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      A chaotic self-oscillating sunlight-driven polymer actuator

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          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.

          Abstract

          It is highly desirable, yet challenging to build actuators in a dry environment that can undergo autonomous oscillation. Here, Kumar et al. achieve this goal in a soft actuator based on the use of a nematic liquid crystal film doped by ortho-fluoroazobenzene that is responsive to sunlight.

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          Fast liquid-crystal elastomer swims into the dark.

          Miguel A. Camacho-López, Heino Finkelmann, Peter Palffy-Muhoray … (2004)
          Liquid-crystal elastomers (LCEs) are rubbers whose constituent molecules are orientationally ordered. Their salient feature is strong coupling between the orientational order and mechanical strain. For example, changing the orientational order gives rise to internal stresses, which lead to strains and change the shape of a sample. Orientational order can be affected by changes in externally applied stimuli such as light. We demonstrate here that by dissolving-rather than covalently bonding-azo dyes into an LCE sample, its mechanical deformation in response to non-uniform illumination by visible light becomes very large (more than 60 degrees bending) and is more than two orders of magnitude faster than previously reported. Rapid light-induced deformations allow LCEs to interact with their environment in new and unexpected ways. When light from above is shone on a dye-doped LCE sample floating on water, the LCE 'swims' away from the light, with an action resembling that of flatfish such as skates or rays. We analyse the propulsion mechanism in terms of momentum transfer.
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            Rapid and reversible shape changes of molecular crystals on photoirradiation.

            Seiya Kobatake, Shizuka Takami, Hiroaki Muto … (2007)
            The development of actuators based on materials that reversibly change shape and/or size in response to external stimuli has attracted interest for some time. A particularly intriguing possibility is offered by light-responsive materials, which allow remote operation without the need for direct contact to the actuator. The photo-response of these materials is based on the photoisomerization of constituent molecules (typically trans-cis isomerization of azobenzene chromophores), which gives rise to molecular motions and thereby deforms the bulk material. This effect has been used to create light-deformable polymer films and gels, but the response of these systems is relatively slow. Here we report that molecular crystals based on diarylethene chromophores and with sizes ranging from 10 to 100 micrometres exhibit rapid and reversible macroscopic changes in shape and size induced by ultraviolet and visible light. We find that on exposure to ultraviolet light, a single crystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene changes from a square shape to a lozenge shape, whereas a rectangular single crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene contracts by about 5-7 per cent. The deformed crystals are thermally stable, and switch back to their original state on irradiation with visible light. We find that our crystals respond in about 25 microseconds (that is, about five orders of magnitude faster than the response time of the azobenzene-based polymer systems) and that they can move microscopic objects, making them promising materials for possible light-driven actuator applications.
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              Self-Walking Gel

              Y Hara, S. Maeda, T Sakai … (2007)
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                Author and article information

                Journal
                Journal ID (nlm-ta): Nat Commun
                Journal ID (iso-abbrev): Nat Commun
                Title: Nature Communications
                Publisher: Nature Publishing Group
                ISSN (Electronic): 2041-1723
                Publication date (Electronic): 04 July 2016
                Publication date Collection: 2016
                Volume: 7
                Electronic Location Identifier: 11975
                Affiliations
                [1 ]Department of Chemical Engineering and Chemistry, Eindhoven University of Technology , PO Box 513, 5600 MB Eindhoven, The Netherlands
                [2 ]Department of Chemistry and IRIS Adlershof, Humboldt-Universitat zu Berlin, Brook-Taylor-Strasse 2 , 12489 Berlin, Germany
                [3 ]Department of Mathematics and Computer Science, Eindhoven University of Technology , PO Box 513, 5600 MB Eindhoven, The Netherlands
                [4 ]Institute for Complex Molecular Systems, Eindhoven University of Technology , PO Box 513, 5600 MB Eindhoven, The Netherlands
                Author notes
                Article
                Publisher Item ID: ncomms11975
                DOI: 10.1038/ncomms11975
                PMC ID: 4932179
                PubMed ID: 27375235
                SO-VID: 8532e6aa-f198-4dc5-8a8b-f205d4c43dde
                Copyright © Copyright © 2016, Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.
                License:

                This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

                History
                Date received : 04 March 2016
                Date accepted : 19 May 2016
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                Subject: Article

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