具階層式結構疏水表面之液滴濕潤現象與凝結機制探討

Abstract

凝結為一種複雜且動態的非均相相變化過程，液滴式凝結主要發生在較疏水的表面上，而表面親疏水性主要以液滴在表面上的接觸角數值為主要依據，故本研究旨在利用接觸角量測與凝結兩種不同面向來探討液滴於表面上的濕潤狀態差異。本研究利用溶膠凝膠法製得一系列固體佔有率為0.28  0.04 的單層微米方柱結構表面，藉由調整方柱維度改變表面粗糙度，並會塗佈不同尺寸(78  9 nm與317  27 nm)的二氧化矽奈米粒子，製得雙層微/奈米結構表面，並進行系統性探討。研究結果顯示，單層微米結構表面表現較大的接觸角遲滯，以粗糙度1.42為分界，液滴濕潤狀態可從Wenzel狀態轉換為Cassie狀態，在凝結過程中卻僅能呈現Wenzel狀態與部分濕潤的混合型狀態(partial Cassie狀態)，且臨界粗糙度提升至1.61。雙層微/奈米結構表面則有效提升表面超疏水性，大為降低接觸角遲滯至約14度，液滴濕潤狀態主要為Cassie狀態，而凝結液滴不僅能呈現Wenzel狀態與partial Cassie狀態，更在臨界粗糙度1.39後，成功觀察到Cassie液滴，取決於凝結過程初期的動態機制，故研究中統計量化不同時間的液滴個數與尺寸，並利用反轉式顯微鏡系統更準確對濕潤狀態與凝結機制進行判定，於微結構底部或側邊成核並成長的Wenzel液滴，能透過與他液滴碰撞而釋放的表面自由能及自身的拉普拉斯壓力差來實現去濕潤轉換，克服Wenzel液滴固液表面間的附著力從而轉變為Cassie液滴，但隨著微米結構高度提升，連帶提升了Wenzel液滴底部的體積與固液接觸面積，降低去濕潤現象發生的機率，對Cassie液滴的形成產生抑制作用。

Condensation is a complex and dynamic heterogeneous phase change process. Droplet condensation mainly occurs on hydrophobic surfaces, and hydrophilicity and hydrophobicity of the surface are mainly based on the measurement of contact angle on the surface. Therefore, the purpose of this research is to observe the difference in the wetting state of droplets on the surface with using two different aspects: contact angle measurement and condensation mechanism. In this study, a series of single-layer micro-square pillar structures with a solid fraction of 0.280.04 were prepared by sol-gel method. The surface roughness was changed by adjusting the dimensions of the square pillars, and coating different size of silica dioxide nanoparticles to prepare with dual-scale micro/nanostructured with the size of 7829 nm and 31727 nm, which will be systematically discussed. The results show that the single-layer microstructure exhibits a large contact angle hysteresis. With a roughness of 1.42 as the boundary, the wetting state of the droplet can be converted from the Wenzel state to the Cassie state, but only the Wenzel state and the partial wetting state can be observed on the surface wetting during the condensation process, and the critical roughness increased to 1.61. The dual-scale micro/nano-structured surface effectively improves the hydrophobicity of the surface and greatly reduces the contact angle hysteresis to about 14 degrees. The wetting state of the droplet is mainly Cassie state, and the condensed droplet can not only show Wenzel state and partial Cassie state, but also the Cassie droplets can be successfully observed with the larger critical roughness which is larger than 1.39. We find this phenomenon depended on the dynamic mechanism of the condensation process in the early stage. Therefore, the number and size of droplets at different times were statistically quantified in the study, and the inverted microscope system was used to more precisely measure the wetting behavior. The Wenzel droplet nucleated and grown at the bottom or side of the microstructure can achieve dewetting conversion through the surface free energy released by the collision with other droplets and its own Laplace pressure difference. The adhesion between the solid-liquid surfaces of Wenzel droplets can be overcame and convert them into Cassie droplets. However, as the height of the microstructure increases, the volume at the bottom of the Wenzel droplet and the solid-liquid contact area are also increased, which reduces the probability of dewetting and inhibits the formation of Cassie droplets.