The food packaging application is one of the promising commercial possibilities for a new type of flexible plastic film developed by scientists at the UK's University of Southampton and the Deutsches Kunststoff-Institut (DKI) in Darmstadt, Germany. The new films have the potential to help food manufacturers prevent recalls and food contamination incidents from affecting their brands. The films combine natural and man-made optical effects to achieve a new way for objects to precisely change their colour, according to research published in the 23 July issue of Optics Express, a journal of the Optical Society of America. The "polymer opal films" belong to a class of materials known as photonic crystals. The crystals are built of many tiny repeating units, and are usually associated with a large contrast in the components' optical properties. The properties can be used to generate a range of frequencies, called a "photonic bandgap", where no light can propagate in any direction. Instead, the researchers have developed the new opal films with a small contrast in their optical properties. Photonic crystals are used in fiber optic telecommunications, and are being considered as a potential replacement for toxic and expensive dyes used for coloring objects, from clothes to buildings. Yet much of their commercial potential has yet to be realised because the colors in manmade films made from photonic crystals depend strongly on viewing angle, the researchers stated. A sheet of the opal film may change to a different colour depending on the angle it is viewed. Photonic crystals are also present in opals, butterfly wings, certain species of beetle, and peacock feathers, which all feature arrays of tiny holes, neatly arranged into patterns. The artificial opal structures are also "self-assembling", the scientists stated. The small constituent particles assemble themselves in a regular structure. But this self-assembly is not perfect, and though meant to be periodic, they have significant irregularities. The interplay between the periodic order, the irregularities, and the scattering of small inclusions can be used to affect the way the light travels through these films, the researchers discovered. "For example, light may be reflected in unexpected directions that depend on the light's wavelength," they stated. Researcher Jeremy Baumberg and his team developed polymer opals to combine the precise structure of manmade photonic crystals with the colour of natural structures. The polymer opal films are made of arrays of spheres stacked in three dimensions, rather than layers. They also contain carbon nanoparticles wedged between the spheres. The light also scatters off the nanoparticles and not just from the interfaces between the plastic spheres and the surrounding materials. The technique makes the film look intensely coloured, even though they are made from only transparent and black components that are environmentally benign.
Additionally, the material can be "tuned" to only scatter certain frequencies of light simply by making the spheres larger or smaller, the scientists stated. Researchers in both the UK and Germany then developed a means to mass produce the photonic crystals. They have developed a manufacturing process that can be successfully applied to photonic crystals to produce long rolls of polymer opal films. The films are "quite stretchy" and when they stretch, they change colour, according to Baumberg.
"This, too, makes them ideal for a wide range of applications, including potential ones in food packaging, counterfeit identification and even defense," he stated.