Researchers at the Indian Institute of Science (IISc) have developed flexible films that exhibit bright colours purely by virtue of their physical structure, without the need for any pigment. When stretched, the films exhibit a change in colour as a response to the mechanical deformation.
“To design these films, the team devised a novel cost-effective and scalable single-step technique that involves evaporating gallium metal to form nano-sized particles on a flexible substrate. Their method allows the simultaneous fabrication of multiple structural colours responsive to mechanical stimuli,” said IISc in a statement.
The team has also shown how these films can be used for a variety of applications, from smart bandages and movement sensors to reflective displays.
“This is the first time that a liquid metal like gallium has been used for photonics,” said Tapajyoti Das Gupta, Assistant Professor in the Department of Instrumentation and Applied Physics (IAP), and corresponding author of the study published in Nature Nanotechnology.
The IISc team began experimenting with gallium, which has not been explored for such applications because its high surface tension hinders the formation of nanoparticles. Gallium is a liquid metal at room temperature and its nanoparticles have been shown to have strong interactions with electromagnetic radiation.
The process developed by the team achieves the feat of overcoming the barrier of surface tension to create gallium nanoparticles, by cleverly using the properties of a substrate called polydimethylsiloxane (PDMS), a biocompatible polymer.
When the substrate was stretched, the researchers noticed something unusual. The material started showing different colours depending on the strain. The researchers theorised that the array of deposited gallium nanoparticles interacts with light in specific ways to generate the colours.
To understand the role of the substrate in colour generation, the team developed a mathematical model.
“We show that the PDMS substrate not only holds the structure, but also plays an active role in determining the structure of gallium nanoparticles and resulting colouration,” said Renu Raman Sahu, PhD student in IAP and lead author. Even after 80,000 cycles of stretching, the material was able to show a repeatable colour change, indicating its reliability.
