Bubble dynamics, shock waves and sonoluminescence By Claus-Dieter Ohl, Thomas Kurz, Reinhard Geisler, Olgert Lindau and Werner Lauterborn Drittes Physikalisches Insitut, Universit ̈ at G ̈ ottingen, B ̈ urgerstrae 42-44, D-37073 G ̈ ottingen, Germany Sound and light emission by bubbles is studied experimentally. Single bubbles kept in a bubble trap and single laser-generated bubbles are investigated using ultrafast and high-speed photography in combination with hydrophones. The optical observation at 20 million frames per second of the shock waves emitted has proven instrumental in revealing the dynamic process upon bubble collapse. When jet formation is initiated by a non-spherically symmetric environment, several distinct shock waves are emitted within a few hundred nanoseconds, originating from di erent sites of the bubble. The counterjet phenomenon is interpreted in this context as a secondary cavitation event. Furthermore, the light emission of laser-generated cavities|termed cavitation bubble luminescence|is studied with respect to the symmetry of collapse. The prospects of optical cavitation and multibubble trapping in the study of few-bubble systems and bubble interactions are briefly discussed. Finally, the behaviour of bubble clouds, their oscillations, acoustic noise and light emission are described. Depending on the strength of the driving sound eld, period doubling and chaotic oscillations of the collective bubble dynamics are observed. Keywords: sonoluminescence, single-bubble; cavitation, laser-induced; shock wave; high-speed photography; intensi ed photography 1. Introduction Bubbles in a liquid, their movement and oscillations are easily amenable to our imag- ination. Nevertheless, as two-phase systems with free boundaries, they constitute a subject notoriously dicult to study scienti cally due to their involved dynamics on largely di erent very short time-scales. In practical applications as well as laboratory experiments, multibubble systems prevail. In fact, they have been the focus of cavitation research for many decades, with a considerable body of knowledge being accumulated over this time. In multibubble cavitation the influence of the environment, i.e. the interaction of bubbles with the applied sound eld, with obstacles or boundaries, and the mutual interaction of bubbles, renders a very complicated situation full of interesting physics: in particular, sound and light emission; free-boundary flow with, e.g. surface oscillations or high- speed liquid jets; shock waves; chemical reactions and structure formation in the bubble eld (Brennen 1995; Leighton 1994). Before one can hope to achieve a better understanding of complex systems com- posed of elementary units, it is necessary to study these units in detail. This scienti c rationale has proven successful in a number of circumstances. For cavitation it means that we have to look thoroughly at the single isolated bubble and its interaction with the environment in a controlled setting. One way of doing so is by means of bubble traps. They have become popular recently through the discovery of the remarkable phenomenon of single-bubble sonoluminescence (SBSL) (Gaitan et al . 1992), which gave new thrust to cavitation physics and, in particular, to the exploration of sono- luminescence. A second complementary method of single-bubble investigation is pro- vided by optic cavitation, i.e. the generation of transient cavities in liquids by strong laser pulses (see Lauterborn 1980). Bubble luminescence is also observed in optic cavitation (Buzukov & Teslenko 1971) and thus it is expected that light-produced bubbles will become increasingly popular for the investigation of this phenomenon as they extend the range of available bubble sizes. The rst part of this paper gives an overview of results on single-bubble dynam- ics obtained by the two methods to discuss the prospects they hold for further exploration, especially of cavitation bubble luminescence. Both approaches will be extended to the systematic study of few-bubble systems with their mutual interac- tion, naturally opening the way for the investigation of more complicated bubble arrangements. In the second part of the paper, we review some modern results on multibub- ble acoustic cavitation and thus come full circle with the historic development of the subject. We believe that, backed up by a better understanding of single- and few-bubble systems, by application of state-of-the-art experimental equipment (e.g. sensitive high-speed gatable cameras or ultrashort pulsed lasers) and by re nement of theoretical models together with their extensive numerical investigation, signi cant progress in this area is possible. 2. Spherical bubble dynamics Optic cavitation and single-bubble acoustic levitation o er methods for studying the `elementary unit of cavitation, the single bubble. In particular, in both cases, spherically symmetric (or radial) bubble dynamics can be realized at least approx- imately, apart from possible deviations from sphericity during bubble collapse due to inherent instabilities. To describe radial bubble dynamics, theoretical models of di erent degrees of complexity have been devised, originating from Lord Rayleighs (1917) treatment of the collapse of an empty cavity. Thus, both methods of investi- gation enable us to assess the various models of radial bubble motion and to clarify which physical processes are important and which ones can be neglected under given experimental conditions. ( a ) Laser-generated bubbles Optic cavitation provides a convenient means of generating a single bubble in a transparent liquid (Lauterborn 1980). For that purpose, a short laser pulse is focused into the liquid. Figure 1 depicts an experimental arrangement for observing the bubble and its shock waves by high-speed photography. The shock waves are also monitored by a fast hydrophone. Almost any laser which can emit short pulses with durations of the order of a few nanoseconds or less, and energies per pulse of a few millijoules, can be used. The light pulses must be tightly focused by aberration-minimized lenses to avoid multiple Phil. Trans. R. Soc. Lond. A (1999) Bubble dynamics 271 flash lamp with diffusor 8 ns Nd:YAG laser laser pulse cuvette long-distance microscope CCD image converter camera hydrophone CCD camera (ICCD) image intensified Figure 1. Typical experimental arrangement for optic cavitation with high-speed photography of bubble dynamics. For the photography of shock waves, the di user is removed and a small aperture of the camera objective is chosen. Figure 2. Bubble generation and shock wave emission upon laser-induced breakdown caused by a focused Nd:YAG laser pulse in water. The CCD photographs of the breakdown were taken with 100 fs pulses for back-illumination, obtained via a pulse picker from a Ti:sapphire laser. The frame size is 0 : 66 0 : 66 mm 2 . From left to right, the following time delays between the generating pulse and the illumination pulse apply (measured from peak to peak): 9.5 ns, 13 ns, 69 ns, 107 ns and 157 ns. The bright light emission is due to the laser-produced plasma. The forming bubble is visible as a dark border around the central spot. In the short time interval covered the bubble does not grow signi cantly. and spatially extended breakdown sites. A planoconvex lens inserted in the wall of the cuvette could serve that purpose as a good rst approximation. In the focal region, a very-high light intensity, associated with a large strength in the electric eld, is achieved. There the laser pulse causes heating of impurities and/or dielectric breakdown with avalanche ionization, and creates a plasma spot. The plasma expands to form the cavity. Figure 2 gives an example of bubble generation by a Nd:YAG laser pulse and the associated shock- wave emission. To stop the rapid expansion of the cavity and the shock propagation, the photographs were taken with single laser pulses of about 100 fs duration. Bubble dynamics is exceedingly fast, and the collapse of a bubble can be captured only with high-speed cameras or short illumination. Figure 3 gives an example of a high-speed photographic sequence which probes the nal collapse stages. This series was taken at 20.8 million frames per second with an image converter camera (Ohl et al . 1995). The maximum number of frames per shot is limited to eight. Therefore, four di erent shots corresponding to nearly identical bubbles have been combined Phil. Trans. R. Soc. Lond. A (1999)
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