| 초록 |
Liquid crystal elastomers (LCEs) are highly attractive as soft actuators for, e.g., robotics, as they can dynamically change their shape in response to temperature changes, light or electric fields.1 The mode of shape change can be powerfully controlled by introducing topological defects, as recently demonstrated by several authors.2,3 Normally this requires advanced patterning of sacrificial alignment layers, which are rejected after production. On the other hand, topological defects arise spontaneously in spherical shells of liquid crystals if the director field has a component in the plane of the shell.4 This has been investigated extensively using low molar mass liquid crystals5-7 but only one study so far8 reported elastomeric liquid crystal shells with topological defects. The LCE shell configuration is interesting, both due to the spontaneous appearance of topological defects, and because—in contrast to LCEs made using standard methods—the ground state is already curved. In [8] the shells were photocrosslinked in-situ during production, locking in a non-equilibrium director field with poor control of the alignment and defect configuration. Two recent papers on LCE shells studied the case of equilibrium director fields,9,10 but in both cases they were radial, thus without topological defects. In the present study,11 we prepare planar-aligned LCE shells, separating in time the production from the crosslinking, in order to allow an equilibrium defect configuration to be established prior to making the configuration permanent in the final LCE. Our focus is particularly set on the interesting case where all defects are concentrated near the thinnest point of the shell. In this work, we show in detail that on crosslinking a shell, the shape and the defect configuration changes. As expected, these shells show a clear actuation behaviour with temperature. This demonstrates that the controlled positioning of topological defects within LCE shells, via manipulation of the equilibrium conditions, can be used to tune the mode of actuation of these unconventional artificial muscles. (1) C. Ohm et al., Adv. Mater., 22, 3366 (2010). (2) T. H. Ware et al., Science, 347, 982 (2015). (3) L.T. de Haan et al., Chem. Int. Ed., 51, 12469 (2012). (4) M. Urbanski et al., J. Phys. Condens. Matter, 29, 133003 (2017). (5) A. Fernandez-Nieves et al., Phys. Rev. Lett., 99, 157801 (2007). (6) T. Lopez-Leon et al., Nat. Phys., 7, 391 (2011). (7) H.-L. Liang et al., Phys. Rev. Lett., 106, 247801 (2011). (8) E.-K. Fleischmann et al., Nat. Commun., 3, 1178 (2012). (9) V. S. R. Jampani et al., Adv. Funct. Mater., 28, 1801209 (2018). (10) V. S. R. Jampani et al., Sci. Adv., 5, eaaw2476 (2019). (11) A. Sharma et al., J. Appl. Phys., 17, 174701 (2021). |
