The work carried out during the NASCAP project involved using a high-power pulsed laser beam on liquid samples to induce the nucleation of nanometric gas cavities. A significant portion of the research focused on understanding the origin of these gaseous objects, specifically the interactions between the laser light and the liquid molecules. This challenging task is further complicated by the fact that the produced bulk nanobubbles are smaller than the optical resolution, making them unobservable with standard microscopy.
To address this, we designed and implemented various opto-acoustic visualisation systems capable of successfully detecting the presence of these bubbles. These systems also confirmed the gaseous nature of the detected objects, thereby avoiding confusion with solid particles or insoluble liquid nanodrops. A key aspect of this study was understanding how the specific size and shape of the liquid container affected bubble seeding and the subsequent dynamics of individual nanobubbles and bubble clouds.
Regarding the diffusional stability of the nanobubbles, we tested the effects of various liquids with diverse physical properties. Our goal was to determine if bubble lifetimes could be extended, meaning they would take longer to dissolve under the influence of Laplace pressure. Concurrently, we explored the tunability of bubble sizes by introducing laser-absorbing nanoparticles into the liquid. Another strategy consisted in appliying a sound field to prevent the bubbles from dissolving. This strategy lead to bubble coalescence but showed us how to manipulate the bubble clouds through the sound signals.
Towards the end of the project, we conducted experiments on the effects of nanobubble nucleation on bacterial colonies of Escherichia coli. These experiments demonstrated that the laser-induced bubbles have a bactericidal effect. However, the specific mechanism behind the damage caused by the bubbles to the microorganisms remains to be determined through further research.
The specific scientific objectives of the NASCAP project can be summarised in four main tasks:
1-Establishing the physical mechanisms behind the nucleation of laser-induced bulk nanobubbles.
2-Developing a reliable bubble detection and size characterisation method.
3-Studying bubble stability and investigating stabilisation methods.
4-Determining the effect of laser-induced bulk nanobubbles on bacteria or pollutants commonly found in grey waters.
Tasks 1 and 2 were highly successful and fully achieved. The bubbles could be detected by optical microscopy by combining acoustic driving and femto-laser illumination. Initially, this was achieved using a medical-grade lithotripter to expand the laser-induced nanobubbles through a rarefaction wave. In later trials, the nanobubbles were seeded in a millimetric droplet, and the negative pressure wave was produced directly by the reflection on the liquid surface of a shockwave emitted when focusing a laser pulse inside the liquid volume.
Alternatively, we proved the possibility of nanobubble detection using high-speed ultrasound imaging. This new method represents a significant step forward in the study of laser nanobubbles, as it not only demonstrates the gaseous nature of the laser-induced cavities but also allows us to study the dissolution of the bubbles in real-time and with a single laser shot, using only a tiny fraction of the time required by the methods implemented at the beginning of the project, which demand around 600 laser shots for a single bubble population decay curve.
The third task, related to the stabilization of the bulk nanobubbles, was only marginally achieved. While the dissolution of the nanobubbles was slowed by the addition of surfactants and salts to the water, this delay was much smaller than expected. These results question previous claims found in the literature about long-term stabilization of bulk nanobubbles generated with other methods, such as sonication of the liquid samples.
The last and fourth objective was partially achieved. On one hand, we proved the effectiveness of the laser-induced bubbles for bacterial eradication; however, the mechanisms behind the inactivation of these microorganisms could not be fully determined. All the evidence indicates that bacteria might be damaged by the shockwaves emitted during bubble nucleation. More targeted experiments are needed to confirm this hypothesis.
