Mechanism of superhydrophilic to superhydrophobic transition of femtosecond laser-induced periodic surface structures on titanium

The wettability is an imperative property of a solid surface. For decades, superhydrophobic and superhydrophilic surfaces have been focus of extensive research owing to their importance for both fundamental research and in many biomedical, aeronautical, and industrial applications. There have been many studies reported on wettability transition of micro- and nanostructured metal surfaces over time, but there seems to be no well-accepted mechanism to address this phenomenon. Therefore, a detailed study of the surface chemical composition of the laser ablated surfaces at different time points is necessary to develop an understanding of the factors resulting in change in wettability over time. The aim of the present work was to investigate the mechanism underlying the wettability transition of femtosecond laser generated periodic surface structures on titanium between the superhydrophilic and superhydrophobic wetting states under ambient conditions. The time-dependent wettability of the laser treated surfaces assessed by the sessile drop method. The samples exhibited superhydrophilic behavior immediately after laser texturing and became superhydrophobic over time. Detailed surface chemical analyses by X-ray photoelectron spectroscopy revealed that the unique electronic structures of Ti2O3 and TiO2 and resulting in hydrophilic and hydrophobic hydration structures, respectively, played a crucial role in the observed wettability transition.