三相線張力於遲滯角之影響與光控仿生偶氮苯花瓣

Abstract

本論文展示花瓣效應是由線張力所控制，並發現表面的結構的特徵長度所扮演的重要角色。藉由線張力所修正的 Wenzle方程式，對應實驗上的數據是較合理的。在這研究中利用高分子材料設計不同的特徵長度的基材表面，進一步去評估在這樣的不同的表面粗糙度與表面接觸線比例對於遲滯角的影響。在遲滯角實驗中，線張力是會轉變的，並且由正轉為負值，轉變的過程，遲滯的狀態也剛好是前進角轉為後退角的狀態。可發現表面粗糙度影響的是在前進角的階段，線張力則是主要影響後退角。當增加特稱長度時，接觸角可從107±1.1度增加為 129.3 ± 0.9度，而遲滯角的部分則是明顯的由23.1±1.3度增加到62.1±2.1度。在這樣的實驗觀察結果下，線張力對於潤濕效應的影響有了清楚的了解，並可以設計所需要的結構表面，這對於生醫上的設計與應用可以有大幅的幫助。 在這研究中，了解表面粗糙度對於潤濕效應的影響，進一步發展表面能量對於潤濕效應的影響。開發一可光控的人造玫瑰花瓣，藉由偶氮本以互穿型網狀結構作為設計。玫瑰花瓣上的微米與奈米結構皆完整的仿生玫瑰花瓣上的微米與奈米結構皆完整的複製。由於偶氮苯是光學幾何異構物，就由不同的波長的光照射，偶氮本的順反異購物有不同的表面能量，使得此仿生花瓣的接觸角度變化範圍99.2±0.9度至 140.9±1.8度，遲滯角也可在99.2±0.9度到140.9±1.8度之間來回變化，比起真實的玫瑰，此仿生花瓣有更高的接觸角與更明顯的遲滯效應，更近一步的是可用非接觸的光學方式進而改變與調整。

This paper experimentally investigated how the petal effect is governed by line tension and showed that the characteristic length of the surface structure plays an important role. Particularly, Wenzel’s equation modified with line tension was used to model the contact angle hysteresis. We designed different surfaces on polydimethylsiloxane with various structure lengths, 15 to 50 um, to evaluate the effect of surface roughness and the actual length of the three-phase contact line on the contact angle hysteresis. The sign of the line tension alters from positive to negative when the droplet changes from advancing to receding state. The surface tension dominates the contact angle in the advancing state while the line tension governs the contact-line movement in the receding state. By increasing the characteristic length of the surface structure, the static contact angle can be altered from 107±1.1o to 129.3 ± 0.9o, and the contact angle hysteresis changes obviously from 23.1±1.3o to 62.1±2.1o. These results facilitate better understanding of the effect of line tension on a wetting surface for a more robust biomimetic design. This paper presents a biomimetic rose petal whose contact angle hysteresis can be reversibly tuned by photo-illumination. With azobenzene interpenetrating polymer network, the micron- and nano-scale surface structure of a rose petal was duplicated. By changing the exposure light wavelength, the surface free energy of the artificial petal was modulated between trans-to-cis azobenzene photo-isomerations. The static contact angle of the artificial petal can be tuned to range from 99.2±0.9o to 140.9±1.8o. The contact angle hysteresis between 50.3 ± 0.6o and greater than 84o can be achieved. Comparing to the value measured on a real rose petal, the fabricated counterpart presents more prominent petal effect in terms of larger static contact angle and greater contact angle hysteresis.