Photoresponsive wettability switching of TiO 2 ‐coated micropillar arrays with different geometries of overhang roofs

Abstract

In this work, micropillar array structures with overhang roofs which increase the switching range of the TiO2 photoresponsive wettability, have been fabricated. The liquid droplet placed on the micropillar array exhibits the Cassie–Baxter superhydrophobic state because of the receding of the liquid–air interface to the pillar top, when its contact angle (θ TiO2) on the TiO2 surface exceeds than the geometrical angle (ψ) between the overhang roof and the vertical micropillar shaft. The Cassie–Baxter state can be transformed into the Wenzel state at a relatively large surface area of the micropillar array when the magnitude of θ TiO2 becomes smaller than ψ after ultraviolet irradiation due to the photoinduced superhydrophilicity of TiO2. The rutile TiO2-coated micropillar array with a small ψ exhibits faster reverse wettability transformation from the Wenzel state to the Cassie–Baxter state despite the relatively small θ TiO2 increase in the dark. In addition, the wettability reversal time was shortened from 180 to 6 min after decreasing ψ from 51° to 9°.