Light-induced swirling and locomotion

Abstract

The design of remotely activated, untethered devices without onboard power is a continuing challenge in soft robotics. This work describes a method of generating a whirling motion in pre-stressed photomechanical liquid crystal elastomer fibres using steady illumination that can be exploited for propulsion and mixing. Photomechanical liquid crystal elastomers (LCEs) can convert light directly into mechanical deformation, making them attractive candidates for soft actuators capable of remote and multi-mode actuation. We propose a three-dimensional multi-scale model of the nonlinear and non-local dynamics of fibres of photomechanical LCEs under illumination. We use the model to show that pre-stressed helix-like fibres immersed in a fluid can undergo a periodic whirling motion under steady illumination. We analyse the photo-driven spatio-temporal pattern and stability of the whirling deformation, and provide a parametric study. Unlike previous work on photo-driven periodic motion, this whirling motion does not exploit instabilities in the form of snap-through phenomena, or unilateral constraints as in rolling. More broadly, our work provides an unusual example of a physical system capable of periodic motion under steady stimulus that does not exploit instabilities. We finally show that such motion can be exploited in developing remote controlled bioinspired microswimmers and novel micromixers.