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.