Hydrogel-based flexible metamaterial for underwater acoustic carpet cloaking

• Proposal for a novel hydrogel-based flexible acoustic metamaterial. • Elucidation of the transverse wave attenuation mechanism in the proposed acoustic metamaterial. • Successful demonstration of the underwater acoustic carpet cloak. Hydrogels uniquely combine near-water acoustic impedance with mechanical load-bearing capacity, displaying significant advantages in underwater acoustics. Through composite material design, hydrogel-based flexible acoustic metamaterials can be developed to achieve diverse underwater wave manipulation. For example, underwater acoustic carpet cloaks require sophisticated wave-path modulation, typically implemented through fluid–solid coupled laminate microstructures. While complete fluid–solid phase isolation eliminates shear wave interference, it inevitably undermines the structural independence of the carpet system. To resolve this trade-off, we present a novel hydrogel-based flexible acoustic metamaterial and its application in the design of the underwater carpet cloak. To elucidate the underlying mechanism of the proposed microstructure, the effects of anisotropic stiffness and anisotropic density are investigated. It is revealed that the efficacy of our novel design arises from suppressed shear wave propagation and an acoustic-solid coupling mode dominated by a single longitudinal wave. Finite element simulations and underwater experiments demonstrated the excellent cloaking performance of this acoustic carpet cloak, which utilizes the novel hydrogel-based flexible acoustic metamaterial. This novel approach expands the application potential of hydrogels in complex underwater acoustic wave-control structures.