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.

http://science.sciencemag.org/content/sci/289/5485/1734.full.pdf

Locally Resonant Sonic Materials

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

저작근 근전도에 관한 정상교합자와 ii급 부정교합자의 비교 연구

The present study demonstrates the creation of a stable, superhydrophobic surface using the nanoscale roughness inherent in a vertically aligned carbon nanotube forest together with a thin, conformal hydrophobic poly(tetrafluoroethylene) (PTFE) coating on the surface of the nanotubes. Superhydrophobicity is achieved down to the microscopic level where essentially spherical, micrometer-sized water droplets can be suspended on top of the nanotube forest.

/pdf/superhydrophobic-carbon-nanotube-forests-4wzbqm5f24.pdf

Superhydrophobic Carbon Nanotube Forests

The emergence of artificially designed subwavelength electromagnetic materials, denoted metamaterials, has significantly broadened the range of material responses found in nature. However, the acoustic analogue to electromagnetic metamaterials has, so far, not been investigated. We report a new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance. These materials have an effective dynamic modulus with negative values near the resonance frequency. As a result, these ultrasonic metamaterials can convey acoustic waves with a group velocity antiparallel to phase velocity, as observed experimentally. On the basis of homogenized-media theory, we calculated the dispersion and transmission, which agrees well with experiments near 30 kHz. As the negative dynamic modulus leads to a richness of surface states with very large wavevectors, this new class of acoustic metamaterials may offer interesting applications, such as acoustic negative refraction and superlensing below the diffraction limit.

Ultrasonic metamaterials with negative modulus

It is well known that the acoustic properties of fluid are characterized by mass density and bulk modulus. Metafluids, the fluid metamaterials, generalize the natural fluid, which can accommodate extreme and/or negative values of these two parameters. Here, we further show that the metafluids, composed of periodic thin-walled hollow cylinders immersed in fluid, can provide not only the designable effective mass density and bulk modulus, but also a completely new effective parameter, which appears in the wave velocities as a role similar to the shear modulus of solid. The new effective parameter, describing the response of the fluid to the quadrupolar component of waves, is obtained by generalizing the effective medium theory (EMT) to include the second-order effects, which is vanishing and neglected in the conventional EMT, but giant here in the metafluids with built-in quadrupolar resonances. With the discovery of the metafluids of shearlike moduli, our Letter extends the concept of metafluids and will have a great significance in the field of metamaterials.

Metafluids beyond the Bulk Modulus.

In this work, we study the acoustically mediated interaction forces among multiple well-separated spherical particles trapped in the same node or antinode plane of a standing wave. An analytical expression of the acoustic interaction force is derived, which is accurate even for the particles beyond the Rayleigh limit. Interestingly, the multi-particle system can be decomposed into a series of independent two-particle systems described by pairwise interactions. Each pairwise interaction is a long-range interaction, as characterized by a soft oscillatory attenuation (at the power exponent of n=-1 or -2). The vector additivity of the acoustic interaction force, which is not well expected considering the nonlinear nature of the acoustic radiation force, is greatly useful for exploring a system consisting of a large number of particles. The capability of self-organizing a big particle cluster can be anticipated through such acoustically controllable long-range interaction.

Acoustically mediated long-range interaction among multiple spherical particles exposed to a plane standing wave

We report on the experimental investigation of the transmission spectra for acoustic waves through temporal phononic crystals (TPCs), which are designed structures with periodically time-varying density and bulk modulus. In our experiment, the TPCs are instanced as the media that swap between air and carbon dioxide repetitively, in between the sound source and the detector. The transmission spectra for monochromatic incident acoustic waves exhibit a series of equidistant peaks, manifesting novel up- and down-conversion effects while not employing material nonlinearity. These experimental results agree with the theoretical calculations based on the plane wave expansion in time domain.

Zhengyou Liu

Papers

Metafluids beyond the Bulk Modulus.

Acoustically mediated long-range interaction among multiple spherical particles exposed to a plane standing wave

Experimental demonstration of the acoustic frequency conversions by temporal phononic crystals

Mechanical properties of an ideal electrorheological fluid

Conductivity effects in electric-field-induced electrorheological solid