Article pubs.acs.org/Langmuir
Spatial Variations and Temporal Metastability of the Self-Cleaning and Superhydrophobic Properties of Damselfly Wings Jafar Hasan,† Hayden K. Webb,† Vi Khanh Truong,† Gregory S. Watson,§ Jolanta A. Watson,§ Mark J. Tobin,∥ Gediminas Gervinskas,‡ Saulius Juodkazis,‡ James Y. Wang,‡ Russell J. Crawford,† and Elena P. Ivanova†,* †
Faculty Life and Social Sciences and ‡Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria, 3122, Australia § Centre for Biodiscovery and Molecular Development of Therapeutics, School of Marine and Tropical Biology, James Cook University, Townsville, QLD, 4811, Australia ∥ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia S Supporting Information *
ABSTRACT: 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.
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numerous plants.8,12−22 Materials inspired by natural superhydrophobic and self-cleaning surfaces show great potential in industrial applications.7,10,23,24 It is well documented that superhydrophobicity (and hence self-cleaning) is determined by a combination of particular surface chemistry and surface architecture.1,3,5,9,11,25−28 Generally, the most hydrophobic surfaces possess a hierarchical topography that consists of multiple levels (i.e., microscale and nanoscale) of roughness and are composed of a material that is intrinsically hydrophobic.7,29,30 In the case of insects and plants, the material that forms the outermost layer is usually a mixture of lipids.31,32 The surface architecture of hierarchically rough surfaces effectively reduces available contact area and
INTRODUCTION Many natural surfaces such as plant leaves and insect wings exhibit excellent water-repelling properties.1−4 One of the best known superhydrophobic surfaces is that of the lotus leaf. Superhydrophobicity is defined as the property of maintaining high water contact angles (>150°) and small roll-off (tilt) angles (
