Making waves in a photoactive polymer film

Oscillating materials1–4 that adapt their shape in response to an external stimulus are of interest for emerging applications in medicine and robotics. Liquid crystal networks have a prominent role in this area because they can be programmed to undergo stimulus-induced deformations in a variety of geometries, including in response to light5,6. In order to make these polymer networks photoresponsive, azobenzene molecules are often incorporated7–11. Most examples in the literature report on bending responses of these azobenzene modified films, where relaxation after photo-isomerization is rather slow. Modification of the core or addition of substituents to the azobenzene moiety can lead to drastic changes in photophysical and photochemical properties12–15 giving opportunity to circumvent the use of a complex set-up. Here we report on the incorporation of azo-derivatives with fast thermal relaxation into liquid crystal network films (LCN), to generate films that can exhibit continuous, directional macroscopic mechanical waves under constant light illumination, with a feedback loop driven by self-shadowing. A theoretical model and numerical simulation demonstrate this mechanism and show good qualitative agreement with experiments. We explore potential applications in light-driven locomotion and self-cleaning surfaces.