For millennia humans have created art with pigment-based paints, inks and dyes. Now researchers have produced tiny plastic paintings whose colors come from variations in microscopic surface features instead.
Pigments are chemicals that absorb certain light wavelengths and reflect others to produce specific colors. But some materials—such as those on morpho butterflies' iridescent blue wings and the striking feathers of some hummingbirds—produce colors based on the size and spacing of microstructures on their surfaces, which interact with light wavelengths of different sizes.
Many plastics form minuscule cracks, called crazes, when put under stress. Typically these fractures occur randomly throughout the material. But first exposing some plastics to light beams can selectively weaken them in places, where crazes will appear when the plastic is stressed. “You can actually control where the cracks form,” says materials scientist and study co-author Andrew Gibbons of Kyoto University in Japan. Depending on size and configuration, these cracks act as microstructures that produce specific colors.
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Gibbons and his colleagues shone powerful LEDs on thin pieces of plastic and then dunked them in acetic acid, generating crazes in the places preweakened by light. These cracks initially reflect the same wavelength of light to which the section of plastic was exposed, according to the study, which was published in June in Nature.
If the plastic is soaked longer or exposed to high temperatures, the cracks can expand to reflect longer wavelengths. The size of each region hit by light and the thickness of the plastic also influence how far the cracks expand. To test their method, the researchers produced miniature renderings of classic paintings and even a Queen album cover. (The smallest was 0.25 millimeter across.)
“It's an innovative twist,” says polymer scientist Christopher Soles of the National Institute of Standards and Technology, who was not involved in the study. “Usually crazing in materials is a very bad thing,” Soles notes, “but here the crazes are useful.” He was surprised the process worked with so many different types of plastics, including polystyrene, polycarbonate and acrylic glass—used in food containers, CD cases and bulletproof glass, respectively.
Gibbons says crazing could potentially create a durable plastic coating for currency or high-end merchandise to discourage counterfeiters. And the microstructures can produce more than pretty pictures. Eventually he hopes the technique could be used to create devices that store microscopic amounts of liquid for medical analysis.