Laser-induced nanoscale superhydrophobic structures on metal surfaces.

The combination of a dual-scale (nano and micro) roughness with an inherent low-surface energy coating material is an essential factor for the development of superhydrophobic surfaces. Ultrashort pulse laser (USPL) machining/structuring is a promising technique for obtaining the dual-scale roughness. Sheets of stainless steel (AISI 304 L SS) and Ti-6Al-4V alloys were laser-machined with ultraviolet laser pulses of 6.7 ps, with different numbers of pulses per irradiated area. The surface energy of the laser-machined samples was reduced via application of a layer of perfluorinated octyltrichlorosilane (FOTS). The influence of the number of pulses per irradiated area on the geometry of the nanostructure and the wetting properties of the laser-machined structures has been studied. The results show that with an increasing number of pulses per irradiated area, the nanoscale structures tend to become predominantly microscale. The top surface of the microscale structures is seen covered with nanoscale protrusions that are most pronounced in Ti-6Al-4V. The laser-machined Ti-6Al-4V surface attained superhydrophobicity, and the improvement in the contact angle was >27% when compared to that of a nontextured surface.