Ridge approximation for thin nematic polymer networks

TL;DR

This paper proposes an approximate method to compute the bending energy of nematic polymer networks (NPNs) by modeling their activated surfaces as flat sectors connected by ridges, aiding understanding of their shape-changing behavior.

Contribution

It introduces a ridge approximation approach to estimate the bending energy in NPN sheets, providing insights into their shape transformation mechanisms.

Findings

The method models the activated surface as flat sectors connected by ridges.

Application to a spiraling hedgehog pattern demonstrates shape-changing ability.

The approach suggests increasing ridges improves approximation accuracy.

Abstract

Nematic polymer networks (NPNs) are nematic elastomers within which the nematic director is enslaved to the elastic deformation. The elastic free energy of a NPN sheet of thickness $h$ has both stretching and bending components (the former scaling like h, the latter scaling like $h^3$). NPN sheets bear a director field $\mathbf{m}$ imprinted in them (usually, uniformly throughout their thickness); they can be activated by changing the nematic order (e.g. by illumination or heating). This paper illustrates an attempt to compute the bending energy of a NPN sheet and to show which role it can play in determining the activated shape. Our approach is approximate: the activated surface consists of flat sectors connected by ridges, where the unit normal jumps and the bending energy is concentrated. By increasing the number of ridges, we should get closer to the real situation, where the activated surface is smooth and the bending energy is distributed on it. The method is applied to a disk with imprinted a spiraling hedgehog. It is shown that upon activation the disk, like a tiny hand, is able to grab a rigid lamina.