Trapping an intensely bright, stable sonoluminescing bubble

Simultaneous High-Speed Recording of Sonoluminescence and Bubble Dynamics in Multibubble Fields

Multibubble sonoluminescence (MBSL) is the emission of light from imploding cavitation bubbles in dense ensembles or clouds. We demonstrate a technique of high-speed recording that allows imaging of bubble oscillations and motion together with emitted light flashes in a nonstationary multibubble environment. Hereby a definite experimental identification of light emitting individual bubbles, as well as details of their collapse dynamics can be obtained. For the extremely bright MBSL of acoustic cavitation in xenon saturated phosphoric acid, we are able to explore effects of bubble translation, deformation, and interaction on MBSL activity. The recordings with up to 0.5 million frames per second show that few and only the largest bubbles in the fields are flashing brightly, and that emission often occurs repetitively. Bubble collisions can lead to coalescence and the start or intensification of the emission, but also to its termination via instabilities and splitting. Bubbles that develop a liquid jet during collapse can flash intensely, but stronger jetting gradually reduces the emissions. Estimates of MBSL collapse temperature peaks are possible by numerical fits of transient bubble dynamics, in one case yielding 38 000 K.

Sonoluminescence and dynamics of cavitation bubble populations in sulfuric acid

The detailed link of liquid phase sonochemical reactions and bubble dynamics is still not sufficiently known. To further clarify this issue, we image sonoluminescence and bubble oscillations, translations, and shapes in an acoustic cavitation setup at 23kHz in sulfuric acid with dissolved sodium sulfate and xenon gas saturation. The colour of sonoluminescence varies in a way that emissions from excited non-volatile sodium atoms are prominently observed far from the acoustic horn emitter ("red region"), while such emissions are nearly absent close to the horn tip ("blue region"). High-speed images reveal the dynamics of distinct bubble populations that can partly be linked to the different emission regions. In particular, we see smaller strongly collapsing spherical bubbles within the blue region, while larger bubbles with a liquid jet during collapse dominate the red region. The jetting is induced by the fast bubble translation, which is a consequence of acoustic (Bjerknes) forces in the ultrasonic field. Numerical simulations with a spherical single bubble model reproduce quantitatively the volume oscillations and fast translation of the sodium emitting bubbles. Additionally, their intermittent stopping is explained by multistability in a hysteretic parameter range. The findings confirm the assumption that bubble deformations are responsible for pronounced sodium sonoluminescence. Notably the observed translation induced jetting appears to serve as efficient mixing mechanism of liquid into the heated gas phase of collapsing bubbles, thus potentially promoting liquid phase sonochemistry in general.

Positional stability and radial dynamics of sonoluminescent bubbles under bi-harmonic driving: Effect of the high-frequency component and its relative phase

The use of bi-frequency driving in sonoluminescence has proved to be an effective way to avoid the spatial instability (pseudo-orbits) developed by bubbles in systems with high viscous liquids like sulfuric or phosphoric acids. In this work, we present extensive experimental and numerical evidence in order to assess the effect of the high frequency component (PAc(HF)) of a bi-harmonic acoustic pressure field on the dynamic of sonoluminescent bubbles in an aqueous solution of sulfuric acid. The present study is mainly focused on the role of the harmonic frequency (Nf0) and the relative phase between the two frequency components (φb) of the acoustic field on the spatial, positional and diffusive stability of the bubbles. The results presented in this work were analyzed by means of three different approaches. First, we discussed some qualitative considerations about the changes observed in the radial dynamics, and the stability of similar bubbles under distinct bi-harmonic drivings. Later, we have investigated, through a series of numerical simulations, how the use of high frequency harmonic components of different order N, affects the positional stability of the SL bubbles. Furthermore, the influence of φb in their radius temporal evolution is systematically explored for harmonics ranging from the second to the fifteenth harmonic (N=2-15). Finally, a multivariate analysis based on the covariance method is performed to study the dependences among the parameters characterizing the SL bubble. Both experimental and numerical results indicate that the impact of PAc(HF) on the positional instability and the radial dynamics turns to be progressively negligible as the order of the high frequency harmonic component grows (i.e. N ≫ 1), however its effectiveness on the reduction of the spatial instability remains unaltered or even improved.

Stable tridimensional bubble clusters in multi-bubble sonoluminescence (MBSL)

In the present work, stable clusters made of multiple sonoluminescent bubbles are experimentally and theoretically studied. Argon bubbles were acoustically generated and trapped using bi-frequency driving within a cylindrical chamber filled with a sulfuric acid aqueous solution (SA85w/w). The intensity of the acoustic pressure field was strong enough to sustain, during several minutes, a large number of positionally and spatially fixed (without pseudo-orbits) sonoluminescent bubbles over an ellipsoidally-shaped tridimensional array. The dimensions of the ellipsoids were studied as a function of the amplitude of the applied low-frequency acoustic pressure (PAc(LF)) and the static pressure in the fluid (P0). In order to explain the size and shape of the bubble clusters, we performed a series of numerical simulations of the hydrodynamic forces acting over the bubbles. In both cases the observed experimental behavior was in excellent agreement with the numerical results. The simulations revealed that the positionally stable region, mainly determined by the null primary Bjerknes force (F→Bj), is defined as the outer perimeter of an axisymmetric ellipsoidal cluster centered in the acoustic field antinode. The role of the high-frequency component of the pressure field and the influence of the secondary Bjerknes force are discussed. We also investigate the effect of a change in the concentration of dissolved gas on the positional and spatial instabilities through the cluster dimensions. The experimental and numerical results presented in this paper are potentially useful for further understanding and modeling numerous current research topics regarding multi-bubble phenomena, e.g. forces acting on the bubbles in multi-frequency acoustic fields, transient acoustic cavitation, bubble interactions, structure formation processes, atomic and molecular emissions of equal bubbles and nonlinear or unsteady acoustic pressure fields in bubbly media.

Energy balance for a sonoluminescence bubble yields a measure of ionization potential lowering

Application of energy conservation between input sound and the microplasma which forms at the moment of sonoluminescence places bounds on the process, whereby the gas is ionized. Detailed pulsed Mie scattering measurements of the radius versus time for a xenon bubble in sulfuric acid provide a complete characterization of the hydrodynamics and minimum radius. For a range of emission intensities, the blackbody spectrum emitted during collapse matches the minimum bubble radius, implying opaque conditions are attained. This requires a degree of ionization >36%. Analysis reveals only 2.1±0.6  eV/atom of energy available during light emission. In order to unbind enough charge, collective processes must therefore reduce the ionization potential by at least 75%. We interpret this as evidence that a phase transition to a highly ionized plasma is occurring during sonoluminescence.

Energy concentration and positional stability of sonoluminescent bubbles in sulfuric acid for different static pressures

In this study we report several experimental and numerical results on the influence of static pressure (P_{0}) over the main parameters in single bubble sonoluminescence (SBSL), using a sulfuric acid aqueous solution (SA) with low concentrations of argon gas dissolved. Bifrequency driving was used in the experiments to enhance spatial stability of the bubbles. The experimental results were compared with simulations provided by a numerical code that models the radial dynamics of the bubbles. The results showed that an increase on the static pressure of the system shifts the Bjerknes instability threshold, allowing the bubble to access higher acoustic pressures (P_{Ac}^{}). Furthermore, a decrease in the measured ambient radius R_{0} and the calculated relative gas concentration c_{∞}/c_{0} were observed. A notorious increment in the bubble collapse violence and energy focusing for P_{0} above 1 bar was achieved. These were mainly indicated by the growth of the bubble expansion ratio (R_{max}/R_{0}), the bubble mechanical energy density, and the maximum bubble wall velocity dR/dt. In agreement with the previous statement, the maximum temperature during the bubble collapse predicted by the model is augmented as well. The use of different harmonics in the ultrasound pressure field regarding energy focusing is also discussed. Finally, we analyzed the stability regions of the R_{0}-P_{Ac}^{} parameter space via numerical predictions for P_{0} above the measured, identifying the shape instabilities as the main limiting agent to obtain further energy concentration in SA systems at high static pressures.

Path suppression of strongly collapsing bubbles at finite and low Reynolds numbers

We study, numerically and experimentally, three different methods to suppress the trajectories of strongly collapsing and sonoluminescent bubbles in a highly viscous sulfuric acid solution. A new numerical scheme based on the window method is proposed to account for the history force acting on a spherical bubble with variable radius. We could quantify the history force, which is not negligible in comparison with the primary Bjerknes force in this type of problem, and results are in agreement with the classical primary Bjerknes force trapping threshold analysis. Moreover, the present numerical implementation reproduces the spatial behavior associated with the positional and path instability of sonoluminescent argon bubbles in strongly gassed and highly degassed sulfuric acid solutions. Finally, the model allows us to demonstrate that spatially stationary bubbles driven by biharmonic excitation could be obtained with a different mode from the one used in previous reported experiments.

Upscaling energy concentration in multifrequency single-bubble sonoluminescence with strongly degassed sulfuric acid

Single-bubble sonoluminescence (SBSL) was explored under a variety of multifrequency excitations. In particular, biharmonic excitation was used to produce SBSL for unprecedented low dissolved noble gas concentrations in a sulfuric acid solution. Reducing the amount of dissolved noble gas makes it possible to reach higher acoustic pressures on the SL bubble, which otherwise are not attainable because of the Bjerknes instability. By using biharmonic excitation, we were able to experimentally trap and to spatially stabilize SL bubbles for xenon pressure overhead as low as 1 mbar. As a result, we have access to regions in phase space where the plasma temperatures are higher than the ones reached before for bubbles driven at ≈30 kHz.

A new experimental device to evaluate eye ulcers using a multispectral electrical impedance technique

We present a novel experimental technique to determine eye ulcers in animals using a spectral electrical impedance technique. We expect that this technique will be useful in dry eye syndrome. We used a sensor that is basically a platinum (Pt) microelectrode electrically insulated by glass from a cylindrical stainless steel counter-electrode. This sensor was applied to the naked eye of New Zealand rabbits (2.0-3.5 kg in weight). Whereas half of the eyes were normal (control), we applied to the remainder a few drops of 20% (v/v) alcohol to produce an ulcer in the eye. Using a multispectral electrical impedance system we measured ulcerated and control eyes and observed significant difference between normal and pathological samples. We also investigated the effects of different applied pressures and natural degradation of initially normal eyes as a function of time. We believe that this technique could be sufficiently sensitive and repetitive to help diagnose ocular surface diseases such as dry eye syndrome.

Analytical study of the acoustic field in a spherical resonator for single bubble sonoluminescence

The acoustic field in the liquid within a spherical solid shell is calculated. The proposed model takes into account Stoke's wave equation in the viscous fluid, the membrane theory to describe the solid shell motion and the energy loss through the external couplings of the system. A point source at the resonator center is included to reproduce the acoustic emission of a sonoluminescence bubble. Particular calculations of the resulting acoustic field are performed for viscous liquids of interest in single bubble sonoluminescence. The model reveals that in case of radially symmetric modes of low frequency, the quality factor is mainly determined by the acoustic energy flowing through the mechanical coupling of the resonator. Alternatively, for high frequency modes the quality factor is mainly determined by the viscous dissipation in the liquid. Furthermore, the interaction between the bubble acoustic emission and the resonator modes is analyzed. It was found that the bubble acoustic emission produces local maxima in the resonator response. The calculated amplitudes and relative phases of the harmonics constituting the bubble acoustic environment can be used to improve multi-frequency driving in sonoluminescence.

Resolving the shape of a sonoluminescence pulse in sulfuric acid by the use of streak camera

A streak camera is used to measure the shape of sonoluminescence pulses from a cavitation bubble levitated stably in a sulfuric acid solution. The shape and response to an acoustic pressure field of the sonoluminescence pulse in 85% by weight sulfuric acid are qualitatively similar to those in water. However, the pulse width in sulfuric acid is wider than that in water by over one order of magnitude. The width of the sonoluminescence pulse is strongly dependent on the concentration of the sulfuric acid solution, while the skewed distribution of the shape remains unchanged.

Dynamics of sonoluminescing bubbles within a liquid hammer device

We studied the dynamics of a single sonoluminescing bubble (SBSL) in a liquid hammer device. In particular, we investigated the phosphoric acid-xenon system, in which pulses up to four orders of magnitude brighter than SBSL in water systems (about 10;{12} photons per pulse) have been previously reported [Chakravarty, Phys. Rev. E 69, 066317 (2004)]. We used stroboscopic photography and a Mie scattering technique in order to measure the radius evolution of the bubbles. Under adequate conditions we may position a bubble at the bottom of the tube (cavity) and a second bubble trapped at the middle of the tube (upper bubble). During its collapse, the cavity produces the compression of the liquid column. This compression drives impulsively the dynamics of the upper bubble. Our measurements reveal that the observed light emissions produced by the upper bubble are generated at its second collapse. We employed a simple numerical model to investigate the conditions that occur during the upper bubble collapse. We found good agreement between numerical and experimental values for the light intensity (fluence) and light pulse widths. Results from the model show that the light emission is increased mainly due to an increase in noble gas ambient radius and not because the maximum temperature increases. Even for the brightest pulses obtained ( 2x10;{13} photons, about 20W of peak power) the maximum temperatures computed for the upper bubble are always lower than 20000K .

Time-resolved spectra of single-bubble sonoluminescence in sulfuric acid with a streak camera

The time-resolved spectra of single-bubble sonoluminescence (SBSL) in sulfuric acid have been observed with a streak camera after a spectrograph. The spectral center evolves from infrared to ultraviolet gradually within a SBSL duration, which corresponds to an increase of temperature. The peak temperature within one sonoluminescence (SL) duration is 5-9 times higher than the average temperature based on the average spectrum in our experiment. Furthermore, the ratio of the peak temperature to average temperature increases with the increase of driving pressure. The SBSL flash dies out after a dramatic heating-up, and there is no cooling procedure observed at the time resolution of 110 SL duration, which is incompatible with the radius-related adiabatic heating model as the mechanism of SBSL.