Birefringent Flow Visualization in a Microfluidic
Date Issued
2014
Date
2014
Author(s)
Huang, Hung-Yen
Abstract
In this study, we construct the microscale flow birefringence instrumentation to facilitate flow visualization in a microfluidic. mPIV diagnosis is also employed to elucidate the corresponding flow field for comparison. The configuration of birefringent flow visualization method is circular polariscope, which is composed of two polarizers and two quarter waveplates. After post processing, the acquired image reveals distribution of retardance, which corresponds to the local stress. We choose three non-Newtonian fluids with birefringence properties, CTAB/NaSal, CpyCl/NaSal, and CpyCl/NaSal/NaCl, as our working fluids.
The microchannels in this study are 1.5 mm in depth with three different designs. The first and second channels both have a cylindrical obstacle in the center, and are 4 mm and 2 mm wide, respectively. The third channel is 2 mm wide and contains an array of seven cylindrical obstacles.
We find that the retardance increases with the increase of the Deborah number. When the Deborah number is small, the retardance is sensitive to the variation of Deborah number and the retardance rises rapidly and linearly with the Deborah number. When the Deborah number is large, on the other hand, the retardance becomes independent of the change of Deborah number and the retardance remains constant. Comparing the birefringent flow visualization to the mPIV results, we find that the distributions of retardance and shear stress are highly similar. Regions with large retardance, usually associate with high shear stress. The flow birefringence successfully visualizes the wake behind the obstacle when flow separation occurs. For narrower channel, the proximity of the walls restrains the occurrence of flow separation, but leads to turbulence at smaller Deborah number. When an array of obstacles is presented, vortices emerge in between the two cylinders. As a result, the distribution of retardance is very different from those in microchannels which only contain a single cylinder. Among the three working fluids, CTAB/NaSal/NaCl usually produces the lowest retardance and maintains stable flow even at high velocity. In contrast, flow turns turbulent more easily for CTAB/NaSal.
In this study, highly simiularity is found between birefringent flow visualization and the distribution of shear strain rate. The results prove that microscale flow birefringence is a non-invasive tool which facilitates flow visualization and provides quantitative information about the details of flow field in a microfluidic.
Subjects
微流道
非牛頓流體
雙折射
偏光
Type
thesis
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