- 學位論文
交叉重疊式凹槽微混合器之設計與流場分析
Design and Analysis of Overlapping-Crisscross Micromixers
摘要
本文特提出一款創新型微混合器，引入交叉重疊式入口之設計理論並製作實體加以驗證，以提昇凹槽式微混合器入口區段的混合效率。該款微型混合器同時整合兩種構成混沌混合模式的重要元素，其一來自於交叉重疊式設計造成局部流場中央存在的雙曲線點，該點兩側流場以共方向旋轉方式流動；其二則由於混合腔壁面包含有不對稱的凹槽結構設計，在凹槽頂點下方存在的拋物線點兩側流場，則沿著斜向溝槽反向進行旋轉。對於這種綜合兩種混沌混合模式的複雜流場，在兩者整合後是否能產生加乘混合的效果，相當值得深入研究。因此本論文研究將分為數值模擬與流場觀測實驗兩部分，深入分析對於交叉重疊式微混合器之複雜流場結構暨其混合機制。 在數值分析方面，本文建立物理問題的流場模型，特別針對常見的數值誤差如網格獨立問題及數值擴散影響進行檢測。確立數值計算的可信度後，便得以開始進行分析。本文針對混合裝置的重點設計(交叉重疊結構及凹槽式微混合腔體)進行系統性的分析，分析結果顯示，前者對於流場的作用主要分為兩方面，（1）交叉重疊結構如同一個交通樞鈕，匯集並重新分配流動方向，（2）滾流作用大幅增進進口流匯集處的對流作用，流場中央靠近鞍點的區域每間隔兩組交叉重疊結構，均勻混合的區域呈現指數型成長。在深入探討凹槽式微混合腔體的混合特性後，也得到幾個設計重點:（1）流體層進入混合腔的型態，會直接影響到下游的混合效率，（2）凹槽結構的配置位置，對於反向螺旋運動打破流體層界面與否息息相關，（3）凹槽的不對稱性造成流體層一旦被打破會形成大小不一的區塊，分割之小型區塊容易在一連串的對流過程中被均勻混合。定量估算流經12個凹槽結構配置後截面濃度場的混合指標趨勢，合理解釋了上述所歸納分析出，影響混合效果的幾何因素。結果顯示凹槽方向呈交錯式配置的SOC μ-mixer混合效果優於OC μ-mixer約21%；至於凹槽方向一致的OC μ-mixer與 staggered herringbone mixer的混合效果，前者則優於後者約46 %。 經過系統性的分析討論數值流場的混合機制，本文發展三種流場可視化實驗，包含染料實驗、化學反應實驗以及共軛焦雷射掃瞄螢光實驗，以驗證數值流場的真實性。由於染料實驗在穿透光的觀測之下對比度不夠強烈，因此在觀測上僅能獲得概略的流場資訊。化學反應實驗以稀釋過的酚酞溶液與氫氧化鈉溶液進行混合實驗，兩流體接觸後產生反應形成紫紅色分布，可用以直接觀測微混合裝置在流場中充份混合區域。為了比對數值分析流場與實驗流場結果，必須試圖修正穿透光觀測之流場影像，造成焦聚面周圍的所有影像投影在成像螢幕上顯示，在檢閱時難以分辨該立體影像的實際位置來源的缺點。為此本文將數值流場沿z軸變化的所有截面濃度圖予以透明化處理並逐層堆疊，截面影像每間隔10 μm擷取一次，形成與化學反應實驗流場之穿透光影像一致的穿透流場圖。另一種解決方法為使用共軛焦雷射掃描顯微技術觀測螢光微粒在混合裝置中的混合散佈行為，由於該技術具有消除非聚焦面雜訊的特性，所擷取的二維影像可被用以驗證數值流場。比對結果顯示數值流場與實驗影像吻合，如此一來將可掌握微混合系統之整體流場特性，預期對於生化檢測系統之微縮化研究具有相當之貢獻。
並列摘要
We successfully develop a novel PDMS-based microfluidics mixer, which incorporates an overlapping crisscross entrance with patterned microchannels, and acts as a high-performance micromixer. Such an entrance design generates significant tumbling and brings about axial advection between mixing fluids. The systematic numerical analysis and experimental investigation have been performed to identify the characteristics of the device. The microfludics-based mixer is recently becoming a prevalent study for widely application in genetic sequencing, environmental monitoring and drug delivery. Among those researches, passive mixing plays one of the major roles due to its low cost and easy fabrication. Microsacle mixing of two aqueous solutions is usually done by advection and diffusion. Generating flow fluctuation significantly enhances flow folding and stretching and results in the improvement of mixing process. For fluids in the Stokes flow field, however, the inertia of flow diminishes and the effective mixing by modulating advection turns into the main challenge of designing new micromixer. In addition, reducing the characteristic length of diffusion or increasing residence time of the fluids evolves the molecular diffusion inside the microchannels. With all these concerns, the present work presents a microfludics configuration comprising two patterned microchannels with one crossing on top of the other. Two chaotic mixing mechanisms are also comprised: (1) twisting and reorienting of the split streams through the first overlapping crisscross junction and (2) merging and re-stretching by the hyperbolic flow at next junction. Both the results of flow visualization and numerical simulation reveal that such a design stimulates significant three-dimensional crossflow at the intersection of two channels and also induces strong helical flow motion in the following mixing channels. The vertical tumbling behavior redistributes the downstream flow pattern to enlarge the interfacial area and to promote axial advection between two mixing fluids. The transversal momentum generated by the device contributes to agitate the streams to mix well. The pressure distribution and mixing performance also become superior as compared with that of T-type grooved micromixer. Numerical analysis shows that although the molecular diffusion is detracted at a greater Reynolds number due to less residence time, the enhancement of the advances between mixing fluids is more overwhelming. This novel design is thus applicable for a much wider range of Reynolds number. The variation of the ratio of two initial flow rates controls the mass transfer rates in the two channels and is possibly extensible to be a satisfactory active micromixer.