Blackbody spectra for sonoluminescing hydrogen bubbles

The dynamical motion of sonoluminescing bubbles formed from a mixture of water and hydrogen gas indicates that these bubbles contain hydrogen. Their spectrum is well matched by an ideal 6000 K blackbody radiating from a surface with a radius less than 1/4 microm. According to this model, the state of matter inside the collapsed bubble is so stressed that the photon mean free path is much smaller than 1 microm. Implications for various theories of the light-emitting mechanism and the role of chemical reactions are discussed.     

Influence of ultrasonic frequency on multibubble sonoluminescence

Computer simulations of bubble oscillations are performed under conditions of multibubble sonoluminescence (MBSL) in water for various ultrasonic frequencies. The range of the ambient bubble radius for sonoluminescing bubbles narrows as the ultrasonic frequency increases; at 20 kHz it is 0.1-100 microm while at 1 MHz it is 0.1-3 microm. At 1 MHz, any sonoluminescing bubble disintegrates into a mass of smaller bubbles in a few or a few tens of acoustic cycles, while at 20 kHz and 140 kHz some sonoluminescing bubbles are shape stable. The mechanism of the light emission also depends on the ultrasonic frequency. As the ultrasonic frequency increases, the amount of water vapor trapped inside bubbles at the collapse decreases. As a result, MBSL originates mainly in plasma emissions at 1 MHz while it originates in chemiluminescence of OH radicals and plasma emissions at 20 kHz.     

Symmetry reduction for molecular dynamics simulation of an imploding gas bubble

We propose a symmetry reduction technique whereby molecular dynamics (MD) simulations for spherically symmetric gas bubbles can be accelerated. Results for an imploding Xenon bubble containing 50 million particles—the smallest measured sonoluminescing system—are presented.     

On the dynamics of moving single bubble sonoluminescence

It is well known that the primary Bjerknes force is the origin of the trapping of sonoluminescing bubble in the sound field in liquid. In the present Letter, the quantitative investigation of the behavior of hydrodynamic force on the moving sonoluminescing (SL) bubble introduces the new role of stabilizing the trajectory motion of the bubble for primary Bjerknes force. Using a complete force balanced radial-translational dynamics, it is analytically discussed that by increasing the bubble distance from the antinode of the sound field the increase of the magnitude of inward Bjerknes force, controls the size of the domain of the bubble trajectory. At this time the wake produced by the rapid variation of the bubble's relative translational velocity to the surrounding liquid, changes the bubble direction of motion through the effect of history force. The required momentum for accelerating the SL bubble around the central antinode is produced by the added mass force at the bubble collapse. It is revealed in a re-examination of the coupled radial-translational dynamics for a trapping bubble that because of the bubble lower translational acceleration caused due to the lower added mass force and the bubble attraction towards the acoustic antinodes in presence of inward Bjerknes force, the small bubble will be trapped at the antinode of the sound field.     

Plasma in sonoluminescing bubble

With the new accommodation coefficient of water vapor evaluated by molecular dynamics model, the maximum temperature of a sonoluminescing bubble calculated with the full partial differential equations easily reaches few tens of thousands degrees. Though at this temperature the gas is weakly ionized (10% or less), the gas density inside a sonoluminescing bubble at the moment of the bubble's flashing is so high that there still forms a dense plasma. The light emission of the bubble is calculated by the plasma model which is compared with that by the bremsstrahlung (electron-ion, electron-neutral atom) and recombination model. The calculation by the two models shows that for the relatively low maximum temperature (< 30,000 K) of the bubble, the pulse width is independent of the wavelength and the spectrum deviates the black body radiation type; while for the relatively high maximum temperature (approximately 60,000 K), the pulse width is dependent of the wavelength and the spectrum is an almost perfect black body radiation spectrum. The maximum temperature calculated by the gas dynamics equations is much higher than the temperature fitted by the black body radiation formula.     

A search for laser heating of a sonoluminescing bubble

Thermodynamic arguments show that a sonoluminescing bubble that acts as a black body can absorb energy from a high power, pulsed laser beam. The mechanism for increasing the temperature of a sonoluminescing bubble is discussed and experiments with a frequency doubled, pulsed Nd:YAG laser are described. A bound for the amount of radiation absorbed is given as no evidence for heating was found at the low radiation intensity employed in the experiments.     

The size of active bubbles for the production of hydrogen in sonochemical reaction field

The sonication of aqueous solution generates microscopic cavitation bubbles that may growth and violently collapse to produce highly reactive species (i.e. OH, HO₂ and H₂O₂), hydrogen and emit light, sonoluminescence. The bubble size is a key parameter that influences the chemical activity of the system. This wok aims to study theoretically the size of active bubbles for the production of hydrogen in ultrasonic cavitation field in water using a single bubble sonochemistry model. The effect of several parameters such as frequency of ultrasound, acoustic intensity and liquid temperature on the range of sonochemically active bubbles for the production of hydrogen was clarified. The numerical simulation results showed that the size of active bubbles is an interval which includes an optimum value at which the production rate of H₂ is maximal. It was shown that the range of ambient radius for an active bubble as well as the optimum bubble radius for the production of hydrogen increased with increasing acoustic intensity and decreased with increasing ultrasound frequency and bulk liquid temperature. It was found that the range of ambient bubble radius dependence of the operational conditions followed the same trend as those reported experimentally for sonoluminescing bubbles. Comparison with literature data showed a good agreement between the theoretical determined optimum bubble sizes for the production of hydrogen and the experimental reported sizes for sonoluminescing bubbles.     

Opacity and transport measurements reveal that dilute plasma models of sonoluminescence are not valid

A strong interaction between a nanosecond laser and a 70?μm radius sonoluminescing plasma is achieved. The overall response of the system results in a factor of 2 increase in temperature as determined by its spectrum. Images of the interaction reveal that light energy is absorbed and trapped in a region smaller than the sonoluminescence emitting region of the bubble for over 100?ns. We interpret this opacity and transport measurement as demonstrating that sonoluminescencing bubbles can be 1000 times more opaque than what follows from the Saha equation of statistical mechanics in the ideal plasma limit. To address this discrepancy, we suggest that the effects of strong Coulomb interactions are an essential component of a first principles theory of sonoluminescence.     

Measurement of pressure and density inside a single sonoluminescing bubble

The average pressure inside a sonoluminescing bubble in sulfuric acid has been determined by two independent techniques: (1) plasma diagnostics applied to Ar atom emission lines, and (2) light scattering measurements of bubble radius vs time. For dimly luminescing bubbles, both methods yield intracavity pressures approximately 1500 bar. Upon stronger acoustic driving of the bubble, the sonoluminescence intensity increases 10,000-fold, spectral lines are no longer resolved, and radius vs time measurements yield internal pressures > 3700 bar. Implications for a hot inner core are discussed.     

An inverse method to fast-track the calculation of phase diagrams for sonoluminescing bubbles

A sound driven air bubble can be transformed into an argon bubble emitting light pulses stably. The very foundation to investigate the sonoluminescing bubble is to accurately determine the ambient radius and gas composition in the interior. The conventional approach is to model the air-to-argon transformation process through a large number of bubble dynamics simulations to obtain the physical parameters of the ultimate argon bubble. In this paper, we propose a highly efficient method to pinpoint this information in a phase diagram. The method is based on the diffusive equilibrium for each species inside the bubble and derives the ambient radius and composition inversely. To calculate the former parameter, the bisection algorithm is employed to consecutively narrow down the searching range until the equilibria is approached. Afterward, several cycles of full dynamics simulations are conducted to refine the composition. The method is validated using published experimental data. The calculated ambient radii deviate from the test results by less than 1 μm, which falls within the margin of measurement error. The advantages of this method over the semi-analytical approach reported by Hilgenfeldt et al. [J. Fluid Mech. 365 (1998)] are also discussed. Our study provides a standard procedure to calculate the ambient radius and composition and is beneficial for the numerical simulation of sonoluminescing bubbles.     

Sonoluminescence from a single bubble driven at 1 megahertz

Measurements of the spectrum of sonoluminescence from an isolated bubble driven at 1 MHz are well fit by assuming thermal bremsstrahlung from a transparent 10(6) degree plasma. According to this interpretation, the photon-matter mean free path is larger than the light-emitting radius of a 1 MHz bubble, but smaller than the light-emitting radius for bubbles driven at approximately 40 kHz, thus accounting for the observed blackbody spectrum at 40 kHz.     

Characterisation of the acoustic cavitation cloud by two laser techniques

An experimental investigation of the size and volumetric concentration of acoustic cavitation bubbles is presented. The cavitation bubble cloud is generated at 20 kHz by an immersed horn in a rectangular glass vessel containing bi-distilled water. Two laser techniques, laser diffraction and phase Doppler interferometry, are implemented and compared. These two techniques are based on different measuring principles. The laser diffraction technique analyses the light pattern scattered by the bubbles along a line-of-sight of the experimental vessel (spatial average). The phase Doppler technique is based on the analysis of the light scattered from single bubbles passing through a set of interference fringes formed by the intersection of two laser beams: bubble size and velocity distributions are extracted from a great number of single-bubble events (local and temporal average) but only size distributions are discussed here. Difficulties arising in the application of the laser diffraction technique are discussed: in particular, the fact that the acoustic wave disturbs the light scattering patterns even when there are no cavitation bubbles along the measurement volume. As a consequence, a procedure has been developed to correct the raw data in order to get a significant bubble size distribution. After this data treatment has been applied the results from the two measurement techniques show good agreement. Under the emitter surface, the Sauter mean diameter D(3, 2) is approximately 10 microm by phase Doppler measurement and 7.5 microm by laser diffraction measurement at 179 W. Note that the mean measured diameter is much smaller than the resonance diameter predicted by the linear theory (about 280 microm). The influence of the acoustic power is investigated. Axial and radial profiles of mean bubble diameters and void fraction are also presented.     

Computational Simulation of Sodium Doublet Line Intensities in Multibubble Sonoluminescence

We perform a computational simulation of the fluid dynamics of sodium doublet(Na-D)line emissions from one sonoluminescing bubble among the cavitation bubbles in argon-saturated Na hydroxide(NaOH)aqueous solutions.Our simulation includes the distributions of acoustic pressures and the dynamics of cavitation bubbles by numerically solving the cavitation dynamic equation and bubble-pulsation equation.The simulation results demonstrate that when the maximum temperature inside a luminescing bubble is relatively low,two emission peaks from excited Na are prominent within the emission spectra,at wavelengths of 589.0 and 589.6 nm.As the maximum temperature of the bubble increases,the two peaks merge into one peak and the full width at half maximum of this peak increases.These calculations match with the observations of Na-D line emissions from MBSL occurring in aqueous solutions of NaOH under an argon gas.     

Resonance frequency of microbubbles: effect of viscosity

The transmitted frequency at which a gas bubble of millimeter or submillimeter size oscillates resonantly in a low-viscosity liquid is approximately equal to the undamped natural frequency (referred to as the Minnaert frequency if surface tension effects are disregarded). Based on a theoretical analysis of bubble oscillation, this paper shows that such an approximation cannot be validated for microbubbles used in contrast-enhanced ultrasound imaging. The contrast-agent microbubbles represent either encapsulated bubbles of size less than 10 microm or free (nonencapsulated) bubbles of submicron size. The resonance frequency of the microbubbles deviates significantly from the undamped natural frequency over the whole range of microbubble sizes due to the increased viscous damping coefficient. The difference between these two frequencies is shown to have a tremendous impact on the resonant backscatter by the microbubbles. In particular, the first and second harmonics of the backscattered signal from the microbubbles are characterized by their own resonance frequencies, equal to neither the microbubble resonance frequency nor the undamped natural frequency.     

Moving single bubble sonoluminescence in phosphoric acid and sulphuric acid solutions

The phenomenon of sonoluminescence still presents some unsolved aspects. Recently [Y.T. Didenko, K. Suslick, Molecular Emission during Single Bubble Sonoluminescence, Nature 407 (2000) 877-879.], it was found that a single cavitating air bubble in polar aprotic liquids (including formamide and adiponitrile) can produce very strong sonoluminescence while undergoing macroscopic translation movements in the resonator, a condition known as moving single bubble sonoluminescing (MSBSL). Here we describe some experiments conducted in aqueous solutions of phosphoric and sulphuric acid. In these liquid media, it is possible to reproduce MSBSL and luminescence is emitted even if a trapped bubble is subjected to a strong shape instability, named in the literature "jittering phase". When a moving and luminescing bubble was present and the acoustic pressure gradually increased, we observed the generation of a discrete lattice of trapped bubbles. The bubbles in the lattice emit very intense light flashes and can change their position while maintaining the overall spatial distribution in time. Some preliminary results, obtained from Mie-scattering and measurements of relative light intensity, are reported.     

声致发光单泡的Mie散射测量参数的理论计算

本文运用Mie散射理论系统地计算了适用于声致发光单泡的Mie散射测量参数，利用Dave倒推算法得出散射光强随半径及散射角的变化曲线I（R），I（θ），结果表明：适合于连续Mie散射测量，脉冲Mie散射测量及Mie散射定标的散射角及接收器张角是各不相同的。文中给出了合适的参数范围。     

Concomitance in single bubble sonoluminescence of period doubling in emission and shape distortion

We report the first direct observation for a single stable sonoluminescing bubble of a shape instability. Furthermore we show that stable saturation of the shape distortion caused by the instability for a certain range of parameters is experimentally possible and furthermore is directly linked to the curious phenomenon of flash by flash period doubling of the sonoluminescent emission as the afterbounce instability causing the shape distortion is always period doubled whenever the emission is & vice versa.     

Electron-probe X-ray microanalysis of individual particles of solid snow sediment with size factor correction

A technique is proposed for X-ray electron-probe microanalysis (EPMA) of individual particles of solid snow sediment. Two sample preparation procedures were used to analyze the sediments. The PAP-method and the original bi-exponential model were used for matrix correction and the calculation of elemental composition. A method of calculating composition has been developed for approximately spherical individual particles, comparable in size with an area of X-ray generation. An analytical expression is proposed for determining the composition of these particles, taking account of their size. Including the size factor reduces the error of composition determination from 0.5-45 to 0.2-22% relative percent, for particles in the size range 1-3 microm.     

Investigation on the cavitation bubble collapse and the movement characteristics near spherical particles based on Weiss theorem

In this paper, the cavitation bubble dynamics near two spherical particles of the same size are investigated theoretically and experimentally. According to the Weiss theorem, the flow characteristics and the Kelvin impulse are obtained and supported by the sufficient experimental data. In terms of the initial bubble position, the bubble size and the distance between the two particles, the collapse morphology and the movement characteristics of the bubble are revealed in detail. The main findings include: (1) Based on a large number of experimental results, it is found that the Kelvin impulse theoretical model established in this paper can effectively predict the movement characteristics of the cavitation bubble near two particles of the same size. (2) When the initial bubble position is gradually away from the particles along the horizontal symmetry axis near two particles of the same size, the movement distance of the bubble centroid in the first period increases first and then decreases. (3) When the initial position of the bubble centroid is at the asymmetric position near the two particles, the movement direction of the bubble centroid is biased towards the particle closer to the bubble, but not towards the center of this particle.