A chaotic self-oscillating sunlight-driven polymer actuator

Abstract

Nature provides much inspiration for the design of materials capable of motion upon exposure to external stimuli, and many examples of such active systems have been created in the laboratory. However, to achieve continuous motion driven by an unchanging, constant stimulus has proven extremely challenging. Here we describe a liquid crystalline polymer film doped with a visible light responsive fluorinated azobenzene capable of continuous chaotic oscillatory motion when exposed to ambient sunlight in air. The presence of simultaneous illumination by blue and green light is necessary for the oscillating behaviour to occur, suggesting that the dynamics of continuous forward and backward switching are causing the observed effect. Our work constitutes an important step towards the realization of autonomous, persistently self-propelling machines and self-cleaning surfaces powered by sunlight.

Figure 1. Components and structure of the F-azo polymer film.

(a) Chemical structures of components used to prepare the nematic liquid crystalline network. (b) Photograph of splay-oriented film after removal from the cell under ambient interior light (homeotropic side on top) and (c) schematic of the LC splay aligned film (grey) containing F-azo molecules (orange).

Figure 2. Continuous chaotic oscillations of the F-azo polymer film in sunlight.

(a) Experimental setup for measurement of oscillatory motion of a photomechanical film during sun exposure. (b) Series of snapshots extracted from the video depicting film oscillations; dotted lines have been added to aid the eye. (c) Plot of the deflection angle versus time during sunlight exposure through the window derived from frame-by-frame analysis of the video: the inset is a blowup showing details from a 10 s period. (d) Frequency spectrum of the angle time series shown in 2c (inset: zoom of frequency spectrum between 0 and 1 Hz).

Figure 3. Solar simulator exposure of the F-azo polymer films.

Plot of the measured deflection angle as a function of time, derived from frame-by-frame analysis of the video (every 30th frame analysed corresponding to a 1 s increment).

Figure 4. Visible light exposure of the F-azo polymer films.

Ultraviolet–vis absorption spectra of the planar aligned polymer film as a function of illumination time when exposed to light of (a) 530 nm and (b) 405 nm wavelength. The UV absorption bands at 430 and 470 nm correspond to the n→π* transitions of cis and trans isomers, respectively. Photographs of the splay-oriented film after illumination by light of (c) 530 nm and (d) 405 nm wavelengths. Below the photographs in e and f are representations of the molecular order of the films showing the mesogen (grey) and the F-azo molecules (orange).

Figure 5. Requirement of simultaneous blue and green light for oscillation.

(a) Plot of the calculated deflection angle versus time of F-azo film during exposure to 157 mW cm⁻² green (530 nm) LED light and simultaneously to blue (405 nm) light of the intensity depicted in the graph, derived from frame-by-frame video analysis. (b) Storage modulus of the polymer film as a function of time during illumination to 157 mW cm⁻² green (green line), 318.8 mW cm⁻² blue (blue line), and both blue and green LEDs simultaneously (dark cyan line).

Figure 6. Structures of H-azo and precursors of F-azo.