Flexible Waterborne Acoustic Metasurface With Tunable Functionality

Developing underwater acoustic devices with flexibility in both shape and functionality is critical for applications in sonar, tracking, and communication, where adaptation to complex environments is required. Among such devices, acoustic metasurfaces have attracted significant attention for their exceptional ability to manipulate acoustic wave propagation. However, most existing metasurfaces are predesigned for specific flat or curved geometries and lack the flexibility to adapt to diverse shapes. Moreover, the strong coupling between acoustic and elastic waves in solid–water systems tightly links device functionality to its shape and deformation, posing major challenges for reconfigurability. Here, we present a design strategy for flexible waterborne acoustic metasurfaces that combine conformability with tunable acoustic-path control. The metasurface comprises Helmholtz resonant unit cells interconnected by soft hydrogel materials. The low stiffness of the hydrogel allows the metasurface to deform freely without inducing mechanical strains in the resonant unit cells. In addition, the hydrogel's low shear modulus suppresses nonlocal acoustic–solid coupling, enabling a discrete analytical design approach. By exploiting local acoustic–solid interactions, each unit cell achieves dual control of phase and amplitude across a broad frequency range. Furthermore, introducing symmetric sliders into the unit cells imparts tunable acoustic functions. The resulting flexible metasurface supports multiple functionalities—including acoustic illusion, wideband diffuse reflection, conversion of propagating waves into surface waves, and acoustic cloaking—demonstrated through simulations and experiments. Our work provides a new design strategy for multifunctional underwater acoustic manipulation by integrating mechanical flexibility with controlled acoustic–solid coupling.