Faculty Life and Social Sciences, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria, 3122, Australia.
Langmuir. 2012 Dec 18;28(50):17404-9. doi: 10.1021/la303560w. Epub 2012 Dec 4.
Self-cleaning surfaces found in nature show great potential for application in many fields, ranging from industry to medicine. The ability for a surface to self-clean is intimately related to the wetting properties of the surface; for a surface to possess self-cleaning ability it must exhibit extremely high water contact angles and low water adhesion. While investigating the self-cleaning properties of damselfly wings, significant spatial variations in surface wettability were observed. Within an area of 100 μm × 100 μm of the wing surface the water contact angle was found to vary up to 17.8°, while remaining consistently superhydrophobic. The contributions of both surface chemistry and topography to the hydrophobicity of the wings were assessed in an effort to explain these variations. Synchrotron-sourced Fourier-transform infrared microspectroscopy revealed that some of the major components of the wing were aliphatic hydrocarbons and esters, which are attributable to epicuticular lipids. The wing topography, as determined by optical profilometry and atomic force microscopy (AFM), also showed only minor levels of heterogeneity arising from irregular ordering of surface nanostructures. The measured contact angle of a single droplet of water was also found to decrease over time as it evaporated, reaching a minimum of 107°. This is well below the threshold value for superhydrophobicity (i.e., 150°), demonstrating that when the surface is in contact with water for a prolonged period, the damselfly wings lose their superhydrophobicity and subsequently their ability to self-clean. This decrease in hydrophobicity over time can be attributed to the surface undergoing a transition from the Cassie-Baxter wettability state toward the Wenzel wettability state.
自然界中存在的自清洁表面在许多领域都有很大的应用潜力，从工业到医学。表面自清洁的能力与表面的润湿性密切相关；为了使表面具有自清洁能力，它必须表现出极高的水接触角和低的水附着力。在研究蜻蜓翅膀的自清洁特性时，观察到表面润湿性存在显著的空间变化。在翅膀表面 100μm×100μm 的区域内，水接触角的变化高达 17.8°，同时仍保持超疏水性。为了解释这些变化，评估了表面化学和形貌对翅膀疏水性的贡献。同步辐射傅里叶变换红外微光谱显示，翅膀的一些主要成分是脂肪族碳氢化合物和酯类，这归因于表皮脂质。通过光学轮廓仪和原子力显微镜（AFM）确定的翅膀形貌也显示出仅存在由表面纳米结构不规则排列引起的微小异质性。还发现，随着时间的推移，单个水滴的接触角也会减小，因为它在蒸发，接触角最小达到 107°。这远低于超疏水性的阈值（即 150°），表明当表面长时间与水接触时，蜻蜓翅膀会失去超疏水性，从而失去自清洁能力。随着时间的推移，疏水性的降低可以归因于表面从 Cassie-Baxter 润湿性状态向 Wenzel 润湿性状态的转变。
