Making waves in a photoactive polymer film

Illumination of thin liquid-crystal polymer films that contain azobenzene derivatives with short thermal relaxation times induces a continuous wave motion throughout the films, owing to a feedback loop driven by material self-shadowing. Liquid-crystal networks are cross-linked polymers made from macromolecules that self-organize into liquid-crystal phases. They can be programmed to respond to stimuli such as heat, electric fields and light, converting these stimuli into mechanical work or macroscopic deformation. Here Dirk Broer and colleagues develop azobenzenes with rapid cis-to-trans thermal relaxation times and incorporate them into liquid-crystal networks to generate photoactive polymer films. The films deform upon irradiation, and this deformation results in part of the film being shaded from further irradiation. The rapid relaxation then sets up a feedback loop, yielding continuous oscillating wave motion throughout the film. The authors take advantage of this oscillatory motion to demonstrate rudimentary self-propelled locomotion and a self-cleaning surface. Oscillating materials1,2,3,4 that adapt their shapes in response to external stimuli are of interest for emerging applications in medicine and robotics. For example, liquid-crystal networks can be programmed to undergo stimulus-induced deformations in various geometries, including in response to light5,6. Azobenzene molecules are often incorporated into liquid-crystal polymer films to make them photoresponsive7,8,9,10,11; however, in most cases only the bending responses of these films have been studied, and relaxation after photo-isomerization is rather slow. Modifying the core or adding substituents to the azobenzene moiety can lead to marked changes in photophysical and photochemical properties12,13,14,15, providing an opportunity to circumvent the use of a complex set-up that involves multiple light sources, lenses or mirrors. Here, by incorporating azobenzene derivatives with fast cis-to-trans thermal relaxation into liquid-crystal networks, we generate photoactive polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination, with a feedback loop that is driven by self-shadowing. We explain the mechanism of wave generation using a theoretical model and numerical simulations, which show good qualitative agreement with our experiments. We also demonstrate the potential application of our photoactive films in light-driven locomotion and self-cleaning surfaces, and anticipate further applications in fields such as photomechanical energy harvesting and miniaturized transport.