液滴在具結構表面的濕潤行為與親疏水表面接觸角定量定義探討

Wetting Behavior of Water Droplet on Patterned Surface and Quantitative Definition of Hydrophilicity and Hydrophobicity

李珊(Shan Li)

指導教授 ： 陳立仁

國立臺灣大學/工學院/化學工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701539

摘要

本研究主要對固體表面親疏水的臨界角度進行探討，選用四種材料表面(PET、nylon 6,6、PEOPDMS、NOA81)，對部分系統進行濕潤性改質，製備七種前進角(90.8、86.3、84.3、62.9、56.6、34.9、31.6度)、四種後退角(58.2、42.5、19.9、0度)的系統。實驗方式包含濕潤現象觀察，及前進角與後退角的測量兩個部分。
液滴於表面的濕潤行為被廣泛地運用在工業上，並在科學研究中被廣泛地探討，傳統上一般以接觸角90度作為親疏水表面的臨界角度，大於90度為疏水表面、小於90度為親水表面，但文獻中對於臨界角度的說法眾說紛紜，其中以Vogler提出的65度新臨界角度最具代表性。本研究選用PET、nylon 6,6、PEOPDMS、NOA81材料表面，使探討系統包含七種前進角與四種後退角，並在表面上製備規則方柱微結構，藉由粗糙度的變化，觀察水在表面上的濕潤行為，並以埋針法測量前進角與後退角。研究顯示Vogler提出的新臨界角度65度與實驗結果不相符，但也未發現一個可以精準劃分親疏水表面的特定角度，從濕潤現象轉換與接觸角的關係，認為可以分別以前進角、後退角對親疏水表面進行定量定義：前進角大於90度或後退角大於55度的表面為疏水表面，前進角小於35度的為親水表面。
此外，對於前進角35-55度的表面，我們一直無法找到在此角度區間可以穩定測量的材料，導致對臨界角度課題的探討不夠完善，對此我們嘗試光接枝丙烯酸於PDMS表面，使表面前進角得以透過紫外光照射時間改變達到連續且精準調控的目的，儘管尚未成功找出可以連續調整前進角的反應條件，但是可以知道反應後接枝表面高度均勻，粗糙度(Ra)落在0-3.5奈米，若將此反應應用在具方柱微結構PDMS，接枝聚丙烯酸分子層不會使樣品規格在巨觀上有太大的變化。而隨反應時間增加，表面後退角先下降至0度後，接著前進角開始下降；在單體濃度較低時，反應40分鐘時達到最高親水性，前進角約為80度，照光時間繼續增加不會使樣品表面前進角繼續下降；在單體濃度較高時，反應較快速，照光10分鐘就可以使表面前進角從100度下降到20度，但是在反應6-9分鐘的區間，前進角數值從70-80度直接下降到20度，而非一個角度連續變化的過程。反應時玻璃與PDMS的距離越大，在相同照射條件下可以達到較小的前進角，表面更親水。

關鍵字

親水 ； 疏水 ； 微結構表面 ； 濕潤轉換 ； 接觸角 ； 表面改質

並列摘要

Our study was focus on the quantitative definition of surface hydrophilicity and hydrophobicity. Four kinds of materials have been chosen, and the surfaces underwent surface modification to enhance their hydrophilicity. Among the surface with different wettability, advancing contact angle of the surfaces ranges from 90.8 degree to 31.6 degree, while receding contact angle of the surfaces ranges from 58.2 degree to 0 degree. Two parts included in our study: observation of wetting phenomena of water droplet deposited on flat and patterned surfaces, and contact angle measurements.
Hydrophilicity and hydrophobicity are among the most important concepts in surface chemistry, and wettability phenomena has been studied for more than 200 years. Conventionally, 90 degree is considered as critical contact angle or, on the other word, quantitative definition of relative terms “hydrophilic” or “hydrophobic”. However, the critical contact angle still remain the subject of some debate in the literature. The most well-known definition is given by Vogler with which is 65 degree. In the experiment, we prepared surfaces with various wettability and modified their surface roughness by creating regular pillar-patterned structure. We observed the wetting phenomena of water droplet, and measured water advancing and receding contact angle. The data reveals that 65 degree given by Vogler does not fit the results, and there is no specific contact angle to precisely define whether the surface is either hydrophilic or hydrophobic. Upon examination of wetting transition and contact angle data between water and a variety of solid surfaces, an improved definition for hydrophilicity and hydrophobicity is proposed. A surface is hydrophobic when advancing contact angle is larger than 90 degree or receding contact angle larger than 55 degree, while a surface is hydrophilic when advancing contact angle lower than 35 degree.
Beside, for surfaces with advancing contact angle ranges from 35 to 55 degree, it was difficult for us to find material that contact angle could be stably measured and with high reproducibility. Therefore, we tried to modify PDMS surface by poly (acrylic acid) photografting polymerization. Hoping that their wettability could be varied in a continuous manner by adjusting UV exposure duration. Though the goal hasn’t been achieved, some surface properties after graft polymerization were provided: (1) surface roughness (Ra) lies in 0-3.5 nm indicates that the surface after modification is almost flat and homogenous. Therefore, the dimension would not change macroscopically if we applied the reaction to pillar-patterned structure surfaces. (2) With increasing exposure duration, receding contact angle dropped to 0 degree first, followed by decrease of advancing contact angle. (3) Lower monomer concentration leads to limit modification with advancing contact angle 80 degree. Further UV exposure would not leads to higher hydrophilicity. Under high monomer concentration, advancing contact angle could drop from 100 degree to 20 degree within 10 min exposure. But the change of contact angle was not in a continuous manner within 6-9 min exposure duration. (4) Under the same reaction condition, larger spacing distance between glass and PDMS surface resulted in lower advancing contact angle.

並列關鍵字

hydrophilicity ； hydrophobicity ； micro-patterned surface ； wetting transition ； contact angle ； surface modification

參考文獻

(70) 楊詠茹; Yang, Y.-J. 利用數值方法模擬親疏水表面接觸角之研究

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