Characterization of the Phononic Landscape of Natural Nacre from Abalone Shells

Abstract Natural design and fabrication strategies have long served as a source of inspiration for novel materials with enhanced properties. Less investigated is the prospect of leveraging the complexity of readily available, naturally occurring micro‐/nanostructures as platforms for investigating functional materials. In the field of phononics, exploiting structural biocomposites is gaining traction; but finding natural phononic structures that interact with ultra‐ and hypersonic acoustic waves remains an open quest. In this context, the phononic behavior of natural Nacre, a biocomposite often looked at for inspiration due to its superlattice‐like architecture of alternating organic and inorganic phases, is here characterized. To such end, a combination of non‐contact pump‐probe laser ultrasonics techniques and Brillouin spectroscopy are employed to interrogate Nacre's hierarchical structure at the micro‐ and nanoscale and measure its phononic dispersion behavior in the MHz and GHz range. It is found that for wavelengths longer than the brick‐and‐mortar characteristic length, Nacre behaves as a dispersionless medium with effective transversely isotropic properties; but as the wavelengths become comparable to its structural periodicity an involved phononic spectrum arises which challenges the notion of a perfectly periodic, high mechanical‐contrast biocomposite.