Superhydrophobic and Antibacterial Hierarchical Surface Fabricated by Femtosecond Laser

Abstract

Superhydrophobic surfaces are important in many applications owing to their special properties such as self-cleaning, anti-icing, antibacterial, and anti-fogging. In this paper, a micro/nano hierarchical superhydrophobic surface with a low roll-off angle was created on 304 stainless steel. The water contact angle was measured to be 152° with a roll-off angle of 7.3°. Firstly, microscale bumps were created by femtosecond laser irradiation. Secondly, zinc oxide (ZnO) nanowires were fabricated on the laser-induced bumps using a hydrothermal synthesis method. Results show that after laser treatment and ZnO nanostructuring, the stainless steel surface became superhydrophobic. However, the roll-off angle of this hierarchical structure surface was larger than 90°. To reduce the surface activity, trimethoxy silane hydrophobic coating was applied. A 7.3° roll-off angle was achieved on the coated surface. The underlying mechanism was discussed. The hydrophobic ZnO structured surface can help prevent bacterial contamination from water, which is important for implants. Thus, for biomedical applications, the antibacterial property of this hierarchical surface was examined. It was found that the antibacterial property of sample surfaces with ZnO nanowires were significantly increased. The optical density (OD) of Escherichia coli (E. coli) attached to the original surface was 0.93. For the micro-structured surface (with bumps), the OD was 0.9, and for the hierarchical surface (with bump & nanowires), it was 0.54. For nanostructured ZnO nanowire surface, the OD was only 0.09. It demonstrates good antibacterial properties of ZnO nanowires.