On-Demand Bulk Nanobubble Generation through Pulsed Laser Illumination

We demonstrate the temporally and spatially controlled nucleation of bulk nanobubbles in water through pulsed laser irradiation with a collimated beam. Transient bubbles appear within the light exposed region once a tension wave passes through. The correlation between illumination and cavitation nucleation provides evidence that gaseous nanobubbles are nucleated in the liquid by a laser pulse with an intensity above $58\mathrm{MW}/{\mathrm{cm}}^2$. We estimate the radius of the nanobubbles through microscopic high-speed imaging and by solving the diffusion equation to be below 420 nm for $\sim 80\%$ of the bubble population. This technique may provide a novel approach to test theories on existence of stable bulk nanobubbles.

Figure 1

Sketch of the experimental method. The nanobubbles are generated by a short and collimated laser pulse illuminating the water. A shock wave is generated after a time $\mathrm{\Delta }t$ focusing a second laser pulse close to the liquid surface, that is reflected at the free surface as a rarefaction wave. Once the rarefaction wave has traveled through the illuminated volume, the nanoscopic gaseous voids are expanded and become visible with optical microscopy.

Figure 2

Laser induced nanobubble cloud. The times in the figure are relative to the firing of the seeding laser. (a) No bubbles are observed outside the seeding laser beam path ($800\mu \mathrm{m}$) indicated by the dashed lines. (b) Detail of a typical nanobubble dynamics captured with an optical resolution of $234\mathrm{nm}/\mathrm{px}$. The bubble size is below the optical resolution prior and after the expansion-collapse cycle. See Supplemental Material [27] for a more detailed sequence.

Figure 3

Detail of the dynamics of a nanobubble. The bubble radius temporal evolution [$R(t)$] is obtained from the video frames (graph inset). A smoothed trace of the pressure variation recorded by the hydrophone (black line) was used as the driving pressure in the numerical fit (red line) of the measured $R(t)$. The best fit was obtained for an ambient radius of $R_0=60\mathrm{nm}$. The measured rebound of the bubble after the collapse is dramatically affected by the water volume entering the gas cavity during the jetting [41]. See Supplemental Material [27] for the complete image sequence.

Figure 4

Top: Percentage of nanobubbles present in the solution as a function of the time $\mathrm{\Delta }t$ between seeding and observation, $N(\mathrm{\Delta }t\to 0)\simeq 112$ [within the ROI in Fig. 2]. The mean amplitude of the tension wave is $-5.0\pm 0.2\mathrm{MPa}$. The dashed horizontal line indicates the mean bubble number in absence of bubble seeding. Bottom: Bubble lifetime as a function of the initial bubble radius according to the Epstein-Plesset model. The dissolution times found indicate that most of the nucleated bubbles have ambient radius below 400 nm.