摘要或動機

本研究的目的在觀察液滴的蒸發過程，探討影響一滴蒸發方式改變的變因。我們利用數位攝影機外加近攝鏡頭，由側面逆光拍攝液滴蒸發過程，並轉錄成電子檔利用電腦協助分析。液滴影像分隔放大投影在方格紙上，以便測量接觸角及高度的變化。結果發現蒸發情形可分為三種：Ⅰ.當接觸角大於九十度時，液滴和界面的接觸線迅速外擴，直到接觸角遞減至90度後定住不動，蒸發終了時，接觸線迅速內縮，留下縮小的實心殘跡。Ⅱ. 當接觸角小於九十度時，接觸線先些微外擴迅速外擴，之後定住不動，直到蒸發終點，液體內縮，留下環狀殘跡。Ⅲ.當接觸角小等於九十度時，接觸線始終定住不變，液滴蒸發後留下原大小的實心班跡。液滴蒸發時接觸角和高度並不是連續變化，而是交互階梯式遞減。本研究的結果可應用在探討噴漆及噴墨印表機的噴墨印刷上。The objective of this investigation is to observe the evaporation of liquid drop and find out the factors that could affect it. A digital camera with micro lens is used to film the process of the evaporation. And a close-up of the sample drop is taken with backlight. The results are analyzed with a personal computer. The magnified images of the drops are projected on grid papers for measuring the changes on contact angles, and heights of drops. We classify the evaporation of a drop into three types: Ⅰ. The contact angle is greater than 90 degrees. At first, the contact line will spread outwardly till the contact angle decrease to the 90 degrees and then be pinned until the evaporation reach its terminal. At last the contact line quickly contract inwardly, and a solid stain remain. Ⅱ. The contact angle is less than 90 degrees. The contact line will spread slightly and then be pinned. At the terminal the liquid contract inward, and a ring-like stain is formed. Ⅲ. The contact angle is equal to 90 degrees. The contact line will always be pinned, and no ring, but spot stain is left. During the evaporation, we discover that the contact angle and the height of the drop do not change continuously but alternatively decrease by steps. The results of our investigation can be applied to the aspects of paint-spraying and jet printers.