液滴在具結構疏水表面上的揮發與濕潤現象

Abstract

本篇研究將分為兩部分進行討論:一是關於水滴在軟硬不同的具結構疏水表面上的揮發現象，而二則是針對Cassie impregnating wetting state這個濕潤狀態進行研究討論。 在同樣的基材結構下，改變基材PDMS(聚二甲基矽氧烷)的軟硬程度，從0度視角及45度視角去觀察水滴在其表面上的揮發現象。水滴在具結構疏水表面的揮發機制大體如下:從定接觸半徑模式開始，當接觸角到達後退角時，轉而進入定接觸角模式。當基材愈軟，其後退角就愈小。而緊接著發生在定接觸角模式之後的是濕潤現象的轉變，水滴從原來的Cassie 狀態變成Wenzel狀態，且當基材愈軟時，濕潤轉換的現象發生的愈早。藉由揮發速率在不同揮發模式下的理論模型不同，可以比較實驗及理論上的水滴的體積變化來檢驗是否符合實驗中觀察到的揮發機制。 Cassie impregnating wetting state的觀察研究是利用酒精液滴在不同具結構的PDMS上產生不同的液滴形狀及濕潤現象。由於這個主題研究的數量比較少，所以我們的主要目的是想找出表面結構及Cassie impregnating wetting state所滲出去的範圍大小之間的關係。實驗觀察發現當表面粗糙度愈大時，滲出去的範圍就越大。然而，到目前為止我們仍無法確切知道他們的關係，僅能對Cassie impregnating wetting state有初步的猜測認為其是處於通往完全濕潤前的半穩定狀態。此半穩定狀態是由酒精在結構中的滲透速率及空氣中的揮發速率兩者間互相拉扯平衡下的結果。此外，也比較Cassie Equation 所預測的接觸角及實驗量測在Cassie impregnating wetting state下的接觸角是否能夠吻合以及酒精在具結構表面的揮發情形也有部分討論以幫助進一步了解Cassie impregnating wetting state。

There are two topics discussed in this study: water evaporation on soft patterned surfaces and the observation of the Cassie impregnating wetting state. First, we demonstrate the evaporation mechanism of water sessile drop on different softness of fixed patterned PDMS (polydimethylsiloxane) substrate and also compare the results from the viewing angle of 0° with that from the viewing angle of 45°. The evaporation mechanism generally starts from the constant contact radius mode and turns into constant contact angle mode when the receding contact angle is reached. The softer the substrate is, the smaller the receding contact angle is. The wetting transition from the Cassie to the Wenzel state is also observed after the constant contact angle mode and the softer substrate will induce an earlier wetting transition due to the softer texture. By comparing the theoretical calculation of evaporation rate in different modes, we can examine whether the expected evaporation mechanism is suitable or not. Second, the Cassie impregnating wetting state is investigated by placing the ethanol drop on different patterned PDMS surfaces. Due to the lack of the knowledge of the Cassie impregnating state, our main purpose is to find out the relationship between the structure of the surfaces and the impregnating region. It is observed that the rougher the substrate is, the larger the impregnating region is. However, so far we cannot specify this phenomenon and its impregnating region. Our preliminary inference is that the Cassie impregnating wetting state is only the metastable state passing to complete wetting and the metastable state is the result of the equilibrium of the imbibition rate and the evaporation rate of ethanol. Besides, the contact angle in the Cassie impregnating wetting state is also examined to see if the Cassie equation can describe and the study of the ethanol drop evaporation on patterned surface is discussed to further understand the Cassie impregnating wetting state.