Approaches to mimic the metallic sheen in beetles

A range of different beetles exhibits brilliant colours and metallic sheen. One of the most spectacular species is the Plusiotis resplendens from Central America with gold metal appearance. The beetle shells are made from chitin and have a number of unique properties that apart from spectacular aesthetic effects include metal sheen from non-metal surfaces combined with electric and thermal insulation. The reflection mechanism has been studied by a number of authors and is well understood. Basically there are 2 different reflection principles. One is the multilayer reflector where alternating layers have high and low refractive index. The other is the Bouligand structure where birefringent chiral nanofibres are organised in spiral structures. The paper describes work done to explore different approaches to mimic these structures using polymer based materials and production methods that are suitable for more complex double curved geometry. One approach is to use alternating layers of 2 different polymers applied by dipping and another is applying cholesteric liquid crystals in paint. However, none of them can yet make the desired metal-looking free-form surfaces.