Topology optimization of microfluidic mixers

This paper demonstrates the application of the topology optimization method as a general and systematic approach for microfluidic mixer design. The mixing process is modeled as convection dominated transport in low Reynolds number incompressible flow. The mixer performance is maximized by altering the layout of flow/non‐flow regions subject to a constraint on the pressure drop between inlet and outlet. For a square cross‐sectioned pipe the mixing is increased by 70% compared with a straight pipe at the cost of a 2.5 fold increase in pressure drop. Another example where only the bottom profile of the channel is a design domain results in intricate herring bone patterns that confirm findings from the literature. Copyright © 2008 John Wiley & Sons, Ltd.     

Formation of artificial micro‐pits on Al alloy with the photon rupture method and the localized corrosion behavior of the formed pits

An in situ artificial micro‐pit fabrication method with an area selective electrochemical measurement technique was applied to investigate the effect of the geometry of artificially formed pits on their localized corrosion behavior in anodized 1000 series aluminum. This technique enables the fabrication of artificial micro‐pits with different aspect ratios (pit depth/pit diameter) in solutions. The aspect ratios of the fabricated artificial micro‐pits in this experiment could be varied from 0.13 to 1.83 by controlling the irradiation time of the focused pulsed YAG laser beam. By applying a constant potential to the final laser‐beam‐irradiated spot in chloride environments, localized dissolution occurred only at the laser beam irradiated area, because the anodic oxide film acted as an insulator. The corrosion current and charge increase with increasing aspect ratio at any applied potential. Copyright © 2010 John Wiley & Sons, Ltd.     

Micro fabrication of microlens arrays by micro dispensing

In this study, master of the microlens arrays is fabricated using micro dispensing technology, and then electroforming technology is employed to replicate the Ni mold insert of the microlens arrays. Finally, micro hot embossing is performed to replicate the molded microlens arrays from the Ni mold insert. The resin material is used as the dispensing material, which is dropped on a glass substrate. The resin is exposed to a 380 W halogen light. It becomes convex under surface tension on the glass substrate. A master for the microlens arrays is then obtained. A 150‐nm‐thick copper layer is sputtered on the master as an electrically conducting layer. The electroforming method replicates the Ni mold insert from the master of the microlens arrays. Finally, micro hot embossing is adopted to replicate the molded microlens arrays. The micro hot embossing experiment employs optical films of polymethylmethacrylate (PMMA) and polycarbonate (PC). The processing parameters of micro hot embossing are processing temperature, embossing pressure, embossing time, and de‐molding temperature. Taguchi's method is applied to optimize the processing parameters of micro hot embossing for molded microlens arrays. An optical microscope and a surface profiler are utilized to measure the surface profile of the master, the Ni mold insert and the molded microlens arrays. AFM is employed to measure the surface roughness of the master, the Ni mold insert and the molded microlens arrays. The sag height and focal length are determined to elucidate the optical characteristics of the molded microlens arrays. Copyright © 2009 John & Sons, Ltd.     

Dual crosslinked pH‐ and temperature‐sensitive hydrogel beads for intestine‐targeted controlled release

Novel drug‐loaded hydrogel beads for intestine‐targeted controlled release were developed by using pH‐ and temperature‐sensitive carboxymethyl chitosan‐graft‐poly(N,N‐diethylacrylamide) (CMCTS‐g‐PDEA) hydrogel as carriers and vitamin B₂ (VB₂) as a model drug. The hydrogel beads were prepared based on Ca²⁺ ionic crosslinking in acidic solution and formed dual crosslinked network structure. The structure of hydrogel and morphology of drug‐loaded beads were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The study about swelling characteristics of hydrogel beads indicated that the beads had obvious pH‐ and temperature‐sensitivity. In vitro release studies of drug‐loaded beads were carried out in pH 1.2 HCl buffer solution and pH 7.4 phosphate buffer solution at 37°C, respectively. The results indicated that the dual crosslinked method could effectively control the drug release rate under gastrointestinal tract (GIT) conditions, which was superior to traditional single crosslinked beads. In addition, the effects of grafting percentage, pH value, and temperature on the release behavior of the VB₂ were investigated. The drug release mechanism of CMCTS‐g‐PDEA drug‐loaded beads was analyzed by Peppa's potential equation. According to this study, the dual crosslinked hydrogel beads based on CMCTS‐g‐PDEA could serve as suitable candidate for drug site‐specific carrier in intestine. Copyright © 2009 John Wiley & Sons, Ltd.     

微型气相色谱仪热导检测器放大电路设计

Agilent公司生产的Agilent 3000+系列色谱仪是微型气相色谱仪(micro GC)的典型代表,其热导检测器的信号放大电路和模数转换器(analog-to-digital convertor, ADC)存在功耗大、工作温度过高等不足.文中分析了micro GC电路的功能需求,从选用低噪声的24 bit Δ-Σ ADC ADS1255入手,设计了高共模电压容限、低噪声的全差分放大电路及其他外围电路,并且对全差分放大电路建立了噪声模型,计算了其噪声理论值,优化了系统设计参数.另外,还设计了一个测试平台,对所设计的全差分放大电路和ADC的性能进行了全面的测试评估,结果表明新设计的热导检测器放大电路与ADC的总噪声(以美国材料与试验协会(ASTM)标准值计)仅为1.25 μV,总功耗降低了3.7 W,满足micro GC的功能需求,而且可靠性高、体积小、结构简单,可用于新一代micro GC的研发和生产.     

Effect of indium content and rapid solidification on microhardness and micro‐creep of Sn‐Zn eutectic lead free solder alloy

The Sn‐Zn alloys have been considered as lead‐free solders. In this paper, the effect of 0.0, 0.5, 1.0, 1.5 and 2.0 wt.% Indium as ternary additions on melting temperature, structure, microhardness and micro‐creep of the Sn‐9Zn lead‐free solders were investigated. It is shown that the alloying additions of Indium to the Sn‐Zn binary system result in a suppression of the melting point to 187.9 °C. From x‐ray diffraction analysis, a new intermetallic compound phase, designated β‐In₃Sn is detected. The formation of an intermetallic compound phase causes a pronounced increase in the electrical resistivity and mechanical strength. Also, an interesting connection between dynamic Young's modulus and the axial ratio (c/a) of the unit cell of the β‐Sn was found in which Young's modulus increases with increasing the axial ratio (c/a). The ternary Sn‐9Zn‐xIn exhibits creep resistance superior to Sn‐9Zn binary alloy. The better creep resistance of the ternary alloy is attributed to solid solution effect and precipitation of In₃Sn in the Sn matrix. The addition of small amounts of In is found to refine the effective grain size and consequently, improves hardness. The 89%Sn‐9%Zn‐2%In alloy is a lead‐free solder designed for possible drop‐in replacement of Pb‐Sn solders. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

双棱镜干涉实验课堂教学演示的可视化研究

传统的双棱镜干涉实验是通过测微目镜来观测实验现象的,其视场很小,不利于课堂教学演示。针对这一问题进行课堂教学演示的可视化研究,引入摄像头观测,增强了课堂教学演示的直观性,提高了课堂教学演示的效率和质量。     

Synthesis, characterization and comparison of sediment electro-codeposited nickel-micro and nano SiC composites

Nickel-silicon carbide composites were produced using 1 μm and 50 nm size powders from a conventional Watt's bath using tetra methyl ammonium hydroxide as the surfactant. Sediment codeposition technique with horizontal electrodes was used. The effect of silicon carbide concentration and bath operating parameters on the volume percents and deposition rates of coatings obtained with the two different particles was studied. Substantial improvements in mechanical properties such as hardness, wear resistance, scratch resistance and roughness were obtained with the nanocomposite material, as compared with composites containing microsized particles.     

微小圆管中分裂火焰的实验与理论研究

对丙烷/空气在内径2 mm的圆管内的预混燃烧进行了实验研究,借助于高速数码摄像机发现了分裂火焰现象,其中一个为向上游传播的较亮的常规火焰,另一个为向下游传播的较暗的微弱火焰。这些火焰先后熄灭,经过一段时间后又重复发生自着火、分裂、反向传播、灭火过程。这种现象在富燃、化学恰当比以及贫燃火焰中都有存在。一维非稳态计算表明化...     

SOFC/MGT混合系统耦合特性研究

在SOFC/MGT混合分布式能源系统中,选择不同的系统参数将对SOFC乃至SOFC/MGT混合系统的输出特性、安全运行等造成很大的影响。本文建立了一个典型的SOFC/MGT混合系统计算模型,研究了透平入口温度及燃气轮机压比对混合系统性能的影响。结果表明,透平入口温度和压比增大时,系统的输出功率和效率都会下降,而透平入口...