Optic cavitation in an ultrasonic field

Cavitation bubbles are generated in water by low-energy femtosecond laser pulses in the presence of an ultrasonic field. Bubble dynamics and cavitation luminescence are investigated by CCD photography and photomultiplier measurements in dependence on the phase of the acoustic cycle at which the bubbles are generated. The experimental results demonstrate that the initially small laser-generated bubbles can be expanded significantly by the sound field and that weak cavitation luminescence can be observed in two small intervals of the seeding phase. The luminescence yield sensitively depends on the degree of sphericity of bubble collapse.

Figure 1

Sketch of the experimental setup.

Figure 2

Synchronization of the acoustic field with the laser oscillator by phase-locking circuitry. The bubbles can be generated at certain phases ${\phi }_s$ of the acoustic cycle by picking and amplifying the corresponding (odd-numbered) pulse of the laser oscillator.

Figure 3

(a) Dynamics of an elongated bubble produced by a $130\text{−}\mathrm{fs}$ laser pulse of energy $0.22\mu \mathrm{J}$ in water without a sound field. The frames are taken at $200\text{−}\mathrm{ns}$ intervals. (b) Bubble generated by a laser pulse with the same parameters in a sound field of amplitude $159\mathrm{kPa}$ at seeding phase 192°. The frame separation is now $800\mathrm{ns}$.

Figure 4

Radius vs time of a bubble without acoustic excitation generated by a laser pulse of energy $0.22\mu \mathrm{J}$. Experimental data (circles) are obtained as averages over three laser shots. The solid line shows a numerical calculation with best-fit initial conditions. The dashed lines represent the radial dynamics at $\pm 4.5\%$ deviation of laser pulse energy.

Figure 5

Collapse time as a function of seeding phase of the bubble (with the same parameters as in Fig. 4). The circles represent experimental data; the solid line shows the numerical calculation with best fit acoustic pressure of $p_a=159\mathrm{kPa}$ and proper phase calibration; the dashed lines correspond to $\pm 4.5\%$ deviation of laser pulse energy.

Figure 6

Experimental data for a bubble shot at time $t=0$ which corresponds to a phase of the sound field ${\phi }_s=184°$. Cavitation luminescence as a function of time is given in the graph at the top, together with the acoustic driving signal. At the bottom two pseudostreak images show the lateral $\left(r\right)$ and longitudinal $\left(z\right)$ extensions of the bubble measured immediately after the photomultiplier tube (PMT) measurements.

Figure 7

Luminescence and dynamics of a bubble with seeding phase ${\phi }_s=192°$.

Figure 8

Luminescence and dynamics of a bubble with seeding phase ${\phi }_s=201°$.

Figure 9

Gray-scale plot of the numerically calculated radius as a function of time (horizontal axis) and seeding phase (vertical axis) for a bubble driven at $f_a=44.6\mathrm{kHz}$ and $p_a=159\mathrm{kPa}$. Initial data and equilibrium radius are given in the text. The superimposed contour lines are labeled by the corresponding radius values in $\mu \mathrm{m}$.

Figure 10

Luminescence and dynamics of a bubble shot at phase ${\phi }_s=323°$. Data are represented as in Fig. 6. The bubble flashes at the delayed strong collapse.

Figure 11

Luminescence and dynamics of a bubble with seeding phase ${\phi }_s=331°$.

Figure 12

Luminescence and dynamics of a bubble with seeding phase ${\phi }_s=340°$.