Wettability of and Water Adhesion on Gradient TiO2 Nanotubes Fabricated via Bipolar Anodization

Abstract

The investigation of wettability and water adhesion of the gradient TiO₂ nanotubes surface can help to understand their manipulation process on droplets. Here, gradient TiO₂ nanotubes were prepared via bipolar anodization. The TiO₂ nanotubes prepared by 98 V 2 min⁻¹, 60 V 10 min⁻¹, and 30 V 30 min⁻¹ showed different gradient diameters. After modification by 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS), the surfaces near the anode edge reached a superhydrophobic state. In the central area, the surfaces gradually became less hydrophobic. Water adhesive forces of the superhydrophobic regions were lower than 20 µN, while they increased as the contact angle decreased. Moreover, the region with high water adhesion could transform into a sliding superhydrophobic state with low adhesion through a heating process. This paper provides some new insights into the wettability and water adhesion of gradient TiO₂ nanotube surfaces.