奈米纖維薄膜覆蓋之微米柱陣列結構的除冰能力

Abstract

冰的形成在生活中隨處可見，但有時卻會影響道路交通、電力設施及航空安全，造成經濟損失或人身安全上的危害。以往研究領域會著重於用超疏水表面來阻止或延緩冰的形成，以及減少冰形成後與表面之間的附著力、增加除冰效率。但近年來也有研究指出，當氣態的水進入超疏水表面其粗糙的結構中形成冰時，會連同表面的冰嵌入結構中並大幅增加冰的附著力。因此目前學術界趨於使用水溶性的高分子聚合物材料填於結構中或表面上，比如含有親水性聚氨酯（PU）或聚丙烯酸（PAA）的塗層，其親水性聚合物中的親水基團會與液態水分子鍵結、降低水分子的冰點，形成不會凍結的含水潤滑層阻隔冰與固體表面的接觸，可以在維持除冰能力的情況下降低冰附著力。本研究使用具有水溶性且常用作潤滑劑的高分子聚合物—聚環氧乙烷（polyethylene oxide, PEO），其經過靜電紡絲加工製成的奈米纖維薄膜具有輕薄、易製備、成本低、親水性的特點，覆於矽微米柱陣列表面上作為除冰能力實驗中的樣本表面。 本研究首先製備四種樣本：平坦矽表面、矽微米柱陣列表面以及各自覆有PEO奈米纖維薄膜的版本，再量測四種樣本表面在不同表面溫度下的冰附著力。實驗結果顯示，冰附著力在表面溫度各為-15 ℃、-20 ℃、-25 ℃時差異甚小；然而在四種樣本中，平坦矽表面的冰附著力最低（316 kPa），覆有PEO奈米纖維薄膜的平坦矽表面為次低（409 kPa），矽微米柱陣列表面為次高（886 kPa），覆有PEO奈米纖維薄膜的矽微米柱陣列表面則有最高的冰附著力（1601 kPa）。此結果顯示，微米柱陣列表面因具備超親水特性，易導致液態水填充於結構間隙形成Wenzel狀態並凍結，產生機械互鎖（Mechanical Interlocking）會使其冰附著力顯著高於平坦表面。針對PEO奈米纖維薄膜未能降低冰附著力反而導致其上升之現象，本研究揭示了其物理性質有許多關鍵限制：首先，PEO薄膜厚度與微米柱陣列的深度存在兩個數量級的巨大落差，吸水後形成的含水層沒有足夠的厚度填平表面粗糙度；其次，液態水容易穿透輕薄又多孔的PEO薄膜並滲入下方結構，導致機械互鎖無法被減緩；再者，PEO奈米纖維的質量相對於水量來說過低，大部分液態水在薄膜飽和後會在低溫下完全凍結，導致預期的含水潤滑(Hydration Lubrication)機制失效；最後，PEO薄膜的高親水性使表面濕潤性增強，更促進了液態水在水平方向的擴散，導致有效接觸面積擴大。綜上所述，本研究闡明了此類親水性奈米纖維在微結構表面應用上的限制，指出在設計此類型的複合表面進行抗冰時，必須有效控制液態水滲透以避免Wenzel狀態的冰形成，並使含水潤滑層維持液態。

Ice formation is a common phenomenon in daily life, but it can sometimes have a negative impact on road traffic, power facilities, and aviation safety, leading to economic losses and potential hazards to personal safety. Previous studies have mainly focused on superhydrophobic surfaces to prevent or delay ice formation, as well as to reduce the adhesion strength between ice and the surface after freezing, thereby improving de-icing efficiency. However, recent research has pointed out that when water vapor condenses and freezes within the rough structures of a superhydrophobic surface, the ice can interlock with the surface, significantly increasing adhesion strength. As a result, current research tends to fill structure or coat surfaces with water-soluble polymers, such as coatings containing hydrophilic polyurethane (PU) or polyacrylic acid (PAA). The hydrophilic groups within these polymers bond with liquid water molecules and lower the freezing point of water, forming a non-freezing lubricating hydration layer that acts as a barrier between the ice and the solid surface. This mechanism can effectively reduce ice adhesion while maintaining de-icing performance. In this study, a water-soluble polymer commonly used as a lubricant—polyethylene oxide (PEO)—was selected. PEO nanofiber membranes were fabricated via electrospinning, providing lightweight, low-cost, easily prepared, and hydrophilic coatings, which were then deposited onto silicon micropillar array surfaces as samples for ice adhesion tests. Four types of samples were prepared: plain silicon, silicon micropillar arrays, and their variants coated with PEO nanofiber membranes. The ice adhesion strength of these surfaces was measured at different surface temperatures. The results showed that the adhesion strength varied little among temperatures of −15 °C, −20 °C, and −25 °C. Among the four samples, plain silicon exhibited the lowest ice adhesion (316 kPa), followed by plain silicon coated with PEO nanofiber membrane (409 kPa), micropillar arrays without coating (886 kPa), and micropillar arrays coated with PEO nanofiber membrane, which exhibited the highest adhesion strength (1601 kPa). These results indicate that the micropillar arrays surface, due to its superhydrophilicity, easily leads to liquid water filling the structural voids, forming a Wenzel state and freezing. This results in mechanical interlocking, causing the ice adhesion strength to be significantly higher than that of plain surfaces. To explain the phenomenon where the PEO nanofiber membrane failed to reduce ice adhesion and instead increased it, this study reveals several critical physical limitations: First, there is a gap of two orders of magnitude between the thickness of the PEO membrane and the depth of the micropillar arrays; thus, the hydration layer formed after water absorption lacks sufficient thickness to smooth out the surface roughness. Second, liquid water easily penetrates the thin and porous PEO membrane, intruding into the underlying structure, which means mechanical interlocking cannot be prevented. Third, the mass of the PEO nanofibers is insufficient compared to the amount of water; consequently, most liquid water freezes completely at low temperatures after the membrane reaches saturation, leading to the failure of the expected hydration lubrication mechanism. Finally, the high hydrophilicity of the PEO membrane enhances surface wettability, promoting the lateral spreading of liquid water and resulting in an expanded effective contact area. In summary, this study clarifies the limitations of applying such hydrophilic nanofibers on microstructured surfaces, pointing out that future designs of this type of composite de-icing surface must effectively control liquid water penetration to avoid Wenzel-state ice formation and ensure the hydration lubrication layer remains in a liquid state.