Composite Pentamode Metamaterials for Enhanced Wave Manipulation by Asymmetric Surface Structure

The pentamode materials (PMs) are metamaterials that connect macroscopic mass blocks through submicroscopic beams. This macro–micro interlaced design gives the PMs a vanishing shear modulus and easily customized elastic tensors, providing them with unique advantages in wave manipulation. However, PMs tend to produce additional scattering that severely disturbs the wavefront in the underwater wave manipulation. Traditional optimization methods generally come at the expense of decreased pressure resistance. In this work, it is found that the scattering is caused by surface bending waves which result from asymmetric loading conditions at the water–PM interface. These surface bending waves alter the fluid‐like properties of the PM and reduce its equivalent bulk modulus. A method is developed to quantify the influence of surface bending waves by strain energy. Based on this method, an asymmetric surface structure is designed to reduce the surface bending waves and achieve the target acoustic equivalent parameter. Simulations and underwater experiments demonstrate that the asymmetric PM metasurface can reduce the scattering from 26.3 to 0.96 dB and multiply the frequency range of wave manipulation. This approach results in a 95% improvement in compressive strength compared to structures with the same acoustic performance, enabling wider deepwater applications.