表面性質對液晶預傾角和不對稱濕潤之影響

Role of Surface Properties in Liquid Crystal Pretilt Angle Control and Asymmetric Wetting

劉邦彥(Bang-Yan Liu)

指導教授 ： 陳立仁

國立臺灣大學/工學院/化學工程學研究所/博士(2018年)

https://doi.org/10.6342/NTU201800305

摘要

此論文之研究旨在探討表面性質對於不同系統之影響。其又可分為三個主題，分別為：一、探討高分子穩定液晶排列系統中表面性質改變對於液晶預傾角控制之效應。二、結構參數對於疏水三角柱結構表面不對稱接觸角之調控。三、親水三角柱結構表面之不對稱滲出現象。
本研究利用雙成分單體系統來控制液晶預傾角，並探討高分子反應對於表面之修飾效應，及研究其與液晶排列之關係。透過改變高分子單體濃度，液晶之預傾角成功地從0.8°調控至90°。在高分子反應之過程中，高分子會沉積在基材表面達到表面修飾之效果，並造成表面能下降和粗糙度增加。兩者對於液晶之預傾角和表面之純水前進角都有增加之效應。並顯示出表面之疏水性變化能有效反應液晶之預傾角變化之趨勢，可以做為預傾角調控之指標。
而在疏水的三角柱結構中則觀察到不對稱之接觸角，在三角形底端方向發現有較高的前進角與後退角。此不對稱性質受到結構排列緊密程度和結構高度影響。當結構排列較緊密時，不對稱性上升；而當柱高增加則會破壞不對稱性。然而當結構密度和高度滿足一定條件時，液體在尖端方向的運動會發生強烈的釘著現象，造成角度的大幅增加並破壞不對稱性質。此研究透過實驗和模擬驗證，提出最佳化不對稱性質結構建議及提供釘著現象之預測。
最後，此研究試圖利用親水之三角柱結構控制液體滲出之方向。液體之滲出行為是透過Surface Evolver計算來做預測，發現三角柱之底端形狀能有效阻止液體前進避免滲出。在對稱排列結構下，尖端和側邊方向之滲出行為同步。欲控制兩個方向分別滲出，則須利用不對稱排列來達成。通過分析滲出條件受結構之影響，不對稱排列之結構能利用相對簡單之方法快速預測。最後計算得到之結果則用實驗來驗證。

關鍵字

液晶 ； 預傾角 ； 前進角 ； 光固化高分子 ； 三角柱 ； 不對稱濕潤 ； 結構表面

並列摘要

This thesis focuses on the effects of surface properties on various systems, and is composed of three parts: 1. Study the surface properties influence on the pretilt angle control of polymer-stabilized liquid crystal system. 2. Control and optimize the contact angle asymmetry on the hydrophobic triangular post patterned surface. 3. Investigate the directional wicking phenomenon on the hydrophilic triangular post structured surface.
This work utilizes two-component monomers to control the LC pretilt angle, and study the surface property change and the influence on the LC pretilt angle. By tuning the monomer concentration, the pretilt angle is successfully tuned from 0.8° to 90°. During the polymerization, the polymer deposits onto the substrates and modifies the surface to a lower surface energy and a higher roughness. These two effects increase both the LC pretilt angle and water advancing contact angle of the substrates. As a consequence, the ACA of substrates reflects the tendency of pretilt angle and can be used as an index for pretilt angle control.
Asymmetric contact angles are observed on the hydrophobic triangular post structure; the advancing and receding contact angles are higher in the base direction than the ones in the tip direction. This asymmetric property is affected by the structure packing density and the post height. The asymmetry is enhanced as the packing density is increased. However, increasing the post height deteriorates the asymmetry. While as the packing density and the post height reach a certain condition, a strong pinning occurs in the tip direction, which greatly raises the contact angle and destroys the asymmetry. This work is studied with both experiments and simulations. A recommendation of structure geometry for optimized asymmetry is given and the strong pinning is predicted.
In the last part of this research, the directional wicking behavior is realized with the hydrophilic triangular post structured surface. Surface Evolver calculates the stability of liquid film on given geometry to predict the conditions for spontaneous wicking. The base direction is found to effectively prevent the liquid spreading. The wicking in the tip and side direction are found simultaneous. To distinguish the wicking conditions for the tip and side direction, an asymmetric arrangement is necessary. By analyzing the structure influence on the wicking conditions, the wicking behaviors on the structure of asymmetric arrangement can be simply and quickly predicted. Finally, the wicking behaviors obtained from the calculation are examined with experiments.

並列關鍵字

Liquid crystal ； Pretilt angle ； Advancing contact angle ； Photocurable polymer ； Triangular post ； Asymmetric wetting ； Structured surface

參考文獻

(1) Barthlott, W.; Neinhuis, C. Purity of the Sacred Lotus; 1997; Vol. 202.

(2) Zheng, Y.; Gao, X.; Jiang, L. Directional Adhesion of Superhydrophobic Butterfly Wings. Soft Matter 2007, 3 (2), 178.

(3) Malik, F. T.; Clement, R. M.; Gethin, D. T.; Krawszik, W.; Parker, A. R. Nature’s Moisture Harvesters: A Comparative Review. Bioinspiration and Biomimetics. 2014, p 31002.

(4) Good, R. J. Contact Angle, Wetting, and Adhesion: A Critical Review. J. Adhes. Sci. Technol. 1992, 6 (12), 1269–1302.

(5) Young, T. An Essay on the Cohesion of Fluids. Philos. Trans. R. Soc. London 1805, 95, 65–87.