Long-term wettability of titanium surfaces by combined femtosecond laser micro/nano structuring and chemical treatments

Two types of titanium surfaces covering with TiO₂ micro/nanostructures were fabricated by femtosecond laser structuring followed by NaOH hydrothermal or H₂O₂ oxidation treatments. After exposure to air, the former surface maintained hydrophilic for 10 days and had a minimum contact angle less than 20° even after 50 days, while the latter surface changed to superhydrophobic with a contact angle more than 150° after 50 days. Detailed analysis in terms of morphology, phase transformation and surface chemistry indicated that the changes of wettability behavior were associated with the adsorption of organic compounds from the surrounding atmosphere. The surface prepared by laser and H₂O₂ oxidation not only had a stronger adsorption ability than the surface prepared by laser and NaOH treatment, but also exhibited a relative higher hydrophobic hydrocarbon tail in the absorbed compounds, which resulted different wettability evolution trend for the two types of titanium surfaces.