濃度梯度液滴陣列微流元件之設計

Design of a Microfluidic to Generate Droplet Array with Concentration Gradient

顏晨恩(Chen-En Yen)

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(2013年)

https://doi.org/10.6342/NTU.2013.00047

摘要

本研究利用純被動式之機制，設計一濃度梯度液滴陣列產生器，目標為連續產生含不同固定濃度之液滴陣列。在此濃度梯度液滴陣列產生器內，首先在入口處利用流體聚焦技術生成液滴，透過不對稱分岔微流道分裂液滴，最後將液滴匯集到融合區，以半深碗形擋體促進液滴融合。在本研究中，透過不同的子流道寬度比與分岔角度探討其對於液滴生成與分裂之投影面積、融合成功率與融合後液滴濃度之影響。從實驗的結果我們可得到多層液滴分裂微流道的設計準則。在此，我們的幾何參數包含90°、180°兩種變化之分岔角度，1.5、2、3組成五種變化之子流道寬度比，總共十種不同的微流道設計。
實驗結果顯示，母液滴之生成頻率與投影面積大小兩者皆隨著Qc/Qd之比值增加而遞減，除了分岔角度為90°，w1/w2=2之設計之外，下游之子流端寬度比對母液滴投影面積皆影響不大。對於子液滴投影面積而言，子流道寬度越大則液滴投影面積越大，而下游端寬度比之設計w3/w4通常不會影響前端之液滴投影面積A1、A2之大小，只有在分岔角度為90°、w1/w2=2的時候，不同的w3/w4之值讓A2之大小產生改變。對於液滴分裂之結果而言，子液滴投影面積比A2/A1隨著Qc/Qd之比值增加而緩慢下降，但子液滴投影面積比A4/A3則無此趨勢，因為融合區內之液滴經過會影響分裂，產生不穩定現象。子流道寬度比w1/w2越大則子液滴面積比A2/ A1越小，w3/w4越大則子液滴面積比A4/ A3越小。w3/w4之值只有在w1/w2 = 3、分岔角度為90°時不影響液滴分裂。
觀察融合區可以發現液滴進入融合區後共有五種不同的形態，而碗形擋體之設計確實為觸發融合之原因。不論是哪一種設計，在Qc/Qd=4和Qc/Qd=5的時候融合成功率都最高，液滴融合成功率最高可達100%；而融合成功率在六成以上相對應之子液滴面積比皆在5到10之間。以投影面積估算，在出口B之液滴融合後濃度最佳可降至70.59%，出口C之液滴融合後濃度最高可達16.36%。若直接由液滴影像灰階值之結果，在出口B之液滴融合後濃度最佳可降至80.86%，出口C之液滴融合後濃度最高可達 50.62%，由影像灰階值計算的濃度值都比透過投影面積計算所得的值還高。檢視能夠完整生成濃度梯度液滴陣列的四種設計，不同出口與液滴濃度之關係呈一反S形曲線，其中當w1/w2 = 1.5，出口B、C之液滴投影面積約為出口A、D之液滴投影面積的1.5至3倍，而在w1/w2 = 2、w1/w2 = 3之設計則相反，在子流道寬度比w3/w4 = 1.5的時候才能在出口B及出口C同時有融合成功的液滴。最佳設計為分岔角度為180°、w1/w2 = 3與w3/w4 = 1.5之組合，在出口B、C之融合成功率分別可達57.1%與98.4%，並連續產生濃度為100%、79.68%、11.04%及0%之液滴。

關鍵字

微液滴 ； 濃度梯度 ； 被動式 ； 液滴融合 ； 液滴分裂 ； 非對稱分岔管 ； 高通量

並列摘要

This study aims at designing a microfulidic device that generates droplet array with concentration gradient. First, droplets are produced by flow focusing method. Then two-step asymmetric bifurcations are employed to split droplets into different sizes. When daughter-droplets enter the fusion region, a bowl-shape obstacle is used to promote droplet fusion. From the images taken, droplet projection area, fusion efficiency and normalized concentration of droplets are determined. Three parameters are considered herein: bifurcation angle (theta = 90°, 180°), width ratio of daughter channels in first the first bifurcation (w1/w2 = 1.5, 2, 3) and the width ratio of daughter channels in the second bifurcation (w3/w4 = 1.5, w3/w4 = w1/w2). The results lead to design guidelines for microfluidics of multi-step droplet fission.
The experimental results reveal that both the projection area and generation frequency of mother droplets decrease as flow rate ratio of the two phases Qc/Qd rises up. Except theta = 90°, width ratio of daughter channels downstream has no effect on the sizes of mother droplets generated upstream. Moreover, the projection area of daughter droplet increases as the width of daughter channels increases. Except theta = 90° and w1/w2 = 2, the projection area of daughter droplets (A1, A2) is independent of the width ratio of daughter channels at the second bifurcation (w3/w4). For droplet fission, the projection area ratio of splitting droplets in the first bifurcation (A2/A1) decreases with the flow rate ratio of the two phases Qc/Qd . Nevertheless A4/A3 in the second bifurcation fluctuates due to droplets passing through the fusion region. Both A2/A1 and A4/A3 decrease as the width ratio of daughter channels heightens. Except theta = 90° and w1/w2 = 3, the width ratio of daughter channels at the second bifurcation (w3/w4) plays an important role in the projection area ratio of splitting droplets (A2/A1) upstream.
We also find that the bowl-shape obstacle is truly beneficial to droplet fusion and four regimes are categorized when droplets enter the fusion region. When the projection area ratio of droplet pairs varies between 5 to 10, the corresponding fusion efficiency can exceed 60%. From the analysis of projection area, normalized concentration is able to drop to as low as 70.59% at outlet B, and rise up to 13.36% at outlet C. On the other hand, grayscale values on images lead to a lowest concentration of 80.86% at outlet B, and a highest concentration of 50.62% at outlet C. Among all design, only with the width ratio of daughter channels (w3/w4) of 1.5 are we able to produce droplet sequence with successfully coalescence for each fusion region. For these 4 designs, normalized concentrations of droplets at outlets correspond to a reverse-S shape. In addition, the droplet projection area of outlet A and D is 1.5~3 times of that of outlet B and C as w1/w2 = 1.5, but for w1/w2 = 2 and w1/w2 = 3 the condition is the exact opposite. For optimal design(theta = 180°, w1/w2 = 3, w3/w4 = 1.5), this microfluidic device can produce droplet sequence with normalized concentrations of 100%, 79.68%, 11.04% and 0%, and the fusion efficiency are 57.1% at outlet B, 98.4% at outlet C.

並列關鍵字

droplet-based microfluidics ； concentration gradient ； passive method ； droplet fusion ； droplet fission ； asymmetric bifurcation ； high throughput

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