Researchers at the Indian Institute of Science (IISc) have developed flexible films that display vibrant colors simply due to their physical structure, without the need for any pigment. When stretched, the film changes color in response to mechanical deformation.

To design these films, the team developed a novel, cost-effective and scalable one-step technique that involves evaporating gallium metal to form nanosized particles on a flexible substrate. Their method allows the simultaneous production of a variety of structural colors that respond to mechanical stimuli. The team also demonstrated how these films can be used for a variety of applications, from smart headbands to motion sensors and reflective displays.

“This is the first time a liquid metal such as gallium has been used for photonics,” says Tapajyoti Das Gupta, an associate professor in the Department of Instrumentation and Applied Physics (IAP) and author of the study published in Nature. Nanotechnology.

Some natural objects, such as gemstones, oyster shells, or peacock feathers, are inherently multicolored. Their colors result from the interaction of light with periodically arranged micro- or nanostructures, such as tiny silica spheres in opal, calcium carbonate-based plates in oyster shells, and segmented stripes on cylindrical structures in peacock feathers.

Structurally colored materials inspired by nature have found widespread use in displays, portable electronic devices, visual sensors, and anti-counterfeiting labels. In recent years, scientists have been trying to create materials that can change color in response to external mechanical stimuli.

The IISc team began experimenting with gallium, which has not yet been explored for such applications because its high surface tension prevents the formation of nanoparticles. Gallium is a liquid metal at room temperature, and its nanoparticles have been shown to interact strongly with electromagnetic radiation.

The process developed by the team achieves the ability to overcome the surface tension barrier to create gallium nanoparticles by cleverly exploiting the properties of a substrate called polydimethylsiloxane (PDMS), a biocompatible polymer.

When the substrate was stretched, the researchers noticed something unusual. The material began to show different colors depending on the voltage. The researchers hypothesized that the deposited array of gallium nanoparticles interacts with light in a specific way to create colors. The team developed a mathematical model to understand the role of the substrate in color production.

PDMS is a polymer made by mixing two liquid components (an oligomer and a crosslinking agent) that react with each other to form a solid polymer. The researchers found that the unreacted portion of the oligomer, which is still in liquid form, plays a crucial role in stabilizing the formation of gallium nanoparticles on the substrate.

When this substrate is then stretched, liquid-like oligomers leak into the gaps between the nanoparticles, changing the size of the gap and their interaction with light, leading to the observed color change. Experiments conducted in the laboratory confirmed the model’s predictions. The researchers achieved a range of colors by adjusting the ratio of oligomer content to cross-linker.

Ph.D. “We have shown that the PDMS substrate not only maintains the structure but also plays an active role in determining the structure and final coloration of gallium nanoparticles,” says Renu Raman Sahu. IAP student and lead author. Even after 80,000 stretching cycles, the material was able to show repeatable color change, demonstrating its reliability.

Traditional methods such as lithography used to make such materials involve many steps and are expensive to scale up. To avoid this, the team developed a one-step physical vapor deposition technique to vaporize and deposit liquid gallium metal onto a PDMS substrate. This allowed them to produce flexible structured color films about half the size of a palm.

There are various options for the use of such films. The team demonstrated one such application: a body motion sensor. A film strip attached to the finger changed color when the finger was bent, helping to detect movement in real time.