We have fabricated sonic crystals, based on the idea of localized resonant structures, that exhibit spectral gaps with a lattice constant two orders of magnitude smaller than the relevant wavelength. Disordered composites made from such localized resonant structures behave as a material with effective negative elastic constants and a total wave reflector within certain tunable sonic frequency ranges. A 2-centimeter slab of this composite material is shown to break the conventional mass-density law of sound transmission by one or more orders of magnitude at 400 hertz.

The geometric-phase concept has far-reaching implications in many branches of physics 1-14. The geometric phase that specifically characterizes the topological property of bulk bands in one-dimensional periodic systems is known as the Zak phase 15,16. Recently, it has been found that topological notions can also characterize the topological phase of mechanical isostatic lattices 13. Here, we present a theoretical framework and two experimental methods to determine the Zak phase in a periodic acoustic system. We constructed a phononic crystal with a topological transition point in the acoustic band structure where the band inverts and the Zak phase in the bulk band changes following a shift in system parameters. As a consequence, the topological characteristics of the bandgap change and interface states form at the boundary separating two phononic crystals having di erent bandgap topological characteristics. Such acoustic interface states with large sound intensity enhancement are observed at the phononic crystal interfaces. We use a simple photonic crystal (PC) system to demonstrate geometric phase (GP) effects and the existence of topological transition points in acoustic systems. The experimental setup is shown in Fig. 1a. The PC is a cylindrical waveguide with periodically alternating cross-sectional areas. Each unit cell has two wider tubes (tube-A) of length (1/2)d A and inner radius r A = 2.4 cm, sandwiching a narrower tube-B of length d B and inner radius r B = 1.5 cm. The tubes are filled with air (mass density ρ = 1.3 kg m −3 , speed of sound v = 343 m s −1) and made of hard plastics to ensure that the inner surfaces meet the sound hardboundary condition. An example of a unit cell, with d A = 3.0 cm and d B = 5.5 cm (we refer to this configuration as 'S1'), is shown in Fig. 1b-d, together with simulated pressure eigenfunctions of the lowest three eigenmodes at k = 0. Their corresponding eigenfrequencies are marked by red dots in Fig. 1e, which shows the acoustic band structure 17-19. It is clear that the lower two modes have their pressure gradient along the propagation direction k (Fig. 1b,c) and negligible pressure variation along the cross-sectional directions. These two modes represent the two band-edge states of the second bandgap between the second and third bands, as shown by black curves in Fig. 1e. The mode shown in Fig. 1d has a clear pressure variation in the cross-sectional planes. The dispersion of this mode is marked by the green line in Fig. 1e. For symmetry reasons, this mode cannot be excited with a plane wave, and is therefore not considered in the following discussions. The two longitudinal modes can be further labelled as even (Fig. 1b) and odd (Fig. 1c) with respect to the centre of tube-A at k = 0. The eigenfrequencies of these two band-edge states

Transmission electron microscopic observation showed that TiO2 nanotubes synthesized via a simple hydrothermal chemical process formed a crystalline structure with open-ended and multiwall morphologies. Unlike multiwalled carbon nanotubes, the TiO2 nanotube walls were not seamless. During alkali treatment, crystalline TiO2 raw material underwent delamination in the alkali solution to produce single-layer TiO2 sheets. TiO2 nanotubes were formed by rolling up the single-layer TiO2 sheets with a rolling-up vector of [001] and attracting other sheets to surround the tubes.

We show experimentally that thin membrane-type acoustic metamaterials can serve as a total reflection nodal surface at certain frequencies. The small decay length of the evanescent waves at these frequencies implies that several membrane panels can be stacked to achieve broad-frequency effectiveness. We report the realization of acoustic metamaterial panels with thickness ≤15 mm and weight ≤3 kg/m2 demonstrating 19.5 dB of internal sound transmission loss (STL) at around 200 Hz, and stacked panels with thickness ≤60 mm and weight ≤15 kg/m2 demonstrating an average STL of >40 dB over a broad range from 50 to 1000 Hz.

Structures of water molecules at water/silica interfaces, in the presence of alkali chloride, were investigated using infrared-visible sum frequency vibrational spectroscopy. Significant perturbations of the interfacial water structure were observed on silica surfaces with the NaCl concentration as low as 1 × 10 -4 M. The cations, which interact with the silica surface via electrostatic interaction, play key roles in perturbing the hydrogen-bond network of water molecules at the water/silica interface. This cation effect becomes saturated at concentrations around 10 -2 to 10 -1 M, where the sum frequency generation peaks at 3200 and 3400 cm -1 decrease by 75%. Different alkali cation species (Li + , Na + , and K + ) produce different magnitudes of perturbation, with K + > Li + > Na + . This order can be explained by considering the effective ionic radii of the hydrated cations and the electrostatic interactions between the hydrated cations and silica surfaces. The interfacial water structure associated with the 3200 cm -1 band is more vulnerable to the cation perturbation, suggesting that the more ordered water structure on silica is likely associated with the vincinal silanol groups, which create a higher local surface electrical field on silica.

No abstract

We demonstrate a class of sonic shield materials based on the principle of locally resonant (LR) microstructures. Each local resonator is found to vibrate almost like an independent unit, and two layers of such resonators can even be regarded as a sonic crystal. By combining several LR layers of different resonant frequencies, a broadband (200–500 Hz) sound shield, with an average transmission intensity 11 dB lower than that dictated by mass density law, has been achieved.

No abstract