Most moth eyes are made up of adjacent hexagonal sectors. Each sector is filled with thousands of orderly rows of miniscule bumps, or nipple-like protrusions. Though formed so perfectly they appear almost manufactured, each protrusion measures less than 300 nanometers, or 300 billionths of a meter – a size that renders them invisible to all but the most powerful electron microscopes.
When moths encounter light, these orderly arrays of protrusions interfere with its transmission and reflection, rendering the light all but invisible. Biologists believe this trait evolved in moths, which are often nocturnal, because it prevents their eyes from reflecting moon or starlight, which would make them easier targets for predators.
Peng Jiang, an assistant professor of chemical engineering, is drawing inspiration from the eyes of moths and the wings of cicadas to create unusual new anti-reflective and water-repellant coatings – coatings that appear to have potential to make solar cells both more efficient and self-cleaning. Windows in cars and homes, computer screens and other consumer products also could improve thanks to the super-transparent coatings.
Jiang said engineers have sought to replicate the eyes’ microscopic structure using a printing technique called lithography, but it is expensive and ill-suited to creating the extremely tiny rows of protrusions that make the moth eyes so effective. To get around this problem, Jiang developed a non-lithographic technique, called spin coating. Unlike lithography, which attempts to carve out the nipple-like pattern on the target surface, spin coating seeks to build the pattern up from scratch on the target.
Jiang places a liquid suspension of nanoparticles on a circular silicon wafer, such as that used in photovoltaic cells. A motor spins the wafer, with centrifugal force distributing the liquid. When it dries, it leaves behind the ordered particles in place.
Cicada wings are amazingly effective at rapidly shedding water and dirt, apparently because the insects often need to fly in humid environments, Jiang said. At the particle level, the wings have a structure very similar to that of the moth eyes – except that rather than deflecting light, tiny pockets of air around each nipple-like protrusion buoy water droplets.
Jiang added the coatings could also improve the performance of ordinary windows on cars or homes, increasing visibility and reducing the need for cleaning. That said, numerous challenges remain, including learning how to “scale up” the spin coating process so that it could be used for industrial production, he said.
Yadong Yin, an assistant chemistry professor at the University of California, Riverside, said Jiang’s research is important in part because it suggests that there is a low-cost alternative to current anti-reflection production techniques. “Importantly,” he said, “the low cost in this case did not lead to low performance.”