Wettability transition of structured steel surface by femtosecond laser after annealing

Steel surfaces with self-organized textures, including LIPSS, lamellar, and strip structures, were fabricated by femtosecond laser ablation. A heat accumulation model was established based on three-dimension heat conduction equation to simulate the heat accumulation temperatures at different repetition frequencies, explaining the formation mechanism of the molten substances on the surface of the self-organized structures. All these surfaces exhibited high hydrophilicity right after laser ablation, but transformed to hydrophobic or superhydrophobic after a low-temperature annealing at 100°C for six hours. The surface chemical characteristics were analyzed by X-ray photoelectron spectroscopy. The results show that the annealing promoted the detachment of polar and hydrophilic substances, such as C–O–C and –OH, and the adsorption of nonpolar and hydrophobic substances, such as C–C, –CH₃, and –CH₂, resulting in the enhanced hydrophobicity of the laser-ablated steel surface. The enhanced hydrophobicity facilitated the formation of gas films in the self-organized structures, which further improved the surface hydrophobicity.