Surface nanostructuring principles and design of extreme wetting treatment for laser textured metal alloys

Avik Samanta

doi:10.17077/etd.005971

Wetting behaviors of structured metal surfaces have received considerable attention due to the wide range of applications for commercial, industrial, and military uses as well as fundamental research interests. Due to its adaptability, precision, and ease of automation, laser-based texturing techniques are desirable platforms to create micro- and nano-structures. However, micro- and nanostructures alone often do not achieve the desired wettability. A subsequent surface chemistry modification method must be performed to attain target extreme wettability for laser textured metal substrates. Specifically, wetting can be modulated from superhydrophobicity to superhydrophilicity by controlling the surface chemistry of a laser textured surface, allowing freedom to achieve complex multi-wettability situations. The present work discovers the fundamental mechanism of novel nanosecond laser-based high-throughput surface nanostructuring (nHSN) process that can simultaneously create random nanostructures and attain desirable surface chemistry over large-area metal alloy surfaces. The work proves that the surface nanostructuring results from a combined effect of chemical etching and attachment of functional groups. nHSN nanostructures with fluorosilane chemistry repel water, while those with nitrile chemistry attract water. Extreme wettabilities, including superhydrophobicity and superhydrophilicity, are assessed for multiple engineering metal alloys. After this, a systematic design approach is developed to modify the dispersive and non-dispersive components of surface chemistry of laser textured metal alloys to achieve various extreme wettabilities. Nanosecond pulsed laser surface texturing is employed to create microscale trenches covered with random nanostructures on the metal alloy. Subsequently, the textured surface is immersion-treated in several chemical solutions to attach target functional groups on the surface to achieve the final extreme wettability. Anchoring fluorinated groups (-CF2- and -CF3) with very low dispersive and non-dispersive surface energy leads to superoleophobicity and superhydrophobicity, resulting in repelling both water and diiodomethane. Attachment of polar nitrile (-C≡N) group with very high non-dispersive and high dispersive surface energy achieves superhydrophilicity and superoleophilicity by drawing water and diiodomethane molecules in the laser textured capillaries. At last, anchoring fluorinated groups (-CF2- and -CF3) and polar sodium carboxylate (-COONa) together leads to very low dispersive and very high non-dispersive surface energy components. It results in the co-existence of superoleophobicity and superhydrophilicity, where the treated surface attracts water but repels diiodomethane. Finally, the thesis introduces capillary driven superwicking behaviors for metal alloys. Dual-scale structures are created on metal alloy using the nHSN process, which consists of microscale trenches and pillars covered with random nanoscale porous structures. Additionally, polar nitrile surface chemistry is imposed on top of those surface structures. The capillary effect of the dual scale surface features and the water affinitive surface nitrile group demonstrate superhydrophilicity and self-propelling superwicking transport behavior uphill against the gravity for alcohol and water. The wicking transport follows the Washburn dynamics for horizontal, angular, and vertical orientation.

Surface nanostructuring principles and design of extreme wetting treatment for laser textured metal alloys

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