Tunable water adhesion on titanium oxide surfaces with different surface structures.

Tunable water adhesion with high static contact angle (SCA) on titanium oxide surfaces was achieved by a two-step process: first, titanium oxide surfaces with different structures were obtained by immersion the titanium alloy substrates into H₂O-H₂O₂-HF solution at 140 °C for different time of 30, 60, and 120 min; then, low-surface-energy molecules of 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (PFOTS) were deposited thereon. SCA for all so-fabricated samples were higher than 150° and sliding angle (SA) for different immersion time of 30 min, 60 min, and 120 min is 180°, 31±2°, and 8±1°, respectively. To analyze the correlation between the surface structures and the dynamic wetting behaviors, we adopted, three contact modes (i.e., Wenzel, Cassie impregnating, and Cassie modes). The analyses showed that the surface adhesion was influenced greatly by water/solid interfacial interaction and could be artificially tuned between Wenzel state with high adhesion to Cassie state with low adhesion through the design of appropriate microstructures.