# Angle-Independent Color Changes in Elastomer-Immobilized Non-Close-Packed Colloidal Amorphous Films Under Stretching

## TL;DR

This paper describes a stretchable material that changes color when stretched, which could be used as a sensor to detect invisible strains.

## Contribution

A new method to create angle-independent color-changing films using elastomer-immobilized colloidal amorphous structures is introduced.

## Key findings

- The prepared film exhibits angle-independent color that changes under stretching.
- Hydrophilic carbon black improves the color saturation of the film.
- The film's flexibility allows it to be applied on curved surfaces for strain sensing.

## Abstract

Colloidal amorphous structures comprise short-range ordered arrays of monodisperse submicrometer-sized particles. They exhibit angle-independent structural color and hence are expected to be promising candidates for advanced color materials. In particular, non-close-packed colloidal amorphous structures embedded in soft polymers can alter the angle-independent color through stimuli-induced volume changes in the polymer. Consequently, such materials should have significant potential for application in sensor devices. This paper reports the preparation of an elastomer-immobilized non-close-packed colloidal amorphous film with an angle-independent color using a hydrogel-immobilized non-close-packed colloidal amorphous film as the starting material. The swelling solvent (i.e., water) in the hydrogel film was replaced with a hydrophilic elastomer precursor solution, which was photopolymerized to immobilize the colloidal amorphous structure with the separated particles within the elastomer film. The color of the elastomer-immobilized non-close-packed colloidal amorphous film was angle-independent and was easily altered under stretching. Furthermore, hydrophilic carbon black dispersed well in the hydrophilic elastomer precursor solution, improving the saturation of the resultant elastomer-immobilized non-close-packed colloidal amorphous film. The flexible nature of the prepared film should allow it to be attached to curved surfaces, thereby promoting its application as a simple strain sensor to express invisible strains through color changes.