Japan’s National Institute for Materials Science (NIMS) exhibited a rubber sheet that shows an iridescent colour at Nano tech 2015, a trade show that took place from Jan 28 to 30, 2015, in Tokyo.
The colour of a jewel beetle’s wing changes depending on how light is applied to it or when viewed from different angles. But the colour of the new sheet sensitively changes in accordance with the degree of stress generated by bending or stretching it.
The rubber sheet was developed by Tsutomu Sawada and Hiroshi Fudoji of Advanced Photonic Materials Unit, Advanced Key Technology Division, NIMS. They dispersed nanoparticles (colloids) with a size equivalent to that of a light wavelength in an elastic rubber sheet and periodically arranged the particles so that they become like a crystal. As a result, the arranged nanoparticles reflect only light within a certain waveband.
Colours created by such factors are called “structural colour.” The colours of the wings of insects such as jewel beetles and butterflies are structural colours.
According to NIMS, the light reflection can be explained as Bragg reflection classically and as the result of the photonic crystal’s forbidden band by a recent theory. NIMS calls the sheet “elastically deformable colloid photonic crystal.”
The colour of the sheet can be changed by applying stress because, for example, the distance between nanoparticles in the depth direction is reduced by stretching the sheet, reflecting light with a shorter wavelength.
NIMS considers that the sheet will be used for (1) toys and ornaments for kids, (2) stress sensors using optical detection, (3) optical filters, (4) spectroscopic elements, etc.
As for the first application, the structural colour of the sheet does not require any paint using an organic solvent that is harmful to humans; therefore, the sheet is highly safe and is not discoloured. For the second application, the sheet can be attached to public infrastructure (e.g. bridge pier) so that stress changes caused by a crack, deformation, etc. can be visualized as colour changes and monitored in real time or used for inspection work.
The third application includes glasses that prevent the light of a laser scalpel from directly entering an eye. And an example of the fourth application is a variable spectroscopic element that enables to select a wavelength of light to be passed through by changing the degree of stress.