Thomas Brunet

Lamb waves in phononic crystal slabs with square or rectangular symmetries

We report on both numerical and experimental results showing the occurrence of band gaps for Lamb waves propagating in phononic crystal plates. The structures are made of centered rectangular and square arrays of holes drilled in a silicon plate. A supercell plane wave expansion method is used to calculate the band structures and to predict the position and the magnitude of the gaps. The band structures of phononic crystal slabs are then measured using a laser ultrasonic technique. Lamb waves in the megahertz range and with wave vectors ranging over more than the first two reduced Brillouin zones are investigated.

Experimental observation of azimuthal shock waves on nonlinear acoustical vortices

Thanks to a new focused array of piezoelectric transducers, experimental results are reported here to evidence helical acoustical shock waves resulting from the nonlinear propagation of acoustical vortices (AVs). These shock waves have a three-dimensional spiral shape, from which both the longitudinal and azimuthal components are studied. The inverse filter technique used to synthesize AVs allows various parameters to be varied, especially the topological charge which is the key parameter describing screw dislocations. Firstly, an analysis of the longitudinal modes in the frequency domain reveals a wide cascade of harmonics (up to the 60th order) leading to the formation of the shock waves. Then, an original measurement in the transverse plane exhibits azimuthal behaviour which has never been observed until now for acoustical shock waves. Finally, these new experimental results suggest interesting potential applications of nonlinear effects in terms of acoustics spanners in order to manipulate small objects.

Soft porous silicone rubbers with ultra-low sound speeds in acoustic metamaterials

Soft porous silicone rubbers are demonstrated to exhibit extremely low sound speeds of tens of m/s for these dense materials, even for low porosities of the order of a few percent. Our ultrasonic experiments show a sudden drop of the longitudinal sound speed with the porosity, while the transverse sound speed remains constant. For such porous elastomeric materials, we propose simple analytical expressions for these two sound speeds, derived in the framework of Kuster and Toksöz, revealing an excellent agreement between the theoretical predictions and the experimental results for both longitudinal and shear waves. Acoustic attenuation measurements also complete the characterization of these soft porous materials.

Negative refraction experiments in soft 3D metafluids

Physics of negative refraction has been intensively studied since the 2000s. Negative refraction is usually evidenced by a Snell’s law experiment using a prism shaped negative-index metamaterial wedge. The first experimental verification of negative index of refraction was reported in 2D resonant structures at microwave frequencies [1]. A few years later, negative refraction was demonstrated in 2D and 3D resonant optical metamaterials [2]. In acoustics, the first experimental demonstration of negative refraction was reported in 3D (non resonant) phononic crystals at ultrasonic frequencies [3]. However, 3D acoustic (random) metamaterials should also offer the possibility to explore this exotic phenomenon since the first 3D locally resonant metamaterial with a negative index has been recently demonstrated [4]. In this talk, we will report on negative refraction experiments performed in these soft 3D metafluids composed of macro-porous micro-beads randomly dispersed in a water-based gel-matrix. Negative refr...

Numerical and experimental observations of azimutal shock waves

Acoustical Vortices (AV) are acoustical beams with a phase singularity of screw type. They possess a phase with an helical structure which is winding around the axis of propagation. These twists of phase engender special properties such as the existence of an associated angular momentum and imply a coupling between the axial and transverse components of the beam. Propagation of AV of finite amplitude follows the classic rule of nonlinear acoustics. Beyond a shock formation distance, AV develop shocks, but the structure of the latters is noticeable. It will be shown by numerical simulations and experimental measurements, that the nonlinear propagation of AV gives birth to 3D shock waves: a classical shock in the direction of propagation plus an azimutal one in the transverse plane. The numerical simulation, based on an original algorithm solving the 3D Khokhlov‐Zabolotskaya equation, is used to investigate the dynamic of the formation of the azimutal shock. Experimental observations made with ultrasonics i...

Multiple scattering in resonant emulsions: Coherent-ballistic propagation and diffusive transport

Ultrasonic pulse propagation experiments are reported on dilute suspensions of fluorinated-oil droplets immersed in a water-based gel matrix. These resonant emulsions are model systems for studying the effects of scattering resonances on wave transport since the large sound-speed contrast between the scatterers and the surrounding medium enhances the Mie resonances of the liquid particles. Measurements of the coherent-ballistic component reveal that both the scattering mean free path and the group velocity strongly depend on the frequency as predicted by the Independent Scattering Approximation. Scattering resonances are also responsible for very slow diffusivity of the multiply scattered ultrasound. This slowing down of the diffusion process due to resonances is well captured by models that include additional scattering delays of the ultrasonic pulses. The relationship between the diffusion coefficient and the ballistic data allow the frequency dependence of energy velocity of diffusing waves to be estim...

Soft porous materials with ultra-low sound speeds in acoustic metamaterials

Porous media have unique acoustic properties that have been intensively studied for many decades and found many applications in various areas. Owing to their low sound speed, porous materials have proven to be interesting key elements for the realization of acoustic metamaterials since they may act as strong Mie-type resonators, when shaped as spherical particles [1]. In that context, soft porous silicone rubbers are ideal “ultra-slow” materials since a sudden drop of the longitudinal sound speed cL with the porosity Φ has been observed in these porous materials (cL < 100 m/s for Φ < 10%). Such unusual behavior is well captured by simple models based on low-frequency approximations of multiple scattering theories [2]. Then, I will discuss the interest of these “ultra-slow” particles for acoustic metamaterials of which the acoustic index has been shown to be negative [3]. At last, I will demonstrate that the exotic values of the acoustic index may be tuned by using other porous particles such as xerogel be...

Diffusive transport and Anderson localization of ultrasonic waves in strongly scattering inhomogeneous media

In inhomogeneous media with constituents having very different acoustic properties, very strong multiple scattering of ultrasonic waves can occur, especially when the wavelength is comparable with the length scales over which the constituent properties vary. Such strong multiple scattering can lead to a long “coda” that dominates the observable behaviour in pulsed experiments, and can dwarf the ballistic pulse that travels coherently through the medium. In many cases, the transport of energy by the multiply scattered waves can be well described using the diffusion approximation, which may even seem quite surprising since all interference effects are ignored. An exception occurs when the return probability that the waves scatter back to the same spot becomes enhanced as a result of very strong multiple scattering; then interference plays an important role and can ultimately lead to Anderson localization and the breakdown of wave propagation. Examples of these wave phenomena will be presented in contrasting...

Acoustic metafluids with multiple negative-index bands

The extraordinary properties of acoustic (random) metamaterials, such as negative refractive index, originate from low frequency resonances of sub-wavelength particles. While most of these functional materials are fabricated by mechanical engineering, we have recently shown that soft matter techniques coupled with microfluidics open a new synthesis route for acoustic metamaterials especially for ultrasonics [1]. As a demonstration, we have achieved 3D-bulk acoustic metafluids with an alternatively low, zero, and negative index by producing large amounts of calibrated soft porous micro-spheres, acting like strong Mie resonators [2]. The wide variety of physico-chemical processes offered by chemical engineering allows for the tuning of the resonant particle properties over a broad range of mechanical/acoustical parameters. In this talk we show that, according to a fine control of the Poisson coefficient of the macro-porous resonators, it is not only possible to achieve soft acoustic metamaterial with one ne...

Experimental measurements of the band structure of Lamb waves in phononic lattices

We have measured the band structure of Lamb waves propagating in 2D phononic crystals. Different cases were investigated. First we have studied the phononic film deposited on a homogeneous substrate. The phononic film is made of cylindrical iron scatterers embedded into a copper background; the substrate is a 700 μm silicon plate. The propagation is along the crystallographic direction [100] of Si. At low filling fraction, a frequency band gap for the antisymmetric mode arises at reduced Brillouin edge. At high filling fraction, a band gap opens also on the symmetric branch. We have also studied the case of phononic slabs with very high contrast between the scatterers and the matrix. The samples are made of 200 μm silicon plates with air holes lattices drilled throughout. Whatever the symmetry of the lattice, square of centered rectangular, broad band gaps open at first and second reduced Brillouin zone edges. Experimental data are then compared to theoretical predictions obtained using a plane wave expan...