Experimental study of microfluidic flow using a Micro-PIV system

Abstract

This thesis presents an experimental study of a creeping flow of water inside a micro-scale rectangular cross-sectional channel (110 µm × 40 µm) using Micro-Particle Image Velocimetry (Micro-PIV). Micro-PIV is designed to achieve the micro-scale resolution required for microfluidic flow investigation. One of the most important modifications is the implementation of volume illumination instead of the thin light sheet used in regular PIV systems. Due to the widespread development of micro-scale devices, many experimental investigations of microfluidic flows using Micro-PIV are reported in the literature. However, relatively few of them have investigated the effects of volume illumination on the measured velocity profiles. In the present study, it became evident that the Depth of Correlation (DOC), which is a characteristic of volume illumination, has a significant effect on the measured velocity profiles. To illustrate this, two objective lenses with different magnifications (10x and 20x) were used for the measurements. The other parameters, i.e. the flow rate (Q = 100 µL/hr), channel geometry, size of the seeding particles (dp = 3 µm) were kept constant during the experiment. An analytical solution for the 3D velocity profile was obtained from the governing equations. The effect of the DOC on the measured velocity profiles appeared as a reduction in the peak velocity compared to the centre-plane value based on the analytical solution. In order to show that this deficiency was due to the volume illumination, a special volume-averaging scheme was applied to the analytical solution. By comparing the experimental and volume-averaged analytical velocity profiles, one could determine the effective value of the DOC (37 µm and 27.5 µm for the 10x and the 20x magnification, respectively). These experimental values were consistent with previously reported DOC values for the 10x and the 20x objectives and for the 3 µm seeding particles. The other objective was to assess the ability of the Micro-PIV to achieve near-wall velocity measurements. A significant improvement was observed in the near-wall resolution by using a higher magnification. This improvement was quantified by calculating the deviation of the measured velocity profiles from the volume-averaged analytical velocity profiles. The deviation exceeded 30% of the peak velocity in the near-wall region for the 10x magnification while it did not go beyond the 10% for the 20x magnification. The smaller interrogation regions in the 20x measurements was introduced as the most likely explanation for this enhancement.