The contact angle of H2O on stoichiometric and hydroxylated rutile (100) surfaces was studied by molecular dynamics simulations. On stoichiometric slabs, TIP3P water forms a highly ordered spreading film containing approximately two monolayers. On top of them, droplets with contact angles of 32-34° were formed. Hydroxylation induced complete spreading, whereas reduction of the partial charges of TiO2 mimicked a hydrophobic nonwetting surface. We postulate that the interface between ordered layers and droplets has an energy similar to that between ice and water. The Young equation is applied to this water/water interface rather than to the contact between H2O and TiO2. This approach reproduces the macroscopic contact angle and is consistent with models for radiation-induced hydrophilicity. UV-induced hydrophilicity, which is well-known for several types of TiO2 films, was observed experimentally for oxide layers on titanium metal. The contact angles of water on different titanium samples exposed to air significantly depended on the surface pretreatment scattering in a range of 30-70°. In all cases, a significant light-induced decrease was observed, the values being in the range of 10-30° immediately after irradiation.
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