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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]

While animals generally move through the aquatic environment by propelling themselves through the water or walking on submerged substrates, some have evolved the unique capacity to move along the water–air interface. This is because cohesive forces between water molecules cause the surface to be in tension, providing a physical substrate that can be used for support and pushed against for horizontal movement. Herein, we report on the use of the underside of the water's surface for locomotion in an unidentified Australian beetle (likely family: Hydrophilidae). Field observations revealed that the beetle was able to rest and move along the surface of an ephemeral pool inverted without penetrating the surface above. The beetle possessed a layer of trapped air along its abdomen and moved forward by walking, with deformations of the water's surface apparent each time a leg made contact. This appears to be the first visual evidence illustrating the ability of aquatic beetles to directly rest and walk on the underside of the water's surface. We propose that the air bubbles located on the abdomen and/or legs may be providing the upward force necessary for the beetle to be pushed against the underside of the water's surface. The water's surface seems to be acting as a support for the beetle, with the force needed for forward movement generated by the beetle's legs as each transition between the stance and swing phases of walking. Our work adds to the small number of observations of animals using the underside of the water's surface to move and highlights the likely importance of surface tension for locomotion on either side of the water–air interface. [ABSTRACT FROM AUTHOR]

Tracy et al. (2015, doi: ) assumed in their recent paper that water velocity at the water-air interface is zero in their pore-scale simulations of water flow in 3-D soil images acquired using X-ray computed tomography. We comment that such a treatment is physically wrong, and explain that it is the water-velocity gradient in the direction normal to the water-air interface, rather than the water velocity, that should be assumed to be zero at the water-air interface if one needs to decouple the water flow and the air flow. We analyze the potential errors caused by incorrectly taking water velocity at the water-air interface zero based on two simple examples, and conclude that it is not physically sound to make such a presumption because its associated errors are unpredictable. [ABSTRACT FROM AUTHOR]

Water interfaces provide the platform for many important biological, chemical, and physical processes. The water-air interface is the most common and simple aqueous interface and serves as a model system for water at a hydrophobic surface. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water at the water-vapor interface is essential for understanding the processes occurring on the water surface. At the water interface the network of very strong intermolecular interactions, hydrogen-bonds, is interrupted and the density of water is reduced. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen-bonded network and thus differ from those of bulk water. Herein, we discuss recent advances in the study of interfacial water at the water-air interface using laser-based surface-specific vibrational spectroscopy. [ABSTRACT FROM AUTHOR]

Measurements of the surface tension of aqueous solutions of ternary and binary surfactant mixtures of p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycols), i.e., ethoxylated tert-octylphenol with 8, 10 and 16 ethylene oxide groups, respectively called TOP8, TOP10 and TOP16, were carried out at 293 K. Based on the surface tension isotherms, the Gibbs surface excess of surfactants concentration at the water-air interface was determined and then recalculated for the Guggenheim-Adam surface excess concentration of these substrates. Next the monomer mole fractions of surfactants in the surface layer were determined and compared with those calculated on the basis of both Rosen's approach and the composition in the bulk phase. Taking into account these different values of composition of the mixed monolayer at the water-air interface, the molecular interaction parameters of surfactants, their activity coefficients and the free enthalpy of their mixing were also calculated. As follows from the measurements and calculations, among other things, by the addition of the differences between the values of the mole fraction of surfactants calculated on the basis of Rosen's approach and those determined on the basis of surface excess concentrations, the mole fraction of the solvent at the water-air interface can be determined. Graphical Abstract: Values of the monomer mole fractions of surfactants in the mixed monolayer calculated on the basis of the Rosen's approach and surface excess concentration determined from the Gibbs ( X( G)) and Guggenheim-Adam ( X( G − A)) equations.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]