Nature of Sonoluminescence: Noble Gas Radiation Excited by Hot Electrons in "Cold" Water

We show that strong electric fields occurring in water near the surface of collapsing gas bubbles because of the flexoelectric effect can provoke dynamic electric breakdown in a micron-size region near the bubble and consider the scenario of the SBSL. The scenario is: (i) at the last stage of incomplete collapse of the bubble the gradient of pressure in water near the bubble surface has such a value and sign that the electric field arising from the flexoelectric effect exceeds the threshold field of the dynamic electrical breakdown of water and is directed to the bubble center; (ii) mobile electrons are generated because of thermal ionization of water molecules near the bubble surface; (iii) these electrons are accelerated in ''cold'' water by the strong electric fields; (iv) these hot electrons transfer noble gas atoms dissolved in water to high-energy excited states and optical transitions between these states produce SBSL UV flashes in the trasparency window of water; (v) the breakdown can be repeated several times and the power and duration of the UV flash are determined by the multiplicity of the breakdowns. The SBSL spectrum is found to resemble a black-body spectrum where temperature is given by the effective temperature of the hot electrons. The pulse energy and some other characteristics of the SBSL are found to be in agreement with the experimental data when realistic estimations are made.Comment: 11 pages (RevTex), 1 figure (.ps

SURFACE OSCILLATIONS AND JET DEVELOPMENT IN PULSATING BUBBLES

Cet article décrit une méthode permettant de provoquer des jets liquides périodiques dans des bulles qui oscillent. Les bulles sont maintenues près d'une paroi plane dans un récipient qu'on fait vibrer. La pression ambiante au-dessus du liquide est réduite jusqu'à une valeur voisine de la pression vapeur. Dans ces conditions les bulles dont la fréquence de résonance est proche de la fréquence excitatrice (60 Hz) présentent des mouvements d'amplitude importante. Ceci est propice à la formation de jets liquides. Des photographies montrent les divers aspects de cette formation ainsi que quelques formes curieuses des bulles.This paper describes a method for producing cyclic liquid jets in pulsating bubbles that have been acoustically trapped near a platform in a vibrating container. The ambient pressure above the liquid is reduced to near that of the vapor pressure of the liquid, and vapor-air bubbles driven near resonance size at 60 Hz develop large pulsations that can readily lead to jet development. Photographs are presented of various aspects of jet production as well as of some intriguing displays of bubble surface oscillations

The underwater sounds produced by impacting snowflakes.

In 1985, Scrimger [Nature 318, 647 (1985)] reported measurements of noise levels significantly above the ambient level for snow falling on a quiet freshwater lake. He examined only the time-averaged sound levels and did not report measurements of individual snowflake impacts. Subsequently, the noise produced by individual and multiple snowflake impacts was examined for a number of different snowfalls. The radiated acoustic signals generated by the impact of individual snowflakes upon a body of water have a remarkable similarity to each other and differ principally in the frequency of the emitted sound wave. The acoustic signal of a snowflake impact thus generates a characteristic signature for snowfall that is clearly distinct from other forms of precipitation noise. Various aspects of this signature suggest that the radiated acoustic waveform from a snowflake impacting with water is due to the entrainment of a gas bubble into the liquid, and the subsequent oscillation of this bubble as it establishes its equilibrium state. Various scenarios are presented for bubble entrainment and approximations to the amplitude of the radiated signal and the acoustic waveform are obtained. </p