According to Science Daily, metamaterials have extraordinary characteristics when it comes to transferring and controlling waves, especially sound waves and light waves: for example, they can hide objects or increase the resolving power of the lens. Researchers at CNRS and the French National Institute of Advanced Chemical Physics at Bordeaux (CNRS/Bordeaux University/National Bordeaux Institute of Integrated Technology/France National Engineering Technology University) developed a combination of physical and chemical composition and microfluidic technology. The first three-dimensional metamaterial.
This is a new generation of soft metamaterials that are more easily plastic. In their experiment, the researchers realized that the ultrasonic oscillation moved backwards, and the energy carried by the acoustic wave moved forward. This study opens up new prospects, especially for high-resolution imaging. The study was published in the December 15 issue of Natural Materials.
The first three-dimensional metamaterial
Since the beginning of the 21st century, the international scientific community’s interest in metamaterials and their unparalleled characteristics has increased exponentially. A metamaterial is a medium in which the phase velocity of an acoustic or optical wave can be negative (ie, the material has a negative refractive index). In such a medium, the phase of the wave (continuous oscillation) and the direction of the energy carried by the wave are opposite. This feature has not been found in any natural homogeneous medium.
In order to obtain a metamaterial, it is necessary to obtain a uniform medium containing a large amount of inclusions (ie, microresonators). Conventional methods use micro-mechanical methods (etching, deposition, etc.) to machine one or two-dimensional carriers with metamaterial properties. However, this method cannot be applied to a micron-sized soft material required for ultrasonics, and currently obtained metamaterials are limited to one-dimensional or two-dimensional.
In the latest study, the researchers invented a fluid-phase metamaterial that was formed from porous silicon particles embedded in water-based gels. This extraordinary fluid is the first three-dimensional metamaterial that can operate at ultrasonic frequencies. In addition, because of its fluid properties, it can also be implemented using physicochemical processes and microfluidics, which are easier to implement than micromechanical methods.
One of the characteristics of porous media is that, compared to water (at a speed of 1,500 meters per second), sound waves pass through them at very slow speeds (more than ten meters per second). Due to this sharp contrast, as long as the droplet concentration is sufficient, the entire suspension has the characteristics of metamaterials. When researchers studied the propagation of ultrasonic waves through this medium, they measured the negative refractive index directly. In such an extraordinary fluid, the energy carried by the wave is transmitted from the transmitting end to the receiving end. This does not depart from the predicted one, but the oscillation seems to move backwards in the opposite direction, just as the dancers dance to the steps of the moonwalk. .
These results have opened up possibilities for a wide range of applications, from high-resolution ultrasound imaging to acoustic insulation and underwater secret acoustics. In addition, the soft-material physico-chemical techniques for making such metamaterials make it possible to manufacture fluids or flexible materials with modifiable shapes on the industrial scale.
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