Modeling Defects, Shape Evolution, and Programmed Auto-origami in Liquid Crystal Elastomers

TL;DR

This paper models the shape-changing behavior of liquid crystal elastomers with complex geometries and defects using nonlinear finite element simulations, revealing how thermal gradients and design motifs influence their actuation and final shapes.

Contribution

It introduces a finite element elastodynamics simulation approach to predict shape evolution in liquid crystal elastomers with various defect patterns and complex structures.

Findings

Finite bending energy affects actuation trajectories.

Thermal gradients enable deterministic shape control.

Design motifs allow complex shape assembly.

Abstract

Liquid crystal elastomers represent a novel class of programmable shape-transforming materials whose shape change trajectory is encoded in the material's nematic director field. Using three-dimensional nonlinear finite element elastodynamics simulation, we model a variety of different actuation geometries and device designs: thin films containing topological defects, patterns that induce formation of folds and twists, and a bas-relief structure. The inclusion of finite bending energy in the simulation model reveals features of actuation trajectory that may be absent when bending energy is neglected. We examine geometries with a director pattern uniform through the film thickness encoding multiple regions of positive Gaussian curvature. Simulations indicate that heating such a system uniformly produces a disordered state with curved regions emerging randomly in both directions due to the film's up-down symmetry. By contrast, applying a thermal gradient by heating the material first on one side breaks up-down symmetry and results in a deterministic trajectory producing a more ordered final shape. We demonstrate that a folding zone design containing cut-out areas accommodates transverse displacements without warping or buckling; and demonstrate that bas-relief and more complex bent-twisted structures can be assembled by combining simple design motifs.