Listening to a resonating bubble

Artistic view of the technique: an air bubble trapped in a cubic cage is scanned over a sample (here an Eiffel Tower motif engraved in a steel plate). The sound emitted by the bubble following an acoustic excitation at resonance is detected by the hydrophone (in black), which also serves as a support for the cage. Image credits: Bruno Peccoud.

Air bubbles in water are excellent acoustic resonators, whose size is very small as compared to the wavelength of the emitted sound waves. Interestingly, these sound waves contains information on the mechanical properties of the materials located in the immediate vicinity of the bubble. In our work, we propose to exploit this phenomenon by moving a bubble in the vicinity of structured samples, in order to make images of these samples.

This approach is directly inspired by an optical technique called scanning near-field optical microscopy. This technique is based on the use of sub-wavelength optical resonators such as gold nanoparticles for example. The main interest of these near-field imaging techniques is to produce super-resolved images, as the resolution is given by the size of the resonating object instead of by the wavelength.

In order to make super-resolved images with acoustic waves, we thus propose to use resonating bubbles instead of resonating nanoparticles. But how can one implement such an approach, when it seems intuitively impossible to manipulate an air bubble in water? We solved this problem by using an air bubble trapped in a cubic cage made by 3D printing. By scanning the caged bubble in the vicinity of structured samples, we are able to reconstruct images from the sound of the resonating bubble, with a resolution two orders of magnitude smaller than the wavelength of the field.

At this stage, we have performed a proof of principle with millimeter-sized bubbles and resonance frequencies of the order of kHz. We are now working on miniaturizing caged bubbles to reach micrometric sizes corresponding to resonances in the MHz domain, which is notably the field of biomedical acoustics.

This study has been published in Nature Communications (Bouchet et al., 2024).

Bubbles are ubiquitous in many research applications ranging from ultrasound imaging and drug delivery to the understanding of volcanic eruptions and water circulation in vascular plants. From an acoustic perspective, bubbles are resonant scatterers with remarkable properties, including a large scattering cross-section and strongly sub-wavelength dimensions. While it is known that the resonance properties of bubbles depend on their local environment, it remains challenging to probe this interaction at the single-bubble level due to the difficulty of manipulating a single resonating bubble in a liquid. Here, we confine a cubic bubble inside a cage using 3D printing technology, and we use this bubble as a local probe to perform scanning near-field acoustic microscopy—an acoustic analog of scanning near-field optical microscopy. By probing the acoustic interaction between a single resonating bubble and its local environment, we demonstrate near-field imaging of complex structures with a resolution that is two orders of magnitudes smaller than the wavelength of the acoustic field. As a potential application, our approach paves the way for the development of low-cost acoustic microscopes based on caged bubbles.