带凹槽的微通道中液滴运动数值模拟

运用改进的耗散粒子动力学方法模拟了液滴在由凹槽所构成的粗糙表面微通道内的运动行为.改进的耗散粒子动力学方法采用新近提出的一种短程排斥、长程吸引相互作用势能函数,从而可以模拟带有自由面的流体,如液滴等.模拟了新势能函数下液滴与固体壁面的静态接触角,并用2次多项式拟合了"接触角-a_wf/a_f"变化曲线.研究了液滴在带凹槽的微通道中运动时,微通道壁面浸润性、外场力、液滴温度对液滴流动特性的影响.研究表明壁面浸润性和外场力对液滴流动特性的影响较大,液滴温度对液滴流动特性的影响较小.研究结果对运用耗散粒子动力学方法模拟并分析微流体在复杂微通道的流动有一定的参考价值.     

Thermal etching process of microscale pits on the GaAs(001) surface

When a GaAs(001) substrate is heated up to 650 °C in a scanning electron microscope (SEM) vacuum chamber with vacuum range from 10^–4 Torr to 10^–5 Torr, real‐time SEM observation reveals microscale pits on GaAs substrate surface. The annealing process of GaAs substrate in vacuum causes excess evaporation of arsenic and accumulation of gallium as liquid droplets on the surface. As the function of electrochemical drills, the gallium droplets etch away GaAs beneath the surface to make microscale holes on GaAs substrate. With small amount of oxygen in the chamber acting as etching catalyst, gallium droplets etch GaAs much faster than in ultra‐high vacuum (UHV) MBE chamber. This process provides an easy technique to fabricate microscale pits on GaAs(001) surface.

     

Efficient and accurate time adaptive multigrid simulations of droplet spreading

An efficient full approximation storage (FAS) Multigrid algorithm is used to solve a range of droplet spreading flows modelled as a coupled set of non‐linear lubrication equations. The algorithm is fully implicit and has embedded within it an adaptive time‐stepping scheme that enables the same to be optimized in a controlled manner subject to a specific error tolerance. The method is first validated against a range of analytical and existing numerical predictions commensurate with droplet spreading and then used to simulate a series of new, three‐dimensional flows consisting of droplet motion on substrates containing topographic and wetting heterogeneities. The latter are of particular interest and reveal how droplets can be made to spread preferentially on substrates owing to an interplay between different topographic and surface wetting characteristics. Copyright © 2004 John Wiley & Sons, Ltd.     

Optimization of dispersive liquid–liquid microextraction based on the solidification of floating organic droplets using an orthogonal array design and its application for the determination of fungicide concentrations in environmental water samples

A dispersive liquid–liquid microextraction method based on the solidification of floating organic droplets was developed as a simple and sensitive method for the simultaneous determination of the concentrations of multiple fungicides (triazolone, chlorothalonil, cyprodinil, and trifloxystrobin) in water by high‐performance liquid chromatography with variable‐wavelength detection. After an approach varying one factor at a time was used, an orthogonal array design [L₂₅ (5⁵)] was employed to optimize the method and to determine the interactions between the parameters. The significance of the effects of the different factors was determined using analysis of variance. The results indicated that the extraction solvent volume significantly affects the efficiency of the extraction. Under optimal conditions, the relative standard deviation (n = 5) varied from 2.3 to 5.5% at 0.1 μg/mL for each analyte. Low limits of detection were obtained and ranged from 0.02 to 0.2 ng/mL. In addition, the proposed method was applied to the analysis of fungicides in real water samples. The results show that the dispersive liquid–liquid microextraction based on the solidification of floating organic droplets is a potential method for detecting fungicides in environmental water samples, with recoveries of the target analytes ranging from 70.1 to 102.5%.     

基于非接触电导检测的PCR液滴检测芯片的研究

数字PCR是继实时定量PCR之后新兴发展起来的一种绝对定量分析技术,广泛应用于疾病早期检测诊断和微生物检测分析等领域。然而,在数字PCR的微反应器检测环节,常规的光学检测方法需要复杂的光学设备,应用环境有限,成本高。因此研究了一种基于非接触电导检测的PCR液滴检测芯片,用电导检测取代传统的光学检测,有效降低了数字PCR结果读出系统的成本。通过优化电极结构与排布,采用三电极平行3D电极结构,减小了相对误差,提高了灵敏度,实现了无标记的PCR反应液滴阴性阳性比例检测。在DNA模板浓度为0.5×10~4～3×10~4 copies/μL内实现了良好的相关性(r~2=0.998),检测通量可达100个/秒。     

Spatially Controlled Supramolecular Polymerization of Peptide Nanotubes by Microfluidics

Despite the importance of spatially resolved self‐assembly for molecular machines, the spatial control of supramolecular polymerization with synthetic monomers had not been experimentally established. Now, a microfluidic‐regulated tandem process of supramolecular polymerization and droplet encapsulation is used to control the position of self‐assembled microfibrillar bundles of cyclic peptide nanotubes in water droplets. This method allows the precise preferential localization of fibers either at the interface or into the core of the droplets. UV absorbance, circular dichroism and fluorescence microscopy indicated that the microfluidic control of the stimuli (changes in pH or ionic strength) can be employed to adjust the packing degree and the spatial position of microfibrillar bundles of cyclic peptide nanotubes. Additionally, this spatially organized supramolecular polymerization of peptide nanotubes was applied in the assembly of highly ordered two‐dimensional droplet networks.     

Formation of Particle Coated Fusiform Droplets via Lowering Interface Tension with Polyaluminum Sulfate

It was found that the interface tension between water and alkenyl succinic anhydride (ASA) was significantly reduced by polyaluminum sulfate (PAS), increased considerably though by TiO₂ nanoparticle. PAS with basicity of 0.75 (PAS-0.75) reduced the interface tension to a larger extent than PAS with basicity of 0.3 (PAS-0.3). By reducing interface tension with PAS-0.75, ASA-in-water emulsion bearing fusiform geometries was constructed. The emulsion stabilized by PAS-0.3 and TiO₂ nanoparticle bore spherical shapes with the exception when mass fraction of TiO₂ nanoparticle was low, in which case fused nonspherical drops were formed. Forming nonspherical emulsion crucially depends on ASA-water interface tension, where a critical interface tension was identified to be 0.6–0.7 mN/m. The fusiform geometries were transformed into spherical shapes when interface tension was higher than 0.7 mN/m. Both the lowering mechanism of the interface tension and the formation mechanism of the fusiform emulsion were proposed.      

Composition Ripening in Mixed Water-in-Oil Emulsions Stabilized with Solid Particles

Water and oil transport in emulsified systems is far from being elucidated. Calorimetric analysis has proved to be an appropriate technique to study composition ripening in mixed water in oil emulsions. In this article, the role of the stabilizing agent is studied and particular attention is given to emulsions stabilized solely with solid particles. Mixed emulsions are prepared by mixing two simple water-in-oil (W/O) emulsions, one with pure water droplets and one with droplets containing an aqueous urea solution. At different time intervals, a sample is introduced in a calorimeter cell and submitted to successive cooling and heating cycles. During the cooling phase, the aqueous internal phase solidifies at a temperature which depends on its composition. Just after the mixed emulsion was prepared, the calorimetric experiment identified two solidification peaks, one corresponding to pure water droplets, and the other one to urea solutions. After a long enough stabilization time, just one peak was observed, showing that the systems evolved toward one type of droplets characterized by a unique composition, due to water transfer between the two aqueous phases. The effect of emulsion stabilizing agent (particles or nonionic emulsifier) on the kinetics of water transfer was investigated.