Metamaterials have long held promise for extraordinary properties when it comes to diverting and controlling waves, especially sound and light: for instance, at the right optical frequency, they can make an object invisible, or increase the resolving power of a lens.

A new project has created three-dimensional metamaterials by combining physico-chemical formulation and microfluidics technology, making soft metamaterials that are easier to shape. In their experiment, the researchers got ultrasonic oscillations to move backwards while the energy carried by the wave moved forwards.

To obtain a metamaterial, it is necessary to make a heterogeneous medium that contains a large number of inclusions (known as microresonators). The usual way is to use micromechanical methods (etching, deposition, etc) to machine solid supports that will have the properties of metamaterials in one or two dimensions. However, this method cannot be used to work with soft matter at the micrometer scales required for ultrasounds, and the materials obtained remain limited to one or two dimensions.


Scanning electron microscope image of porous microbeads with diameters of approximately 300 µm.© CRPP

In this study, the researchers developed a new type of metamaterial, in the fluid phase, formed of porous silicone microbeads embedded in a water-based gel. This metafluid is the first three-dimensional metamaterial to work at ultrasonic frequencies. In addition, due to its fluid nature, it can be made using physico-chemical processes and microfluidics technologies, which are much easier to implement than micromechanical methods.

One of the properties of porous media is that sound travels through them at very low speed (a few tens of meters per second) compared to water (1500 meters per second). Due to this sharp contrast, the whole suspension has the properties of a metamaterial provided the bead concentration is sufficient: when the researchers studied the propagation of ultrasonic waves through this medium, they directly measured a negative refractive index.

Within such a metafluid, the energy carried by the wave travels from the emitter to the receiver, as expected, whereas the oscillations appear to move backwards in the opposite direction, rather like a dancer doing the 'moonwalk'.

These results open the way to numerous applications ranging from high-resolution ultrasound imaging to sound insulation and stealth in underwater acoustics. In addition, the soft-matter physico-chemical techniques used to make this metamaterial makes it possible to produce fluid or flexible materials with adaptable shapes, potentially at the industrial scale.

Soft 3D acoustic metamaterial with negative index, Thomas Brunet, Aurore Merlin, Benoit Mascaro, Kevin Zimny, Jacques Leng, Olivier Poncelet, Christophe Aristégui, Olivier Mondain-Monval. Nature Materials, 15 December 2014. DOI: 10.1038/nmat4164. Source: CNRS (Délégation Paris Michel-Ange)