No abstract

No abstract

We studied the dependence of thermodynamic variables in a sonoluminescing ͑SL͒ bubble on various physical factors, which include viscosity, thermal conductivity, surface tension, the equation of state of the gas inside the bubble, as well as the compressibility of the surrounding liquid. The numerical solutions show that the existence of shock waves in the SL parameter regime is very sensitive to these factors. Furthermore, we show that even without shock waves, the reflection of continuous compressional waves at the bubble center can produce the high temperature and picosecond time scale light pulse of the SL bubble, which implies that SL may not necessarily be due to shock waves. ͓S1063-651X͑98͒09504-X͔ PACS number͑s͒: 78.60. Mq, 47.40.Ϫx, 44.10.ϩi, 43.25.ϩy

By solving numerically the full set of hydrodynamic equations governing the pulsation of a bubble, we show that shock waves are often absent in a stable sonoluminescing bubble. Nevertheless, for a wide range of physical parameters, a continuous compressional wave emerges and heats up the bubble, and the resulting black-body radiations have pulse heights and widths that agree with experimental data. Shock waves, being much less robust, are not essential for stable single-bubble sonoluminescence. ͓S1063-651X͑98͒50908-7͔ PACS number͑s͒: 78.60. Mq, 47.40.Ϫx, 44.10.ϩi, 43.25.ϩy The remarkable discovery that acoustic energy can be converted to light through an oscillating air bubble trapped in water has ignited widespread interest in single-bubble sonoluminescence ͑SBSL͒. Under certain conditions, a narrow and regular flash of light with a width of about 50-250 ps is emitted in each cycle of the bubble oscillation ͓1,2͔. Such a conversion of sound into light represents a high concentration of energy, and it has been speculated that the temperature and pressure at the center of the bubble could be high enough to ignite thermonuclear fusion ͓3͔.Previous calculations based on inviscid spherical hydrodynamics ͓3-7͔ suggested that a converging shock produced high temperature and pressure and the reflected diverging shock quenched them in picosecond time scale ͓3,4,8͔. This model could explain the emergence of a picosecond time scale as well as the large energy concentration, but there are still many unanswered questions, such as whether the shock waves are stable and robust. The effects of transport processes, surface tension, the equation of state ͑EOS͒, and the compressibility of liquid were also largely unknown and ignored.An important clue to the mechanism of SBSL is its sensitivity to the gas content: apparently the presence of noble gas is essential. This can be explained by the argonrectification theory ͓9͔, in which air molecules dissociate and react and leave the bubble in the pre-SBSL stage, and argon becomes the dominant bubble content when stable lightemission takes place. If shock-heating is essential to SBSL, then one expects that shock waves should develop easily in an argon bubble. A major goal in this Rapid Communication is to study and compare the relevance of shock waves in both air and argon SBSL.To determine whether shock waves are important to SBSL, and to study the sensitivity of the process to the experimental conditions, a reliable hydrodynamic simulation that takes into account various physical processes affecting the bubble, including diffusive transport and different bubble content, surface tension, and liquid compressibility, is essential. In this Rapid Communication, based on a comprehensive numerical study of the bubble hydrodynamics, we show that shock waves are absent in a large parameter range of argon SBSL, in agreement with an earlier calculation by Vuong and Szeri ͓10͔. Rather, a smooth compressional wave emerges naturally in a collapsing bubble, and a large fraction of the bubble co...

We present an exact analytic solution of a nonlinear Schrödinger field interacting with a moving potential (obstacle) at supersonic speed. We show that the field forms a stable shape-invariant structure localized around the obstacle -a dressing effect that protects the field against excitations by the obstacle.PACS numbers: 03.75. Fi, 67.57.De One of the most intriguing phenomena in superfluid is frictionless flow below a critical velocity v c [1]. This phenomenon, first observed in liquid helium, demonstrates key features of collective behavior in macroscopic quantum coherent systems. Recently, experimental evidence of the existence of a critical velocity in a Bose-Einstein condensed gas has been found [2]. It is known that atoms in such a dilute system interact weakly with each other, and so it justifies the use of the nonlinear Schrödinger equation (NLSE) to study the quantum dynamics of the condensate. Indeed, direct numerical solutions of the NLSE have indicated a distinct transition from superfluid flow to resistive flow at a critical velocity [3,4]. Recent studies have also addressed dynamic features in solutions of the NLSE, such as phonon and vortex emissions [5][6][7][8], in order to understand the dissipation mechanisms.As an obstacle moves through a Bose condensate, the regime of supersonic speed is often considered as a dissipation domain where frictionless motion disappears. This is understood from Landau's argument that energymomentum conservation forbids phonon excitations unless the obstacle's velocity is at least the speed of sound c, i.e., the critical velocity v c equals c for systems with a pure phonon spectrum [9]. In fact, for realistic systems obeying the NLSE, v c is less than c because of the reduced fluid densities appearing at the boundary of the obstacle [3][4][5]10]. Therefore the speed of sound seems to be an upper limit beyond which dissipation occurs generally. However, we shall show that there exist interesting exceptions at least in one dimensional systems. Such exceptions exhibit rich dynamic features resulted from coherent field-obstacle interactions, which cannot be captured by the concept of critical velocities alone.In this paper we present an exact analytic solution of a nonlinear Schrödinger field interacting with a moving repulsive potential (obstacle) in one dimension. Our solution represents a family of motional dressed states in which the field organizes itself as a stable and shape invariant structure localized at the moving obstacle. Once the dressing is fully developed, there is no energy transfer from the obstacle to the field. We show that this happens when the velocity of the potential is greater than the speed of sound in the field. Therefore dressed states formation is a novel coherent feature that can be maintained in a condensate at high speeds. The existence of dressed states at supersonic speed requires a specific matching between the field and the potential. Our solution provides a prescription for the matching potentials. This opens possibilities of ...