具階層式結構超疏水表面的研究

Abstract

我們以軟性模板壓印法(soft embossing)製備規則柱狀表面，經由PDMS (polydimethylsiloxane)轉印半導體微影蝕刻技術製造出一系列各種尺寸及不同間距的方形柱狀突起矽晶母片，再經過溶膠凝膠溶液(sol-gel precursor)及PET (poly(ethyleneterephthalate))溶液以旋轉塗布方式塗布在基材上，直接以PDMS 印章壓印在塗布二氧化矽溶膠凝膠溶液及PET溶液的基材上，加熱以除去溶液中的溶劑，進而進行熱固化，再移去PDMS 印章後，在表面即留下具有微結構圖案的無機二氧化矽薄膜及有機PET薄膜，完成矽晶母片圖案的轉移。在疏水的自聚性單分子膜處理下，由接觸角的量測結果發現，水滴於這些具結構表面的理論Wenzel狀態及Cassie狀態數值相符。由於藉由製備階層式結構可以更有效地達到超疏水的特性。在此研究中，我們主要提供3種不同建構微/奈米階層式結構的方法，在表面修飾後進行濕潤行為的探討: (1) 利用軟壓印溶膠凝膠與二氧化矽奈米粒子混合液製備微/奈米階層式結構，由前進角與後退角的結果中證明由於二氧化矽奈米粒子的添加，有助於增加表面的粗糙度，水滴於規則柱狀結構由原本處於Wenzel狀態，於適當粗糙度的微/奈米階層式結構下濕潤轉換為Cassie狀態，此結果顯示微/奈米階層式結構有助於增加表面的疏水性。如此一來，我們可以藉由微/奈米階層式結構當作模板，利用二次軟壓印溶膠凝膠的方式，快速製備具階層式結構的超疏水表面。 (2) 旋轉塗布二氧化矽奈米粒子於規則柱狀二氧化矽薄膜上製備二氧化矽微/奈米階層式結構，由前進角/後退角與滑動角的量測結果，顯示不同的表面濕潤行為。我們發現旋轉塗布二氧化矽奈米粒子於原本處在Wenzel狀態的結構，將有助於粗糙度的增加，濕潤轉換為黏附型超疏水表面。再經由二次旋轉塗布二氧化矽奈米粒子後，建構的複合式粗糙結構可以製備易滑動型超疏水表面。此外，為了將這些表面更具有廣泛的應用性，我們改善並檢驗二氧化矽奈米粒子與基材的黏附測試，在結果中進行討論。 (3) 射頻式電容耦合電漿蝕刻規則柱狀PET薄膜製備微/奈米階層式結構，經由簡單的控制電漿蝕刻時間，達到不同的濕潤行為(Wenzel 狀態、玫瑰花瓣狀態與Cassie 狀態)。由前進角/後退角與滑動角的量測結果，蝕刻後具奈米特定紋路結構為主宰黏附型超疏水表面/易滑動型超疏水表面的重要因素。我們發現只需電漿蝕刻規則柱狀PET薄膜1分鐘，即可以得到易滑動型超疏水表面。因此，經由電漿蝕刻方式，可以有效地製備黏附型超疏水表面/易滑動型超疏水表面。 此外，我們製備一系列不同尺寸具有不同軟硬程度的PDMS具規則柱狀結構，觀察表面的形貌及疏水影響性，我們亦觀察水滴在表面上的動態行為。控制不同比例主劑與硬化劑溶液比例的PDMS製備規則柱狀結構，可以得到不同的濕潤行為。由前進角/後退角與滑動角的量測結果，我們發現當主劑與硬化劑溶液比例為5:1與10:1時符合理論預測的Wenzel 狀態與Cassie 狀態。然而，當主劑與硬化劑溶液比例為20:1時，濕潤行為在量測前進角/後退角時有較大的改變，規則柱狀結構易倒塌或黏附在一起而造成濕潤行為的改變。此外，水滴在其表面經由光學顯微鏡的動態現象觀察，當方柱小、材質軟且柱高較高時，水滴會往結構底部開始滲透，由Cassie狀態濕潤轉換為Wenzel狀態。接著，由於水滴重力誘發像骨牌般倒塌的魚脊狀特定紋路產生。

A series of pillar-like patterns silicon wafer with different pillar sizes and spacing are fabricated by photolithography and further modified by a self-assembled fluorosilaned monolayer. The regular pillar-like structure silica and Poly(ethylene terephthalate) (PET) surface is fabricated by embossing silica sol-gel precursor and PET precursor on glass substrates with an elastomeric mold. Both advancing and receding contact angles of water on these surfaces are carefully measured and found to be consistent with the theoretical predictions of the Cassie model and of the Wenzel model after surface modification. It is well understood that the superhydrophobicity could be achieved much more effectively by applying a hierarchical structure. The wetting behaviors of three different methods to construct hierarchical structures after surface modification will be discussed: (1) The hierarchical structure silica surface of inlaying silica nanoparticles along a regular pillar-like pattern is fabricated by embossing silica sol-gel precursor mixed with silica nanoparticles on glass substrates with an elastomeric mold. The advancing/ receding contact angle measurements are performed to demonstrate that a water droplet on these surfaces under an appropriate roughness can exhibit a transition from the Wenzel state to the Cassie state due to the addition of silica nanoparticles to enhance its surface roughness. Thus, the hierarchical structure is followed to develop the PDMS mold from the substrate as the template, and then, the substrate with hierarchical structure of bumpy surfaces along the pillar-like pattern is fabricated by simply embossing the PDMS mold on the silica sol-gel precursor (with no nanoparticles) directly for forming patterns over large areas in a facile fabrication. (2) Silica nanoparticles were spin-coated onto the flat/patterned (regular pillar-like) substrate to enhance the surface roughness. The advancing/receding contact angle and sliding angle measurements were performed to determine the wetting behavior of a water droplet on the surface. It is interesting to find out that a transition from the Wenzel surface to the sticky superhydrophobic surface is observed due to the spin-coating silica nanoparticles. The slippery superhydrophobic surface can be further obtained after secondary spin-coating silica nanoparticles to generate the multi-scale roughness structure. The prepared superhydrophobic substrates should be robust for practical applications. The adhesion between the substrate and nanoparticles is also examined and discussed. (3) Dual-scale roughness superhydrophobic surfaces are prepared by rf-capacitively coupled plasma etching the regular pillar-like patterned poly(ethyleneterephthalate) (PET) surfaces. Different wetting behaviors (Wenzel state, Petal state and Cassie state) can be achieved on the prepared dual-scale roughness surfaces by simply controlling the plasma etching time. It is found that a slippery superhydrophobic surface of patterned PET structures can be obtained by plasma etching simply for 1 minute. The advancing/receding contact angles and the sliding angle are systematically performed and applied to distinguish the difference between the slippery and sticky superhydrophobic surfaces. The nanoscale texture created by plasma etching on top of the patterned surfaces is an important factor governing the sticky/slippery surface. As a consequence, slippery/sticky superhydrophobic surfaces could be achieved much more effectively by plasma etching process. In addition, we prepare a series of pillar-like patterns PDMS with different pillar sizes and spacing, and we demonstrate that different wetting behavior of regular pillar-like patterned PDMS can be achieved by controlling the mixing ratio of polydimethylsiloxane’s (PDMS’s) prepolymer base and curing agent. The dynamic contact angles and sliding angles of water droplet on these patterned surfaces are carefully measured. It is found that the experimental results in PDMS substrates prepared at the mixing ratio of base to curing agent 5:1 and 10:1 are consistent with the theoretical predictions of the Wenzel model and Cassie model. However, for the PDMS substrates prepared at the mixing ratio of base to curing agent 20:1, the wetting behavior would change due to the deformation of the structure—collapse of pillars due to the softness. Besides, the dynamic phenomenon of the wetting transition process of water penetrating into the pillars is observed through the optical microscope when a water droplet is placed on the regular pillar-like patterned PDMS substrates. The pillars are pushed down by water one after one like dominoes at higher ratio of base to curing agent. The featured pattern of the impregnating process is discussed when the collapse is occured.