4 pagesInternational audienceWe 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

Spherical silica xerogels are efficient acoustic Mie resonators. When these sub-wavelength inclusions are dispersed in a matrix, the final metafluid may display a negative acoustic refractive index upon a set of precise constraints concerning material properties, concentration, size, and dispersity of the inclusions. Because xerogels may sustain both pressure and shear waves, several bands with negative index can be tailored.

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.