For easy-to-clean and self-cleaning applications, it is not only the photocatalytic activity of titanium dioxide surfaces which is utilized but also the photo-induced hydrophilic properties. Irradiation of TiO2 surfaces with UV light results in the water contact angle (Θ) becoming as small as desired (superhydrophilic properties). On storage in the dark for longer periods the water contact angle Θ slowly increases again. A hydrophilic surface can be relatively easily cleaned by water.
On the other hand, controlled and, if possible, space-resolved switching of the surface between a very hydrophilic and a less hydrophilic to hydrophobic state can open up possibilities for other applications (printing processes, microfluidics). For this it is however necessary to increase the water contact angle with more effective and faster methods than dark storage. Numerous studies have already been undertaken involving chemical hydrophobicization, ultrasound treatment in water, rubbing with damp paper, and heat input via various methods.