Air-gap-enhanced pearlescent effect in periodic stratified bilayers of Perna viridis shell

Pearlescent effect is a unique optical phenomenon originated by an interference of visible light with biomaterial having periodic structure such as pearls, mother-of-pearls, mollusk shells, beetle wings, and butterfly wings. The pearlescent effect exhibits series of vivid colors at a particular spot as an angle of observation is changed. In this paper, the pearlescent effect in Asian green mussel (Perna viridis) shell was investigated. The nacreous layer of the shells is an assembly of stratified bilayers of alternate aragonite calcium carbonate and organic matrix. A consecutive chemical/thermal treatment was employed to remove the organic layer. The enhanced pearlescent effect, indicated by more vivid and brighter colors, was due to a greater refractive index difference within the treated structure as the organic layers were replaced by air gaps of the same thickness. The modified transfer matrix method developed for the stratified bilayers system was employed to verify enhanced pearlescent effect. The results from theoretical simulation corroborated the visual observation as a 4-time full-width-at-half-maximum increment of the reflectance peak, and a broader spectral coverage was achieved after the replacement of organic layers by air gaps.