Optical control of droplet motion on Fe-doped lithium niobate crystals via photovoltaic effect

This thesis presents the innovative study on the local modification of surface wettability actuated by light by exploiting the photovoltaic properties of a photorefractive material, a lithium niobate crystal doped with iron. The project advances the first step towards the improvement of the already available electrowetting technology which uses electric voltages to manipulate aqueous droplets over metallic electrodes. The novel optowetting technique is aimed to get rid of fixed electrodes and hence to provide multi-purpose devices. Lithium niobate samples were properly selected and characterized and a search for the most suitable dielectric film was conducted in order to optimize the optowetting performances of the system. The wettability of the functionalized surfaces was evaluated by means of the contact angle of 1 $\mu$L water droplets and, using a systematic measurement protocol, the dynamics of the phenomenon was investigated. The results obtained shows that the virtual electrodes induced on the crystal surface by a laser source have a temporal duration of several hours in the case of specific compositional properties of the substrate coupled with a thin dielectric layer of PDMS. In addiction, the characteristic time of the optowetting effect can be tuned by regulating the intensity of excitation light, the thickness of the dielectric layer and by varying the reduction degree of iron of the lithium niobate crystal. A final experiment demonstrated the feasibility of such a device. A virtual electrode was traced as an optical path using a 532 nm laser on the treated surface sample and it was observed that the motion of a water droplet driven by gravity on the sample was modified as it ran into the optical path.