Modeling continuous light-driven mechanical waves in a photoactive, nematic polymer film

Photoactive materials that oscillate when illuminated are of interest for applications such as self-cleaning surfaces. Using finite element simulation, we model actuation of such a device, a "blueprinted" liquid crystal polymer film with nematic director planar on one side and homeotropic on the other, as demonstrated by Gelebart et al [1]. The polymer is doped with an azoderivate that undergoes trans-cis isomerisation when illuminated and thermal relaxation when in shadow, both on the time scale of one second. When fixed to a substrate and illuminated with UV light from an oblique angle, the film generates continuous mechanical waves. Curiously, the direction of wave propagation depends on which surface is illuminated. To model actuation, we use a simplified ray tracing algorithm and a kinetic model for light-induced isomerization/relaxation. Changes in nematic order induce dynamic motion. We find the directionality of wave propagation is controlled by a self-shadowing mechanism and periodic pop-through transitions. We compare to experiment and discuss applications such as autonomous light-driven robotics. [1] AH Gelebart, DJ Mulder, M Varga, A Konya, G Vantomme, EW Meijer, RLB Selinger & DJ Broer, Nature 546, 632 (2017).