Oscurato, Stefano Luigi (2017) Light-driven directional mass transport in azobenzene containing materials for complex textures on surfaces. [Tesi di dottorato]

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Azobenzene-containing materials are one of the most investigated photo-responsive material classes over the last decades. The main reason of such huge interest is their ability to develop superficial reliefs in response to the irradiation with spatially structured optical fields in the UV/visible optical range. This phenomenon has been understood as generated by a light-driven macroscopic mass transport of the host material (typically an amorphous polymer) driven in non-trivial way by the microscopic photo-isomerization dynamics of the azobenzene molecules embedded into it. Even if the exact physical link between the light-induced molecular dynamics and the macroscopic mass displacement is still debated, some of the fingerprints of the phenomenon are fully established. The mass migration, indeed, happens only in illuminated areas of the material and it is highly directional, with a very peculiar sensitivity to the intensity and polarization distributions of the irradiating light field. Since its discovery in 1995, the possibilities offered by this effect for superficial pattering have been largely exploited and recent advances in this field are now oriented toward the realization of complex superficial textures. In the present thesis are proposed two main ideas to accomplish this complex light-driven structuration onto the azopolymer surfaces. The first idea is based on the use of complex structured intensity patterns to irradiate a plane azopolymer film. Such approach can have a two-fold relevance in the azobenzene related research fields. If, on one hand, the use of complex illumination patterns has already been demonstrated to be a fundamental tool in order to highlight new aspects of the mass migration phenomenon, on the other hand the possibility to achieve a precise control on the complex illumination patterns allows the actual employment of the azomaterials as versatile platform in photo-lithographic applications. In particular, the holographic illumination technique described in this thesis opens unprecedented possibilities in both the mentioned research areas. The second idea is instead based on the light-driven reconfiguration of azopolymer surfaces presenting a pre-patterned micro texture. In this situation the illumination pattern can be maintained as simple as conceivable, being constituted even only by a single polarized light beam. However, a great variety of three-dimensional micro-architectures can be obtained using this approach. In particular, azopolymer surfaces having a directional and reversible geometrical asymmetry are achieved by tuning few illumination parameters. These asymmetric microstructures, furthermore, have the ability to tailor several physical macroscopic features of the surfaces, as for example the wettability properties of the azomaterial films. In this thesis is reported a detailed study of such light-controlled wettability tuning, highlighting once more the possibilities offered by this unique photo-responsive material framework for applications in many fields of science.

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