液滴於具結構親溶液表面之蒸發現象及濕潤行為

Vaporization and Wetting Behaviors of Liquid Droplet on Solventphilic Patterned Surface

陳定隆(Ding-Long Chen)

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(2014年)

https://doi.org/10.6342/NTU.2014.00911

摘要

本研究主要對溶液與其相親的結構表面進行濕潤行為的探討，此處指稱的相親代表液滴於表面上形成小於90度的接觸角。我們選用正辛醇作為液體、PDMS作為基材，正辛醇於PDMS的前進角為38度、後退角25度，從接觸角可看出PDMS跟正辛醇的相親性。實驗方式包含觀察不同幾何結構表面上液滴蒸發行為，及前進角與後退角的測量兩個部份。
我們以PDMS製備不同大小、間隔的規則排列方柱結構，進行蒸發行為觀察時，可以發現結構不同所觀察到的蒸發型式也相異，我們可以將蒸發性態歸納為三種：Wenzel state、Cassie impregnating wetting state及Mixed state，三者差異可由半毛細現象是否發生及液滴蒸發方式看出差異。Wenzel state 的樣品不發生半毛細現象，並維持固定後退角度值；Cassie impregnating wetting state的樣品最大特色是半毛細現象，過程中幾乎觀察不到後退角；Mixed state則有上述兩種狀態的部份特色，其不發生半毛細現象，但過程中後退角近乎觀察不到。此三種狀態的出現與結構有相關性，蒸發狀態的變化趨勢因柱狀結構變高、柱子間隔變窄，從Wenzel state轉為Mixed state，最後變為Cassie impregnating wetting state。
本文用兩種測量角度的方式：貼泡法與埋針法，測量樣品的前進角與後退角。我們發現貼泡法得到的角度趨勢則與埋針法不同，前者隨粗糙度增加前進角下降，粗糙度變高後，前進角轉變為定值；後者隨粗糙度增加前進角增加，粗糙度變高後，轉變為定值，進入Cassie impregnating wetting state。後者變化雖不符Wenzel模型預測，但是經由置換液體可以證明具有再現性，對於貼泡法與埋針法不同的角度變化化趨勢目前沒有適合的解釋方式，這也表示使用貼泡法測量前進角與後退角仍有許多我們許要釐清的問題。
本研究提供接觸角小於90度時，結構變化對表面濕潤行為及角度的影響，希望能對溶液與表面相親的濕潤狀態，提供不一樣的看法。

關鍵字

接觸角 ； 前進角 ； 親溶液 ； 蒸發 ； 結構表面

並列摘要

Our research was focus on the wetting behavior of liquid droplet on solventphilic patterned surface. Contact angle of liquid droplet on a surface below than 90° was referred to the solventphilic surface. We chose octanol as the liquid droplet, PDMS as the solid surface. The advancing contact angle of octanol was 38°, the receding contact angle of octanol was 25°. The contact angle showed the affinity between the PDMS and octanol. The experiment included two parts: vaporization process of an octanol droplet deposited on the patterned PDMS surfaces and contact angle measurement.
We used PDMS as the substrate to create series of different width and spacing pillar surface. When we observed the vaporization process of an octanol, it could be found that different pattern geometry showed different vaporization process. The vaporization process could be characterized into three kinds: Wenzel state, Cassie impregnating wetting state and Mixed state. Hemi-wicking and vaporization sequence of droplet differed from each other. The hemi-wicking wasn’t found in the process and receding with a constant angle when droplet was in Wenzel state. Hemi-wicking was the key feature of Cassie impregnating wetting state. But receding contact angle couldn’t be seen. Mixed state had both characteristics of Wenzel state and Cassie impregnating wetting state. No hemi-wicking and receding contact angle couldn’t be observed when droplet in Mixed state. From the observation of vaporization process, we know pattern geometry could affect the wetting states. Narrower pillar spacing and taller pillar would change the vaporization process from Wenzel state to Mixed state then the Cassie impregnating wetting state.
We used two kinds of method to measure the advancing and receding contact angle. One was captive bubble method, the other was embedded-needle method. We got different tendency in contact angle with these two methods. When captive bubble was conducted, the advancing contact angle decreased as the surface roughness was increased. Eventually, the advancing contact angles become a constant when the surface roughness is further increased, i.e., the region of Cassie impregnating wetting state. However, when embedded-needle method was used, the advancing contact angle increased as the surface roughness was increased. Eventually, the advancing contact angles became constant when the surface roughness was further increased. The advancing contact angle measured by embedded-needle method didn’t meet the Wenzel model, but the result can be reproduced even we changed other kind of liquid. There was still no clear explanation to why captive bubble method could get the tendency of advancing contact angle decreased as the roughness increased. We still have long way to clarify the captive bubble method.
This research provides another opinion when we deal with the wetting behavior and vaporization process in solventphilic surface.

並列關鍵字

contact angle ； advancing contact angle ； solventphilic ； vaporization ； patterned surface

參考文獻

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