Professor Xie Tao from Zhejiang University proposed a method to accurately 'encode' in plastic products, digitally regulate the internal 'stress' of plastic products, and implant a delicate 'invisible' pattern to realize the invisible storage of information. Related papers Published in the journal Nature Communications.
The first author of the paper, Zhang Guogao, a doctoral student at Zhejiang University, showed a few coins of transparent plastic sheets. Under the polarized lenses, these transparent plastic sheets clearly showed two-dimensional codes, colored butterflies, Mona Lisa portraits and other patterns. .
'We did not add pigment to the material, nor did it change the microstructure of the surface of the material. The pattern is due to stress. ' Zhang Guogao said that stress is brought in by process reasons and is an inevitable ubiquity in plastic products. It will be released in the form of warpage, deformation, and even cracking, which is one of the reasons why plastic products are not durable.
Because of the existence of stress, the refractive index of the transparent material is different in all directions. Under the polarized lens, the interior of the plastic product will show a colorful color.
The research team found a way to digitally control stress. The researchers first uniformly stretched a polymer plastic film to store stress at around 60 degrees Celsius, and then printed a different grayscale pattern on the polymer film with a laser printer. Infrared light illumination, on the plain film, fixed point 'eliminate' stress. The difference in temperature makes the release of stress at each pixel different, so the stress shows a fine gradient change. In this way, the pattern is grayed out. 'Transcoding' is the stress distribution, which in turn forms a preset 'invisible' pattern in the material.
'In materials, stress is usually a passively introduced, uncontrollable factor. And we control this force and perform 'encoding' operations to produce more functions. 'Xie Tao said that the first one is visible. The application is to achieve the invisible storage of information. 'Only by means of polarized lenses, we can see the pattern of material storage.'
Zhang Guogao believes that stress and optical properties of materials, electrical properties and structure are related. By 'coding' stress, programmable plastics will show more functions. 'For example, all 3D printing now is from liquid materials to solids. Our method provides a possibility from solid to solid. '