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Faceted bubbles, or how to stabilize bubbles with 3D printing

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Air bubbles in water are excellent acoustic resonators, good candidates for the manufacturing acoustic metamaterials, but have the disadvantage of being difficult to keep in place, in addition to dissolving quickly. Grenoble physicists have shown that it is possible to create "faceted bubbles" attached to 3D printed substrates to solve the issues of conventional spherical bubbles, while preserving their acoustic properties.

Take an air bubble in water and send a sound on it: for a very particular frequency — Minnaert’s frequency — the bubble will resonate and start oscillating very strongly, and re-emit a sound in all directions. A peculiarity of the re-emitted wave is that its wavelength is about 500 times larger than the size of the bubble. This feature makes the bubble an excellent candidate for « acoustic metamaterials », innovative materials with original properties such as an increased absorption, a negative refractive index or the ability to focus fields over distances shorter than the wavelength. The difficulty is that metamaterials require a very precise arrangement of resonators, while a bubble in water has the unfortunate tendency to move and dissolve. Several attempts have been made in recent years to fix the bubbles, such as putting them in a gel or under a net. While these solutions are interesting, they have some flaws: 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 Laboratoire Interdisciplinaire de Physique in Grenoble have devised a new and very clever method to overcome these problems. This consists in fixing a bubble to a cubic millimetric support made using 3D printing (see image). By immersing this support intowater, air is trapped in it by capillarity. The bubble thus created is then fixed to the support, preventing it from rising to the surface as is usually the case. Moreover, by adopting the cubic shape of the support, the bubble is in fact "faceted", with six almost flat water-air interfaces. The flatness of the interfaces results in the fact the bubble dissolve more slowly than a spherical bubble of the same size. In addition, the researchers showed, through experiments, simulations and a theoretical model, that the cubic bubble thus created had a beautiful resonance, similar to the Minnaert resonance of cubic bubbles, but with the complexity that the six faces interact with each other via air, but also via water. Therefore, surprisingly, it is possible to modify the resonance frequency of the cubic bubble by changing its number of open faces!
By proposing an original method of fixing a bubble without preventing it from oscillating, these results open the way to a new way of easily creating bubble metamaterials.

On the cover !!

View online : Maxime Harazi, Matthieu Rupin, Olivier Stephan, Emmanuel Bossy, and Philippe Marmottant, “Acoustics of cubic bubbles: six coupled oscillators” Phys. Rev. Lett. , december 2019