Shock Wave Emissions of a Sonoluminescing Bubble

A single bubble in water is excited by a standing ultrasound wave. At high intensity the bubble starts to emit light. Together with the emitted light pulse, a shock wave is generated in the liquid at collapse time. The time-dependent velocity of the outward-traveling shock is measured with an imaging technique. The pressure in the shock and in the bubble is shown to have a lower limit of 5500 bars. Visualization of the shock and the bubble at different phases of the acoustic cycle reveals previously unobserved dynamics during stable and unstable sonoluminescence. [S0031-9007(98)07896-X] When intense ultrasound is applied to water, bubbles appear in the liquid. Among the properties they exhibit is sound radiation and emission of photons [1]. In a controlled experiment, a single bubble alone may be driven stably in a standing ultrasound field. Here, intense light pulses of very short duration may be observed. Since this discovery [2] experimental work on the so-called single bubble sonoluminescence (SBSL) has been extensively carried out to explain the phenomenon and the interesting features it displays: The energy is focused by 12 orders of magnitude [3], the light pulses are of picosecond duration [4], the emitted light energy per pulse is in the MeV range, and the blackbodylike spectrum peaks in the ultraviolet. The interpulse synchronicity can be accurate on the picosecond scale [3] or chaotic on the microsecond scale [5]. Parameter studies have been done showing the region of stable SBSL lying on the boundary of a dissolution island [6]. Advanced driving of the bubble is employed to increase the light output [7]. Theoretical and numerical work [8 ‐10] has been done to explain SBSL but so far few basic assumptions of the different theories could be verified experimentally. An inner shock wave launched in the interior of the bubble upon collapse has theoretically been assumed to account for the observed short SBSL light pulse and its spectrum [9,11,12]. Our experimental and numerical work focuses on the observation of shock waves being emitted into the liquid at bubble collapse [13]. The shock waves are visualized, the velocity of the front as it travels outwards is measured, and the peak pressure of the shock is deduced. Effects appearing at unstable SBSL are analyzed. The experiments are consistent with numerical simulations. In the experiment (Fig. 1) the standing ultrasound wave is produced in a cylindrical cell filled with water of ambient temperature, distilled, and degassed to 10% ‐40% of ambient gas pressure. The cell consists of two piezoceramic cylinders connected by a glass tube [14] of 2.9 cm radius (overall height 12 cm). An optical glass plate closes the bottom, and the top remains open. The driving frequency is 23.5 kHz and the driving amplitude is 1.2 to 1.5 bars. A bubble is inserted into the liquid with a syringe. The oscillating bubble is illuminated from the top by a copper vapor laser by light pulses of 7 ns duration (FWHM) followed by a low intensity tail of 30 ns. The wavelength is 511 nm. The repetition rate of one-half the driving frequency is adjusted via a controllable delay to accommodate locking to the driving signal at a preset phase. Because shock waves modulate the phase of the laser light, optical filtering is used to transform this information into intensity modulations. Therefore the bubble image is passed through a (magnifying) 4f spatial Fourier filter. Specifically, a dark-ground method [15] is used that removes the zeroth order in the Fourier plane with a thin metal stick. Subsequently the image is picked up by a video camera delivering 25 framesys. The shutter opening time is 0.25 ms such that the average image of 2 ‐3 shock waves is seen. Because of the stable repetitive bubble collapse a slow motion video of the oscillations [14,16] and the shedding of shock waves is produced by slightly detuning the laser flash frequency. The images of the shock waves are digitized in a computer, and their radiusytime curves can be plotted. Because of their submicron size, sonoluminescing bubbles are hard to detect at collapse. But by recording the center of the shock wave the bubble position can be determined. Figure 2 shows the images of shock waves at different times. The shock is emitted at the main collapse of the bubble. The shock front shows