Papilio palinurus uses micro-concavities to exhibit dual colour on its wing scales. Image: Wikipedia

Colour-changing material inspired by butterfly wings

25 September 2023

A team of scientists at the University of Hong Kong has designed an innovative colour-changing system with potential for wearable devices, anti-counterfeiting measures and robotic applications. 

The colour-changing system which the team has called a morphable concavity array (MoCA), was inspired by the dual-colour concavities on butterfly wings. The colour alteration is achieved through the pixelation of structural color of a two-dimensional photonic crystal elastomer. The morphable concavity can shift its surface between concave and flat based on changes in temperature or solvents, resulting in angle-dependent colour-shifting. Using multichannel microfluidics, the colour of each concavity can be independently controlled, offering a significant advance over existing technologies.

The potential applications include dynamic displays, anti-counterfeiting and encryption. The technology could also inspire the design of new transformable optical devices such as artificial compound eyes or crystalline lenses for biomimetic and robotic applications.

MoCA was inspired by the stuctures on butterfly wings called dual-colour micro-concavities that produce vibrant, irredescent colours. Dual-colour micro-concavities are tiny pits which block certain wavelength of light, producing two different colors depending on the angle of light and the viewer’s perspective. On butterfly wings these pits are arranged into a regular structures known as photonic crystals.

In nature, the Papilio palinurus butterfly displays dual colour on its wing scales due to micro-concavities with photonic crystal structure. Specifically, when a beam of light is incident on the concavity, the disparity in crystal orientation between the centre and edge leads to different angles of incidence and, thus, distinct reflection wavelengths. Therefore, even though the photonic crystal structures in the butterfly’s wings are consistent, variations in surface topography can result in distinct colours.

MoCA reproduces these crystals and is the first example of a pixelated colour-changing system that relies on flat and concave structures to produce different colours.

“We use multi-channel microfluidics to introduce and remove solvents to manipulate MoCA, offering a complementary approach to the conventional electrochromic methods,” explained Dr Yi Pan of the Department of Mechanical Engineering, HKU, the leader of the research team. 

The dynamic pixelated display system of the MoCA system enables the continuous output of different patterns, which can be combined in a specific sequence to form a more complex pattern containing richer information. As a demonstration, the researchers used MoCA to forming a pixelated version of the Mona Lisa.

Anti-counterfeiting and encryption applications

The dynamic display capability of the MoCA system can be used as a distinctive anti-counterfeiting technique, providing covert protection unbeknownst to counterfeiters. The ethanol-triggered morphing and the angle-dependent colouration provide the first layer of protection. 

In its anti-counterfeiting mode the system could displace QR codes that are not perceivable to the human eye but are machine-readable. The team demonstrated a QR code composed of an array of 29 × 29 pixels which could be read accurately and efficiently by a custom-designed scanner with an appropriate filter.

Compound lenses

The team’s long-term goal is to use the principles behind the technology to construct optical devices to mimic, and potentially surpass, the capabilities of the compound eyes of insects.

Compound eyes contain multiple light-processing structures and offer several advantages over non-compound vision, such as wider field of vision and the ability to focus on multiple objects at the same time.

Deformation of the crystalline lens in the human eye allows us to focus on different distances, explains Professor Shum (Croucher Senior Research Fellow 2020) who co-directed the work with Professor Mingzhu Li from the Institute of Chemistry, Chinese Academy of Sciences,.

The MoCA system can be harnessed to create multiple lenses that can change focus individually. Crystalline lenses have their own adavantages such as greater focusing ability, higher resolution, and better colour perception.

“Optical devices with a combination of the compound eye and the crystalline lens, would not only imitate nature but transcend it,” said Professor Shum.

To view Professor Shum's Croucher profile click here.