Role of chemistry changes in the hydrophobic transition following femtosecond laser texturing of stainless steel

Hydrophobicity is a targeted surface property in many industries due to the variety of functionalities (anti-corrosion, antibacterial, anti-icing, etc.) it enables. Femtosecond laser, as part of the ultra-short pulsed laser category, can be used to design such surfaces. After the texturing process, metallic surfaces exhibit a peculiar phenomenon of wetting transition from hydrophilicity to hydrophobicity. Even though lots of authors have attributed this behaviour to an accumulation of organic molecules on the surface, the exact nature of the relation between molecules adsorbed during ageing and static contact angle still needs to be clarified. In this paper, non-textured stainless steel surfaces, used as reference, and laser textured ones presenting Laser Induced Periodic Surface Structures (LIPSS) were studied through ageing. XPS analyses performed on day 0, have made possible to demonstrate that the femtosecond laser process can lead to a chemical reorganization of the native oxide layer of the sample resulting in the formation of manganese and nickel oxides at the surface in addition to the iron and chromium oxides present before texturing. Then, by simultaneously monitoring the evolution of the wetting properties and surface chemistry of the samples, a threshold concentration of apolar carbon controlling the hydrophobic behavior of the surface was identified.