以水滴之前進角來區分親水與疏水表面

Abstract

本研究以平坦表面上的前進角作為指標，定量探討了在同一系統下，親水表面及疏水表面的定義。此研究的實驗基材選用具有規則方柱陣列結構的聚多巴胺塗覆之NOA81表面。藉由改變NOA81在多巴胺溶液中的塗覆時間，可以調控基材的固有接觸角。實驗中，三組NOA81基材的固體覆蓋率分別維持定值在0.479±0.020、0.270±0.018和0.147±0.012，並藉由提高結構粗糙度以觀察水滴在結構表面上濕潤狀態的改變。此研究中，水滴的濕潤行為被分為三種狀態：Wenzel狀態、Cassie狀態，以及Cassie浸潤狀態。水滴在實驗中的濕潤狀態轉換也嘗試與三種分別根據熱力學觀點、幾何關係，和能量最小化觀點的濕潤轉換準則進行比較。 本研究結果中，藉由改變多巴胺塗覆時間及結構參數的製程方式，成功的調控出了前進角從22度到158度的廣泛區間。另外，由本研究的實驗結果可歸納出，當平坦表面上的前進角大於76o±4o時，Wenzel狀態到Cassie狀態的濕潤狀態轉換可以隨著粗糙度的增加而被觀察到，因此在此角度下的平坦表面可以被歸類為「疏水」表面。另一方面，若平坦表面上的前進角小於56o±4o，粗糙度的增加可以使水滴從Wenzel 狀態轉變到Cassie浸潤狀態，因此在此角度下的平坦表面則可以被定義為「親水」表面。另外，本研究也根據實驗結果，提出了能夠發生「超親水」現象的條件。而至於當平坦表面上的前進角介於56o±4o和76o±4o之間時，隨著粗糙度的增加，水滴即便在很高的粗糙度時 (r>4)，仍維持在Wenzel 狀態，並沒有濕潤現象的轉換能夠被觀察到，因此我們歸納前進角界於此範圍的平坦表面時，處在一個親疏水性的介穩地帶。

In order to quantitatively distinguish between hydrophobicity and hydrophilicity, advancing contact angle of a water droplet on the flat surface is chosen as an index. The polydopamine-coated NOA81 surfaces with regular square pillar arrays are utilized as the model substrates. The intrinsic contact angles of NOA81 surfaces are varied by dopamine coating time. Also, the roughness of the substrate is adjusted while the solid fraction for each series of structures is fixed at 0.479±0.020, 0.270±0.01 or 0.147±0.012. Besides, the wetting behavior of a water droplet is observed on the patterned surfaces and classified into three wetting states: Wenzel state, Cassie state, and Cassie impregnating wetting state. The wetting transition in this study is compared with different criteria, which are based on the viewpoints of thermodynamics, geometric relationships, and energy minimization. In this study, a wide range of advancing contact angle, from 22o to 158o, can be successfully tuned on the NOA81 substrates by varying dopamine coating time and structure parameters. Concluded by the experiments, if the advancing contact angle θ_A on the flat surface is larger than 76o±4o, the transition from Wenzel state to Cassie state can be observed when the roughness increases. Therefore, it is classified as a hydrophobic surface. On the other hand, when θ_A on the flat surface is smaller than 56o±4o, the transition from Wenzel state to Cassie impregnating wetting state is observed with the increasing roughness. Thus, the surface with θ_A < 56o±4o can be defined as a hydrophilic surface. A condition for the occurrence of superhydrophilic behavior is also proposed in this study. For the flat surface with 56o±4o < θ_A < 76o±4o, a water droplet keeps at Wenzel state and no transition can be observed. As a result, it is classified as a hydrophobic/hydrophilic metastable zone