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Sunlight readable displays take cue from low-reflection butterfly wings

06 May 2015

The glasswing butterfly (Greta Oto) hardly reflects any light in spite of its transparent wings. Scientists studying this phenomenon believe the wing structure could be synthesised to produce a range of anti-reflective surfaces, including sunlight readable displays.

The glasswing butterfly - Greta Oto (photo: Radwanul Hasan Siddique/KIT)

Researchers at the Karlsruhe Institute of Technology (KIT), under the direction of Hendrik Hölscher, have found that irregular nanostructures on the surface of the glasswing butterfly wing are responsible for this low reflection. In theoretical experiments, they have succeeded in reproducing the effect, which opens up application possibilities, such as sunlight readable displays for handheld devices.

Transparent materials such as glass always reflect part of the incident light. Some animals with transparent surfaces, such as a moth’s eyes, succeed in keeping those reflections small, but only when the view angle is vertical to the surface. However, the wings of the glasswing butterfly, with a habitat mainly in Central America, also have a very low reflection when viewed from different angles.

Indeed, depending on the angle of view, specular reflection varies only between two and five percent. A flat glass plate, on the other hand, reflects between eight and 100 percent, depending on the viewing angle. Interestingly, the glasswing butterfly's wing not only achieves low reflection across the visible spectrum, but also suppresses the infrared and ultraviolet radiation that can be perceived by animals. This is important for its survival.

Using scanning electron microscopy, earlier studies have already revealed that regular pillar-like nanostructures are responsible for the low reflections from other animals. While these nanopillars have now been discovered on the glasswing butterfly's wings, they are arranged irregularly and are of random heights. The typical height of the pillars varies between 400 and 600nm and the distance between the pillars ranges from 100 to 140nm.

In simulations, the researchers have modelled this irregularity in terms of height and arrangement, and found that the calculated reflected amount of light exactly corresponds to the observed amount at variable view angles. The low reflection at variable viewing angles is thus caused by the irregularity of the nanopillar height and distribution.

Hölscher’s research assistant, doctoral student, Radwanul Hasan Siddique, who discovered the effect, considers the object of their research a fascinating animal, and not just in terms of its transparent wings. “In contrast to other natural phenomena, where regularity is of top priority, the glasswing butterfly uses an apparent chaos to reach effects that are also fascinating for us humans,” he says.

The findings open up a range of applications where low-reflection surfaces are desirable - lenses or the displays of mobile phones, for example. Apart from the theoretical studies, KIT is also seeking practical implementation. These are at the conception stage at the moment, but prototype experiments have already revealed that this type of surface also has water-repellent and self-cleaning properties.

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