Comparison of detection efficiencies of negatively charged gold-alkanethiolate-, gold-sulfur- and gold-clusters in ToF-SIMS

In order to improve quantification of high mass ions, the influence of cluster composition on detection efficiencies has been studied using a TOF-SIMS IV with the extended capability of postaccelerating ions up to 20 keV. In this experimental study, we focus on the comparison of detection efficiencies for three types of negatively charged secondary cluster ions: gold-alkanethiolate-clusters (Au_xM_y), gold-sulfur-clusters (Au_xS_y) and gold-clusters (Au_x). The clusters were sputtered from self-assembled monolayers of hexadecanethiols on gold substrates using 10 keV Ar⁺ primary ions. The detection efficiencies were derived on the basis of a function for the secondary electron yield and a fourth-order approximated Poisson probability distribution for electron propagation and amplification within the microchannel plate.In addition to the well-known dependence of detection efficiencies on ion mass and energy, which has already been studied for positively charged ions, we were able to show a similar behaviour for the investigated negatively charged secondary ions. We have observed major variations among the three types of clusters at similar mass and energy as predicted in a theoretical approach. The observed differences are due to the different composition of the investigated clusters which has a major influence on the kinetic ion induced electron emission within the microchannel plate. For the first time it was possible to experimentally verify these predictions for detection efficiencies.     

Numerical analysis of roughness effect on microtube heat transfer

Roughness effect on the heat transfer and pressure loss performances of microscale tubes and channels are investigated through a finite element CFD code. Surface roughness is explicitly modelled through a set of random generated peaks along the ideal smooth surface. Different peak shapes and distributions are considered; geometrical parameters are representative of tubes in the diameter range from 50 to 150 μm. The use of a fine enough mesh allows the direct computation of tube performances under the assumption of incompressible fully developed flow. As a result, a significant increase in Poiseuille number is detected for all of the configurations considered, while the effect of roughness on heat transfer rate is smaller and highly dependent on the tube shape.     

Influence of the three-dimensional heterogeneous roughness on electrokinetic transport in microchannels

Surface roughness has been considered as a passive means of enhancing species mixing in electroosmotic flow through microfluidic systems. It is highly desirable to understand the synergetic effect of three-dimensional (3D) roughness and surface heterogeneity on the electrokinetic flow through microchannels. In this study, we developed a three-dimensional finite-volume-based numerical model to simulate electroosmotic transport in a slit microchannel (formed between two parallel plates) with numerous heterogeneous prismatic roughness elements arranged symmetrically and asymmetrically on the microchannel walls. We consider that all 3D prismatic rough elements have the same surface charge or zeta potential, the substrate (the microchannel wall) surface has a different zeta potential. The results showed that the rough channel's geometry and the electroosmotic mobility ratio of the roughness elements' surface to that of the substrate, epsilon(mu), have a dramatic influence on the induced-pressure field, the electroosmotic flow patterns, and the electroosmotic flow rate in the heterogeneous rough microchannels. The associated sample-species transport presents a tidal-wave-like concentration field at the intersection between four neighboring rough elements under low epsilon(mu) values and has a concentration field similar to that of the smooth channels under high epsilon(mu) values.     

一步法高通量可控制备生物相容水/水微囊及其响应释放

生物相容水/水微囊在药物递送、 医学治疗等领域具有重要应用. 本文通过设计同轴微流控器件, 结合数值模拟优化和流动阻力分析, 实现一步法高通量可控制备大小均匀、 尺寸可控、 壁厚可调、 生物相容的水/水微囊. 采用实验研究和数值模拟相结合的方式, 研究了微流控器件结构、 内相流速、 外相流速、 外相/空气界面张力、 内相/外相界面张力、 内相黏度和外相黏度等参数对水/水微囊直径和壁厚的调控规律. 通过微通道流动阻力分析, 设计多通道平行放大微流控器件, 实现尺寸均匀可控水/水微囊的高通量制备. 验证了生物相容水/水微囊作为活性物质的理想载体, 可以通过改变pH或溶解囊壁释放载体, 进而实现活性物质的pH响应释放, 为其实际应用奠定了基础.     

金阴极微通道板能谱响应的理论研究

分析了金阴极微通道板在X射线段（0.1～10 keV）的能谱响应,从阴极量子效率,X射线在通道材料中的衰减,微通道壁的铅层的光电效应,微通道板通道增益等多个方面进行综合计算,结果表明:得出较完善的阴极型微通道板能谱响应理论公式及其数值模拟曲线.在只考虑一个通道,增益值为1时,微通道板的能谱响应完全取决于金阴极的量子效率,若考虑多通道效应,微通道板的能谱响应受通道材料元素吸收边的影响发生突变,且通道数目越多,影响越显著;能谱响应随电压增大呈增长趋势,但会受到微通道板饱和电流的限制.实验给出了微通道板能谱响应与入射角的关系曲线,确定了能获得增益的最小入射角.     

Polyelectrolyte-modified short microchannel for cation separation

Three alkali cations, potassium, sodium, and lithium, have been separated within 15 s in a 1 cm long polymer microchip. The separation microchannel is modified by a polycation, poly(allylammonium chloride), which makes the channel surfaces positively charged leading to a reversed electroosmotic flow (EOF) when compared to bare channels. Due to the decreased apparent mobility of the cations, the separation resolution is improved allowing the use of shorter channels.     

Replication of microchannel structures in polymers using laser fabricated glass–ceramic stamp

A new microreplication process with photo-etchable glass–ceramic stamps and polymers is presented. This process has two main advantages: a rapid master fabrication with a laser process and a flexible replication process compared with conventional nano- or microreplication technique on polymers. Photo-etchable glass–ceramics are used for the master stamp. Micropatterns can be rapidly transferred with a laser direct writing process and the removal of the glass–ceramics can be efficiently achieved with a wet etching process. Therefore, microstructures with flat bottom surfaces and straight sidewall structures can be obtained, which is difficult in the laser direct writing process. A microstamping process of applying heat and pressure, also referred to as hot embossing lithography or microstamping, can replicate microstructures on polymer surfaces. In this work, the fabricated glass–ceramic stamps are used for the replication process and various replicated polymer microstructures are presented.     

One-stage fabrication of sub-micron hydrophilic microchannels on PDMS

A sub-micron hydrophilic microchannel was fabricated on poly(dimethylsiloxane) (PDMS) in one step using vacuum ultraviolet light (VUV) lithography in vacuum. The topographies and properties of the irradiated PDMS surface were characterized and analyzed by atomic force microscopy (AFM), and the chemical composition changes on VUV-treated PDMS analyzed by X-ray photoelectron spectroscopy (XPS). The hydrophilic stability of irradiated PDMS surface was studied by static water contact angle. As demonstrated, the hydrophilicity on surface of PDMS microchannel can be kept for a longer term even three months after the treatment.     

A Lattice Boltzmann model with high Reynolds number in the presence of external forces to describe microfluidics

We present here a lattice Boltzmann model with high Reynolds number in the presence of external force fields to describe electrokinetic phenomena in microfluidics, by considering pressure as the only external force for liquid flow. Our results from a 9-bit square lattice Boltzmann model are in excellent agreement with experimental data in pressure-driven microchannel flow that could not be fully described by electrokinetic theory. The difference between the predicted and experimental Reynolds numbers from pressure gradients are well within 5%. Our results suggest that the lattice Boltzmann model described here is an effective computational tool to predict the more complex microfluidic systems that might be problematic using conventional methods.     

Two-phase annular flow and evaporative heat transfer in a microchannel

A physical and mathematical model has been developed to predict the two-phase flow and heat transfer in a microchannel with evaporative heat transfer. Sample solutions to the model were obtained for both constant wall temperature and constant wall heat flux conditions. Results are provided for evaporation rate, liquid film thickness, liquid and vapor phase pressure and temperature distributions. In addition to the sample calculations that were used to illustrate the transport characteristics, computations based on the current model were performed to generate results for comparisons with the experimental results of Qu and Mudawar (2004) where two different mass flow rates of the working fluid were used in the experiment. The comparisons of total pressure drops with the experimental data of Qu and Mudawar (2004) cover the wall heat flux range of 142.71-240 W/cm² with a total channel mass flux of 400.1 kg/m² s and also the wall heat flu range of 99.54-204.39 W/cm² with total channel mass flux of 401.9 kg/m² s. The calculated results from the current model match closely with those of Qu and Mudawar (2004).