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.