When ultrasounds walk the Moonwalk
Metamaterials are artificial composite materials dedicated to the full control of wave propagation. Many potential applications are expected depending on their composition and structure: ultra-damping, cloaking, super focalization devices… Using fabrication techniques originating from soft matter science (formulation and microfluidics), a team from the Labex AMADEUS (I2M, LOF, CRPP) achieved the first 3D acoustic metamaterials with negative refractive index for which the wave energy and the phase propagate with opposite directions. This work has recently been accepted for publication in Nature Materials.
Since the beginning of the 2000s, there has been a growing interest for metamaterials and their extraordinary properties. In such materials, the refractive index (the physical quantity which governs the wave propagation) can become negative. In that case, the wave energy and the phase propagate in opposite directions. Following several spectacular experimental demonstrations of negative index metamaterials with electromagnetic waves, several smart technical realizations of 1D or 2D acoustic metamaterials have been achieved, mostly active in the audible frequency range. These solid metamaterials were obtained through micro-mechanical engineering, with typical frequency ranges around few kilohertz, limited by the minimal possible structuration length that can be achieved with such fabrication techniques.
Using an emulsion-templating approach and microfluidics, the Labex team did synthesize a new generation of fluid ultrasonic metamaterials, also referred to as metafluids. By studying the propagation properties of acoustic pulses in suspension of both dense and soft macroporous microbeads, the authors directly measured a negative acoustic refractive index at frequencies close to those used in echography techniques (the megahertz range). In such suspensions, while the wave energy does propagate forward from the emitter to the receptor as expected, the angular phase propagates backward reminding the famous “Moonwalk”.
These fundamental findings pave the way to numerous applications, especially in the field of high-resolution ultrasonic imaging techniques. Moreover, the used synthesis techniques, based on a soft-matter approach, allow the realization of fluid or soft materials with tunable shapes, which might be potentially scaled up for industrial purposes.