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  • 學位論文

液滴潤濕行為中的接觸角遲滯研究

Contact Angle Hysteresis in Droplet Wetting

指導教授 : 諶玉真
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摘要


在日常生活、工程程序、與科學研究中,液滴潤濕固體表面的現象都是顯而易見的,這個論文將探討表面粗糙度對液滴潤濕行為的影響,分成四個部分。 (1) 因為光滑無缺陷的液−液界面,接觸角遲滯在易滑的液體填充多孔表面上實質地消失。我們將含氟的潤滑油填充在拉伸的聚四氟乙烯止洩帶表面,方便又快速地製造液體填充表面,因為聚四氟乙烯膜很薄,液體填充表面呈現透明,並且可彎曲。接觸角或基底直徑的遲滯圈消失,代表液體填充表面具有可忽略的接觸角遲滯。最後,我們用兩個實驗彰顯液體填充表面的優異抗污性能,第一個為傾斜基材上的滑移液滴移除髒汙,第二個為液體填充表面與疏油表面對十六烷油滴的潤濕競爭。 (2) 利用多體耗散粒子動力學探討無遲滯表面上奈米液滴與溝槽的互動。自由液滴展現布朗運動且其擴散率隨著液−固接觸面指數下降,相反地,在溝槽上的液滴則展現被限制的隨機運動。自由態與受限態的轉換需要做功克服能量障礙,根據施力−位移圖,勢能景圖可以被建立,其結果顯示,當液滴要滲入溝槽時,溝槽表現的像疏水的汙點,反之,當液滴要脫離溝槽時,溝槽表現的像親水的汙點。藉由液體填充表面,我們也做了巨觀的實驗,其結果與模擬結果定性上一致。 (3) 利用多體耗散粒子動力學探討奈米液滴在規則粗糙表面上的行為。在粗糙表面上,接觸線會被釘住,且因為表面粗糙度,可以得到不同於本質接觸角的最穩定接觸角,接觸角遲滯的程度可以用類似於實驗中的微量針頭法與斜板法的兩個方法來決定,遲滯圈可以被得到且兩個方法的結果一致。接著,我們驗證了潤濕性與表面粗糙度對最穩定接觸角與接觸角遲滯的影響,最穩定接觸角與Wenzel或Cassie-Baxter推估的理論值之間的偏差可以藉由滲入程度來解釋,滲入程度會隨著溝槽位置與潤濕性變化,另外,接觸角遲滯受到溝槽寬度的影響大於溝槽深度。 (4) 溝槽中的氣袋穩定度與接觸角遲滯有關,這部分利用滲入實驗與表面演化器模擬研究氣袋穩定度,我們將液滴放置在壓克力基材上的圓錐形孔洞以形成氣袋,其穩定性與表面潤濕性有關。液體滲入分成四種行為,且透過使用橄欖油液滴,滲入的路徑也被清楚觀察到。藉由表面演化器得到與滲入路徑有關的能量−障礙圖,並且可以解釋不同的滲入行為。另外,也量測了不同液體在具有多鑽孔的壓克力基材上的接觸角遲滯,其潤濕行為分成三類,並且可以合理解釋實驗結果。

並列摘要


A liquid drop wetting a solid surface is everywhere in our daily life as well as in engineering and science. In this thesis, there are four major parts to investigate the effect of surface roughness on droplet wetting. (1) Contact angle hysteresis (CAH) on slippery liquid-infused porous surfaces (SLIPS) is essentially absent due to the smooth and defectless liquid-liquid interface. Our SLIPS is facilely fabricated by a stretched polytetrafluoroethylene (PTFE) tape infused with fluorinated lubricant. Because the PTFE film is thin, SLIPS can be transparent and flexible. The absence of hysteresis loops of contact angle (CA) or base diameter for both water and hexadecane shows neglected CAH of SLIPS. The excellent anti-smudge performance of our SLIPS is exhibited by two experiments: the removal of stains by sliding drops at an inclined plane and the wetting competition of a hexadecane drop between SLIPS and a lipophobic surface. (2) The encounter of a nanodrop with a trench on a CAH-free surface is explored by many-body dissipative particle dynamics (MDPD). A free nanodrop exhibits Brownian motion and the diffusivity decays exponentially with the liquid-solid contact area. In contrast, a nanodrop sitting on a trench shows the restricted random motion. Work must be done to overcome the energy barriers for the transition between free and trapped states. According to the force-displacement plot, the potential energy landscape is constructed. It is shown that the trench acts as a hydrophobic blemish for capture but like a hydrophilic blemish for escape. The macroscopic experiments are performed by SLIPS. The experimental observations agrees qualitatively with simulation outcomes. (3) Nanodrops on patterned rough surfaces are explored by MDPD. On a rough surface, the contact line is pinned and the most stable CA different from the intrinsic CA is acquired due to surface roughness. The extent of CAH is determined by two approaches which resemble the inflation/deflation method and inclined plane method for experiments. The hysteresis loop is acquired and both approaches yield consistent results. The influences of wettability and surface roughness on the most stable CA and extent of CAH are examined. The deviation of the most stable CA from that estimated by the Wenzel or Cassie-Baxter models is explained by the extent of impregnation which varies with the groove position and wettability. Moreover, the extent of CAH depends more on the groove width than the depth. (4) The stability of air pockets formed in grooves on a surface is relevant to CAH and it is investigated by imbibition experiments and Surface Evolver (SE) simulations. The air pocket formed by placing a liquid drop atop a conical hole on a polymethyl methacrylate (PMMA) substrate and its stability depends on surface wettability. Four kinds of imbibition behaviors are observed. The imbibition pathway is clearly observed by using the olive oil drop. The energy-barrier profile associated with the imbibition pathway acquired by SE can interpret the outcome of imbibition. CAH of various liquids on a PMMA substrate with drilled holes is also determined. Their wetting behaviors are categorized into three types and can explain the experimental results reasonably.

參考文獻


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