局部熱源引致自然對流之圖案化流場及其解析能力

Flow Patterning Capability of Localized Natural Convection

黃凌霆(Ling-Ting Huang)

指導教授 ： 趙玲

國立臺灣大學/工學院/化學工程學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201603403

摘要

發展控制流體流動圖形以產生材料圖案化排列的技術，在光學元件、電子儀器以及生物感測器上能有許多的應用。 在這篇研究中，我們藉由控制熱源的幾何形狀及位置，產生我們所想要的自然對流的流場圖案，進而達成材料的圖案化排列。自然對流的產生是由於我們在空間中製造了溫度梯度的分佈，而導致在流體中有密度梯度分佈的狀況，進而驅動流體的流動。 為了分析這項圖案化技術的解析能力，我們透過數學模型以及無因次分析，找出圖案解析度與實驗操作參數之間的關係。 從無因次分析的結果來看，在我們感興趣的條件下，能夠影響流場圖形及其解析度的無因次參數群只有兩個，分別是1/√Gr 以及 1/Pr√Gr，其中Gr (Grashof number)的物理意義是流體浮力(Buoyancy force)與黏滯力(viscous force)的比值；而Pr (Prandtl number) 的物理意義是動量擴散(momentum diffusivity)與熱擴散(thermal diffusivity)的比值。 此外，我們利用這個模型分析出所有可能的流動型態，並將流動型態與這兩個無因次參數群的關係製成圖表，進而找出最適合用來產生圖案化流場的實驗操作條件。 為了能夠有效率地取得解析度的資訊，我們發展出V字型的熱源系統，來讓我們不僅在模擬上或是實驗上，都能顯著地簡化獲得解析度的程序。由V字型的熱源系統實驗所得之圖案解析度，的確與利用模擬之方法所得的圖案解析度相符。實驗和模擬上結果的一致性，佐證我們發展的模擬模型可以適當地描述此圖案化系統的狀態，也因此支持我們用模擬模型得到的「流場型態圖」與「適當操作條件」這兩樣資訊對未來的應用能有所助益。 最後我們利用這項圖案化系統成功地排列銀奈米線，展現出這項圖案化技術的應用性及可行性。

關鍵字

局部型態自然對流 ； 流體控制 ； 圖案化流場 ； 解析度 ； 奈米線排列

並列摘要

Controlling flow patterns to align materials can have various applications in optics, electronics, and biosciences. In this study, we developed a natural-convection-based method to create desirable spatial flow patterns by controlling the locations of heat sources. Fluid motion in natural convection is induced by the spatial fluid density gradient that is caused by the established spatial temperature gradient. To analyze the patterning resolution capability of this method, we used a mathematical model combined with nondimensionalization to correlate the flow patterning resolution with experimental operating conditions. The nondimensionalized model suggests that the flow pattern and resolution is only influenced by two dimensionless parameters, 1/√Gr and 1/Pr√Gr, where Gr is Grashof number, representing the ratio of buoyancy to the viscous force acting on a fluid, and Pr is the Prandtl number, representing the ratio of momentum diffusivity to thermal diffusivity. We used the model to examine all of the flow behaviors in a wide range of the two dimensionless parameter group and proposed a flow pattern state diagram which suggests a suitable range of operating conditions for flow patterning. In addition, we developed a heating wire with an angular configuration, which enabled us to efficiently examine the pattern resolution capability numerically and experimentally. Consistent resolutions were obtained between the experimental results and model predictions, suggesting that the state diagram and the identified operating range can be used for further application. Finally, the success in aligning silver nanowires by using our patterning device implies the applicability and potential of this technique.

並列關鍵字

localized natural convection ； flow control ； patterning control ； resolution ； nanowire alignment

參考文獻

26 Van Dyke, M. & Van Dyke, M. An album of fluid motion. Vol. 176 (Parabolic Press Stanford, 1982).

1 Arpaci, V. S., Kao, S.-H. & Selamet, A. Introduction to heat transfer. (Prentice Hall, 1999).

3 Lamberti, G. Flow induced crystallisation of polymers. Chemical Society Reviews (2014).

4 Lukaschek, M., Grabowski, D. A. & Schmidt, C. Shear-induced alignment of a hexagonal lyotropic liquid crystal as studied by rheo-NMR. Langmuir 11, 3590-3594 (1995).

5 Jackson, C. L. et al. A shear-induced martensitic-like transformation in a block copolymer melt. Macromolecules 28, 713-722 (1995).