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Super-hydrophobic nanopatterned interfaces : optimization and manufacturing Moradi, Sona

Abstract

This work studies in detail the effect of femtosecond laser irradiation process parameters (fluence, scanning speed and scanning overlap) on the wettability of the resulted micro/nano-patterned morphologies on stainless steel. Depending on the laser parameters, four distinctly different nano-patterns were produced, namely nano-rippled, parabolic-pillared, elongated sinusoidal-pillared and triple roughness nanostructures. All of the produced structures were classified according to a newly defined parameter, the Laser Intensity Factor (LIF) that is a function of scanning speed and fluence of laser. By increasing LIF, the ablation rate and the periodicity of the asperities increase. In order to decrease the surface energy, all of the surfaces were coated with a fluorinated alkylsilane agent. Analysis of the wettability in terms of contact angle (CA) and contact angle hysteresis (CAH) revealed enhanced superhydrophobicity for most of these structures, particularly that possessing triple roughness pattern. This also exhibited a low CAH. The high permanent superhydrophobicity of this pattern is due to the special micro-nano structure of the surface that facilitates the Cassie-Baxter state. A new two-dimensional (2D) thermodynamic model is developed to predict the contact angle (CA) and contact angle hysteresis (CAH) of all types of surface geometries, particularly those with asperities having non-flattened tops. The model is evaluated by micro/nano sinusoidal and parabolic patterns fabricated by laser ablation. These microstructures are analyzed thermodynamically through the use of the Gibbs free energy to obtain the equilibrium CA and CAH. The effects of the geometrical details on maximizing the superhydrophobicity of the nano-patterned surface are also discussed in an attempt to design surfaces with desired and/or optimum wetting characteristics. The analysis of the various surfaces reveals the important geometrical parameters, which may lead to lotus effect (high CA>150° and low CAH150° and high CAH>>10°).

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Attribution-NonCommercial-NoDerivs 2.5 Canada

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