Researchers mimic colour and texture of butterfly wings
16 October 2012
The colours of a butterfly’s wings are the result of an unusual trait: the way they reflect light is fundamentally different from how colour works most of the time.
A team of researchers at the University of Pennsylvania has found a way to generate this kind of 'structural colour' that has the added benefit of another trait of butterfly wings: super-hydrophobicity, or the ability to strongly repel water.
The research was led by Shu Yang, associate professor in the Department of Materials Science and Engineering at Penn’s School of Engineering and Applied Science, and included other members of her group: Jie Li, Guanquan Liang and Xuelian Zhu.
“A lot of research over the last ten years has gone into trying to create structural colours like those found in nature, in things like butterfly wings and opals,” Yang said.” People have also been interested in creating superhydrophobic surfaces which is found in things like lotus leaves, and in butterfly wings, too, since they couldn’t stay in air with raindrops clinging to them.”
The two qualities — structural colour and superhydrophobicity — are related by structures. Structural colour is the result of periodic patterns, while superhydrophobicity is the result of surface roughness
When light strikes the surface of a periodic lattice, it’s scattered, interfered or diffracted at a wavelength comparable to the lattice size, producing a particularly bright and intense colour that is much stronger than colour obtained from pigments or dyes.
When water lands on a hydrophobic surface, its roughness reduces the effective contact area between water and a solid area where it can adhere, resulting in an increase of water contact angle and water droplet mobility on such surface.
While trying to combine these traits, engineers have to go through complicated, multi-step processes, first to create colour-providing 3D structures out of a polymer, followed by additional steps to make them rough in the nanoscale. These secondary steps, such as nanoparticle assembly, or plasma etching, must be performed very carefully so as not to vary the optical property determined by the 3D periodic lattice created in the first step.
Yang’s method begins with a non-conventional photolithography technique, holographic lithography, where a laser creates a cross-linked 3D network from a material called a photoresist. The photoresist material in the regions that are not exposed to the laser light are later removed by a solvent, leaving the “holes” in the 3D lattice that provides structural colour.
Instead of using nanoparticles or plasma etching, Yang’s team was able to add the desired nano-roughness to the structures by simply changing solvents after washing away the photoresist. The trick was to use a poor solvent; the better a solvent is, the more it tries to maximize the contact with the material. Bad solvents have the opposite effect, which the team used to its advantage at the end of the photolithography step.
“The good solvent causes the structure to swell,” Yang said. “Once it has swollen, we put in the poor solvent. Because the polymer hates the poor solvent, it crunches in and shrivels, forming nanospheres within the 3D lattice.
“We found that the worse the solvent we used, the more rough we could make the structures,” Yang said.
Both superhydrophobicity and structural colour are in high demand for a variety of applications. Materials with structural colour could be used in as light-based analogues of semiconductors - for example, for light guiding, lasing and sensing. As they repel liquids, superhydrophobic coatings are self-cleaning and waterproof.
Since optical devices are highly dependent on their degree of light transmission, the ability to maintain the device surface’s dryness and cleanliness will minimise the energy consumption and negative environmental impact without chemicals and in a less labour-intensive way. Yang has recently received a grant to develop such coatings for solar panels.
This research is published in the journal Advanced Functional Materials.