A plastic film that changes color under stress could provide more detailed information in product safety testing and other areas where pressure and strain are measured. A lab at the University of California, Riverside, used a self-assembly method to string together gold nanoparticles embedded in a polymer film. The system takes advantage of the plasmonic shift that results when nanoparticle chains are pulled apart. “When linked together, the gold nanoparticles originally appear blue,” said Yadong Yin, an associate professor of chemistry whose lab led the research. “But they gradually change to red with increasing pressure as the nanoparticles start disassembling. This easily and visually helps us figure out how much pressure has been applied.”  The color change persists even after the stress is removed, prompting researchers to dub the material a “colorimetric stress memory sensor.” The team’s film differs from commercially available pressure sensor films, which indicate pressure by changing the intensity of just one color. It can potentially be used for revealing pressure distribution over even very complex surfaces, such as automobile crash test dummies.  “The many electronic stress sensors commercially available are bulky and not suitable for certain applications,” Yin said. “For example, it is difficult to tell the stress distribution over a particular area if the contact surfaces are not flat and uniform. Our sensor films can be painted on the contact surfaces so that the color variance in different areas clearly shows the stress distribution over the contact surface.” The university is seeking to patent the discovery.

Some of the most vividly colored materials in nature, including opals do not obtain their color from pigment. Instead, their internal structure reflects light at a given wavelength, producing a specific color. In collaboration with Germany’s Fraunhofer Institute for Structural Durability and System Reliability, scientists from the University of Cambridge have now copied the colorful nanostructure of the opal. The result is a flexible, colorful material that will not fade over time, that changes color when stretched, and that could have many applications. Natural opal stones are formed when water evaporates, leaving behind silica spheres that were suspended in it, that settle into hard-packed layers. In order to make the so-called “polymer opals,” the silica spheres are replaced with nanoparticles that have a rubbery outer shell. When these particles are uniformly linked together in large quantities to form a thin sheet, their internal structure reflects light to produce a single desired color, while their outer coating gives the material an elastic quality. The exact color produced by the polymer opals can be determined by the size of the nanoparticles used. Whatever color is chosen, it can be temporarily changed by stretching or twisting the material. This is because deforming the material causes the spacing between the nanoparticles to change, altering the wavelength at which they reflect light. Stretching the material causes it to shift towards the blue end of the color spectrum, whereas compressing it causes it to get redder. This particular quality could make the material useful for mechanical strain sensors, as its color would reveal how much stress an attached item was undergoing. Additionally, it could be used to replace some of the toxic dyes currently used to color fabrics in the textile industry. Not only would the polymer opals be more environmentally-friendly and safer to work with, but also, unlike dye, their color wouldn’t fade or run. It has been suggested that the technology could be used as a more difficult-to-copy, lower-cost alternative to the holograms presently incorporated into forge-proof banknotes. The scientists are now looking into ways in which a single sheet of the polymer opal material could be manufactured so that it produced different colors in different areas – currently, each sheet is one uniform color. The university is presently looking for an industrial partner, to commercialize the technology.