From Superhydrophilicity to Superhydrophobicity: Laser-Induced Wettability Control of 7075 Aluminum via Chemical-Free Surface Texting and Air Ageing

This study presents a reagent-free route to engineer robust superhydrophobicity on 7075 aluminum alloy substrates through fiber laser surface texturing. Uniform parallel microgroove arrays were inscribed using a fiber laser marking system at optimized parameters (8 W power, 10-50 mm/s scan speed, 20 kHz repetition frequency, 0.05 mm focal spot, two-pass scan). The as-processed surface initially displayed complete wettability (water contact angle, CA ≈ 0°), consistent with freshly ablated metal behavior. Upon 30 days of ambient air ageing, the CA climbed to 154.5 ± 1.5° with a sliding angle of 4.5 ± 1.1°, establishing a pronounced superhydrophobic character. X-ray photoelectron spectroscopy (XPS) revealed that air exposure drove the formation of a low-surface-energy silica overlayer on the laser-structured surface, progressively shifting the wetting state from the Wenzel to the Cassie–Baxter regime. Scanning electron microscopy (SEM) confirmed a dual-scale rough texture comprising macro-scale groove channels (width: 35–40 μm, depth: ~20 μm) densely decorated with re-solidified granules (0.8–5.2 μm diameter), capable of entrapping substantial air volumes beneath water droplets. Mechanical robustness testing under cyclic compression demonstrated a contact angle retention of ~98.9%, validating the structural integrity of the superhydrophobic state. The process requires no supplementary chemicals, is technically straightforward, and offers a scalable path for imparting water-repellent, low-adhesion, and self-cleaning functionality to aluminum alloy surfaces.