Superhydrophobic Nanocrystalline Nickel Films Inspired by Lotus Leaf
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1132-Z07-17
Superhydrophobic Nanocrystalline Nickel Films Inspired by Lotus Leaf Mehdi Shafiei1 and Ahmet T. Alpas1 1 Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, ON N9b3P4, Canada. ABSTRACT A new method to fabricate superhydrophobic hard films is described. Surface texture of lotus leaf was replicated on an acetate film, on which a nanocrystalline (NC) Ni coating with a grain size of 30 ± 4 nm and a hardness of 4.42 GPa was electrodeposited. The surface texture consisted of conical protuberances with a height of 10.0 ± 2.0 µm and a tip radius of 2.5 ± 0.5 µm. An additional electrodeposition for 120 s and 300 s was used to locally modify the surface structure by depositing ‘Ni crowns’ on the protuberances that increased their height to 14.0 ± 2.0 µm and their tip radius to 6.0 ± 0.5 µm. The modified structures were then treated with a perfluoropolyether (PFPE) solution, which provided a high water contact angle of 156°, i.e., comparable to the naturally superhydrophobic lotus leaf. The increased hydrophobicity as a result of surface structure and chemistry modifications was evident compared to a smooth NC Ni sample, which had a contact angle of 64°. INTRODUCTION The commercialization of micro-electro-mechanical systems (MEMS) with miniature moving parts (e.g. micromotors, gears and transmissions, mechanical discriminators, optical microswitches) relies on the development of new materials and surfaces with high hydrophobicity and low adhesion [1]. Surface roughness has a significant effect on wettability, as it affects the surface area underneath the liquid. Therefore, the actual contact angle (θA) of a rough surface with a liquid droplet can be determined using [2]:
cos θ A = r cos θ T
(1)
where r is the roughness ratio (the actual surface area divided by the apparent surface area) and θT is the thermodynamic contact angle defined by [3]: cos θ T =
γ sv − γ sl γ lv
(2)
where γsv is the solid-vapor surface energy, γsl is the solid-liquid interfacial energy and γlv is the liquid-vapor surface energy. As roughness increases, air can be locally trapped underneath the liquid, resulting in the formation of a composite surface with a large contact angle [4], a theory that is expressed by the following equation [5]: cos θ A = f S cos θ T − f air
(3)
where fS is the fractional contact area of the liquid with the surface, and fair is the fractional contact area with air underneath the droplet. On the other hand, structuring a surface with sufficient hardness may also result in a reduction in friction by reducing the real area of contact with the counterface [6-9]. Therefore, by choosing the right material and surface structure, it is possible to design and fabricate superhydrophobic surfaces with low friction. The surface texture of the lotus leaf consists of microscopic protuberances covered in a needle-like nanostructure with a waxy surface composition. This multilevel surface roughness is known as the source of the lotus leaf’s superhydrophobic property [10-12]. A simple repli
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