Overcoming Van der Waals Forces in reconfigurable nanostructures

TL;DR

This paper presents a new acrylic acid-based material with high stiffness and self-recovery capabilities, enabling 3D printed nanostructures to overcome Van der Waals forces and recover shape after collapse, advancing reconfigurable metamaterials.

Contribution

Introduction of a high-modulus, self-recovering polymer system and a high-resolution 3D printing process for nanostructures that can overcome stiction caused by Van der Waals forces.

Findings

Demonstrated self-recovery of collapsed nanostructures in experiments.

Developed a high-resolution resin enabling 3D printing of high-aspect ratio nanopillars.

Showcased reconfigurable structural color prints and holograms.

Abstract

Reconfigurable metamaterials require constituent nanostructures to demonstrate switching of shapes with external stimuli. For generality, such nanostructures would touch and stick to other surfaces in one of its configurations. Yet, a longstanding challenge is in overcoming this stiction caused by Van der Waals forces, which impedes shape recovery. Here, we introduce a stiff yet self-recovering material system based on acrylic acid, and tested it in high-aspect ratio structures, where recovery is weak. This designer material has a storage modulus of ~5.2 GPa at room temperature and ~90 MPa in the rubbery state at 150 Celsius, an order of magnitude higher than previous reports. A high-resolution resin for two-photon lithography was developed based on this polymer system, enabling 3D printing of nanopillars with diameters of ~400 nm and aspect ratio as high as ~10. Experimentally, we observed self-recovery as collapsed and touching structures overcome stiction to stand back up. We developed a theoretical model to explain the recoverability of these sub-micron structures. Reconfigurable structural colour prints and holograms were demonstrated, indicating potential applications of the material system as a shape memory polymer suitable for sub-micron reconfigurable metamaterials.