Effects of contact-stress on hot-embossed PMMA microchannel wall profile

Hot-embossing (thermal-compression) based microchannel fabrication techniques have gained much attention recently due to their low-cost setup and ease of implementation. However, not much effects have been attempted in trying to understand or characterize the mechanics of the hot-embossing process in fabricating microchannels. Most research groups still rely on trial-and-error processes to hot-emboss microchannels for microfluidic control applications. The present paper describes the application of the contact-stress analysis to understand the mechanism of using molds with micro-features to hot-emboss PMMA substrates. Experimental results showing that the resulting microchannel wall profile can be predicted with good accuracy via a close-form solution of the analysis are also presented.     

Flows in micro fluidic networks: From theory to experiment

When complex flow structures are designed, such as in DNA computing, it is essential to be able to predict the flow pattern of the solutions in the fluidic network. A model based on the resistance of the channels and flow velocities of the inlets can eliminated re-iterative design steps. We have constructed a symbolic model using Mathematica ® to determine the desired flow pattern based on the equations of Ohm and Kirchoff. The values from this simulation were used in a flow simulation program and then tested in a microflow network. Results show that the simulation and calculation match very well, while the experiments in the fluidic network show a flow pattern as predicted by the model.     

Optimality of the Münch mechanism for translocation of sugars in plants

Plants require effective vascular systems for the transport of water and dissolved molecules between distal regions. Their survival depends on the ability to transport sugars from the leaves where they are produced to sites of active growth; a flow driven, according to the Münch hypothesis, by osmotic gradients generated by differences in sugar concentration. The length scales over which sugars are produced (L_leaf) and over which they are transported (L_stem), as well as the radius r of the cylindrical phloem cells through which the transport takes place, vary among species over several orders of magnitude; a major unsettled question is whether the Münch transport mechanism is effective over this wide range of sizes. Optimization of translocation speed predicts a scaling relation between radius r and the characteristic lengths as r ∼ (L_leaf L_stem)^1/3. Direct measurements using novel in vivo techniques and biomimicking microfluidic devices support this scaling relation and provide the first quantitative support for a unified mechanism of sugar translocation in plants spanning several orders of magnitude in size. The existence of a general scaling law for phloem dimensions provides a new framework for investigating the physical principles governing the morphological diversity of plants.     

内置周期挡板的T-型微混合器

设计了一种流道内布置周期挡板结构的高效T-型微混合器来提高微流控系统的混合效率。该微混合器结构简单,周期布置的挡板可以有效地缩短流体混合所需的流道长度和时间,混合效率高。安排了正交实验组,利用计算流体力学软件ANSYS CFX研究了流道结构参数对混合效果的影响。采用静态田口分析法对数值模拟结果进行分析。结果表明:流道结构参数对混合效果的相对影响程度排列如下:挡板攻角(θ)流道高度(H)挡板宽度(L)相邻混合单元之间距离(D)。根据结构参数对混合效果的影响程度,得出研究参数范围内的最优组合为:θ=75°,H=0.4 Wm,L=0.7 Wm,D=0.6 Wm(这里Wm为流道宽度,等于200μm)。实验显示,结构参数符合最优参数组合的微混合器的混合效果提升显著,雷诺数Re=54时即可实现完全混合(混合指标M95%)。文中研究了流道结构对进出口压降的影响,结果显示,攻角θ对进出口压降的影响趋势在不同雷诺数下相同,参数H,D亦如此。     

Decoding the Chemical Language of Motile Bacteria by Using High‐Throughput Microfluidic Assays

Motile bacteria navigate chemical environments by using chemoreceptors. The output of these protein sensors is linked to motility machinery and enables bacteria to follow chemical gradients. Understanding the chemical specificity of different families of chemoreceptors is essential for predicting and controlling bacterial behavior in ecological niches, including symbiotic and pathogenic interactions with plants and mammals. The identification of chemical(s) recognized by specific families of receptors is limited by the low throughput and complexity of chemotaxis assays. To address this challenge, we developed a microfluidic‐based chemotaxis assay that is quantitative, simple, and enables high‐throughput measurements of bacterial response to different chemicals. Using the model bacterium Escherichia coli, we demonstrated a strategy for identifying molecules that activate chemoreceptors from a diverse compound library and for determining how global behavioral strategies are tuned to chemical environments.     

Foldable Printed Circuit Boards on Paper Substrates

This paper describes several low‐cost methods for fabricating flexible electronic circuits on paper. The circuits comprise i) metallic wires (e.g., tin or zinc) that are deposited on the substrate by evaporation, sputtering, or airbrushing, and ii) discrete surface‐mountable electronic components that are fastened with conductive adhesive directly to the wires. These electronic circuits—like conventional printed circuit boards—can be produced with electronic components that connect on both sides of the substrate. Unlike printed circuit boards made from fiberglass, ceramics, or polyimides, however, paper can be folded and creased (repeatedly), shaped to form three‐dimensional structures, trimmed using scissors, used to wick fluids (e.g., for microfluidic applications) and disposed of by incineration. Paper‐based electronic circuits are thin and lightweight; they should be useful for applications in consumer electronics and packaging, for disposable systems for uses in the military and homeland security, for applications in medical sensing or low‐cost portable diagnostics, for paper‐based microelectromechanical systems, and for applications involving textiles.