The liquid crystal monomer can transform between cis state and trans state under illumination, which results in large deformation of the LCE, ,, ,,. The LCE is a network structure with liquid crystal monomers connected by cross-linked molecules, and deforms under light, ,, heat, , and electric fields, etc. There is a similar phenomenon existing in liquid crystal elastomer (LCE) rod subjected to light or heat. The bending direction is the same as the rolling direction for the thermal-expansive material, but opposite for the thermal-shrinkable material. The bending state of the rod is related to the material properties. A straight rod placed on a hot platform and applied with initial force, will become bent and roll continuously,. In recent years, another mode of self-excited oscillation has gained widespread attentions. As well as the coupling mechanism of droplet evaporation, motion and film buckling leads to the periodic oscillation of droplets. The coupling mechanism of chemical diffusion, reaction and large deformation realizes the periodic swelling-shrinking of ionic gels. Self shading mechanism could convert light energy into mechanical energy to realize self-excited motion, such as torsion, vibration, rolling and buckling. In order to realize and sustain self-excited oscillation, various mechanisms have been proposed to compensate the energy from damping dissipation by inputting net energy, ,, ,, ,, ,,. Self-excited oscillation system is usually accompanied by damping dissipation, and its motion is a non-equilibrium dissipation process. In addition, self-excited oscillation can ensure the stability and normal operation of various systems due to its robust characteristics.īased on active materials, self-excited oscillation systems with various motion modes under different stimuli have been constructed, ,, ,, ,. And the frequency and amplitude of self-excited oscillation mainly depend on the system parameters and can be self-control without additional controller. For example, the self-excited oscillation can directly collect energy from the environment to maintain motions of the active machine and avoid carrying heavy power equipment. It has great potential applications in the field of active machines such as soft robots, medical devices and micro nano devices due to its many advantages, ,, ,,. Self-excited oscillation is a phenomenon of periodic motion under external constant stimulus, ,, ,, ,. This study may guide the potential applications of the optically-responsive rod in soft robots, harvesting energy and active machinery. These predictions are consistent with the existing experimental results. In addition, the rolling velocity of the LCE rod can be easily controlled by the light intensity and illumination angle. It is found that the rolling of straight LCE rod can be triggered from rest without external disturbance under the inclined light, which is different from the case of the vertical light illumination. In this paper, we adopt an inclined light to manipulate the rolling of a straight LCE rod, and theoretically study its rolling behaviors to obtain the illumination angle range for triggering the self-sustained rolling. Recent experiment has discovered that a straight liquid crystal elastomer (LCE) rod on a horizontal surface can roll continuously under the vertical light illumination, but the start of rolling requires external disturbance. The self-sustained motion of active materials has the great application values in the fields of soft robots, medical devices, micro nano devices, etc., due to its advantage of directly harvesting energy from the environment.
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