利用流體雙折射法進行微流道內之流場可視化

Birefringent Flow Visualization in a Microfluidic

黃宏彥(Hung-Yen Huang)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.02696

摘要

本研究中使用流體雙折射法與mPIV觀察及量測不同之流體在微流道中之微觀流場比較其結果。流體雙折射法之實驗架構是利用兩片偏光片及兩片1/4波片建立circular polariscope架構，所得到之影像經後處理可得流場中的相位角延遲。實驗之工作流體為CTAB/NaSal，CpyCl/NaSal，CpyCl/NaSal/NaCl，此三種流體皆為具雙折射性質的非牛頓流體。實驗中所使用之微流道深度為1.5 mm，流道設計有三種，一號流道寬4 mm，流道中央有一顆圓柱形擋體，二號流道寬2 mm，流道中央有一顆圓柱形擋體，三號流道寬2 mm，流道中央有七顆圓柱形擋體陣列。此外，我們利用mPIV量測微流場的流速，並計算拉伸應變率及剪應變率的分佈，用來與雙折射性質造成的相位角延遲做比較。

實驗結果顯示，相位角延遲會隨Deborah number 上升而增加。Deborah number較小時，相位角延遲對於Deborah number的變化較敏感，且彼此關係近乎線性。當Deborah number較大，相位角延遲隨Deborah number的變化較不明顯，曲線較為平緩。比較nPIV與流體雙折射法的結果可發現，相位角延遲的分布與剪應變率的分布非常的相似，在相位角延遲較大的地方，有較大的剪應變率。若流體發生分離現象，我們亦可以用流體雙折射法觀察到擋體後方流場產生的尾流區。此外，若流道較窄，壁面較靠近擋體時，可以抑制分離流動的發生，但會使流場在較小的Deborah number時就進入不穩定的狀況；在包含擋體陣列的微流道中，兩顆擋體之間可能產生渦流，使得雙折射現象的分布與含單一擋體的微流道大為不同。不同的工作流體方面，我們發現在相同的Deborah number下，CpyCl/NaSal/NaCl所發生的相位角延遲較其他兩種流體為小，即使流速很大，流場也可以維持穩定的狀態，而CTAB/NaSal的流場則較容易發生不穩定的擾動。

在本研究中，我們發現流體雙折射法所得之流場可視化結果與剪應變率分布有很高的相似性;在相位角延遲較大的地方，亦具有較大的剪應變率。在擋體後方若發生流體分離，流體雙折射法亦可觀察到尾流區的發生。因此，流體雙折射法提供了一非侵入式的微觀流場可視化及診斷方式，可由其結果影像推得剪應變率分佈與流場狀況。

關鍵字

微流道 ； 非牛頓流體 ； 雙折射 ； 偏光

並列摘要

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.

並列關鍵字

microfluidic ； non-Newtonian fluid ； birefringence ； polarizer

參考文獻

[3] S. Yoneyama, J. Gotoh, and M. Takashi, "Tricolor photoviscoelastic technique and its application to moving contact," Experimental mechanics, vol. 38, pp. 211-217, 1998.

[4] M. Fukuzawa and M. Yamada, "Photoelastic characterization of Si wafers by scanning infrared polariscope," Journal of Crystal Growth, vol. 229, pp. 22-25, 2001.

[5] D. A. Jenkins and J. J. Hren, "Quantitative piezobirefringence studies of dislocations in transparent crystals," Philosophical Magazine, vol. 33, pp. 173-180, 1976.

[6] H. Kotake, K. Hirahara, and M. Watanabe, "Quantitative photoelastic measurement of residual stress in LEC grown GaP crystals," Journal of Crystal Growth, vol. 50, pp. 743-751, 1980.

[7] T. J. Ober, J. Soulages, and G. H. McKinley, "Spatially resolved quantitative rheo-optics of complex fluids in a microfluidic device," Journal of Rheology, vol. 55, pp. 1127-1159, 2011.