Inverse effects of the gas feed positioning on sonochemistry and sonoluminescence

Purging of solutions to enhance sonochemical reactions is a common practice. A fundamental study combining sonoluminescence spectroscopy and sonochemical activity is adopted to study the effects of continuous Ar gas flow in the solution and of the position of the gas inlet tube on high-frequency sonolysis of aqueous solutions. It has been observed that neither sonochemical activity nor sonoluminescence intensity is controlled by the gas solubility only. Besides, the change in position of the gas inlet tube leads to opposite effects in sonoluminescence intensity and sonochemical activity: while the former increases, the latter decreases. Such an observation has never been reported despite sonochemical reactions have been carried out under different gas environments. Sonoluminescence spectroscopy indicates that more extreme conditions are reached at collapse with the gas inlet on the side, which could be explained by a more symmetrical collapse. Finally, it is shown in certain conditions that it is possible to favor the formation of some sonochemical products simply by positioning the gas inlet at different positions, which has practical significance in designing large scale sonochemical reactors for industrial applications.     

Theory of sonoluminescence in single bubbles: Flexoelectric energy contribution

An equation of motion for a cavitating gas bubble immersed in a liquid is introduced which includes a flexoelectric energy term. This energy is deduced from the electric field produced by the bubble wall acceleration (pressure gradient) in the fluid (the flexoelectric effect). We show that under conditions of sonoluminescence, this electric field reaches values typical of the electric breakdown field in water. Our theoretical results are consistent with the duration of light emission, minimum bubble radius, and energy release as measured in sonoluminescence experiments in water. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 442–448 (25 September 1998)     

Enhancement of sonoluminescence emission from a multibubble cavitation zone

Investigations have been performed on various methods of increasing cavitation activity measured by the intensity of sonoluminescence. It is shown that the effect of the combined action of (a) pulsed modulation of an acoustic field, (b) liquid degassing and cooling and (c) increasing the static pressure considerably exceeds the sum of the effects achieved by each of these methods individually. A more than 250-fold increase of the sonoluminescence intensity has been attained compared with continuous irradiation under normal conditions (room temperature, atmospheric pressure, gas-saturated liquid). An interpretation of the results obtained is proposed.     

Luminescence mechanism of acoustic and laser-induced cavitation

A new approach is proposed for explaining the experimental data on sonoluminescence of acoustic and laser-induced cavitation bubbles. It is suggested that two different sonoluminescence mechanisms, namely, thermal and electric ones, are possible and that they manifest themselves depending on the bubble dynamics. An intense thermal luminescence occurs as a result of compression of an individual stationary spherical bubble; a weak electric luminescence accompanies the deformation and splitting of the bubble when thermal luminescence is suppressed (for example, in the case of multibubble sonoluminescence). It is shown that, when an individual bubble loses its spherical shape under the effect of different actions (change in the acoustic pressure, artificial deformation, translatory motion, etc.) or when a laser-induced bubble undergoes fragmentation, the sonoluminescence spectrum exhibits specific bands that are similar to the bands in the multibubble sonoluminescence spectrum. The appearance of these bands is attributed to the suppression of the thermal sonoluminescence mechanism and the manifestation of the electric mechanism. It is shown that the maximum temperature T _max characterizing the compression of a laser-induced bubble is primarily determined by the temperature of the plasma at the instant of the laser-induced breakdown, whereas, for an acoustic bubble, T _max is primarily determined by the acoustic and hydrostatic pressures and by the saturation vapor pressure of the liquid.     

Multi-bubble sonoluminescence of phosphoric acid

Multi-bubble sonoluminescence spectra of 85% H₃PO₄ and the dependences of sonoluminescence intensity on the acid concentration and temperature are obtained. The spectra contain a weakly structured 300–600-nm band formed by the superposition of radiation from several emitters (presumably, oxygencontaining products of acid sonolysis, viz., PO, HOPO, and PO₂). Weak luminescence at a wavelength exceeding 600 nm can be due to emission from excited O* and Ar* atoms. The shape of the fundamental band changes upon a transition from multi-bubble sonolysis to sonolysis in the setup for one-bubble sonoluminescence, in which several clusters of cavitation bubbles are formed in a spherical flask at ultrasonic frequencies multiple of the first acoustic resonance frequency (multi-cluster sonoluminescence). The form of the temperature dependence of the sonoluminescence intensity depends on the detection regime: for natural heating of 85% acid under the action of ultrasound, a curve with a luminescence peak at 40°C is observed, while in detection with preliminary thermostating “over points,” only an inflection exists on a monotonic curve describing a decrease of intensity upon heating. An analogous curve for acids with a lower viscosity (hydrochloric and nitric acids) has neither a peak nor inflection irrespective of the detection regime. It is concluded that the viscosity of phosphoric acid plays a decisive role in the evolution of cavitation and in obtaining intense sonoluminescence.     

Optimization of a sonochemical reactor using a pulsing operation

It is known that sonochemical reactions are enhanced by pulsing ultrasound. A method to optimize a sonochemical reactor using a pulsing operation was studied through the measurement of changes in sonoluminescence (SL) intensity from distilled water under various experimental conditions. It was confirmed that pulsing with a constant input power level enhanced SL intensity at lower power levels because of the higher amplitude of ultrasound. In contrast to this, a quenching effect due to excessive sound pressure appeared at higher power levels, and the pulsing operation was not effective under these conditions. Pulsing is more effective at higher frequency than at lower frequency.     

Influence of frequency sweep on sonochemiluminescence and sonoluminescence

Bubbles generated by acoustic cavitation may be efficient in light production by direct emission (sonoluminescence) or indirect emission (sonochemiluminescence) depending on operating parameters such as acoustic pressure and surface tension. These conditions are quite difficult to reach at very high frequencies, even by concentrating the acoustic power at a given location via focusing the acoustic field thanks to the transducer shape (High Intensity Focused Ultrasound). The current work aims at probing the cavitation bubble behaviour under short frequency sweeps by monitoring sonochemiluminescence and sonoluminescence activities. When the frequency was swept in reverse (negative sweep), an enhancement in the SCL, relative to the SCL observed under a single frequency irradiation, was observed. Conversely, a positive frequency sweep resulted in the quenching of SCL intensity. The degree of SCL enhancement and quenching was also dependent on the rate at which the frequency was being swept and on the change in the size of cavitation bubbles. The size of cavitation bubbles varied with varying starting sweep frequency (3.4, 3.6 and 4.2 MHz), affecting both SCL and sonoluminescence (SL) emissions. The addition of a surfactant (sodium dodecyl sulphate) affected the observed results, possibly due to its influence on coalescence between cavitation bubbles. The results suggest that the enhancement and quenching are related to the response of bubbles generated by the starting frequency to the direction of the frequency sweep and the influence of the sweep rate on growth and coalescence of bubbles, which affected the population of the active bubbles.     

Sonoluminescence

The results of our studies of sonoluminescence are summarized. Sonoluminescence spectra are interpreted in terms of the distribution of spectral intensity with wavelength, the spectral changes that occur due to scavenging of radiative species, and the shift and the broadening of emission bands. Estimates of conditions within a bubble during collapse are obtained from the spectral features. There is considerable evidence that sonoluminescence is produced by chemical processes induced by thermal mechanism.     

Computational simulation of ionization processes in single-bubble and multi-bubble sonoluminescence

The most recent spectroscopic studies of moving-single bubble sonoluminescence (MSBSL) and multi-bubble sonoluminescence (MBSL) have revealed that hydrated electrons (e$_{{\rm aq}}^{-}$) are generated in MSBSL but absent in MBSL. To explore the mechanism of this phenomenon, we numerically simulate the ionization processes in single- and multi-bubble sonoluminescence in aqueous solution of terbium chloride (TbCl$_{3}$). The results show that the maximum degree of ionization of single-bubble sonoluminescence (SBSL) is approximately 10000 times greater than that of MBSL under certain special physical parameters. The hydrated electrons (e$_{{{\rm aq}}}^{-}$) formed in SBSL are far more than those in MBSL provided these electrons are ejected from a bubble into a liquid. Therefore, the quenching of e$_{{{\rm aq}}}^{-}$ to SBSL spectrum is stronger than that of the MBSL spectrum. This may be the reason that the trivalent terbium [Tb(III)] ion line intensities from SBSL in the TbCl$_{3}$ aqueous solutions with the acceptor of e$_{{{\rm aq}}}^{-}$ are stronger than those of TbCl$_{3}$ aqueous solutions without the acceptor of e$_{{{\rm aq}}}^{-}$. Whereas the Tb(III) ion line intensities from MBSL are not variational, which is significant for exploring the mechanism behind the cavitation and sonoluminescence.     

A fundamental study on the degradation of paracetamol under single- and dual-frequency ultrasound

The degradation of paracetamol, a widely found emerging pharmaceutical contaminant, was investigated under a wide range of single-frequency and dual-frequency ultrasonic irradiations. For single-frequency ultrasonic irradiation, plate transducers of 22, 98, 200, 300, 400, 500, 760, 850, 1000, and 2000 kHz were employed and for dual-frequency ultrasonic irradiation, the plate transducers were coupled with a 20 kHz ultrasonic horn in opposing configuration. The sonochemical activity was quantified using two dosimetry methods to measure the yield of HO• and H₂O₂ separately, as well as sonochemiluminescence measurement. Moreover, the severity of the bubble collapses as well as the spatial and size distribution of the cavitation bubbles were evaluated via sonoluminescence measurement. The paracetamol degradation rate was maximised at 850 kHz, in both single and dual-frequency ultrasonic irradiation. A synergistic index higher than 1 was observed for all degrading frequencies (200 – 1000 kHz) under dual-frequency ultrasound irradiation, showing the capability of dual-frequency system for enhancing pollutant degradation. A comparison of the results of degradation, dosimetry, and sonoluminescence intensity measurement revealed the stronger dependency of the degradation on the yield of HO• for both single and dual-frequency systems, which confirms degradation by HO• as the main removal mechanism. However, an enhanced degradation for frequencies higher than 500 kHz was observed despite a lower HO• yield, which could be attributed to the improved mass transfer of hydrophilic compounds at higher frequencies. The sonoluminescence intensity measurements showed that applying dual-frequency ultrasonic irradiation for 200 and 400 kHz made the bubbles larger and less uniform in size, with a portion of which not contributing to the yield of reactive oxidant species, whereas for the rest of the frequencies, dual-frequency ultrasound irradiation made the cavitation bubbles smaller and more uniform, resulting in a linear correlation between the overall sonoluminescence intensity and the yield of reactive oxidant species.     

Bubble levitation and translation under single-bubble sonoluminescence conditions

Bubble levitation in an acoustic standing wave is re-examined for conditions relevant to single-bubble sonoluminescence. Unlike a previous examination [Matula et al., J. Acoust. Soc. Am. 102, 1522-1527 (1997)], the stable parameter space [Pa,R0] is accounted for in this realization. Forces such as the added mass force and drag are included, and the results are compared with a simple force balance that equates the Bjerknes force to the buoyancy force. Under normal sonoluminescence conditions, the comparison is quite favorable. A more complete accounting of the forces shows that a stably levitated bubble does undergo periodic translational motion. The asymmetries associated with translational motion are hypothesized to generate instabilities in the spherical shape of the bubble. A reduction in gravity results in reduced translational motion. It is hypothesized that such conditions may lead to increased light output from sonoluminescing bubbles.     

Experimental and theoretical investigations on sonoluminescence under dual frequency conditions

The multibubble sonoluminescence (MBSL) intensities from water exposed to the simultaneous ultrasonic irradiation from 20 kHz (fixed at 6.3 W) and 355 kHz (variable power) ultrasound sources have been compared to the MBSL from the individual ultrasound sources under the same power conditions. A synergistic enhancement of the sonoluminescence (SL) signal, >30-fold, at low powers (4.6 W) of the higher frequency was observed. At a higher acoustic power level (15.8 W) the dual frequency operation produced a decrease in the SL signal. These results are in agreement with previously reported data [P. Ciuti, N.V. Dezhkunov, A. Francescutto, F. Calligaris, F. Sturman, Ultrasonics Sonochem. 10 (2003) 337; N.V. Dezhkunov, J. Eng. Phys. Therm. 76 (2003) 142] under similar experimental conditions. Numerical single bubble (SB) dynamics calculations have been used to help interpret the experimental results. It is suggested that the observed effects are caused by a combination of changes to the peak collapse temperature of individual bubbles as well as to changes in the active bubble population.     

Optoacoustic phenomena in insulating oils

Sonoluminescence is a process by which light is emitted from collapsing ultrasound-driven gas bubbles in a liquid. Recent works on sonoluminescence have shown that many parameters of the dissolved gas, surrounding liquid and external conditions influences this phenomenon [10]. The light intensity and emitted light spectra depends mainly on the fluid and dissolved gases properties [9,13]. These features indicate the possibility of estimating the amount of dissolved chemical compounds in liquids. The use of sonoluminescence for aging properties diagnostic of insulation oils was proposed. This article presents the schematic of used measurement setup and points out the difficulties in the research resulted from subtleness of the process and no fully accepted sonoluminescence theory.     

Temperature and pressure dependence of sonoluminescence

The dependence of sonoluminescence on ambient pressure and temperature is measured. As water is cooled, there occurs a 100-fold increase in light emission which can be accompanied by only slight changes in the ambient radius of the pulsating bubble. This suggests that water vapor trapped in the collapsing bubble is a key parameter for this system. For fixed concentration of gases in water, the maximum intensity of sonoluminescence decreases as the ambient pressure is lowered below 1 atm.     

Sonoluminescence

Abstract

Sonoluminescence is the light emission phenomenon from collapsing bubbles in liquid irradiated by an ultrasonic wave. In the present review, theoretical and experimental studies of the two types of sonoluminescence [single‐bubble sonoluminescence (SBSL) and multibubble sonoluminescence (MBSL)] are described. SBSL is a sonoluminescence from a single stably pulsating bubble trapped at the pressure antinode of a standing ultrasonic wave. MBSL is a sonoluminescence occurring from many bubbles in liquid irradiated by an ultrasonic wave. The theoretical and experimental studies suggest that SBSL originates in emissions from plasma inside the heated bubble at the bubble collapse, whereas MBSL originates both in emissions from plasma and in chemiluminescence inside heated bubbles at the bubble collapse. Unsolved problems of sonoluminescence have also been explained in detail.     

Mie scattering and the physical mechanisms of sonoluminescence

We propose an experimental procedure to investigate possible mechanisms for radiation emission in sonoluminescence. Our analysis is based on Mie's theory of light scattering for a coated sphere in an external medium. Depending on the physical mechanism responsible of sonoluminescence, the dielectric constant of the hot spot changes. As a case study we consider the problem of the detection of an inner plasma core in sonoluminescent bubbles. Our results show that polarization measurements of scattered light should discern the presence of a plasma provided that light detectors are fast enough. Extensions to other emission mechanisms are briefly discussed.     

The effect of fluorocarbon gases on sonoluminescence: a failure of the electrical hypothesis

Ultrasonic irradiation of solutions containing volatile organometallic complexes results in intense emission from excited-state metal atoms. We have determined the effect of dissolved gases (Xe, Kr, Ar, Ne, He, CF₄, C₂F₆, CO, N₂) on the intensity of the sonoluminescence resulting from ultrasonic irradiation of silicone oil solutions of Cr(CO)₆. This provides a well-defined, spectrally resolved probe of sonoluminescence with emission resulting from a single species, the chromium atom excited states. As predicted by the hot-spot, thermal mechanisms of sonoluminescence, the intensity of excited-state Cr emission decreases with increasing thermal conductivity of the noble gases. The intensity of sonoluminescence increases with increasing γ (i.e. C_p/C_v), which is also in accord with a thermal mechanism. Sonoluminescence is substantially diminished by the addition of even small amounts (≈ 1%) of CF₄ or C₂F₆, even though they are capable of supporting electrical discharge. This is in agreement with a thermal mechanism, but is in direct conflict with electrical theories of sonoluminescence.     

Sonoluminescence of luminol-sodium carbonate solution

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Influence of dissolved oxygen content on multibubble sonoluminescence with ambient-pressure reduction

The efficiency of chemical reactions in the presence of ultrasound at reduced pressures has been monitored using the influence of dissolved oxygen (DO) content on a luminol solution undergoing multibubble sonoluminescence. From these measurements under the condition of constant ultrasonic frequency and constant amplitude of sound pressure, it is shown that the intensity of sonoluminescence is higher at subatmospheric ambient pressure than at atmospheric pressure under the same degree of saturation. Also, it is found that there is an appropriate content of DO to produce the highest intensity of the luminescence and its value varies with ambient pressure.     

Using phase space diagrams to interpret multiple frequency drive sonoluminescence

The recent experimental results of J. Holzfuss, M. Ruggeberg, and R. Mettin [Phys. Rev. Lett. 81, 1961 (1998)] in which a second harmonic drive system was used to generate sonoluminescence (SL) have been analyzed in the context of the dissociation hypothesis (DH) of D. Lohse and S. Hilgenfeldt [J. Chem. Phys. 107, 6986 (1997)]. The second harmonic introduces two more variables that are under experimental control: a phase and an additional pressure term to the acoustic drive pressure. Diffusive equilibrium curves for a fixed gas concentration were calculated as was the Mach criterion. A phase space diagram was constructed to permit the prediction of regions of stable SL, unstable SL, stable non-SL, and unstable non-SL. These were compared to Holzfuss' experimental observations, and excellent quantitative agreement was seen. The results provide further evidence that the underlying assumptions of DH are sound. They also indicate the utility of DH for determining appropriate experimental conditions to achieve SL and for optimizing an experimental system.