Time-dependent wettability of nano-patterned surfaces fabricated by femtosecond laser with high efficiency

High efficient fabrication of nanoscale surface structure has been achieved by line focusing a low repetition (1kHz) femtosecond laser using a cylindrical lens. The results showed that this cylindrical lens increased fabrication rate by 10 times compared with a spherical lens. Both periodic ripple and random nanoparticle covered surface structures were obtained by optimizing the defocus distance and scanning speed. Strong time-dependent water contact angle (CA) was found in both laser patterned Cu and Ni surfaces. The random nanoparticle covered surface was superhydrophilic (around 1°) immediately after laser processing, but increased to ∼25° in the first day after exposed in air. The CA of periodic ripple surface showed a more obvious time dependency, in which a significant CA increasing (50° for Cu and 100° for Ni) was observed. The surface chemical analysis showed that absorption of hydrophobic functional groups changed the surface from the Wenzel state wetting to the Cassie state wetting, which was responsiblefor the hydrophilic to hydrophobic transition.