PHILADELPHIA — The colors of a butterfly’s wings are unusually bright and beautiful and are the result of an unusual trait; the way they reflect light is fundamentally different from how color works most of the time.
A team of researchers at the University of Pennsylvania has found a way to generate this kind of “structural color” that has the added benefit of another trait of butterfly wings: super-hydrophobicity, or the ability to strongly repel water.
[...] the team exploited microphase separation of crosslinked polymer chains from nonsolvents to generate nanoroughness (≤120 nm) on holographically patterned diamond photonic crystals.
The process of formation of these nanoroughened patterns consists out of spin-coating, pre-exposure bake, exposure, post-exposure bake (PEB), development, solvent rinsing and critical-point drying (CPD). The pattern is etched with the use of a laser which etches a 3D cross-linked pattern in a kind of material called photoresist. A solvent then washes away all the photoresist untouched by the laser, creating the 3D structure that affects light to create the color effects.
Bio-organisms often exhibit an exquisite array of hierarchical organization with multiscale structures as exemplified by the iridescence in blue Morpho rhetenor butterflies, the waveguiding properties in diatom exoskeletons, the self-cleaning ability of lotus leaves, and the dry adhesion of Gecko foot hairs. These examples provide inspiration for the development of new functional hybrid materials. To mimic hierarchical organization in Nature, one of the emerging strategies is the convergence of top-down microfabrication and bottom-up nanoassembly.
RO: Can you give us an idea of what this material would actually look like when applied to a large surface like, say, an office building or a house? Would it really have that same intense, shimmering quality that we associate with peacock feathers and butterfly wings? Yang: Yes. Since the structural color is a reflective color that is dependent on the structure, it does not suffer photobleaching like pigmentation. As long as the structure maintains its integrity, we will always see the intense shiny color from these materials. However, to fabricate the 3D photonic structures reported in our paper, we used a state-of-art non-conventional 3D lithography technique. So it is not intended for low-cost, large area fabrication. We believe that the concept we demonstrated here is applicable to other fabrication methods. RO: Aside from its potential use in beautifying the outsides of buildings, have you imagined any other potential uses for such this material, or is that something you plan on leaving to the marketing experts? Yang: It could be used as a traffic sign, which needs to be shiny and clean in the rainy or snowy days. It could be used as a bulletin board on the highway or on the building. It could be used as a fancy, protective cover of the iPhone or iPad. It could also be used as camouflage or something that could be worn by the soldiers, for example, as blast injury dosimeters.
We are currently looking into new methods that will allow us to mass-produce these materials for potential commercialization. Of course, we welcome any suggestion from experts about market needs.