Counterfeiters be warned: banknotes may soon be harder to fake thanks to a new iridescent film that changes colour when you turn or twist it.
The polymer film mimics the structure of naturally occurring opals (pictured) and could be used to make eye-catching paint that shimmers when viewed from any direction, or food packaging that changes colour if its contents spoil.
Butterfly wings and opals have already inspired the creation of make-up and paints with the same flickering, iridescent colours. These qualities spring from photonic crystals, whose micro-spheres sit in a three-dimensional repeating pattern, similar to a stack of egg boxes. This structure blocks certain wavelengths of light at some viewing angles, while other wavelengths zip through. As a result, the crystals change colour, or shimmer, as the viewer鈥檚 perspective changes. Not only that, when flexible photonic crystals are stretched, the spacing between the 鈥渆gg holes鈥� changes, causing further colour variation.
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Some teams have successfully created photonic crystals by etching patterns into various materials, but that is expensive and the colours can look dull or milky if viewed from the wrong angle. Now a collaboration between the University of Southampton, UK, the pharmaceutical company Merck and the German Institute for Polymers, both in Darmstadt, has developed a photonic crystal film that shimmers from virtually every angle. Not only that, it can be mass-produced cheaply because it鈥檚 partly self-assembling ().
鈥淭he crystals can be cheaply mass-produced because they self-assemble鈥�
To make the film, polystyrene spheres are grown to a diameter of 200 nanometres, hardened with a blast of heat or light, and then coated with a second, sticky polymer. During further heating, natural shear forces nudge the spheres into the repeating 3D structure of an iridescent photonic crystal.
Carbon nanoparticles were thrown into the mix to make the film shimmer when viewed from a greater range of angles. The nanoparticles nestle into the spaces between the polystyrene spheres 鈥� without altering the crystal structure 鈥� and scatter light in all directions, allowing the iridescence to be seen from virtually any viewing angle. The researchers say the nanoparticles could be further engineered to change shape in response to specific toxins, altering the colour of the film, which could one day prove useful in food packaging. And using the material in banknotes would give forgers a new hurdle to overcome if they want to create credible fakes.
