Physico-chemical engineering of ultrasonic acoustic metafluids

The field of metamaterials is no longer entirely new, although it remains extraordinarily active. First imagined in the 1960s by V. Veselago, who questioned the possibility of obtaining a negative refractive index for the propagation of an electromagnetic wave, the concept became a reality in the early 2000s thanks to J. Pendry and D. Smith, who designed and produced a specific structure for which the refractive index of the material is negative in a small frequency range in the microwave domain. The key to this development lies in the introduction of micro-resonators, which are very small compared to the length of the incident wave, so that the wave perceives a medium that appears homogeneous (so-called effective), but whose constituents enter into resonance at a certain frequency. Pendry and Smith used millimetre-scale RLC circuits fabricated on conventional printed circuit boards, opening up a phenomenal field of conceptual, technological and experimental study. Since then, the race for miniaturization has led to the realization of metamaterials right up to the field of visible optics. A few years later, the concept of metamaterials was generalized to other propagative phenomena. Whether we're talking about acoustic, seismic, thermal or marine phenomena, it's possible to envisage materials that considerably modify the way a "wave" propagates through them, and as a corollary, promising structures: perfect absorbers, flat, thin and aberration-free lenses, optical or acoustic components to overcome diffraction limits and shape wave fronts, or even camouflage structures to protect offshore platforms from tsunamis, for example.

The scope of this article is deliberately very limited: a few examples of metamaterial applications are given in the field of ultrasonic acoustics in aqueous media. The principle is based on the seeding of a matrix with inclusions of small size relative to the incident ultrasonic wavelength. These inclusions are able to resonate "strongly" when their sound celerity is much lower than that of the surrounding matrix; these Mie-type resonances modify the ultrasound phase velocity, and therefore the acoustic refractive index, which can potentially be negative.

Proof-of-concept was first provided by dispersing drops of a so-called slow oil in an aqueous gel. The microfluidic tool ensures that the inclusions are highly calibrated, and the oil is chosen with a lower sound velocity than in the aqueous gel, so that Mie-type resonances are very pronounced. As a result, the drops resonate in a very piquant way at particular frequencies corresponding to the resonances of the different modes of vibration of the drops. The most interesting materials were then produced by increasing the sound celerity contrast between matrix and inclusion, using microfluidic...