研究流場紊變及營養鹽梯度對浮游生物生態影響之微流元件設計

Design a Microfludic Platform to Study the Influence of Flow Fluctuation and Nutrient Gradient in Plankton Ｅcology

蕭博仁(Po-Jen Hsiao)

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.2014.00308

摘要

本研究目的為設計一微流元件，用以研究流場紊變及營養鹽梯度對浮游生物生態之影響。研究中紊流流道設計理念為主流道旁附一圓型孔穴，圓型孔穴有六種不同的尺寸，最小的尺寸半徑為250 μm，最大的為5000 μm，而主流道與孔穴接合處可分無縮口及有縮口兩種設計，無縮口設計是孔穴與主流道直接相接，而縮口設計是將主流道與孔穴用內凹之半圓相接，半圓的半徑皆為孔穴半徑的0.15倍，共有十二種不同的微流道設計。而營養鹽梯度的設計主要是利用水膠層擴散的原理造成濃度梯度場，此一微流元件共有三層，最上層是兩水平對稱的流道，此二流道分別通入染劑水溶液與純水，中間層是水膠，最下層為紊流流道。
首先，我們觀察紊流流道中流道幾何形狀以及Reynolds number對流速、紊流強度及Kolmogorov length scale的影響。從μPIV的速度分佈結果可發現，尺寸越大的孔穴，孔穴內的速度越慢，且縮口對速度的影響也相當明顯，有縮口的情況下孔穴內的流速會慢許多。流場在不同尺寸的孔穴及縮口設計下有不同的結果，在無縮口設計的孔穴下，當Reynolds number為1、5、10，孔穴半徑為1000 μm、1500 μm時，孔穴中無法形成渦旋，當孔穴半徑縮小至750 μm，流場為多顆渦旋，最後當孔穴半徑為500 μm時，流場為單顆渦旋；然而，當Reynolds number大於50時，任何尺寸孔穴均可產生單顆渦旋的情況。在有縮口情況下，當Reynolds number大於100時，孔穴皆會出現兩顆渦旋，除了孔穴半徑為500 μm時例外；隨著Reynolds number降低至100後，流場均是單顆渦旋，除了孔穴半徑為1500 μm的尺寸例外。紊流強度在無縮口設計時值比較小，而有縮口設計時值比較大，且在任何情形下，紊流強度在渦旋中心處最小。在有縮口設計下，紊流強度隨著Reynolds number上升而下降，而無縮口設計時，紊流強度依然隨著Reynolds number上升而下降，只是趨勢沒有縮口設計時明顯。此外，在同樣Reynolds number及孔穴尺寸下，無縮口孔穴之Kolmogorov length scale的值比較小，而有縮口時值則比較大，在同樣縮口與Reynolds number下，尺寸越大的孔穴Kolmogorov length scale的值越大，最後，不論有無孔穴的設計，Kolmogorov length scale均隨Reynolds number上升而下降。
經由本實驗所設計之微流元件，可以在孔穴中得到一營養鹽梯度，在未來可用於研究浮游生物在紊變流場及濃度梯度之生態。

關鍵字

流場紊變 ； 營養鹽梯度 ； 微流元件

並列摘要

This study aims at designing a microfluidic platform to study the influences of flow fluctuation and nutrient gradient in plankton ecology. This microfluidic platform has three different layers. The bottom layer is a turbulent channel which serves to generate turbulence in a cavity, the middle layer is a 3 mm thick agarose gel and the top layer is a concentration-gradient channel. The turbulent channel consists of a round cavity which attaches to a straight main channel. Two geometrical parameters are varied herein: the cavity radius (R = 250 μm, 500 μm, 750 μm, 1000 μm, 1500 μm, 5000 μm) and the design of the cavity mouth. The throatless design is the cavity connected to main channel directly and the throat design is the cavity and main channel connected to main channel with a pair of semicircle. The concentration-gradient channel consists of two symmetric channels which are fed with water and dye. The two solutions diffuse through the agarose gel into the turbulent channel, creating concentration gradient in the cavity.
From the μPIV results, we find that both the size and the mouth design affect the velocity field significantly. At the same Re, velocity decreases as the size of cavity grows. Moreover, throatless design leads to faster velocity than the throat design. Viewing from the streamline, the occurrence of vortices take place at radius of the cavity reaches 500 μm, whereas the number of vortex remains 1 from Reynolds number 1 to 5. As the radius of the cavity reaches 750 μm, the number of vortices remains 2 from Reynolds number 1 to 10, When the radius of the cavity exceeds 1000 μm, no vortex occurs in the cavity under the same Reynolds number. For Reynolds number larger than 50, there is only one vortex in the cavity regardless of cavity size . Under the same Reynolds number, turbulence intensity of the throat design is larger than of the throatless design. We also find that the turbulent intensity decreases as Reynolds number increases. Comparing to throatless design, throat design tends to increase the Kolmogorov length scale. In addition, the Kolmogorov length scale increase with cavity size, but decrease with Reynolds number.
In our microfluidic platform, we create a flow-free environment in the cavity with constant-concentration boundary condition. The agarose gel permits the passage of molecules by diffusion, but completely blocks advection. This advantage allowing as to study plankton ecology just by turbulent flow field and flow-free concentration gradient.

並列關鍵字

Microfluidic Platform ； Velocity Fluctuation ； Nutrient Gradient

參考文獻

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