The particle image velocimetry (PIV) technique was used to measure the flow field around a micro-pillar. Sequential images of fluorescent tracer particles with a diameter of 1 µm were obtained using a high-speed camera (DFK33UX174, The Imaging Source Co., Ltd., Germany) via a high NA objective lens (UCPlanFLN 20x/0.70, Olympus, Inc., Japan) and were stored in a PC. Because the frequency of the micropillar was above 500 Hz, the exposure time for each frame was set to 1/50000 s (20 μs) such that the instantaneous particle images could be resolved. The image acquisition frame rate was set to 100 fps. Based on the acquired images, the average flow field was obtained by correlating the particle images obtained at nearly identical phases in periodic motion using a PIV software package (Flow Expert, KATO Koken, Japan). Two-dimensional velocity vectors were obtained by correlating the particle images of successive frames using 64 × 64 pixels (20 µm × 20 µm in the physical dimension) interrogation windows.