Benoit Mascaro

Sharp acoustic multipolar-resonances in highly monodisperse emulsions

We report the achievement of highly monodisperse emulsions exhibiting about ten acoustic Mie resonances. Thanks to robotics, the effective acoustic properties of such strongly scattering media can be precisely targeted by means of the production of calibrated (random) liquid-droplets. Ultrasonic experiments are compared, with an excellent quantitative agreement, to theoretical predictions derived within the framework of the independent scattering approximation. The dependence of the sound speed and of the acoustic attenuation on both the size and the volume fraction of droplets is quantitatively examined for dilute and more concentrated emulsions, and is presented in a dimensionless way.

Impact of polydispersity on multipolar resonant scattering in emulsions

The influence of size polydispersity on the resonant acoustic properties of dilute emulsions, made of fluorinated-oil droplets, is quantitatively investigated. Ultrasound attenuation and dispersion measurements on various samples with controlled size polydispersities, ranging from 1% to 13%, are found to be in excellent agreement with predictions based on the independent scattering approximation. By relating the particle-size distribution of the synthesized emulsions to the quality factor of the predicted multipolar resonances, the number of observable acoustic resonances is shown to be imposed by the sample polydispersity. These results are briefly discussed into the context of metamaterials for which scattering resonances are central to their effective properties.

Dual-band negative index ultrasonic metafluids

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 [1]. As a demonstration, we have achieved soft 3D ultrasonic metafluids with negative index by producing large amounts of calibrated soft porous microspheres, acting like strong Mie resonators [2]. The wide variety of physico-chemical processes offered by chemical engineering allows for the full-control of the mechanical/acoustical parameters (elasticities/celerities) of these resonant micro-particles. Therefore, the number and the characteristics (width, depth) of the “negative bands” can be precisely tuned over a broad range. As an example, we have recently demonstrated that it is not only possible to achieve soft 3D ultrasonic metafluids with one negative band [2], but also with two separate ones [3]. In this talk, we show that the emergence of the second negative band is due to shear waves that propagate within the resonant micro-beads. While they are often neglected in theoretical works, shear wave may induce a dipolar transverse resonance that leads to a negative index when it overlaps the monopolar (longitudinal) resonance. We demonstrate that the Poisson coefficient, which parametrizes the ratio of transverse-to-longitudinal sound celerities, is a relevant mechanical parameter to anticipate whether or not the second negative band will emerge.