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Faceted bubbles for new acoustic materials

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Air bubbles in water are excellent resonators under acoustic frequencies. They are an avenue for new underwater acoustic materials, provided they are stabilized over time and their spatial position is controlled. Using 3D printed supports, physicists from Grenoble have demonstrated that it is possible to generate "faceted bubbles", which are more durable and efficient than conventional spherical bubbles.

When an air bubble in water receives a sound wave of a very specific frequency - the Minnaert frequency - it enters into resonance and starts to oscillate very strongly. It then re-emits the sound in all directions. Since the sound is scattered in all directions, its intensity can be very attenuated during multiple reverberations, allowing for very effective sound absorption. The wavelength of the resonant re-emitted wavelength is about 500 times greater than the bubble. This difference in lengthscale makes the bubble an excellent candidate for the design of acoustic metamaterials, inspired from optical metamaterials, with many original applications. Several attempts have been made in recent years to fix bubbles, such as putting them in a gel or under a net. While these solutions are interesting, they have some drawbacks : in a gel, bubbles cannot oscillate as freely as in water ; and under a net, bubbles are not easy to organize.

Grenoble researchers from the Interdisciplinary Laboratory of Physics (LIPhy, CNRS/University Grenoble Alpes) have devised a clever method to get around these problems. This consists in fixing each bubble to a cubic frame of a few millimetres in size made in 3D printing with a standard polymer (figure 1). By submerging this frame into the water, air is trapped by capillary forces. The bubble thus created is then fixed to the frame, preventing it from rising to the surface as is usually the case. Furthermore, by adopting the cubic shape of the frame, the bubble is in fact "faceted", with six almost flat water-air interfaces. The flatness of the interfaces causes the bubble to dissolve more slowly than a spherical bubble of the same size. In addition, researchers have shown, through experiments, simulations and a theoretical model, that the resulting cubic bubble has a beautiful resonance, similar to the Minnaert resonance of spherical bubbles. Large assemblies in which series of caged bubbles are connected by posts can thus be created at will (Figure 2), resulting in metamaterials.
It has also been observed that the bubbles, by vibrating, influence each other when they have many neighbours. Collective vibration modes then appear at lower frequencies, where all the bubbles resonate in phase, which absorbs the acoustic waves and redistributes them into internal vibrations.
By proposing an original method to fix a bubble without preventing it from oscillating, these results open the way to new easily created bubble metamaterials.

Corresponding publications :
Acoustic interaction between 3D fabricated cubic bubbles. Thomas Combriat, Philippine Rouby-Poizat, Alexander A. Doinikov, Olivier Stephan, Philippe Marmottant, Soft Matter, le 14 février 2020.

Acoustics of cubic bubbles : six coupled oscillators. Maxime Harazi, Matthieu Rupin, Olivier Stephan, Emmanuel Bossy, Philippe Marmottant, Physical Review Letters, le 16 décembre 2019.