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Bidirectional controllable propagation of waterborne sound holds significant importance in developing multifunctional underwater acoustic and mechanical devices. However, the existing waterborne acoustic metasurfaces have rarely explored the bidirectional sound modulations. Here, a class of Janus waterborne acoustic metasurface, enabling two‐faced arbitrarily asymmetric wavefront manipulations is reported. A three‐degree‐of‐freedom mechanical system facilitated by acoustic‐structure interaction underwater is proposed to introduce bianisotropic responses of unit cells. Monolayer ultrathin Janus metasurface is inversely designed by utilizing a function‐structure integrated topology optimization framework. Distinct underwater acoustic functionalities, including axial and oblique focusing, beam splitting, and sound diffusion, are successfully demonstrated. Underwater experiments are further conducted to validate the concept of Janus metasurface. The good consistency between experimental and simulated results confirms the excellent two‐faced asymmetric acoustic focusing performance. The proposed Janus metasurface opens up a new dimension for designing advanced waterborne acoustic devices with two‐faced multifunctional wavefront manipulations. [ABSTRACT FROM AUTHOR]

Exceptional point (EP), a special degeneracy in non‐Hermitian systems, has exhibited various distinctive wave characteristics. However, the conventional synthesis of scattering EPs in acoustics is confined to rather limited methods for breaking Hermiticity, typically requiring intrinsic losses or open interfaces. Here, the concept of synthesizing scattering EPs is theoretically and experimentally demonstrated by leveraging a metasurface with fluid‐solid interaction (FSI) in water. The incorporation of FSI offers a novel mechanism to customize natural radiation losses. Simulations and experiments consistently confirm that synthesized EPs result in extremely asymmetric scattering patterns, even in the case of spatially impenetrable metasurfaces. This enhanced spatial symmetry breaking benefits from the FSI on the interface and the nonlocal interaction between unit cells. The proposed non‐Hermitian framework, involving the interplay of sound in fluids and solids, is expected to open up new possibilities for exploring unique non‐Hermitian physics and underwater acoustic applications. [ABSTRACT FROM AUTHOR]

Extreme impedance mismatch causes sound insulation at water–air interfaces, limiting numerous cross‐media applications such as ocean‐air wireless acoustic communication. Although quarter‐wave impedance transformers can improve transmission, they are not readily available for acoustics and are restricted by the fixed phase shift at full transmission. Here, this limitation is broken through impedance‐matched hybrid metasurfaces assisted by topology optimization. Sound transmission enhancement and phase modulation across the water–air interface are achieved independently. Compared to the bare water–air interface, it is experimentally observed that the average transmitted amplitude through an impedance‐matched metasurface at the peak frequency is enhanced by ≈25.9 dB, close to the limit of the perfect transmission 30 dB. And nearly 42 dB amplitude enhancement is measured by the hybrid metasurfaces with axial focusing function. Various customized vortex beams are experimentally realized to promote applications in ocean‐air communication. The physical mechanisms of sound transmission enhancement for broadband and wide‐angle incidences are revealed. The proposed concept has potential applications in efficient transmission and free communication across dissimilar media. [ABSTRACT FROM AUTHOR]

Potassium perfluorooctane sulfonate (KPFOS) and sodium perfluorooctane sulfonate (NaPFOS) exhibit poor surface activities in aqueous solution at room temperature because of their high Krafft points. In this work, we attempted to increase the solubility of KPFOS and NaPFOS and consequently improve their surface activities at room temperature with sodium p-methylbenzene sulfonate (BS) and urea, which are typical hydrotropes in industrial applications. The effects of BS and urea on the surface tension of the aqueous solutions of KPFOS and NaPFOS were investigated at 25 °C. When the hydrotropes were added, the effectiveness of KPFOS and NaPFOS in surface tension reduction was greatly enhanced and KPFOS showed higher efficiency in surface tension reduction than NaPFOS. On the other hand, BS had much stronger ability than urea to reduce the surface tension of KPFOS and NaPFOS in water. In particular, with the assistance of BS the minimum surface tension of KPFOS approached 19 mN/m at 25 °C. It was worth noting that in the presence of BS, the surface tension of an apparently 'saturated' solution (i.e., with coexisting surfactant solid) continuously decreased with increasing surfactant concentration. This behavior was ascribed to enrichment of branched PFOS isomers in aqueous phase with the assistance of BS, as evidenced by high-resolution F NMR. Hydrotropes were able to recover the inherent character of KPFOS and NaPFOS as highly surface-active fluorinated surfactants by increasing the solubility of branched isomers. This is an easy way to enhance the effectiveness in surface tension reduction at room temperature for fluorinated surfactants with high Krafft points. [ABSTRACT FROM AUTHOR]