在親溶液柱狀微結構表面上液滴的濕潤與蒸發行為

Abstract

觀察正丁醇在親液性的柱狀微結構聚二甲基矽氧烷材質表面上的濕潤行為，其液滴狀態、形狀在不同的結構接觸表面積比與粗糙度的表面上的變化。運用由結構上方、下方的影像與接觸角的測量取得液滴的形狀並分辨處於Wenzel狀態、Cassie浸潤狀態或是介於兩者之間的過渡情形。隨著表面結構粗糙度增加，可以看到由Wenzel狀態經過過度情形往Cassie浸潤狀態的變化。控制結構接觸表面積比增加粗糙度使濕潤狀態由Wenzel狀態往Cassie浸潤狀態過度時，液滴的形狀會由粗糙度決定，以圓形、八邊形至方形的順序變化；而進入Cassie浸潤狀態以後，液膜擴散的情況則會因結構的固體覆蓋率(solid fraction)由低到高增加，呈現方形、八角形至圓形的變動。液滴是否會擴散出液膜，進入Cassie浸潤狀態的條件是由柱子高度、間距與接觸角的幾何關係來決定的；但液滴蒸發時一部分的結構雖然不會有向外擴散的液膜，在液滴縮小時仍會有液膜停留在液滴原本所在位置。這種介於Wenzel和Cassie浸潤狀態之間的狀態稱為混和狀態。液滴蒸發的時候液膜的存在與粗糙度的上升都會使蒸發的速度增快。另外，結構的前進角會影響液滴放置在表面上時的起始角度，進而影響蒸發過程的恆定接觸半徑階段時間。最終整個蒸發過程的耗時是由多種因素共同造成的結果。

We observed the wetting behavior of 1-butanol drop sitting on a series of regular pillar-like patterned lyophilic PDMS surfaces with different surface roughness and solid fraction. The contact angles measurement verified whether a liquid drop sitting on the patterned lyophilic surface exhibits Wenzel state, Cassie impregnating wetting state or transition state between them. The wetting transition from the Wenzel state to the Cassie impregnating wetting state occurs on a surface with a certain surface roughness. In addition, those patterned surfaces with liquid drops exhibiting the Cassie impregnating wetting state demonstrate the final shape of the wetted area during imbibition of the texture, e.g., octagons, squares and circles. The final shape of the imbibed area depends on the surface solid fraction. Under the condition of a fixed solid fraction when the roughness increasing, a liquid drop would exhibit a transition from Wenzel state to the Cassie impregnation state. The final shape of the imbibed area would exhibit a series of transitions: square → octagon → circle in sequence. We showed the relation of advancing/receding contact angles and roughness of groups of solid fraction. The advancing boundary of Wenzel state and Cassie impregnating wetting state was found as a geometry limit consist of pillar height/distance ratio and liquid equilibrium contact angle, and receding one was critical contact angle. The combination of multiple effects results evaporating time of drop on the substrate. Liquid film enhances the evaporating rate and consequently we lined Wenzel state, mixed state I, mixed state II and Cassie impregnating wetting state up in sequence of decreasing evaporating time. However, advancing contact angle also affects duration of constant contact radius stage.