對微流體系統而言，有效且快速地達到混合的目的是一相當重要的課題。本研究利用圓形凹槽流道結構來探討幾何形狀及流體流動條件對流動特性的影響，並利用這些結果進一步設計出理想的混合器。

　　本實驗利用微型模造技術並以PDMS為材料來製作微流道系統。藉螢光粒子並利用顯微鏡觀察流體在微流道內的流動情形。由實驗結果得知，在改變流道幾何形狀的情況下，當凹槽開口角度越小，漩渦越容易形成，而流道管徑寬度越小，也越容易形成漩渦。在圓形凹槽開口角度越小跟流道管徑寬度越大的情況下，越容易形成低雷諾數漩渦。但是當開口角度大於210°，要形成漩渦就會比較困難。而改變流體流動條件，則可以造成漩渦位置偏移。適度地控制流量大小，可以使漩渦位置不斷地偏移，讓圓形凹槽內的流體產生擾動，造成較好的混合效果。

　　最後，我們根據流道幾何形狀與流體操作條件的對應關係，設計兩组不同的混合器。測試的結果顯示，凹槽內的流動型態會隨流動條件明顯變化，預期對於混合效果的改善可能會有幫助。所以對於未來如何設計簡單而理想的微混合器，本研究結果可提供有效指引。

　Effective and rapid mixing is essential to microfluidic systems. In this work, a microchannel with a circular cavity is designed for assessing how flows are characterized by microchannel geometries and flow conditions.

　We utilize the PDMS-based microfabrication techniques to construct microchannels. Flow patterns are observed under a microscope with aid of fluorescent particles. The results reveal that the smaller the opening angle of the circular cavity and the width of the microchannel, the more susceptible vortex formation. As long as the opening angle is sufficiently small or the channel width is large enough, a low Reynolds-number vortex forms. On the other hand, as the opening angle is larger than 210°, it is difficult to form vortex. Changing the flow rate provides a control of the size and the location of a vortex.

　Finally, we design different types of micromixers based on the knowledge of flow characterization obtained above. We find that it might be more appealing to improve the performance of mixing by modulating flow structures through regulating the flow rate. This research can provide useful guidance for developing optimal strategies of designing micromixers.

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