A new method has been devised by researchers for arranging tiny marbles into standard layers, thereby forming exciting materials, which disperse light into vivid colors, and which alters its color on stretching or twisting.
University of Cambridge have led a team who has invented a method to produce such sheets on large industrial scales, which paves the way for applications spanning from banknote security to smart clothing for people or buildings.
The scientists were able to construct many meters of these materials referred to as ‘polymer opals’ on a roll-to-roll process using a novel technique called Bend-Induced-Oscillatory-Shearing (BIOS). Their findings have been published in the Nature Communications journal.
Opal gemstones, beetles, and butterfly wings contain the brightest colours in nature. These obtain their color from their systematically-arranged microstructures and not from pigments or dyes.
Cambridge’s Cavendish Laboratory has a research team who has been studying about techniques by which this ‘structural color’ could be artificially reproduced for several years; however, till date it has not been possible to produce the materials using techniques that are relatively low cost to enable their extensive use.
The group started growing vats of plastic transparent nanospheres to produce the polymer opals. All these microscopic spheres are sticky outside but solid at the center. Then these spheres were dried into a firm mass, and the balls were forced to form precisely ordered stacks by bending the sheets comprising alternating layers of spheres and rollers, at the end of which they have intense color.
Various colors of light are reflected by altering the sizes of the initial nanospheres. When the material is stretched and twisted, the distance between the spheres is changed as it has rubber-like consistency, resulting in a color change of the material. The material turns to the blue range of the spectrum when it is stretched and towards the red range when it is contracted. The material reverts to its original color when it is released. These materials that change their color like a chameleon could be used in building coatings, which reflect infrared thermal radiation, or color changing wallpapers.
Finding a way to coax objects a billionth of a metre across into perfect formation over kilometre scales is a miracle. But spheres are only the first step, as it should be applicable to more complex architectures on tiny scales.
Professor Jeremy Baumberg, senior author of the paper
The group had to first comprehend the internal structure of the polymer opals, so that it can be replicated and produced in large quantities. The scientists were able to visualize the 3D position of spheres within the material, estimate how the spheres could slide against one another, and how the colors change using a range of techniques like x-ray scattering, electron microscopy, optical spectroscopy, and rheology.
It’s wonderful to finally understand the secrets of these attractive films.
Qibin Zhao, PhD student, main author of the paper
The University’s commercialization arm, Cambridge Enterprise, is assisting in commercializing this material. It has been approached by over 100 companies that were interested in utilizing polymer opals, and a new division called Phomera Technologies has also been set up. Phomera will explore means to scale up the manufacture of polymer opals, and market the material to probable customers. Some of the applications the company is focusing on include smart clothing and footwear, coatings for buildings to reflect heat, or for packaging applications and banknote security.
The study is supported as part of a UK Engineering and Physical Sciences Research Council (EPSRC) investment in the Cambridge NanoPhotonics Centre, and the European Research Council (ERC).