TiO₂ patterns with wide photo-induced wettability change by a combination of reactive sputtering process and surface modification in a microfluidic channel

This paper reports the formation of TiO₂ patterns with a wide range of photo-induced wettability switching from high hydrophobic to superhydrophilic states for on-chip liquid manipulation. TiO₂ thin films with rough surface morphology were formed by a combination of optimised reactive sputtering and CF₄ plasma etching. Octadecylphosphonic acid self-assembled monolayer (ODP-SAM) surface modification was applied to the surface-roughened TiO₂ thin films in order to obtain a highly hydrophobic surface initially. Photocatalytic decomposition of ODP-SAM on the surface-roughened TiO₂ by ultraviolet (UV) irradiation caused a wetting transition from the Cassie–Baxter state to the Wenzel state. Switching of the flow direction into branch channels was also demonstrated by utilising the photoresponsive wettability of the surface-modified TiO₂ patterns on a fluidic chip. (paper)