Underwater invisibility: pushing pentamode cloaks into 3D
Sebastiano Cominelli · 2025
In this talk, I will present the key developments of my PhD research on the design and implementation of 3D underwater acoustic cloaks using pentamode metamaterials (PMs). These engineered structures mimic the acoustic properties of fluids, exhibiting a high bulk modulus and nearly vanishing shear resistance. While PM-based cloaking has been successfully demonstrated in two dimensions, extending these concepts to realistic 3D applications poses significant challenges. The high bulk modulus of water, in particular, requires a fundamentally different microstructural kinematics. I will first present a novel 3D PM geometry that closely mimics the acoustic behaviour of water and serves as the basic building block for the cloak. The second part of the talk will focus on the critical challenge of wrapping a curved 3D surface—such as a sphere—with a cloak. Unlike 2D scenarios, there is no feasible conformal mapping in three dimensions, resulting in unavoidable geometric distortions. I will discuss our strategies to accommodate these deformations and their effect on the local material behaviour. Finally, I will describe the discretisation of the cloak into a lattice of unit cells and the complexities of wrapping a 3D object. Emphasis will be placed on maintaining lattice continuity and ensuring reliable inter-cell connectivity—key requirements for the transition from theory to experiment.