Abstract: Although preparation methods of super-hydrophobic surfaces are extensively studied and reported, investigation into micro-nano structures' contribution to Cassie state stability is comparatively limited. The microstructures with dense nano-nickel-pyramids are fabricated on the copper surface by laser ablation and electrodeposition. The laser scanning intervals can be adjusted to produce microstructures with different scales, and the produced surfaces show different wettability at normal temperature (25℃) and low temperature (5℃). More specifically, the modified surface with micro-nano hierarchical structures is associated with better hydrophobic stability than that with only micro-structures. When the laser scanning interval is equivalent to the laser spot diameter, the contact angles of super-hydrophobic surface peak both at normal temperature and low temperature, and the corresponding maximum angles are 161° and 151°,respectively. The sliding angles decrease both at normal temperature and low temperature to the bottom, the corresponding minimum angles are 1° and 5°,respectively. Meanwhile,the most stable Cassie state is achieved, as represented in the droplets stamping experiments, where the droplets rebound completely for 3 or 4 times, indicating the droplet does not transit from the Cassie state to the Wenzel state during the stamping process. Condensation experiments also demonstrate that the stable Cassie state is closely related to the micro-nano hierarchical structures. Specifically, much smaller water droplets generate on surface with dense nanostructures,and therefore jump away easily from the surface. In addition, when the scanning interval is equivalent to the spot diameter, the microstructures distribute evenly and compactly, resulting in the condensed droplets smaller than the critical size for jumping, and hence a more stable Cassie state. The study carried out here not only focuses on efficient fabrication of superhydrophobic surfaces with dense nanostructures, but also amply discusses the Cassie state stability of the produced surface, making it promising for practical applications.

Key words: picosecond laser ablation, electrodeposition, micro-nano structures, superhydrophobic, Cassie state stability