純液滴和含奈米粒子的液滴在同質與異質性條紋表面的揮發現象

Abstract

本篇研究將分為兩部分討論：第一部分是研究添加奈米粒子的水滴在化學同質性的平坦表面的揮發現象，第二部分是觀察液滴在化學異質性條紋表面的濕潤狀態與其揮發現象。 藉由在矽晶片表面接上不同官能基的自聚合單分子膜，可以改變固體表面的親疏水性。利用光學上拍與側拍的方式，比較純水滴與添加了二氧化矽奈米粒子的水滴在不同親疏水性表面的揮發現象。兩者揮發的機制大致相同：由定接觸半徑模式開始，當接觸角到達後退角，轉而進入定接觸角模式，最後進入半徑和角度皆同時下降的混合模式。兩者差異發生在揮發的末期，添加了奈米粒子的水滴，會在混合模式的過程中，產生第二次定半徑的揮發並留下環形殘跡。對於相同表面，其發生第二次定半徑時的濃度約略為定值且與起始濃度無關。此外，我們發現該濃度與表面的親疏水性有很大的關聯性。 在第二部分，我們利用微觸印刷的方式製造親疏水線條相間的化學異質性表面，並同樣藉由光學儀器，從平行與垂直條紋的視角觀察水滴在此種表面的揮發現象。兩個視角的揮發均可以觀察到定半徑、定接觸角與混合模式。但從平行方向觀察，在定接觸角模式時，可以發現角度呈現明顯且穩定的週期震盪，而半徑呈現階梯式的下降，然而在垂直方向上卻觀察不到此現象。此外，利用酒精液滴，可以觀察到三相線在此表面上扭曲的情形。我們亦嘗試將添加了二氧化矽奈米粒子的酒精滴在此表面上揮發，可以發現親水的線條上會留下奈米粒子的沉積。

There are two topics discussed in this study: evaporation of water droplet containing silicon dioxide nanoparticles on homogeneous surfaces with different hydrophobicities and evaporation of liquid droplet on chemically heterogeneous striped surface. First, the evaporation mechanism of pure water droplet and water droplet containing silica nanoparticles on silicon wafer with different self-assembled-monolayers were observed and compared from top and side view in this study. The evaporation mechanism generally starts with constant contact radius mode and turns into constant contact angle mode when the receding contact angle is reached. Finally, the mechanism enters the mixed mode. The major differences of the evaporation process are an additional contact line pinning during the mixed mode and a ring-like structure left on the surface after the droplet drying. For each surface, it is found that the concentration at pinning moment is rather a constant and independent of initial concentration. Moreover, the concentration is highly related to the hydrophobicity of the surface. Second, chemically heterogeneous striped surfaces consisting of alternating and parallel hydrophobic/hydrophilic self-assembled monolayer were prepared on silicon substrate by micro-contact printing. The evaporation of a sessile droplet on the heterogeneous surface was studied by enhanced video microscopy system to record the evolution of contact angle and contact radius along two different viewing angles (normal or parallel to the stripe) in the evaporation process. At the very beginning of the evaporation process in these two viewing angles, the contact angle decreases dramatically while the radius of three phase contact line remains almost constant. Then, the angle decreases slowly while the radius decreases. It is interesting to note that the angle in the direction parallel to the stripe exhibits the stick-slip-jump phenomenon and the radius decreases with a step jump while the contact angle and radius in the direction normal to stripe decrease disorderly. In addition, the corrugation of the three phase contact line of ethanol droplet and dynamic shrinkage of contact line during evaporation are observed. Besides, in an appropriate initial concentration of ethanol-based droplet containing silica nanoparticle, a regular pattern of nanoparticles on hydrophilic lines can be observed after the droplet drying.