds is proposed. According to the flow behaviors analyzed by Fluent, the manifolds are divided into several approximate rectangular channels, and then an equivalent simplified resistance network model is developed to establish the relationships between the velocities and pressure drops in microchannels and approximate rectangular channels. The velocity distributions are calculated under two situations, respectively, considering and ignoring singular losses. The outcomes of the present study are compared with Fluent's simulated results to analyze the effects of singular losses on the velocity distributions. It indicates that the proposed model is suitable for the calculation of velocity distribution among microchannels with obtuse angled or right triangle manifolds under low Reynolds numbers. The premise of ignoring singular losses is that the frictional pressure drops are three times larger than the singular pressure drops in each flow loop. The manifold optimization results indicate that the velocity distribution among microchannels with right triangle manifolds is much more uniform than that of the corresponding one with obtuse angled manifolds. © 2009 American Institute of Chemical Engineers AIChE J, 55: 1969–1982, 2009     

微分散设备数量放大方式研究进展

单分散液滴是生产球形颗粒和微胶囊的模板，相关材料广泛用于光学显示、药物递送、生物测定、食品加工等领域。微分散技术通过精确控制微米级通道内的多相流体流动获得尺寸小、粒径分布窄、结构复杂可控的微液滴，是一种公认的新方法。但由于单个微通道的低生产率，其大规模的工业和商业应用受到限制。为了实现产品的高通量、易控制的规模化生产，将大量液滴生成单元和流体分配通道网络并行到单个芯片上进行数量放大是微分散设备放大的基本模式。为了保持液滴的单分散性和设备的鲁棒性，流体在并联微通道内的均匀分布是关键科学问题。系统综述了微分散设备数量放大的最新进展，对加工技术和材料进行了相关分析，并在此基础上对微分散设备放大的未来研究方向进行展望。     

Production of high-aspect-ratio rectangular-lumen microtube by gas-assisted microextrusion

Flexible polymeric microtubes with rectangular-lumen are useful in chemical and bioengineering. However, improving the aspect ratio of the rectangular lumen is still a tough task. In this study, numerical simulations are carried out to study the influences of the processing parameters and the design of the extrusion die on the aspect ratio of the rectangular microchannels. Increasing the length of the inner die and reducing its width are critical to improving the aspect ratio of channels. The pressure difference is also important for controlling the sizes and shapes of channels. A linear mathematical relationship is built between the aspect ratio of channels and processing parameters and the sizes of the die. According to the simulating results, the geometry of the inner die is optimized and manufactured experimentally. The flexible rectangular microtubes are produced with well-controlled channel sizes. The highest aspect ratio of the channel we achieve is 87.     

Surface texturing inside ceramic macro/micro channels

Surface textures (roughness) inside of macro/micro channels have a number of potential uses in advanced ceramic applications, including areas such as bioceramic, cooling, microreactors or friction control. The presented microtemplating method is dealing with templating of structured fibres, which results in formation of variation in surface textures and roughness. Effect of the microtemplate skin texture and ceramic slurry behaviour has been investigated and discussed. Porosity and defects of the microtemplate skin play a key role for roughness/texture of sintered microchannels if their size is larger than the ceramic grain size. The surface roughness inside the micro/macro channel can be easily changed from R_a = 0.8 μm to R_a = 4.9 μm just by changing the microtemplate skin porosity. The results demonstrate possibilities to use this method to tailor surface texture inside microchannels in a one step procedure.     

Immiscible liquid–liquid slug flow characteristics in the generation of aqueous drops within a rectangular microchannel for preparation of poly(2-hydroxyethylmethacrylate) cryogel beads

The microchannel liquid-flow focusing and cryo-polymerization is an efficient method for the preparation of cryogel beads with a narrow diameter distribution. In order to prepare cryogel beads with expected diameters, it is necessary to get insights in the liquid–liquid immiscible flow characteristics of the flow-focusing fluid and the monomer solution in microchannels. In this work, the slug flow behaviors of two immiscible liquids regarding the preparation of poly(2-hydroxyethylmethacrylate) (pHEMA) cryogel beads in a rectangular cross-junction microchannel were investigated experimentally by the high-speed imaging method. Correlations of the immiscible liquid–liquid slug flow parameters like the aqueous slug velocity and length, the aqueous slug nose and tail lengths, the water-immiscible slug length as well as the aqueous droplet size were obtained. The pHEMA cryogel beads were prepared under certain flow conditions and the bead sizes were measured by laser particle size analyzer. The obtained correlations were then employed to estimate the bead sizes and compared with those obtained experimentally. The results showed that the present correlations gave reasonable estimations of the mean bead diameters at various conditions and thus, could be useful and helpful in the preparation of cryogel beads with expected size distributions in rectangular microchannels.     

The effect of gelation on statically and dynamically freeze-cast structures

Freeze-casting is a technique used to produce structures with anisotropic porosity in the form of well-defined microchannels throughout a sample. Here, this technique is used on the magnetocaloric ceramic La_0.66Ca_0.26Sr_0.07 Mn_1.05O₃. We show that a dynamic freezing profile, where the temperature is decreased continuously at −10 K/min, results in homogeneous, lamellar channels with widths of ∼15 µm, while static freezing, where the temperature is kept constant at 177 K, results in channels of increasing size away from the initial ice crystal nucleation site. The effect of gelation before freeze-casting is also investigated. Gelation inhibits ice crystal growth, which significantly changes the morphology by making channel cross sections less elongated, while additionally introducing more dendrites and ceramic bridges in the structure. The latter significantly dominates the flow path through the gelated structures, affecting the calculated tortuosity, which increases to τ ≈ 4 when compared to non-gelated samples where calculated tortuosities are in the range of ∼1.3 to ∼3. Finally, we present a systematic and automatic approach for evaluating channel and wall sizes and calculating tortuosities. This is based on analysis of images obtained by scanning electron microscopy using a continuous particle size distribution method and the TauFactor application in MATLAB^®.     

微通道内载紫杉醇固体脂质纳米粒制备及工艺优化

目的:制备载紫杉醇固体脂质纳米粒。方法:微通道内采用溶剂扩散法制备脂质纳米粒,并通过正交优化制备工艺。结果:制备的纳米粒稳定性良好,平均粒径为(129.87±2.91)nm,包封率为(3.11±0.06)%,载药率为(43.67±0.24)%。结论:本研究制备的载药固体脂质纳米粒载药特性与重复性良好。     

Performance characteristics of the low-cost Plantower PMS optical sensor

Abstract

Low-cost sensors have become very popular in recent years for monitoring air pollutants. Commonly, they are calibrated by correlating their signals with reference instrument measurements and using a machine learning model to account for the influence of air properties. As particle properties vary over location, such calibration models are only relevant to measurements made at the calibration location during a limited time period. For a more general operation of these sensors it is critical that their measurement performance is established using the calibration approaches commonly for research grade instruments. Without loss the generality, here we conducted an experimental study with size-classified, composition and concentration varied particles to determine the response function of a popular low-cost sensor, Plantower PMS5003. The sensor response in all the size channels is analyzed using Tikhonov regularization and quadratic programing method with the constraints of nonnegative and monotonic response with particle size. We show that the shape of the response function is closely related to the light scattering response, consistent with what might be expected for an optical sensor. The response function shows that signals in all size channels have a complex dependence on particle material and size distribution. Accurate determination of particle mass and number distributions from the sensor signals in different channels is, thus, not straightforward. The response function calculation is validated by comparing sensor measured and predicted signals using polydispersed particles. The obtained response functions provide critical insight into the operation of a popular low-cost sensor and guidance on interpretation of its results.

Copyright © 2019 American Association for Aerosol Research     

Hydrodynamic feedback on bubble breakup at a T‐junction within an asymmetric loop

Bubble breakup at a microfluidic T‐junction by taking into consideration the hydrodynamic feedback at the downstream channels is presented. Experiments are conducted in square microchannels with 400 μm in width. The splitting ratio of the bubble size in the bifurcations varies nonmonotonically with the flow rate ratio of gas/liquid phases, and it is also affected by the liquid viscosity. A critical size of the mother bubble determines the variation trend of the splitting ratio of bubble size with flow rates of both phases and the liquid viscosity, which is related to the different breakup mechanisms for long and short bubbles at the junction and the different additional resistances induced by long and short bubbles in downstream channels. A theoretical model is proposed to predict the tailoring size of bubbles at the T‐junction by taking into account of the additional resistance in the presence of bubbles in downstream channels. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1920–1929, 2014     

A study on the fabrication of rounded patterns by spin coating of photoresist on silicon substrate with microstructures

This article describes a spin coating process for making micropatterns with curved surfaces. Liquid photoresist was used to compensate etched profile of microchannels on a silicon substrate. Due to the surface tension of liquid, curved surfaces or slopes could be obtained by coating photoresist on the rectangular microstructures. The results depend on properties of photoresist, geometry of microstructures and coating conditions. In this study, revolution speed of spin coater is a major coating condition. And tests were done for various microchannels with different size and direction. After coating the photoresist layer, it was heated on a hot plate so that its shape could be fixed. The final profiles of curved structures were examined with a polymer replica which was applied on the photoresist surface. The profile was controlled by changing revolution speed in spin coating process, but it also depended on the size and direction of microchannels. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Aerosol penetration in microchannels

New applications involving aerosol transport in microscale configurations requires the derivation of the penetration efficiency of aerosols in microchannels. Although many analytical solutions for the aerosol penetration in channels have been investigated, none of them are applicable for microchannels. Previously, the no-slip condition for the gas velocity and the zero particle concentration boundary condition have been applied to the convection diffusion equation. However, recent studies show these boundary conditions may not be appropriate for microscale geometries. The particle penetration through rectangular microchannels and cylindrical microtubes has been obtained using the numerical Crank Nicolson method with slip flow at the walls. Existing correlations for the aerosol penetration have been modified for the slip flow regime based on an optimization method. These correlations give the penetration as a function of the dimensionless deposition factor and Knudsen number of the gas. At large Knudsen numbers the penetration decreases relative to the case with continuum flow. Therefore, the aerosol penetration decreases in the slip flow regime. However, the non-zero boundary condition for the particle concentration at the walls does not have any significant effect on the model results of the particle penetration.     

Generation of highly uniform droplets using asymmetric microchannels fabricated on a single crystal silicon plate: Effect of emulsifier and oil types

Uniform droplets of soybean oil, MCT (medium-chain fatty acid triglyceride) oil and n-tetradecane with a mean diameter of 26-29 μm have been generated using a silicon 24 × 24 mm microchip consisting of 23,489 asymmetric microchannels fabricated by photolitography and deep-reactive ion etching. Each microchannel consisted of a circular 10-μm diameter straight hole with a length of 70 μm and a 50 × 10 μm rectangular microslot with a depth of 30 μm. At the constant oil flux of 10 L m^− 2 h^− 1, the percent of active channels increased with increasing the oil viscosity and ranged from 4% for n-tetradecane to 48% for soybean oil. The size distribution span for SDS (sodium dodecyl sulphate)- and Tween 20 (polyoxyethylene (20) sorbitan monolaurate)-stabilized soybean and MCT oil droplets was 0.21-022. The ability of asymmetric microchannels to generate monodisperse soybean oil droplets at the very low SDS concentration of 0.01 wt.% has been demonstrated. At the SDS concentration below the CMC, the generated droplets tend to attach to the plate surface, whereas at the higher SDS concentration they detach from the plate as soon as they are formed. The agreement between the experimental and CFD (Computational Fluid Dynamics) simulation results was excellent for soybean oil and the poorest for n-tetradecane.     

Flow and Heat Transfer Characteristics of Confined Noncircular Turbulent Impinging Jets

Abstract

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layer κ–η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number.     

Fabrication of 3D Microfluidic Channels and In‐Channel Features Using 3D Printed,Water‐Soluble Sacrificial Mold

Recent advent of additive manufacturing potentiates the fabrication of microchannels, albeit with limitations in resolution of printed structures, freedom of geometry, and choice of printable materials. Herein, a method is developed by sacrificial molding to fabricate microchannels in various polymer matrices and geometries. This method allows for rapid fabrication of 3D microchannels and channels harboring intricate in‐channel features. The method uses commercially available fused deposition modeling 3D printer and filament made of polyvinyl alcohol (PVA). Mechanically stable molds are fabricated for 3D microchannels that can be completely removed in water. Importantly, the PVA mold is stable and resilient in hydrogels despite being hygroscopic. Perfusion channels are fabricated in biocompatible substrates such as gelatin and poly(ethylene glycol) diacrylate. Fabrication of the network of 3D multilayer microchannels is demonstrated by preassembling sacrificial molds from modular pieces of molds. Intricate staggered‐herringbones grooves (SHGs) are also fabricated within microchannels to produce micromixers. The versatility and resilience of the method developed here is advantageous for biological and chemical applications that require 3D configurations of microchannels in various matrices, which would not be compatible with fabrication by direct 3D printing and softlithography.     

Confinement effect and spreading of water into microchannels fabricated on the VACNT surfaces

An easy and fast method to produce microchannels on the vertically aligned multiwalled carbon nanotube (VACNT) surfaces is described. Alternating superhydrophilic and superhydrophobic channels were built on VACNT films, using oxygen plasma functionalization and CO₂ laser treatment, respectively. A combined effect of wettability and capillary forces promotes an effective spreading, and confinement of water microdroplets on the superhydrophilic channels.     

微通道内液滴/气泡破裂动力学分析

微化学工程与技术是现代化学工程学科的前沿领域。微通道内液滴及气泡破裂动力学是决定多相过程并行微通道数目放大的基础与难点。破裂流型转换条件、界面动力学和尺寸调控等三方面是微通道内液滴与气泡破裂动力学的主要研究对象。讨论了对称微通道、非对称微通道、多级微通道、旁路微通道、含有障碍物的微通道内气泡和液滴破裂行为及影响因素,指出了目前微尺度下气泡与液滴破裂行为相关研究工作存在的不足,并对该领域未来的发展进行了展望。     

Characterisation and analysis of microchannels and submicrometre surface roughness of injection moulded microfluidic systems using optical metrology

Abstract

Precision injection moulding of miniaturised products with microfeatures, such as channels for microfluidic applications, poses the greatest challenges in terms of tooling technology and process optimisation. The injection moulding process window of polypropylene was validated using a metrological approach for the production of a microfluidic substrate. The dimensional accuracy of microchannels that are 48 μm wide and 110 μm deep and the quality surface topography replication (surface roughness from 30 to 360 nm) were investigated using non-contact measuring instruments, such as an optical coordinate measuring machine and a white light interferometer respectively. The effect of the dimensional scale range on the micro-/nanofeature replication was evaluated, and it was found to be the dominant parameter if compared with the effect of the other process related parameters investigated (melt and mould temperatures and injection speed).     

High-Resolution Reconstruction Method for Presenting and Manipulating Particle Histogram Data

Modern electronic aerosol instruments such as the scanning mobility analyzer or the aerodynamic particle sizer provide size information at much higher resolution than older impactors or optical particle counters do. Whereas an impactor may span a decade of particle diameter with two or three stages, an aerodynamic instrument may do so with 25-30 channels and a scanning mobility spectrometer with 64 channels. Whether or not the channels represent real particle size discrimination, comparison of data between high- and low-resolution instruments would be made easier if the data resolution could be made comparable. Moreover, the problems of converting mass concentration measurements to number concentrations (and vice versa) would be facilitated if there were high resolution size bins to use for computations, rather than having to rely on some average size to represent all the particle sizes within a size band. We present here an interpolation method for redistributing coarse histogram data into finer size bins without compromising the original data and including information gleaned from experience with multiple aerosol measurements. It is based upon a simple smoothness criterion and can be implemented in many computer spreadsheet programs.     

Orthogonal Curvilinear Cake Filtration

Abstract

Conventional analysis of cake filtration considers cakes formed under one-dimensional rectilinear flow conditions. Other cake geometries are often encountered in practice, and more generalized equations are needed. An orthogonal curvilinear filter cake equation is derived here for an arbitrary orthogonal cake geometry. This equation is applied to rectangular, cylindrical, oblate-spheroidal, and elliptic-cylindrical geometry cakes through the coordinate scale factors. Experiments are run on Lucite cakes in rectangular and elliptic-cylindrical coordinates. The constitutive parameter, Λ, is determined from experiments in rectangular coordinates and is used to predict the performance of an elliptic-cylindrical cake. Comparison of the predicted results with the experimental results of the elliptic-cylindrical cake shows the consistency of the equation for different geometries. The filter assembly used in the experiments was designed to be pressurized without being submerged within a larger slurry agitation vessel. This allows ready access to the assembly walls for placement of probes for measuring local pressure and porosity in rectangular and elliptic-cylindrical geometries.