Velocity profile of thin film flows measured using a confocal microscopy particle image velocimetry system with simultaneous multi depth position

In this paper, we report a technique for simultaneously visualizing flows near walls at nanodepth positions. To achieve such a high interval of depth gradient, we developed a tilted observation technique in a particle image velocimetry (PIV) system based on confocal microscopy. The focal plane along the bottom of the flow channel was tilted by tilting the micro-channel, enabling depth scanning in the microscopic field of view. Our system is suitable for measuring 3D two-component flow fields. The depth interval was approximately 220 nm over a depth range of 10 mu m, depending on the tilt angle of the micro-channel. Applying the proposed system, we visualized the near-wall flow in a drainage film flow under laminar conditions to the depth of approximately 30 mu m via vertical scanning from the bottom to the free surface. The velocity gradient was proportional to the distance from the wall, consistent with theoretical predictions. From the measured near-wall velocity gradient, we calculated the wall shear stress. The measurement accuracy was approximately 1.3 times higher in our proposed method than in the conventional confocal micro-PIV method.