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.
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This work was supported financially by the Netherlands Organization for Scientific Research (NWO; TOP PUNT grant 10018944), the European Research Council (Vibrate ERC, grant 669991), and US National Science Foundation grants DMR 1409658 and CMMI 1436565. A.H.G. acknowledges funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7-2013, grant number 607602. Computing resources provided by the Ohio Supercomputer Center (M.V., A.K., R.L.B.S.) R.L.B.S. acknowledges F. Nazarov for discussions and B. L. Mbanga for his role in developing the Finite Element Method algorithm. The work of D.J.M. forms part of the research programme of the Dutch Polymer Institute (DPI), project 776n.
The authors declare no competing financial interests.
Reviewer Information Nature thanks T. Ikeda, R. Verduzco and Y. Yu for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Components of the LCN films include AzoPy, I and II.
a, Differential scanning calorimetry scans (second runs, exotherm downwards) showing the phase behaviour of all mixtures investigated. The nematic-to-isotropic transition occurs at 90 °C. b, Differential scanning calorimetry scan of a polymerized sample showing the change in specific heat at the glass transition temperature (Tg). The table summarizes the Tg data of the various polymerized compositions. c, Normalized absorption spectra of the various mixtures investigated.
a–d, Thermal relaxation from the photostationary cis state to the trans state of A6MA (a), I (b), II (c) and DR1A (d) at various temperatures. Here and .
Extended Data Figure 4 Pictures taken at different angles showing the curvatures that were created, inducing the self-shadowing effect.
Scale bar, 5 mm. At 90°, the bump is formed (indicated by the arrow), but because no shadow is created the wave cannot propagate and the film remains in that position.
a, Influence of the intensity on the temperature increase at the front of the wave. The red shaded region is a guide to help to visualize the glass transition region. b, Measured temperature for the uniaxially oriented sample. Despite the rubbery character of the films, no motion was observed.
a, c, Thermal pictures of the wave taken at different times t. b, Temperature profile over the length of the film (along the black line in a) for the homeotropic-up sample during wave propagation at t = 0 s (black line), t = 0.67 s (dark grey line) and t = 1.40 s (light grey line). d, Temperature profiles over the length of the film (along the black line in c) for the planar-up sample at t = 0 s (black line), t = 0.11 s (dark grey line) and t = 0.22 s (light grey line).
a, 1H NMR of the compound AzoPy, which was used to form compound I. b, 1H NMR of compound II.
Transmissions (T, expressed as percentages) for compound I (green), compound II (black), A6MA (red), AzoPy (pink) and DR1A (blue) are shown. Thickness, 20 μm. The films containing A6MA, compound I and AzoPy are actuated with 405-nm light. At this wavelength, the transmissions are 6.3%, 4.1% and 8.9%, respectively. The samples containing DR1A and compound II are illuminated with 455-nm light. At this wavelength, the initial transmissions are 26% and 13%, respectively.
Comparison between a splay aligned sample (left) and two uniaxially aligned samples (planar and homeotropic). Videos are played in real time. (MOV 9844 kb)
Type of deformation obtained when the LCN films is exposed to UV light without any constraint (worm like displacement) and when one constraint point is used (oscillation and tube).
Type of deformation obtained when the LCN films is exposed to UV light without any constraint (worm like displacement) and when one constraint point is used (oscillation and tube). Videos are played in real time. (MOV 18846 kb)
Comparison between the different molecules investigated. Videos are played in real time. (MOV 14673 kb)
The incident light comes from the left and the wave propagates toward the opposite direction. Model and experiment are placed side to side to show good agreement. In the second part of the video the thermal effects are shown. Videos are played in real time. (MOV 23391 kb)
The incident light comes from the left and the wave propagates toward the light. Model and experiment are placed side to side to show good agreement. In the second part of the video the thermal effects are shown. Videos are played in real time. (MOV 25149 kb)
The first part of the video shows that the LCN film carry uphill object much heavier and much bigger than its own dimension. The second part of the video shows the use of the wave for self-cleaning surfaces with the ejection of sand placed at the surface. Videos are played in real time. (MOV 20362 kb)
The light comes from the left side and the polymeric film is constrained in a non-responsive frame. Videos are played in real time. (MOV 10702 kb)
The light comes from the left side and the polymeric film is constrained in a non-responsive frame. Videos are played in real time. (MOV 27077 kb)
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Gelebart, A., Jan Mulder, D., Varga, M. et al. Making waves in a photoactive polymer film. Nature 546, 632–636 (2017). https://doi.org/10.1038/nature22987
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