A dynamic nanoindentation technique to investigate the nanomechanical properties of a colored beetle

Abstract

Special internal structures of the cuticle of a beetle have multifunctionality including self-protection and attraction of mates using structural coloration, and being lightweight with high strength, which can protect the body and membranous hindwings. Special optical properties were found in the black spots region (BSR) of the cuticle of the multicolored Asian lady beetle (Harmonia axyridis). Both the BSR and the orange region (OR) have alternating layers of chitin and melanoprotein, as detected using field emission scanning electron microscopy (FESEM). However, a parallel wavy line structure was found in the BSR via laser scanning confocal microscopy (LSCM). This special structural color may arise via a diffraction grating mechanism. To explore the relationship between the material and its optical properties, the dynamic nanoindentation approach (nano-DMA) was used. The viscoelastic properties of the cuticle were assessed including the storage modulus (E′), loss modulus (E′′) and loss tangent (tan δ), in the differently colored zones. The extent of protein cross-linking affects the cuticle's mechanical properties. We then demonstrated the applicability of a power-law frequency dependence analysis of E′ and tan δ. Furthermore, the frequency exponent of n and tan δ were discussed in relation to the BSR and OR. A lower wavelength of maximum reflectance was found in the BSR that had little frequency dependence on n and a larger tan δ value related to its extent of protein cross-linking. This is contrary to the results obtained in the OR. The results will help in designing lightweight, high strength, and color-changing micro air vehicles (MAVs).