OCT for 2D and 3D velocity measurement in micro-fluidic flows

本项目旨在利用光学相干层析成像（OCT）测量亚毫米流道的速度。在这种情况下，流量测量的一个主要应用是在微流体系统的设计和评估中，它体现了技术许多方面日益增长的小型化趋势。在流体流动工程中，小型化仪器减少了实验室占地面积，减少了对能源和昂贵或危险试剂的需求，并且能够在单个仪器中同时从多个系统获取数据。微流体是一个快速发展的领域，在医疗保健（快速，芯片实验室，技术，如免疫测定和生物相分离），能源技术（膜燃料电池）和高通量，便携式化学分析系统中都有好处。目前，研究微流体流动的主要技术是微粒子图像测速法，它使用高分辨率摄影来确定“种子”粒子在薄片激光照射下的平面内的位置。快速连续获取的两幅图像允许计算帧之间粒子移动的距离，从而得到图像平面上的速度分量。在非常小的管道中，不能产生足够薄的光片。小体积的照明，通过显微镜的安排，是通常的，测量平面的厚度是用光学定义的，从焦平面以外的区域排除光。对于最小的通道，种子粒子必须小于照明波长（约为1）。1微米)。这就造成了困难，因为光散射随着粒子直径的减小而迅速减少，并且可能下降得很低，以至于信噪比（SNR）不足以产生可接受的图像。然后需要荧光粒子和强化照相机，并通过滤波将荧光信号从背景中分离出来。OCT主要用于医疗环境，用于详细的生物组织成像。然而，它的高空间分辨率，结合从PIV继承下来的粒子跟踪技术，提供了在二维或三维区域测量2或3分量速度的可能性，用于微流体系统中经历的流速。经典流动理论对微流体流动的描述不完善，在混合器、热交换器和燃料电池等微流体装置的设计中，需要实验技术对模型进行验证。例如，消除流动中的“死区”以及理解弯曲或分叉微通道中的流体运动是很重要的。经过适当的处理，OCT可以获取三个垂直平面的图像，并且只能从一个方向访问；这是微流体的一大优势，当进入受限时。通道的高分辨率结构成像和速度测量是可能的，这将有助于检测对流动有很大影响的壁面结构的微小变化或缺陷。OCT具有优异的光学切片能力，成像平面厚度仅为几微米。对来自测量区域外的散射光的强烈抑制消除了对荧光颗粒的需要，并简化了近壁测量。OCT的信噪比使得信号可以从浑浊生物组织中数百微米的深度中获得，这表明与PIV的类似实现相比，在更高的播种密度或更大的深度下可以进行流量测量。通常，二维OCT图像的更新速率为几十赫兹。虽然微流体速度通常较低，但更新间隔将可测量的速度限制在几毫米/秒。较短的间隔对提高这个极限是非常有利的。为了减少图像间的间隔，允许同时定义多个图像平面，本文提出了对多个照明光束获取的图像进行复用的方法

This project aims to use optical coherence tomography (OCT) for velocity measurement in sub-millimetre flow channels. A major application for flow-measurement in this regime is in the design and assessment of microfluidic systems, which exemplify an increasing trend for miniaturization pervading many aspects of technology. In fluid-flow engineering, miniaturised instruments offer a reduced laboratory footprint, reduced requirements for energy and expensive or hazardous reagents and the ability to acquire data simultaneously from many systems within a single instrument. Microfluidics is a rapidly-growing field with benefits in healthcare (rapid, lab-on-a-chip, techniques for, e.g. immunoassays and biological phase separation), energy technology (membrane fuel cells) and high-throughput, portable systems for chemical analysis.Currently, the main technique for investigation of micro-fluidic flows is micro particle image velocimetry, which uses high-resolution photography to determine positions of 'seed' particles within a plane illuminated by a thin sheet of laser light. Two images, acquired in rapid succession, allow the distance moved by particles between frames to be calculated, yielding velocity components in the image plane.In very small ducts, a sufficiently thin light sheet cannot be generated. Illumination of a small volume, through a microscope arrangement, is usual, and the measurement plane thickness is defined using optics that exclude light from regions outside the focal plane. For the smallest channels, seed particles must be smaller than the illumination wavelength (approx. 1 micron). This causes difficulties, because light scattering decreases rapidly as particle diameter drops, and can fall so low that the signal-to-noise (SNR) is inadequate for acceptable images. Fluorescent particles and intensified cameras are then required, with filtering to separate the fluorescent signal from the background.OCT is used mainly in medical environments, for detailed biological tissue imaging. However, its high spatial resolution, combined with particle-tracking techniques carried over from PIV, offer the possibility of 2- or 3-component velocity measurement in 2-D or 3-D regions, for flow velocities experienced in micro-fluidic systems. Micro-fluidic flow is not well described by classical flow theory, and experimental techniques are needed to validate models in designing micro-fluidic devices such as mixers, heat exchangers and fuel cells. It is important, for example, to eliminate 'dead zones' in the flow, and to understand the fluid motion in curved or bifurcated micro-channels. With appropriate processing, OCT can acquire images in three perpendicular planes with access from only one direction; a big advantage in micro-fluidics, when access is necessarily limited. High-resolution structural imaging of the channels is possible, alongside velocity measurement, which will help in detecting small variations or defects in wall structure that can have a large effect on flow.OCT offers excellent optical sectioning capability, the image plane thickness being a few micrometres. Strong rejection of scattered light from outside the measurement region eliminates the need for fluorescent particles and eases near-wall measurements. The SNR of OCT is such that signals can be obtained from depths of hundreds of micrometres within turbid biological tissue, which suggests that flow measurements will be possible at higher seeding densities, or greater depths, than for comparable implementations of PIV. Typically, update rates for 2D OCT images are a few tens of Hz. Although micro-fluidic velocities are generally low, the update interval limits measurable velocities to a few mm/s. A shorter interval would be very advantageous in raising this limit. Multiplexing of images acquired from multiple illumination beams is proposed here, to reduce the inter-image interval and allow multiple image planes to be defined simultaneously