Velocity measurement in microchannels with a laser confocal microscope and particle linear image velocimetry

Particle linear image velocimetry (PLIV), a novel velocity measurement method for microfluidic characterization, is reported. The method records a series of one-dimensional images that represent the trace of particles in the flow across a one-dimensional imager. Linear imaging results in a faster frame rate than planar imaging, allowing observations at larger magnifications, or the measurement of faster flow in real-time than comparable techniques. In contrast to particle image velocimetry (PIV), PLIV captures fast flow dynamics and does not require high-speed cameras or shutters. Furthermore, PLIV is adaptable to multiple linear imager formats and, as one example, can use laser scanning confocal microscopes (LSCMs) that acquire images slowly, but with high optical sectioning resolutions. A higher spatial resolution can be obtained for flows where the in-plane velocity gradient in the direction of the optical path (z direction) is important. The PLIV algorithm is fully described and its utility is demonstrated by the measurement of: a steady Poiseuille flow with 1-m spatial resolution in a microfluidic environment; dynamic measurement of transient flows with 250-ms temporal resolution; and the simultaneous calculation of particle dimension statistics.     

Experimental and numerical investigation of unsteady cavitating flows through a 2D hydrofoil

In the present study, firstly, the unsteady cavitating flows around a hydrofoil are studied based on the flow visualization and detail velocity measurement, a high-speed video camera is used to visualize the flow structures, and a particle image velocimetry (PIV) technique is applied to the measurement of the time-averaged and instantaneous velocity and vorticity fields. The results show that the unsteadiness of mass transfer process between the vapor and the two-phase regions is substantial, a self-oscilla...     

Multispectral processing for color particle image velocimetry

Since the adoption of digital video cameras and cross-correlation methods for particle image velocimetry (PIV), the use of color images has largely been abandoned. Recently, however, with the re-emergence of color-based stereo and volumetric techniques, and the extensive use of color microscopy, color imaging for PIV has again become relevant. In this work, we explore the potential advantages of color PIV processing by developing and proposing new methods for handling multi-color images. The first method uses cross-correlation of every color channel independently to build a color vector cross-correlation plane. The vector cross-correlation can then be searched for one or more peaks corresponding to either the average displacement of several flow components using a color ensemble operation, or for the individual motion of colored particles, each with a different behavior. In the latter case, linear unmixing is used on the correlation plane to separate each known particle type as captured by the different color channels. The second method introduces the use of quaternions to encode the color data, and the cross-correlation is carried out simultaneously on all colors. The resulting correlation plane can be searched either for a single peak, corresponding to the mean flow or for multiple peaks, with velocity phase separation to determine which velocity corresponds to which particle type. Each of these methods was tested using synthetic images simulating the color recording of noisy particle fields both with and without the use of a Bayer filter and demosaicing operation. It was determined that for single-phase flow, both color methods decreased random errors by approximately a factor of two due to the noise signal being uncorrelated between color channels, while maintaining similar bias errors as compared to traditional monochrome PIV processing. In multi-component flows, the color vector correlation technique was able to successfully resolve displacements of two distinct yet coupled flow components with errors similar to traditional grayscale PIV processing of a single phase. It should be noted that traditional PIV processing is bound to fail entirely under such processing conditions. In contrast, the quaternion methods frequently failed to properly identify the correct velocity and phase and showed significant cross talk in the measurements between particle types. Finally, the color vector method was applied to experimental color images of a microchannel designed for contactless dielectrophoresis particle separation, and good results were obtained for both instantaneous and ensemble PIV processing. However, in both the synthetic color images that were generated using a Bayer filter and the experimental data, a significant peak-locking effect with a period of two pixels was observed. This effect is attributed to the inherent architecture of the Bayer filter. In order to mitigate this detrimental artifact, it is suggested that improved image interpolation or demosaicing algorithms tuned for use in PIV be developed and applied on the color images before processing, or that cameras that do not use a Bayer filter and therefore do not require a demosaicing algorithm be used for color PIV.     

Optical measurement of pore scale velocity field inside microporous media

An experimental technique to quantify velocity field at pore scale with in microporous media, formed by packing of microglass spheres of size 200?μm inside a glass micro-model, is presented. A microparticle image velocimetry (μ-PIV) system is used to study velocity fields at four different spatial regions in the microporous medium. A combined particle image velocimetry (PIV) and particle tracking velocimetry (PTV) scheme is used to quantify velocity within a typical pore size of 10–50?μm. The experiments are conducted at four different flow rates. Two different measurement planes are selected for obtaining the detailed pore scale velocity field—one close to the glass wall and the other inside the porous medium at a distance 100?μm below the glass wall. The image processing technique for dealing with noisy data and sparse vector field has been discussed in detail. Probability density functions of transverse and axial velocity components are compared with available results in literature. The pore scale velocity field obtained can provide insight to flow properties in microporous media and can be a powerful tool to validate existing numerical results for flow through porous media.     

Heat transfer in microchannel devices using DSMC

The heat transfer characteristics of supersonic flows in microchannels is studied using direct simulation Monte Carlo (DSMC) method. The velocity components and the spatial coordinates of the simulated particles are calculated and recorded by using a variable-hard-sphere (VHS) collision model. The effects of Knudsen number (Kn) on the heat transfer of the microchannel flows are examined. The results show that the magnitude of the temperature jump at the wall increases with increasing Kn. The heat transfer to the isothermal wall is found to increase significantly with Kn. The possible causes for the increase of wall heat transfer are discussed     

Experimental research on resist filling behavior in microimprint lithography by using defocusing digital particle image velocimetry

The resist filling behavior is crucial to the quality of the final imprinted patterns in microimprint lithography. To achieve the microscale velocity field of resist, a 3-D defocusing digital particle image velocimetry (DDPIV) system was established. The spatial coordinates of 500 nm fluorescent tracer particles were inferred from their DDPIV images generated by a mask with three apertures forming an equilateral triangle. Time-resolved 3-D particle field inside the resist was obtained with the spatial coordinates of particles. Particle tracking velocimetry was utilized to derive the velocity field from the particle spatial position in the imprinting sequence. Velocity history of particles along the extracted track was described. The results showed that the maximum velocity of the resist was always located in the region between the mold corner and the supporting substrate; meanwhile, the velocity component of the resist in the vertical direction is approximate to zero. The volume transfer characteristics of resist was illustrated from the view of particle field and velocity field, which will help reveal the resist filling mechanism and provide useful guidance for the mold structure optimization.     

结合人眼亮度调制传递函数的形态学锐化算法

针对常规彩色图像锐化方法存在的局限性,提出一种空间域锐化算法,其中包括数学形态学运算,利用高斯滤波器分离图像的低频成分和高频成分,强化图像高频成分,调整图像低频成分,将两者合成为新的输出图像。由于形态学开运算和闭运算分别具有自适应极大值滤波和极小值滤波的特点,因此将开闭组合运算用于图像高频成分,自适应地突出图像局部细节,再根据人眼亮度调制传递函数模型,对图像的低频成分进行全局调整,从而进一步改善图像的整体视觉效果。结果证明,该方法在RGB彩色空间和色调/亮度/饱和度感知彩色空间内的锐化效果都好于常规锐化方法。     

颜色传输算法的研究

研究了一种彩色图像颜色传输算法,对于给定两幅彩色图像,首先把计算机表示的RGB颜色空间转换为LMS空间,并通过线性变换把LMS基变换到一组正交基来消除空间内各个分量之间的强相关性,从而把RGB颜色空间转换到一个正交空间,然后利用统计学方法进行局部和整体相结合的自适应采样分析,提取一幅图像的颜色信息和另一幅图像的形状信息,合成一幅崭新的图像,实现颜色样本图像到目标图像的颜色传输过程。实验结果表明,取得了较好的效果。     

A Particle Flow Velocity Profiler Using In-Channel Electrodes With Unevenly Divided Interelectrode Gaps

This paper presents a particle flow velocity profiler that employs in-channel electrodes with unevenly divided interelectrode gaps. The proposed electrical method measures both the particle position and velocity from the voltage signals generated by particles passing across three sensing electrodes, thus obtaining the flow velocity profile of the particles in a microfluidic channel. In this paper, we use polystyrene microparticles to characterize the performance of the present particle flow velocity profiler. At flow rates of 1.85, 2.68, and 3.60 muL/min, a flow velocity profile of 6.59-mum-diameter particles is measured with an uncertainty of 5.44%, which is comparable to the uncertainty (5%) in a previous microparticle image velocimetry. From the voltage signals for 6.59- and 5.47-mum particles, we also verify that the present device detects the particle position showing less sensitivity to particle size variation than an existing particle impedance spectroscopy. In addition, in-channel clogging detection using the present electrical method is demonstrated. The present particle flow velocity profiler offers advantages of simpler structure, cheaper cost, and higher measurement stability that is insensitive to particle size for use in integrated microbiofluidic systems.     

基于Kriging插值的PIV算法在污水流速测量中的应用研究

针对污水管道中流体成分的多样性和流动型态的复杂性,借鉴粒子图像测速中的流场测量思想,提出了基于PIV(粒子图像测速)算法的管道污水流速测量方法.将该算法应用于污水流速测量时,发现由于伪矢量和缺失矢量的存在,导致测量结果精确度降低的问题,进而提出应用Kriging插值的方法对PIV算法进行改进,最终实现管道污水流速的有效测量.最后,在实验室搭建了实验模拟平台,对提出方法进行了仿真测试,测试结果表明,与传统的PIV测速方法相比,该方法能有效提高测量精度,保证测量结果的准确性.     

Methods for counting particles in microfluidic applications

Microfluidic particle counters are important tools in biomedical diagnostic applications such as flow cytometry analysis. Major methods of counting particles in microfluidic devices are reviewed in this paper. The microfluidic resistive pulse sensor advances in sensitivity over the traditional Coulter counter by improving signal amplification and noise reduction techniques. Nanopore-based methods are used for single DNA molecule analysis and the capacitance counter is useful in liquids of low electrical conductivity and in sensing the changes of cell contents. Light-scattering and light-blocking counters are better for detecting larger particles or concentrated particles. Methods of using fluorescence detection have the capability for differentiating particles of similar sizes but different types that are labeled with different fluorescent dyes. The micro particle image velocimetry method has also been used for detecting and analyzing particles in a flow field. The general limitation of microfluidic particle counters is the low throughput which needs to be improved in the future. The integration of two or more existing microfluidic particle counting techniques is required for many practical on-chip applications.     

A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up

A novel design for the classical microfluidic device known as T-junction is proposed with the purpose of obtaining a simultaneous measurement of the in-plane velocity components in two orthogonal planes. A crucial feature of the proposed configuration is that all three velocity components are available along the intersection of the two planes. A dedicated optical set-up is developed to convey the two simultaneous views from the orthogonal planes into the sensor of a single camera, where a compound image is formed showing on either half one of the two views. A commercial micro-particle image velocimetry system is used to measure the velocity in the two planes. Feeding the T-junction with a liquid continuous phase and a dispersed gas phase, the velocity is measured by phase averaging along the bubble formation and break-up process showing the potentialities of the new design. The accuracy analysis shows that the error is dominated by a systematic component due to the thickness of the measurement slice. The error can be reduced by applying confocal microscopy to the present system with no further modifications so as to reduce the thickness of the measurement slab thereby reducing the error. Moreover, by sweeping the planes across the region of interest, a full three-dimensional reconstruction of the velocity field can be readily obtained. Finally, the simultaneous views offer the possibility to extract the principal curvatures of the bubble meniscus thereby providing access to the Laplace pressure.     

Ultrafast measurement of transient electroosmotic flow in microfluidics

We present a non-intrusive molecular dye based method, i.e., laser-induced fluorescence photobleaching anemometer (LIFPA), to significantly increase temporal resolution (TR) for velocity measurement of fast transient electrokinetic flows. To our knowledge, the TR has been for the first time achieved to 5–10 μs, about 100 times better than that published from state-of-the-art micro particle image velocimetry (μPIV), which is currently the most widely used velocimetry in the microfluidics community. The new method provides us with new opportunities to study experimentally the fundamental phenomena of unsteady electrokinetics (EK) and to validate relevant theoretical models. One application of the new method is demonstrated by measuring the rise time of DC electroosmotic flows (EOFs) in a microcapillary of 10 μm in diameter.     

Advanced particle-based velocimetry techniques for microscale flows

Recently, microscale flows have been receiving large attention in various research areas. However, most conventional imaging techniques are unsatisfactory due to difficulties encountered in the visualization of microscale flows. Recent advances in optics and digital image processing techniques have made it possible to develop several advanced micro-PIV/PTV techniques. They can be used to obtain quantitative velocity field information of various microscale flows from visualized images of tracer particles. As new advanced micro-PIV techniques, the basic principle and typical application of the time-resolved micro-PIV and X-ray micro-PTV methods are explained in this review. As three-dimensional (3D) velocity field measurement techniques that can be used to measure microscale flows, the stereoscopic micro-PTV, defocusing micro-PTV and holographic micro-PTV methods are introduced. These advanced PIV/PTV techniques can be used to reveal the basic physics of various microscale flows and will play an important role in visualizing hidden microscale flow phenomena, for which conventional methods face several difficulties in analysis.     

基于HSI颜色空间的蔗糖结晶图像分割方法

针对煮糖过程蔗糖结晶图像的特点,采用颜色空间转换的方法,将图像从RGB颜色空间转换到HSI颜色空间,利用HSI颜色空间各分量相对独立性以及结晶颗粒和糖浆溶液的色调差异通过改进的大津法对H分量进行阈值分割,再通过数学形态学,中值滤波,孔洞填充,去除噪声颗粒,去除图像边界的非完整颗粒等方法进行后续处理,最终将结晶颗粒从复杂的图像信息中分割出来。实验证明该方法分割效果好,切实可行。     

Computational simulation of gas flow and heat transfer near an immersed object in fluidized beds

To improve the understanding of the heat transfer mechanism and to find a reliable and simple heat-transfer model, the gas flow and heat transfer between fluidized beds and the surfaces of an immersed object is numerically simulated based on a double particle-layer and porous medium model. The velocity field and temperature distribution of the gas and particles are analysed during the heat transfer process. The simulation shows that the change of gas velocity with the distance from immersed surface is consistent with the variation of bed voidage, and is used to validate approximately dimensional analysing result that the gas velocity between immersed surface and particles is 4.6U_mf/ε_mf. The effects of particle size and particle residence time on the thermal penetration depth and the heat-transfer coefficients are also discussed.     

基于模糊域的彩色图像增强

本文提出了一种新的基干模糊域的彩色图像增强方法。先把图像从RGB模型转换为HSV模型,引入了一个新的对比度增强算子——模糊对比度,采用高斯分布函数,在模糊域对V分量进行图像增强,然后再将图像转换回RGB模型。实验证明该方法对干彩色图像的增强能达到令人满意的效果。     

基于色彩空间自然场景统计的无参考图像质量评价

RGB色彩空间中各色彩分量间存在强相关性, 图像发生失真会改变各分量间的相关性. 基于此, 本文提出了一种新的通用无参考图像质量评价方法. 首先, 根据人类视觉对RGB色彩空间中绿色分量更为敏感的颜色感知特性, 提取了G分量MSCN系数及其4方向邻域系数的统计特征; 其次, 在分析RGB色彩空间中R、G及B分量间相关性的基础上, 分别计算RGB色彩空间各色彩分量及其纹理、相位间的互信息, 利用互信息作为统计特征来描述其各分量间的相关性; 进而, 结合上述统计特征, 分别利用SVR和SVC构建无参考图像质量评价模型和图像失真类型识别模型; 最后, 在LIVE、CSIQ 及TID2008图像质量评价数据库上进行了算法与DMOS (Different mean opinion score)的相关性、失真类型识别及计算复杂性等方面的实验. 实验结果表明, 本文方法的评价结果与人类主观评价具有高度的一致性, 在LIVE 数据库上的斯皮尔曼等级相关系数和皮尔逊线性相关系数均在0.942以上; 而且, 图像失真类型识别模型的识别准确率也高达93.59%, 明显高于当今主流无参考图像质量评价方法.     

Combined measurement of concentration distribution and velocity field of two components in a micromixing process

A lot of production processes involve mixing steps. The understanding of fluid flows in mixing processes of liquid components is needed in order to develop appropriate mixers for the chemical and pharmaceutical industry. Especially mixing in microfluidic systems is a challenge due to the diffusion-based processes. A multi-lamination micromixer with chessboard outlet geometry is used to induce the mixing process. To get comprehensive information about the mixing process, the velocity profile of the fluid flow and the species concentration distribution during the mixing process should be measured. Thus, we have combined particle image velocimetry (PIV) and Raman scattering. To enable rapid detection, the Raman imaging mode is used to visualise the concentration distribution. By this setup light sheets along and orthogonal to the outlet of the micromixer are recorded and synchronized with PIV measurement. As a model system we have used water and ethanol/methanol, enabling a selective monitoring of the substances by choosing appropriate spectral areas. The PIV is recorded based on Mie scattering and fluorescence using microsphere tracers. In this study, we present a setup for determination of the velocity profile field and the spatial concentration distribution of water and ethanol/methanol in a micromixer.     

Velocity measurement of pneumatically conveyed particles through digital imaging

A digital imaging based method for measuring the velocity of pneumatically conveyed particles is described in this paper. The measurement system consists of a laser sheet for illumination, a CCD camera for particle image acquisition, and bespoke image processing software. By controlling the exposure time of the camera, blurred images of moving particles are captured, which contain velocity information of the particles. The relationship between the motion blur length and the particle velocity is established using a lens model of the camera. To improve the robustness of the system to noise in the acquired particle images, a traveling wave equation (TWE) method is used to estimate the motion blur length and then deduce the particle velocity. In addition to computer simulations experimental work on a free fall particle flow rig and pneumatically conveying test rig is carried out. The results indicate that the proposed method is feasible for the velocity measurement of pneumatically conveyed particles.