水滴於單ㄧ結構疏水表面之濕潤現象

Abstract

一般認為玫瑰花瓣效應是由於雙層結構(奈米加微米結構)所造成，但是以水滴滲入結構的情況，微米結構會比奈米結構還來得好，這造成我們看到玫瑰花瓣同時具有疏水性質(接觸角大於150∘)及對水有很高的附著力，基於這個現象我們推測只要改變單ㄧ結構尺寸就能看到玫瑰花瓣效應，於是我們利用曝光顯影製程作出了ㄧ系列不同高度的PDMS柱狀結構，並分別以埋針法測量前進角、揮發法觀測水滴在結構上的揮發情形、用倒立埋針法觀察水滴底部在接觸線向外擴時滲入結構的情況及作滑動角測試觀察水滴對於結構的附著程度，我們發現隨著PDMS柱狀結構粗糙度增加，水在表面上的濕潤情形會呈Wenzel→Petal (具黏滯性的超疏水區域) →Cassie (具滑動的超疏水區域)變化；至於水在結構上的附著程度則是呈現高附著力→低附著力。

It is generally believed that the surface of petal effect possess both nano-and micro-structure. It has been proposed that water droplet penetrate into micro-structure much more than nano- structure. That makes the rose petals have the characteristics of superhydrophobicity (contact angle larger than 150o) and strong adhesion to pin water drops. Based on this phenomenon, we conjecture that changing the roughness of micro-structure can make surface exhibit petal effect, so we make a series of micropillar-like patterned PDMS surfaces with different pillar sizes and spacing are fabricated via soft lithography. In this study, embedded needle method and evaporation method are used to measure advancing and receding angle respectively. An inverted needle method is applied to observe and identify whether water penetrates into the bottom of the substrate when the water droplet contact line is expanding and we also measure sliding angle to observe the ability of water droplet to adhere to micro-structure. We find out that a sequence of wetting transitions: Wenzel→ petal (sticky super-hydrophobic region) → Cassie (slippery super-hydrophobic state) would be consistently observed along with an increase in surface roughness for these micro-structure PDMS substrates. In addition, the ability of water droplet to adhere to micro-structure changes from high adhesion to low adhesion with increasing surface roughness.