in alphabetical order of speakers

Recent Work in Sonoluminescence at Yale

R. Apfel, J. Ketterling, Y. Tian, J. Jankovsky, and T. Stottlemyer

Faculty of Engineering, Yale University,

New Haven, CT 06520

Yale's involvement in SL began three years ago. In that time, we have made several small contributions to the understanding of SL. Our first accomplishment was a stroboscopic method of viewing bubble dynamics [Tian, et al., 1996 J. Acoust. Soc. Am. 100, 3976]. Tian backlit a levitated bubble with a LED which was strobed at a frequency just off of the acoustics drive frequency (by 0.1 Hz). A microscope was used to view the strobed bubble, and the image was recorded with a VCR. The video can be used to obtain R-t curves and to examine when shape instabilities set in. Our group was also the first to study surfactant effects on the parameter space of SL. Stottlemyer [PhD Thesis, Yale University 1996; J. Acoust. Soc. Am., in Press] discovered that with the presence in water of Bovine Serum Albumin, an insoluble surfactant, the pressure range and light intensity of SL was shifted above that of pure water. His work indicated certain surfactants can be used to delay the onset of surface instabilities at higher acoustic drive pressures. We hope to exploit this effect in our current research effort which aims at achieving greater energy focusing on the bubble.

Bubble Dynamics and Sonoluminescence at Cryogenic Temperatures

O. Baghdassarian, H. Cho, E. Varoquaux, and G. A. Williams

Department of Physics and Astronomy, University of California, Los Angeles

Los Angeles, CA 90095

Initial studies of the properties of bubbles acoustically trapped in cryo-genic fluids have been carried out. Helium bubbles have been trapped in liquid nitrogen over the temperature range of 65-72 K. The bubble oscillations, monitored with Mie scattering, show relatively strong damping compared with that of bubble in water. This may arise from evaporative dissipation, since the vapor pressure of liquid nitrogen is considerably higher than that of water. Although sonoluminescence was previously observed in this apparatus from xenon bubbles in ethanol cooled to 165 K, no SL has yet been observed from the helium bubble in liquid nitrogen.

Work supported by: U. S. National Science Foundation, Grant DMR-9500653.

A Fully Coupled Radiation-Hydrodynamic Model for SBSL

Lawrence Bernstein

Spectral Science, Inc., 99 South Bedford Street, Burlington, MA 01803

Many theories have been advanced to explain sonoluminescence (SL), but none have quantitatively encompassed the richly varied spectral, dynamical, and chemical sensitivities displayed by this phenomenon. The recently reported Confined Electron Model (CEM) has demonstrated the potential to do so. It has been applied to analysis of the ultraviolet-visible continuum emission observed in both SBSL and MBSL and has shown consistency with the high peak power levels, fast emission time scale, and high sensitivity of the emitted power and spectral distribution to bubble gas composition for rare gas (Rg) SBSL. CEM originates from thermally excited electrons confined to voids in the bubble fluid formed during the late states of collapse. Treatment of the electron-void system as a particle in a box accounts well for the observed spectral features of Rg SBSL. In the current study we have fully coupled the CEM radiation model to a hydrodynamic model of the bubble expansion and contraction. The hydrodynamic model is based on isothermal expansion and adiabatic contraction thermodynamics and uses a modified form of the Rayleigh-Plesset equation. Parametric SBSL spectral predictions are performed for an Ar bubble over a range of forcing pressures and ambient radii corresponding to the phase space of stable SBSL. The model calculations are consistent with the observed spectral and dynamical properties of Rg SBSL; they also demonstrate that shock waves, which may be generated during bubble collapse, are not necessarily required to drive the light generation mechanism.

Comparative Study of Hydrodynamic models for Single Bubble Sonoluminescence

M.C. Chu

Department of Physics

The Chinese University of Hong Kong

We present a comparative study of hydrodynamic models of a sonoluminescing bubble. Three forms of the Rayleigh-Plesset equations describing the bubble wall motion are coupled to the Euler or Navier-Stokes equations for the gas motion inside the bubble. The first form (RP1) includes surface tension, viscosity and acoustic radiation but does not explicitly include the compressibility of the liquid. The second form (RP2) is the Keller formulation, which approximates the liquid compressibility to first order accuracy in the bubble wall Mach number. The third form (RP3) is the Gilmore ormulation modified by us to include acoustic radiation. We found that for a range of driving pressure, shock waves are generated in RP1 but not RP2 and RP3. This difference suggests that the compressibility of the liquid plays a crucial role in the reducing the violence of bubble motions. We solve the energy equation in the surrounding liquid to determine the heat transfer across the bubble wall, and we show that heat conduction during the expansion phase of the bubble increases the violence of the bubble collapse. We also discuss the dependence of the hydrodynamics on the surface tension.

Sonoluminescence and Collision-Induced Emission

Lothar Frommhold

Physics Department, University of Texas

If one assumes that sonoluminescence (SL) involves some mechanism generating shockwaves (as is usually assumed), one must consider collision-induced emission (CIE) as the possible light emitting mechanism of SL. CIE arises from the well known transient electric dipoles induced in intermolecular interactions (collisions) by exchange, dispersion and multipole forces, and also by molecular frame distortions during collisions [1]. CIE is a prime source of radiation in compressed, mostly neutral gases composed of nonpolar molecules (air, etc.) if i.the gas densities are high enough and ii.the translational and rotovibrational energies available in a collision are commensurate with the energy of the observable photons: CIE involves always two (or more) molecules and CIE intensities, therefore, increase as density s_q_u_a_r_e_d, i.e., much faster than monomolecular radiation. For emission in the visible the sum of translational and rotovibrational energies, e.g., (3/2)kT+(1 or more times)(2/2)kT, of an average collisional complex must amount to a few eVolt; a simple estimate has shown that temperatures around 10,000 K explain not only the various types of spectral profiles observed in SL studies, but also the correct order of magnitude of the intensities observed [2]. Moreover, CIE provides very strong (at present still mostly qualitative) arguments why an addition of a small amount (1% or so) of argon to nitrogen, etc., should increase the intensities very much [3]: dipoles induced in dissimilar systems (such as N2-Ar) are much stronger than dipoles induced in similar pairs (e.g., N2-N2). Efforts are underway to compute CIE spectra of colliding, highly rotovibrationally excited molecules of interest in SL from first principles, to put the above mentioned estimate [2] on a quantitative, solid basis. The results obtained thus far indicate that the SL light emission is substantially due to CIE if two assumptions are true: i.that there be some sort of shockwave mechanism heating the gas, and ii.the dense environment thus created is basically neutral, i.e., with temperatures in the 6,000 to 12,000 K range (but not much higher). The CIE light source is optically thin, given the small physical dimensions of the SL light source, so that Planck's radiation law cannot be used for an analysis of the SL spectra [2,3].

[1] L. Frommhold. Collision-induced Absorption in Gases. Cambridge

University Press, Cambridge and New York, 1993.

[2] L. Frommhold and A.A. Atchley. Phys. Rev. Letters 73 (1994) 2883.

[3] L. Frommhold and W. Meyer. In M. Zoppi, ed., Spectral Line Shapes,

vol.9, A.I.P., New York, 1996.

Anomalous Mass Flux and The Threshold For Light Emission In Single-Bubble Sonoluminescence

D. Felipe Gaitan

National Center for Physical Acoustics, University, MS 38677

R. Glynn Holt

Boston University, Dept. of Aerospace and Mechanical Engineering, Boston, MA 02215

In a previous page by the authors (Phys. Rev. Letts. 77, 3791), the region of parameter space (acoustic pressure Pa, bubble radius Ro) in which stable single bubble sonoluminescence (SBSL) occurs in an air-water system driven at 20.6 kHz was described as a function of the dissolved gas concentration (Ci/C01). In this paper, we show that diffusive dynamics apparently governs the mass flux stability, despite quantitative disagreement of measurements reported in the previous work. We present the first measurements of absolute (Rmax) and normalized (Rmax/Ro ) response for bubbles near the threshold for light emission. The results suggest that bubble interfacial response is an insufficient criterion for the onset of light emission, and we present evidence for the dependence of the emitted light on bubble volume and compression ratio.

Single Bubble Sonoluminescence: Acoustic emission measurements with a

fiber optic probe Hydrophone

B. Gompf, R. Günther*, R. Pecha, W. Eisenmenger

1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart

*Natural and Medical Science Institut, Eberhardstr. 29, D-72762 Reutlingen

In Single Bubble Sonoluminescence additionally to short light pulses, the bubble emits in the collapse phase a shockwave which can be measured with a fiber optic probe hydrophone [1]. This type of hydrophone is an absolute ultrasonic wide band reference standard with an accuracy of about 5%. Compared to piezoelectric hydrophones it allows measurements with higher spatial (100 m and temporal (10ns) resolution. The intensity of the emitted shock wave increases with increasing driving pressure of the sound field up to about 8 bar at a distance of 1mm, but is independent of the water temperature in contrast to the emitted light intensity. The total radiated energy of the shock wave is below 10% of the initial energy of the bubble. The results are compared with earlier measurements on transient cavitation bubbles and with new theoretical results.

[1] J. Staudenraus, W. Eisenmenger, Ulltrasonics 31 (1993) 267

Resolving Sonoluminescence Pulse Width with Time-Correlated Single Photon Counting

B. Gompf, R. Gunther*, G. Nick, R. Pecha, W. Eisenmenger

1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart

*Natural and Medical Science Institute, Eberhardstr. 29, D-72762 Reutlingen

Until now one of the unsolved questions on single bubble sonoluminescence (SBSL) is the width of the emitted light pulses. We demonstrate that time-correlated single photon counting (TC-SPC), normally used for fluorescence life-time measurements, is an ideal method to investigate SBSL pulse width and shape even under conditions where the emission is extremely dim. Because of the low intensity required in TC-SPC experiments it is possible to discriminate different pulse forms in different regions of the broad band SL spectrum. Time and frequency resolved measurements are on the other hand essential for testing the predictions of the theoretical models under discussion describing the light emitting process. The TC-SPC experiments show that the pulse width at room temperature increases from about 60ps at low gas concentrations and low driving pressures to more than 250ps at high gas concentrations and driving pressures at the upper SL threshold. The pulse shape is nearly Gaussian and the width is identical in the red and UV part of the spectrum. A comparison of the experimental values with a simple model exclude that SL is black body radiation, but indicates that Bremsstrahlung could be the underlying light emitting process.

Analytical and Numerical Results for Pressure, Temperature and

Light Emission at the Collapse of SBSL Bubbles

R. Günther, B. Gompf*, W. Eisenmenger*

Natural and Medical Science Institute, Eberhardstr. 29, D-72762 Reutlingen

*1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart

We present analytical approximations as well as numerical calculations of the hydrodynamic equations describing the bubble wall motion and the physical conditions inside and outside the bubble. Using an expansion of the fields inside and outside the bubble, we derive a set of dimensionless equations, which allows perturbative treatment during different stages of the bubble collapse. Taking into account the thermal coupling between liquid and bubble content and viscous effects, our results show that there exist smooth solutions for the pressure and temperature fields inside the bubble.

In addition it can be shown that the pulse length of light emission lies in the range from 50-250 ps, in agreement with new experimental results. The last result depends crucially on the use of Gilmores equation for the description of the bubble wall dynamics [1]. Comparison of our results with experimental data seem to rule equilibrium radiation and strongly suggest bremsstrahlung as the mechanism behind light emission.

[1] F. R. Gilmore, Cal. Inst. of Tech. Hydrodynamics Laboratory Report NO. 26-4 (1952).

Analysis of Rayleigh-Plesset dynamics for sonoluminescing bubbles

S. Hilgenfeldt

University of Marburg, Renthof 6, 35032, Marburg, Germany

M. Brenner,

Department of Mathematics, MIT, Cambridge, MA 02139

S. Grossmann, and D. Lohse

University of Marburg, Renthof 6, 35032, Marburg, Germany

Recent work on single bubble sonoluminescence (SBSL) has shown that many features of this phenomenon, especially the dependence of SBSL intensity and stability on experimental parameters, can be explained from an analysis ofbubble wall dynamics, which is conveniently described by the Rayleigh-Plesset (RP) equation. In this work, analytical approximations for RP dynamics and subsequent analytical laws for parameter dependences are derived. Taking into account bubble instability against surface oscillations, we conclude that the ideal liquid for violently collapsing, surface stable SL bubbles should have small surface tension and large viscosity.

On the relationship between nonlinear bubble dynamics and light

emission

R. Glynn Holt

Boston University, Dept. of Aerospace and Mechanical Engineering, Boston, MA 02215

D. Felipe Gaitan

National Center for Physical Acoustics, University, MS 38677

The paradigm system for studying light emission from bubbles has been a single, acoustically levitated air bubble in water. Such a bubble, whose volume pulsations are forced by the time-varying portion of the levitating acoustic field, has a rich dynamic life over a large range of the relevant parameter space. We have made measurements of bubble dynamics over a large part of a bubble's parameter space, including the light emission regime. In the context of these measurements, we will discuss the relationship between observed light emission and the activity of the oscillating bubble. In particular, we will focus on how the dual constraints of mechanical stability and mass flux stability dictate what all experiments in SBSL have seen, namely a very small window of parameter space where light is observed to be emitted. Our aim is to provide enough experimental evidence to make a clear distinction between features of SL which are intrinsic (and thus crucial to any theory of light emission) and those features which SL merely 'illuminates' (which are nonetheless important as boundary conditions for the light emission).

Shock Wave Emissions of Sonoluminescing Bubbles

Joachim Holzfuss, Andreas Billo, Matthias Rüggeberg

Institut für Angewandte Physik, TH Darmstadt, Schlossgartenstr. 7, 64289 Darmstadt, Germany

A single bubble is acoustically levitated in a cylindrical cell. The bubble responds to the sound field with nonlinear oscillations. At high enough sound pressures the bubble starts to emit light flashes upon collapse. This phenomenon is called single bubble sonoluminescence (SBSL) and has attracted great attention over the last years. It is characterized by stable oscillations over thousands of cycles, an ultrashort (picosecond) duration of the flash and an enormous energy concentration from low energy acoustic waves to visible photons. In this paper the acoustic emissions of sonoluminescing bubbles are analyzed. The results are visualized with a video showing the shedding of shock waves upon the collapse of the oscillating bubble. Results for stable and unstable SBSL are presented.

In the experiment a sonoluminescing bubble is illuminated with a CU-vapor laser. The laser is capable of producing light pulses of 10 ns duration with a repetition frequency of up to 14 Khz. Every second cycle of the driving sound field a picture of the bubble is captured with a video camera using a schlieren-type imaging technique. Because of the stability of the oscillations, the shedding of shock waves can be visualized. It is seen, that at the first collapse a shock wave is emanating from the bubble into the liquid. The motion can be visually halted by locking the laser flashes to the driving sound pressure phase thus demonstrating the stability of the bubble motion. Also unstable SBSL can be analyzed. Unstable SBSL is characterized by a slow growth of the bubble due to rectified diffusion and subsequent split-off of micro bubbles. The phenomenon can be measured by a change of phase of the SBSL flash. In our experiment the phase locked laser flashes reveal a change of radius of the spherical shock wave surrounding the bubble. Because a growing bubble shifts it's collapse to later times, the time between collapse and the laser flash changes, too. A growing bubble thus results in a smaller radius of the shock wave and vice versa. The results of the shift of the phase show a slow growth of the bubble, a fast, but not immediate split-off accompanied by a sudden dislocation and diffusion oscillations.

Creations of a variety of sonoluminescing bubbles in low-Q cavities.

Takao Kokubun and Shigeo Hayashi

University of Electro-Communications, Department of Applied Physics and Chemistry

1-5-T Fugaoka, Chofu, Tokyo 182, Japan

For the purpose of exploring possible frameworks for both of single-bubble sonoluminescence (SBSL) and multibubble sonoluminescence (MBSL), a spherical acoustic resonator was driven by a pair of Langevin transducers at a fixed, near-resonant frequency. Higher angular mode could be excited, producing four off-center sonoluminescing spots. Upper threshold of oxygen content was about 55% of saturation, which was practically the same as that for the ordinary, cavity-centered SBSL. Above the threshold, unstable SBSL could be observed when the drive voltage was increased and MBSL was sustained in the spherically symmetric mode when the drive voltage was increased further.

Why is ambient pressure important?

Ljubinko Kondic

Courant Institute of Mathematical Sciences

New York University, New York City

We present a theoretical analysis of the influence of ambient pressure on single bubble sonoluminescence. Variation of ambient pressure modifies the bubble dynamics and furthermore it changes the equilibrium bubble size through modification of the diffusion (?) mass flow between the bubble and the surrounding liquid. We show that a simple measurement of the equilibrium bubble size would give a clear answer about the validity of the diffusion theory and various mass-ejection hypotheses. In particular, we explore the recent suggestion that nitrogen is being ejected from an air light-emitting bubble and predict the corresponding change of equilibrium bubble size with variation of ambient pressure. As a further test of the theory, our results predict that stable SL is not possible for very low values of ambient pressure ($< 0.3 - 0.4$ atm). The stability of the bubble with respect to surface instabilities is explored as well, and it is found that there is a narrow region in the parameter space defined by ambient and acoustic pressure, where stable SL is achievable. Finally, based on our shock theory approach, we calculate SL radiation. The results are compared with preliminary experimental data, and a good agreement is found in the experimentally explored part of parameter space. The theory predicts strong increase of SL radiation if the ambient pressure is decreased. This prediction remains to be verified experimentally. We suggest that variation of ambient pressure provides simple and interesting test for a validity of various SL theories, diffusive or non-diffusive mass flow ideas and stability analyses.

Bubble Dynamics and Sonoluminescence in Multi-Bubble Cavitation Fields

H. Kuttruff

Institute of Technical Acoustics, Aachen University of Technology

In this paper several cavitation experiments (mainly earlier ones), are described along with the luminescence phenomena observed with them. The frequencies of acoustic excitation range from 0 to 260 kHz. They include the observation of single bubble implosions as well as of cavitation next to a solid surface. In most of these experiments more or less extended cavitation fields are employed consisting of many cavitation bubbles which, of course, interact with each other. Possible mechanisms of interaction between the bubbles will be discussed.

Theory of Ultrafast Temperature Measurement of

Sonoluminescent Implosion Using Inelastic Light Scattering

R.B. Laughlin

Department of Physics, Stanford University, Stanford, CA 94305

Shock pulse from sonoluminescing gas bubble

Yoon-Pyo Lee and Sarng Woo Karng

Korea Institute of Science and Technology, P. O. Box 131, Cheongryang, Seoul, Korea

and

Jin-Seok Jeon and Ho-Young Kwak

Mechanical Engineering Department, Chung-Ang University, Seoul, 156-756 Korea

The shock pulse emanating from the sonoluminescing gas bubble was calculated by using the analytical solutions for the gas inside bubble and Kirkwood-Bethe hypothesis for outgoing wave. The magnitude of the shock profile decreases as 1/ characterizes the spherical shock propagation in the medium. The rise time of the shock pulse is about 5 ns, which is close to the instrument limited rise time of 10 ns. The pulse signal calculated at 1 mm from the bubble center is about 7.5 atm, which is also comparable with the observed value of 3 atm. The elapsed time for the shock to arrive at the hydrophone is about 0.68 after the flash, that is exactly the time required for sound to propagate 1 mm in water. It has been found that the measurement of the shock pulse signal provides the approximate value of the gas pressure at/near the bubble collapse: that is 10,000 bar from theoretical estimation.

Mechanisms of Luminescence

Ritva Lofstedt

Institute for Theoretical Physics, University of California, Santa Barbara

Santa Barbara, CA 93106

Much of the debate about sonoluminescence has addressed the nature of the light emitting mechanism. Insight into this process might be gained from a study of other forms of luminescence such as triboluminescence and electroluminescence, the discovery of which dates back over 300 years. This talk will present a general overview and potential new avenues for probing the paradigm underlying sonoluminescence will be discussed.

Sonoluminescing air bubbles rectify argon

Detlef Lohse and Sascha Hilgenfeldt

University of Marburg, Renthof 6, 35032, Marburg, Germany

Michael Brenner

Department of Mathematics, MIT, Cambridge, MA 02139

We review the hydrodynamic approach towards sonoluminescence we elaborated in the last two years. For SL to occur, the bubble collapse has to be violent enough to ensure energy transfer from the fluid to the gas in the bubble. Moreover, three kinds of instabilities have to be considered: (i) The bubble has to be shape stable. (ii) Diffusive stability to distinguish between unstable and stable SL. (iii) Chemical stability, i.e., molecular gases dissociate, react to water soluble gases and only inert gases remain into the bubble. We give further experimental and theoretical evidence for these claims and extend the theory to other gas mixtures.

Particle drift near an oscillating bubble*

Michael S. Longuet-Higgins

Institute for Nonlinear Science, University of California, San Diego, La Jolla, CA 92093-0402

The drift of fluid particles induced by a bubble oscillating with motions typical of a single sonoluminescent bubble are considered. In the first calculation the bubble is assumed to remain spherical and to oscillate both radially, with large amplitude motion, and laterally, with a smaller amplitude. Viscosity is neglected. The paths of marked particles are traced by numerical integration of the velocity field. It is found that the resulting drift motion at large distances from the oscillating sphere is similar to a steady dipole flow. The strength of the dipole, however is typically seven orders of magnitude less than the streaming motion observed in the laboratory by Lepoint-Mullie et al (1977).

A second calculations supposes that the bubble does not remain spherical but collapses asymmetrically, with one side falling inwards towards the other at a greater speed. The resulting impulse again induces a dipole motion in the far field, whose strength may be estimated on certain assumptions. It is found to be quite comparable to that observed.

*To appear in Proc. R. Soc. Lond. A (1997)

Viscous streaming from an oscillating spherical bubble

Michael S. Longuet-Higgins

Institute for Nonlinear Science, University of California,San Diego, La Jolla, CA 92093-0402

Asymmetric patterns of streaming around a single sonoluminescent bubble have been observed by Lepoint-Mullie et al. (1997). The present paper will consider theoretically the viscous streaming from a spherical bubble undergoing small radial and lateral oscillations simultaneously, using two different approaches: (1) an extension of the method of Davidson and Riley (1971) so as to include radial oscillations, and (2) the alternative approach by Wu and Du (1997). Results from the two calculations will be compared with each other, and with the experiments. The theoretical streaming falls short of that observed.

Nucleation of Bubbles in Superfluid helium: Quantum Tunneling and Exploding Electrons

Humphrey Maris

Department of Physics, Brown University, Providence, RI 02912

I will describe experiments to study the nucleation of bubbles in superfluid helium at negative pressures. We have seen bubbles that form as a result of the explosion of electrons, and at low temperatures we have observed quantum tunneling through the nucleation barrier. In these experiments absolutely no light has been detected.

Sonoluminescence Experiments

Thomas J. Matula and Lawrence A. Crum

Applied Physics Laboratory, University of Washington, 1013 NE 40th St., Seattle, WA 98105

The discovery of Single-Bubble Sonoluminescence has given researchers the ability to probe the mysteries of sonoluminescence from a single micron-sized bubble, removed from boundaries and the influences of other bubbles. In this talk we will discuss recent experiments from our lab on single cavitation bubbles covering such issues as bubble levitation, radial and nonradial oscillations of a bubble via light-scattering techniques, and acoustic emissions from cavitation bubbles. The emphasis will be on cycle-to-cycle occurrences, both below and above the threshold for sonoluminescence.

Metal Atom Emission from Multi-Bubble Sonoluminescence

William B. McNamara III, Yuri Didenko, and Kenneth S. Suslick

Department of Chemistry

University of Illinois at Urbana-Champaign

601 S. Goodwin Avenue

Urbana, Il 61801

Acoustic cavitation results in extraordinary transient conditions inside the collapsing bubble. In addition to interesting chemical effects (sonochemistry,, cavitation also produces light emission. Such sonoluminescence, from cavitating clouds of bubbles ("multi-bubble sonoluminescence", MBSL) in room temperature liquids closely resembles flame emission, and we have used this to measure the effective temperature and pressure of species formed during cavitation.

Effective emission temperatures have been obtained for MBSL from excited state metal atom emission (from sonolysis of several volatile metal carbonyls). From the relative intensities of multiple line emissions from Cr, Mo, and Fe, emission temperature have been calculated and are all in close agreement with each other. The effects of bubble contents can alter the observed temperatures and this has been now directly observed for the first time. In addition, effective pressures can be estimated from line broadening and line shifts of the metal atom emission. The effective transient conditions formed during cavitation of bubble clouds are ~5000K, 500 atm which implies heating and cooling rate in excess of 10¹⁰K/s. Temperatures reached during single bubble sonoluminescence (SBSL) are likely to be very much higher. Differences between MBSL and SBSL will be discussed. The chemistry generated by cavitation hot-spots often differs from either ordinary thermal or photochemical processes, and sonochemistry represents a fundamentally unique interaction of energy and matter.

Sonoluminescence as a cooperative Many Body Phenomenon

P. Mohanty & S.V. Khare

Department of Physics, University of Maryland, College Park MD

We propose that sonoluminescence occurs due to the cooperative interaction of matter with a radiation field in a cavitating bubble. We show that the collective spontaneous emission of population inverted atomic or molecular states leads to a timescale of the radiated light pulse which is consistent with that observed in experiments. Pumping by an ultrasound source provides the necessary condition for the inverted states to be correlated over a small volume to trigger sonoluminescence. The experimentally observed role of trace impurities is shown to be consistent in this picture.

Temporal Characteristics of Sonoluminescence

Michael J. Moran and Daren Sweider

Lawrence Livermore National Laboratory, Livermore, CA 94550

The weak emission energies and short durations of sonoluminescence flashes have frustrated numerous attempts to measure details of the single-pulse emission histories. Even a parameter as simple as the FWHM of the emission duration has remained open to question. Photomultipliers now can be used to measure the emission duration with improved resolution. Conventional correlated photon counting with two photomultipliers can provide temporal resolution better than 100 picoseconds, although the results are averaged over many flashes. A variation on the correlated counting techniques can provide resolution that is improved by more than 30%. Finally, single pulse measurements with fast photomultipliers can measure pulsewidths in the 100 picosecond regime. These measurements now are suggesting that sonoluminescence pulses have duration in the 100 picosecond regime. Results of these measurements will be presented and discussed.

Sonoluminescence: The Effect of Magnetic Fields in the Planck Theory

T. V. Prevenslik

2E Greenery Court, Discovery Bay, Hong Kong

Sonoluminescence (SL) observed in the cavitation of water is explained by the Planck theory of SL that treats the bubbles as miniature masers converting the velocity of bubble collapse to electromagnetic (EM) waves at microwave (MW) frequencies. The Planck theory of SLP is consistent with historical experimental data that shows MW's concurrent with SL are produced in bubble cavitation. The MW's are absorbed through the rotation quantum state of clusters of bubble wall molecules. However, the emission of both MW and optical radiation requires a threshold collapse velocity to be exceeded. Under these conditions, optical SL radiation comprising visible (VIS) and ultraviolet (UV) photons are emitted as the Planck energy accumulates to ~ 3 - 6ev. Recent data on the effect of applied magnetic fields on SL is reviewed and found consistent with the Planck theory of SL..

``OLD-FASHIONED'' BUBBLE DYNAMICS

A. Prosperetti

Department of Mechanical Engineering, Johns Hopkins University, Baltimore 21218 USA

A review of the basic physics of pulsating gas bubbles in liquids will be presented. Particular attention will be paid to the fluid dynamic aspects of the liquid motion and to the phenomena having a bearing on the shape of the bubble. In the second part of the talk, a new theory of light emission from a pulsating bubble [J. Acoust. Soc. Am. 101, 2003 (1997).] will be described.

[Study supported by the Office of Naval Research]

Defining the Unknowns of Sonoluminescence

Seth Putterman

Department of Physics, UCLA, Los Angeles, CA 90095

The passage of sound through a fluid with a trapped bubble leads to the clock-like emission of picosecond flashes of ultra-violet light. In this phenomenon, sonoluminescence (or SL), acoustic energy focuses by over twelve orders of magnitude. SL is extremely sensitive to ambient temperature, acoustic drive and doping with noble gases. Despite a plethora of theoretic publications the most basic aspects of SL remain unexplained. Neither the light emitting mechanism nor the size of the bubble nor the range of acoustic drives at which SL can be achieved are understood. It is also a mystery as to why water is the friendliest fluid for SL and it would be most valuable to understand why pure diatomic gases are very weak, unstable sources of SL. The attainment of stable synchronous SL from pure diatomic would constitute an experimental breakthrough. A reasonable picture of the energy concentrating mechanism starts from Rayleigh's 1917 work on the high pressure developed in a collapsing bubble. Femtosecond light scattering from an SL bubble indicates that the collapse is strongly supersonic and suggests the formation of an imploding shock wave that further focuses the acoustic field. Experiments, however, have not proven the shock wave hypothesis and shortcomings of this model will be emphasized. Measurements find that the flash of SL is emitted at the minimum radius where the acceleration exceeds 10^11g. The upper limit of energy focusing can be achieved with SL has not yet been determined.

Multi-Bubble Sonoluminescence

Kenneth S. Suslick

Department of Chemistry and of Material Science & Engineering

University of Illinois at Urbana-Champaign

601 S. Goodwin Avenue

Urbana, IL 61801

High Intensity ultrasound has found a variety of new application in chemistry. The chemical effects of ultrasound originate from acoustic cavitation, which produces extremely energetic local transient conditions. In cavitating clouds of bubbles, both sonochemistry and sonoluminescence occur. Spectroscopic analysis of multi-bubble sonoluminescence from hydrocarbons and from metal carbonyls reveal temperatures of ~5000K, ~500 atm, with heating and cooling rate that exceeds 1e10 K/s. Single bubble sonoluminescence produces much more symmetric bubble collapse with subsequently much higher effective temperatures during collapse.

In cold liquids, bubble cloud cavitation is able to drive reactions that normally occur only under extreme conditions. Examples include activation of liquid-solid reactions and synthesis of amorphous and nanophase metals, alloys, metal carbides, and nano-colloids.

Another remarkable phenomena occurs during ultrasonic irradiation of liquid-solid slurries: extremely high speed inter-particle collisions. Turbulent flow and shock waves produced by acoustic cavitation can drive metal particles together at sufficiently high velocities to induce melting upon collision. Metal particles that are irradiated in hydrocarbon liquids with ultrasound undergo collisions at roughly half the speed of sound and generate localized effective temperatures of ~3000 K at the point of impact. Both stoichiometric and catalytic liquid-solid reactions can be tremendously enhanced.

Shock formation within sonoluminescence bubbles

Andrew J. Szeri,

University of California, Berkeley

Vi Q. Vuong,

University of California, Irvine

A strong case has been made by several authors that sharp, spherically symmetric shocks converging on the center of a strongly driven bubble give rise to rapid heating and compression that leads to the production of light. The formation of such shocks is considered. It is found that, although at the main collapse the bubble wall does indeed launch a wavy disturbance that propagates into the center, and although the subsequent reflection of the disturbance at the center produces a very rapid temperature and pressure spike, the wavy disturbance can be prevented from steepening into a sharp shock by an adverse gradient in the sound speed. Heat transfer out of the bubble leads to a zone of reduced gas temperature near the bubble surface. Because the bubble is nevertheless being heated rapidly by the collapse, especially if it is a noble gas bubble, a significant radial temperature gradient develops. This corresponds to a significant radial gradient in sound speed. Hence, it is into a region of ever-increasing sound speed that the wavy disturbance propagates. It is shown that the mathematical characteristics of the flow can be prevented from accumulating into a shock front by this adverse sound speed gradient. Both rapid compression heating of the bubble contents and heat flux between the gas and liquid are required to engender an adverse gradient in sound speed; therefore it is argued that shock formation cannot be considered without taking account of heat transfer.

Constraints on quantum vacuum radiation models of

sonoluminescence from experiments

C. S. Unnikrishnan

Gravitation Experiments Group, Tata Institute of Fundamental Research

Homi Bhabha Road, Mumbai (Bombay) - 400 005, INDIA

Dynamically modified electromagnetic vacuum has been put forward as the cause of emission in sonoluminescence by some authors, following ideas pioneered by J. Schwinger. This is an interesting idea, though the parameters of such a model have to be generally on the unrealistic side to reproduce the correct emission power. We have realized that existing results from experiments already contain enough evidence to rule out the vacuum radiation models. We present a detailed analysis of these models in the light of the results on dependence of emission on the amount of dissolved gases, and on the ambient temperature. We also comment on the temporal scales needed in these models, on the basis of what is actually known on the bubble dynamics from the experiments. We argue that the values used in the models are not supported by any experiments, though not ruled out entirely at present. Our results suggest that only an insignificant fraction of the emission, if at all, can be contributed by processes like the dynamic Casimir effect.

New experiments to measure the time scale and other properties of

light emission in sonoluminescence

C. S. Unnikrishnan

Gravitation Experiments Group, Tata Institute of Fundamental Research

Homi Bhabha Road, Mumbai (Bombay) - 400 005, INDIA

The phenomenon of sonoluminescence has presented many puzzling aspects to physicists and chemists, and some of these remain unresolved. One of the most important puzzles in sonoluminescence is the extremely short pulse width of light emission, in the range of a few picoseconds. Precision measurement of pulse characteristics is very difficult and remains a challenge. We have set up an experiment to infer the pulse characteristics from studying the temporal coherence of the pulse in an interferometric configuration. This involves studying the first order and second order interferences of photons from the pulse. This task is generally very complicated for the pulsed weak sources, since nonlinear techniques cannot be used, and coherence is normally not enough to get stable first order interference. Intensity interferometry is difficult for the pulsed source with extremely short pulse widths. In this paper we describe the various attempts to derive useful constraints on the pulse properties, and also present the preliminary results.

Sonoluminescence from an Isolated Hemispherical Bubble on a Solid Surface

K. Weninger

Dept. of Physics, UCLA, Los Angeles, CA 90095

We have studied a regime of sonoluminescence (SL) from an isolated cavity which spontaneously sets up as one increases the amplitude of sound in a resonator that contains a defect such as a wire or other solid body. Within experimental accuracy this cavity is a hemisphere attached to the defect. The hemispherical bubbles emit a sub nanosecond flash of light with each cycle of sound and furthermore the spectrum is strongly weighted in the ulraviolet. The presence, in this complicated geometry of these signatures of the extreme conditions expected to exist in SL from single spherical pulsating bubbles indicates the robust nature of the energy focusing mechanism at play in the fluid mechanics of bubbles. As the hemispherical bubbles are about ten times larger than the spherical bubbles they are more amenable to experimental probes of the energy focusing and light emitting mechanisms. In particular the hemispherical bubbles damage the boundary on which they sit and it may be possible to determine (with photography?) whether this effect is due to jets or shocks or some other mechanism.

A New Model of Single-Bubble Sonoluminescence

Kyuichi Yasui

Department of Physics, Waseda University, 3-4-1 Ohkubo, Shimjuku, Tokyo, Japan

Numerical calculations of bubble oscillations under a condition of single-bubble sonoluminescence (SBSL) are performed, being based on the assumption by Lohse et al. that a SBSL bubble consists of argon in water in which air is dissolved. In the model, effect of non-equilibrium evaporation and condensation of water vapor and that of thermal conduction both inside and outside a bubble are taken into account as in the previous papers by the author (K. Yasui: J. Acoust. Soc. Am. 98 (1995) 2772, K. Yasui: J. Phys. Soc. Jpn. 65 (1996) 2830). In the present calculations, effect of kinetic energy of gas is newly taken into account, which is changed to be heat in a bubble at the final stage of the strong collapse. It is clarified that this effect increases the bubble temperature considerably. It is clarified that the bubble temperature increases to be over 10,000 K though the effect of shock wave is neglected, partly due to the small molar heat of argon compared with air. The black-body radiation is examined as a possible mechanism of SBSL.

Magnetic Field Study of Sonoluminescence

J.B. Young, H.J. Cho, and W. Kang

Department of Physics, University of Chicago, Chicago, IL 60637

T. Schmiedel

National High Magnetic Field Laboratory, Tallahassee, FL 32306

We present results from our on-going study of sonoluminescence in high magnetic fields. In magnetic field sweeps at constant levels of acoustic drive, SL intensity decreases with increasing magnetic field and disappears altogether above a pressure-dependent threshold magnetic field. Sweeps of acoustic drive at fixed magnetic fields show that the upper and lower bounds of forcing pressure that determine the region of SL increase dramatically with magnetic field. An update on our recent work in magnetic field will be presented.