Bubble nucleation and growth in fluids

The nucleation and growth of CO₂ bubbles in non-Newtonian and Newtonian fluids that were initially supersaturated under different pressures are investigated in the present work. Quantitative information by means of two cameras reveals that at an immobile nucleation site the bubble grows rapidly followed by a linear increase in bubble diameter with time. After reaching a critical size, the bubble detaches from the stagnant site to rise in liquids with an exponential temporary increase for both the diameter and distance. A simple physical reasoning was proposed to qualitatively explain these observed phenomena. Recently, the growth rate and flow fields around a CO₂ micro-bubble were measured in a microdevice by a micro-Particle Image Velocimetry in water. This information at microscale gives new insight into the complex mechanism of bubble nucleation and growth in fluids and could help to develop a rigorous theoretical modelling and numerical simulation such as the Lattice Boltzmann approach.     

典型微管道流场数值模拟与Micro-PIV检测研究

针对微流体器件中几种典型微管道进行了理论与试验研究,采用CFD流场仿真技术对压力驱动方式下90°折管、突然扩散管、弯管三种流道的流态进行数值模拟,重点介绍了电渗驱动的流场仿真技术,并以十字交叉微管道为对象进行电渗流场仿真;同时采用玻璃微加工工艺制造了上述四种微流道模型,并通过Micro-PIV技术对其相应条件下的流场进行试验测试,并定量对比与分析了仿真与试验结果,结论显示Micro-PIV是适合于微米级流场检测的最有效试验手段之一,同时通过试验也验证了N-S方程在微米级流场数值计算中是依然适用的。     

Dynamics of bubble breakup in a microfluidic T-junction divergence

The aim of this work is to investigate experimentally the bubble breakup in a microfluidic T-junction divergence using a high-speed digital camera and a micro-Particle Image Velocimetry (micro-PIV) system. The breakup and non-breakup of N₂ bubbles in glycerol–water mixtures with several concentrations of sodium dodecyl sulphate (SDS) as surfactant were studied with capillary number ranging from 0.001 to 0.1. The cross section of PMMA square microchannel is 400 μm wide and 400 μm deep. Four various flow patterns were observed at the T-junction by changing gas and liquid flow rates. The dynamics of three various types of symmetric breakup of bubbles were investigated. The symmetric breakup of bubbles type I is mainly controlled by the augmented pressure in liquid phase. The symmetric breakup of bubbles type II is controlled by both the increased pressure and viscous forces. In the symmetric breakup of bubbles type III, a scaling law for the minimum bubble neck and the remaining time during bubble breaking process were found. The transitions between breakup and non-breakup of bubbles were investigated, and a power–law relationship between bubble extension and capillary number was proposed to predict the transitions between adjacent regimes. Our experimental results reveal that the bubble breakup in a microfluidic T-junction divergence is similar to the droplet behaviours in such a device ( [Jullien et al., 2009] , [Leshansky and Pismen, 2009] and [Link et al., 2004] ).     

非牛顿流体微流场测量Micro PIV系统研究

介绍了一种专用于非牛顿流体微流动测量Micro-PIV系统,主要包括微流场激发、微流动观察记录与数据处理三大部分。针对非牛顿流体流动激发形式多样性的特点,微流场激发部分包括显微镜冷热台、波形发生器和磁场发生仪,可以产生激发非牛顿流体为流场所需的电、磁及温度场。观察记录部分主要包括荧光显微镜、CCD等,用于观察并记录荧光示踪粒子的运动,获取荧光示踪粒子运动的视频文件。数据处理部分用于将视频文件进行图像处理以得到所需流场数据。应用该系统对非牛顿流体5CB液晶在电场作用下所激发的微流动进行了测量,重点测量了液晶盒侧面的速度剖面图,所得到的试验结果与计算结果非常吻合,且比传统测量方法更加快速、准确。此外,对温度场变化产生的液晶缺陷进行了试验研究,得到了连续加热冷却状态下的液晶缺陷形成温度变化趋势,即形成缺陷的温度逐渐升高,从开始的31.25℃逐渐升高至34.4℃后保持平稳。     

基于Micro-PIV的微观流场检测技术

介绍了一种针对微观流场检测数字粒子图像测速技术,通过添加荧光显微装置,改进激光入射与照明方式、聚焦平面控制以及纳米级示踪粒子布朗运动误差消除算法等软硬件方面的技术创新,解决了微观检测的关键问题。针对微流体特征尺度小、精度要求高的特点,建立了具有自主知识产权的Micro－PIV微流场测试平台,并就几种典型微观流场进行了检测。试验结果表明该测试技术可对微米级稳态、瞬变流场进行全流场速度分布的检测,各项性能指标达到国际先进水平。     

Micro_PIV三维测量图像处理及优化研究

Micro-PIV是一种重要的微尺度流动测速技术,可获得示踪颗粒速度和绘制流体速度场.在实验室已有显微测量技术基础上,发展了Micro-PIV三维测量图像处理技术,并对图像处理进行了优化和修正.颗粒图像中心二维坐标的精确性关键在于如何根据图像中心主要特征进行识别定位.荧光颗粒的三维识别是基于颗粒衍射图像随离焦量变化而变化,通过待测实验图像与数值模板的匹配实现Micro-PIV三维测量,数值图像的离焦量就是实验图像的离焦量.在分析实验图像和数值模板基础上分别对实验图像进行预处理和对数值模板进行修正,使得实验荧光颗粒图像与数值模板匹配效果更好.     

Effects of miter bend on pressure drop and flow structure in micro-fluidic channels

The characteristics of a pressure-driven water flow including flow micro-structures and pressure drops are investigated in serpentine micro-channels with miter bends. The micro-channels have rectangular cross-sections with hydraulic diameters of 0.209 mm, 0.412 mm and 0.622 mm, respectively. To segregate the bends and entrance effects individually from the total pressure drop, for each size three types of micro-channels: straight short, straight long and long serpentine, were fabricated to get the reliable pressure data without entrance effect. A micron-resolution particle image velocimetry system (micro-PIV) was develop and used to obtain the detailed velocity vector field. The experimental results show that the vortices around the outer and inner walls of the bend do not form when Re < 100. Those vortices appear and continue to develop with increasing Re number when Re is larger than a value around 100, and the shape and size of the vortices almost remain constant when the Re is larger than a value around 1000. The experimentally observed additional pressure drop due to the bend is commensurate with the strength of the calculated vortices. The bend loss coefficient K_b was observed to be related only with the Re number when Re < 100, but with the Re number and channel size when Re > 100. It almost keeps constant and changes in the range of ±10% when the Re is larger than a threshold value somewhere in 1000–1500.     

Biomolecular Nano-Flow-Sensor to Measure Near-Surface Flow

We have proposed and experimentally demonstrated that the measurement of the near-surface flow at the interface between a liquid and solid using a 10 nm-sized biomolecular motor of F₁-ATPase as a nano-flow-sensor. For this purpose, we developed a microfluidic test-bed chip to precisely control the liquid flow acting on the F₁-ATPase. In order to visualize the rotation of F₁-ATPase, several hundreds nanometer-sized particle was immobilized at the rotational axis of F₁-ATPase to enhance the rotation to be detected by optical microscopy. The rotational motion of F₁-ATPase, which was immobilized on an inner surface of the test-bed chip, was measured to obtain the correlation between the near-surface flow and the rotation speed of F₁-ATPase. As a result, we obtained the relationship that the rotation speed of F₁-ATPase was linearly decelerated with increasing flow velocity. The mechanism of the correlation between the rotation speed and the near-surface flow remains unclear, however the concept to use biomolecule as a nano-flow-sensor was proofed successfully.     

A microfluidic flow-converter based on a double-chamber planar micropump

A microfluidic flow-converter that transforms an oscillatory flow into a steady-like flow in a reciprocating-type pumping device is successfully developed in this study. The flow quality at the outlet is found to be significantly improved. The present micro-device is composed of two single-chamber PZT micropumps in parallel arrangement and can be fabricated using simple micro-electro-mechanical-system (MEMS) techniques. Based on the concept of the electronic bridge converter, the flow rectification is supported by four passive planar valves. Two operation modes, in-phase and anti-phase, were used to test the performance of the present device. In addition, the flow characteristics at the outlet were examined by an externally triggered micro-PIV system. The results reveal that the current flow-converter provided both high volume and smoothly continuous flow rates at the outlet when it was in anti-phase mode. Moreover, the volume flow rate was linearly proportional to the excitation frequency within a specific frequency regime. This indicates that the flow-converter was easily operated and controlled. The present microfluidic flow-converter has great potential for integration into future portable micro- or bio-fluidic systems.     

微管道内湍流转捩的实验研究

研究旨在确定微管道内流动从层流到湍流转捩的临界雷诺数。利用微观粒子图像测速技术(Micro-PIV)研究了去离子水在内径为230μm的圆形截面玻璃微管道内的流场结构,得到了从层流到充分发展湍流各流动状态下的轴向平均速度分布和湍流度分布,实验雷诺数为1020~3145,同时研究了微管道内的流动阻力特性。平均速度场和脉动速度场的实验结果表明微管道内从层流到湍流的转捩发生在Re=1800~1900左右,与流动阻力的测量结果一致,与宏观流动比较,并未发现微管道内的流动转捩有明显提前。实验结果还显示,当Re>2700时,微管道内的平均流速分布和相对湍流度分布呈现典型的充分发展湍流状态特征。