Material Could Provide Self-Cleaning, Waterproof Optics

Inspired by butterfly wings, a new material that repels water and shines with beautiful, brilliant hues could provide the foundation for optical devices that are self-cleaning and waterproof.

“A lot of research over the last ten years has gone into trying to create structural colors like those found in nature, in things like butterfly wings and opals,” said associate professor Shu Yang of the University of Pennsylvania. “People have also been interested in creating superhydrophobic [super water-repellant] 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.”

These two qualities are related by structures. Structural color is the result of periodic patterns, while superhydrophobicity results from surface roughness. Trying to combine these traits, however, is a complicated, multistep process.

Yang first used a holographic lithography technique to recreate the wings’ reflective properties, using a laser to make a three-dimensional cross-linked pattern in a photoresist material. A solvent was then used to wash away all the photoresist untouched by the laser, creating a 3-D lattice that provided structural color.

The material provide a more energy-efficient, environmentally friendly alternative to conventional building paints, Yang said, because “the structural color we can produce is bright and highly decorative, and it won’t fade away like conventional pigmentation color dies.”

The surface’s texture was made to exhibit the water-resistant property of butterfly wings by roughening it with a poor solvent, instead of through other methods such as using nanoparticles or plasma etching.

“Because the polymer hates the poor solvent, it crunches in and shrivels, forming nanospheres within the 3-D lattice,” Yang said.

Both qualities are in demand for a variety of applications.

“It could be a high-end façade for the aesthetics alone, in addition to the appeal of its self-cleaning properties,” Yang said of UPenn’s material.

ShuYang’s group at the University of Pennsylvania has a new way of combining the structural color and superhydrophobicity that characterize butterfly wings into a new material. This drop of water sits on a wafer made with their process. Courtesy of Advanced Functional Materials.

Materials with structural color could be used as light-based analogs of semiconductors, for example, or for light guiding, lasing or sensing. As they repel liquids, superhydrophobic coatings are self-cleaning and waterproof. Such qualities would be useful for optical devices that are highly dependent on their degree of light transmission, and on their ability to maintain surface dryness and cleanliness. Yang recently received a grant to develop such coatings for solar panels.

While applications abound for how the traits could be used separately, Yang also has an application idea for how the two qualities could be combined.

“Specifically, we’re interested in putting this kind of material on the outside of buildings,” she said. “We are also developing energy-efficient building skins that will integrate such materials in optical sensors.”

The findings appeared in Advanced Functional Materials (doi: 10.1002/adfm.201200013).

For more information, visit: www.upenn.edu