Reversible wetting of titanium dioxide films

Titanium dioxide (TiO₂) films were rendered hydrophilic through ultraviolet (UV) light irradiation (254nm) and returned to their previous hydrophobic condition when exposed to a sealed pressurized nitrogen atmosphere. UV light irradiation on TiO₂ films resulted in super-hydrophilic surfaces with water contact angles of <5°. Alternatively, exposure of the films to an N₂ environment resulted in relatively hydrophobic surfaces with water contact angles of >40°. The switching of TiO ₂ surface wettability could be repeated on the same surface with little hysteresis in water contact angle values. The mechanism behind the hydrophilic and hydrophobic reversal in TiO₂ surfaces is proposed to be due to UV light mediated photocatalysis and physio- adsorption of N ₂ molecules respectively. The non-intrusive control of TiO ₂ surface wettability could be an attractive alternative to other wettability-based microfluidic valving strategies like electrowetting and photochromic wetting variation. The above results are discussed in terms of the potential use of the films in wettability based valving and repeated wettability patterning of TiO₂ surfaces for open and sealed microfluidic systems.