Sonoluminescence Spectra in the First Tens of Seconds of Sonolysis of [BEPip][NTf2], at 20 kHz under Ar.

Following recent works on the sonochemical degradation of butyl ethyl piperidinium bis-(trifluoromethylsulfonyl)imide ([BEPip][NTf2]), monitoring of sonoluminescence (SL) spectra in the first tens of seconds of sonolysis was needed to better characterize the formed plasma and to question the correlation of the SL spectra with the viscosity. A very dry [BEPip][NTf2] ionic liquid (IL) and a water-saturated liquid are studied in this paper. In both cases, IL degradation is observed as soon as SL emission appears. It is confirmed that the initial evolution of the SL intensity is closely linked to the liquid viscosity that impacts the number of bubbles; however, other parameters can also play a role, such as the presence of water. The water-saturated IL shows more intense SL and faster degradation. In addition to the expected bands, new emission bands are detected and attributed to the S2 B-X emission, which is favored in the water-saturated ionic liquid.

Diagnosing the plasma formed during acoustic cavitation in [BEPip][NTf2] ionic liquid.

Sonoluminescence (SL) spectra of a very dry [BEPip][NTf2] ionic liquid were measured in the first minutes of sonication under Ar. The intense sonoluminescence allowed us to monitor the time-evolution of the SL spectra. Several molecular emissions were observed. Rovibronic temperatures of C2 and CN were determined giving vibrational temperatures of 5800 ± 500 K and 6000 ± 500 K and rotational temperatures (i.e. translational or gas temperatures) of 4000 ± 500 K. These temperatures stay remarkably constant during the sonolysis, while SL spectra undergo strong changes that illustrate the very fast evolution of the plasma during the first minutes of sonication. The expected strong decrease in the plasma electron energy also reflects in the evolution of the populations of CH electronically excited states. The physical meaning of temperatures derived from molecular emissions in SL spectra is discussed.

Use of NH (A3Π-X3Σ-) sonoluminescence for diagnostics of nonequilibrium plasma produced by multibubble cavitation.

In this work, the sonoluminescence of NH radicals has been evaluated as a new spectroscopic probe for the nonequilibrium plasma produced by multibubble cavitation in liquids. The experiments were performed in aqueous ammonia solutions subjected to power ultrasound at low and high frequencies and under two different rare gases (Ar and Xe). Sonoluminescence (SL) spectroscopy focuses on the emission of the two present systems: NH (A3Π-X3Σ-) and OH (A2Σ+-X2Π). Both spectroscopic systems indicate the absence of thermal equilibrium during bubble collapse (Tv > Tr) irrespective of the saturating gas. When Ar is used as the saturating gas, these emissions can be fitted using Specair software and the corresponding rovibronic temperatures are derived. Both species indicate a net increase in vibrational temperatures with the US frequency. In Xe, the SL spectra exhibit OH (C2Σ+-A2Σ+) and NH (c1Π-a1Δ) emission bands indicating a higher electron temperature compared to Ar. However, in Xe, the SL spectra cannot be satisfactorily fitted because of significant line broadening. The estimation of the intrabubble pressure via SL simulation using Specair software is discussed. Monitoring of the sonochemical activity indicates the formation of H2 and N2H4, while no H2O2 accumulates under these conditions. In the presence of Xe, NO is also formed as a sonolysis product. The appearance of new possible reaction pathways under Xe is made possible by the higher plasma electron density and correlates with SL data.

Magnesium and magnesium alloy dissolution by high intensity focused ultrasound: erosion/cavitation vs. Wave propagation.

The dissolution of metals, influenced by mechanical and chemical factors, plays a crucial role in various applications. Ultrasonic irradiation has been explored for its ability to enhance dissolution rates and modify surface characteristics. In this study, we investigate the dissolution of magnesium (Mg) and magnesium alloys under high-intensity focused ultrasound (HIFU) conditions with frequency sweeping (wobbling). Our findings reveal distinct effects of cavitation and acoustic streaming on the dissolution process. For pure magnesium, ultrasonic treatment significantly increases dissolution rates compared to silent conditions. Negative frequency sweeps result in the highest dissolution rates, linked to increased cavitation activity, while positive sweeps reduce dissolution rates but maintain acoustic streaming effects. The removal of surface oxides is accelerated in all sonication conditions. Macro- and micro-roughness patterns on the surface correspond to the wobbling frequency range, with wavelengths matching the average ultrasonic frequency. However, dissolution is not uniform across the sample, and preferential attack occurs at the focal point during negative frequency sweeps. In contrast, magnesium alloys exhibit lower dissolution rates than pure Mg. The alloy's mechanical properties make it less susceptible to cavitation erosion but more sensitive to acoustic streaming-induced dissolution. Grain boundaries are preferentially attacked, revealing differences between ductile pure Mg and the harder, more cavitation-resistant, alloy. This study highlights the complex interplay between cavitation and acoustic streaming in the dissolution of magnesium and its alloys under HIFU conditions, shedding light on the limits and potential applications of this technique, particularly in microstructure analysis.

The origin of isotope effects in sonoluminescence spectra of heavy and light water.

Bubble and peak: The isotope effects in the sonoluminescence spectra of light and heavy water under ultrasound indicate the formation of a non-equilibrium plasma inside the collapsing cavitation bubbles. The picture demonstrates the active cavitation zones in water at 204 kHz.

The effect of bulk viscosity on single bubble dynamics and sonoluminescence.

In our previous paper, we derived a new single bubble model including the effect of bulk viscosity. To confront it to experiments, single bubble dynamics was measured here in 30% (v/v) glycerol-water mixture under different acoustic amplitudes and compared to models including or not the effect of bulk viscosity. The results showed that calculated bubble dynamics were not significantly affected by the bulk viscosity within the experimental conditions used in this study. However, there was a noticeable delay for the first rebound when bulk viscosity was considered. The corresponding sonoluminescence intensities were collected and compared with theoretical predictions. The results did not allow to discriminate between the two models (one includes the effect of bulk viscosity, the other does not), confirming the negligible effect of bulk viscosity in this condition (30% (v/v) glycerol-water mixture). Due to the instability of a single bubble in higher viscosity solutions, we could not implement experiments that can discriminate between the two models.

Ultrasonics and sonochemistry: Editors' perspective.

Ultrasonic waves can induce physical and chemical changes in liquid media via acoustic cavitation. Various applications have benefitted from utilizing these effects, including but not limited to the synthesis of functional materials, emulsification, cleaning, and processing. Several books and review articles in the public domain cover both fundamental and applied aspects of ultrasonics and sonochemistry. The Editors of the Ultrasonics Sonochemistry journal possess diverse expertise in this field, from theoretical and experimental aspects of acoustic cavitation to materials synthesis, environmental remediation, and sonoprocessing. This article provides Editors' perspectives on various aspects of ultrasonics and sonochemistry that may benefit students and early career researchers.

Nonequilibrium vibrational excitation of OH radicals generated during multibubble cavitation in water.

The sonoluminescence (SL) spectra of OH(A(2)Σ(+)) excited state produced during the sonolysis of water sparged with argon were measured and analyzed at various ultrasonic frequencies (20, 204, 362, 609, and 1057 kHz) in order to determine the intrabubble conditions created by multibubble cavitation. The relative populations of the OH(A(2)Σ(+)) v' = 1-4 vibrational states as well as the vibronic temperatures (T(v), T(e)) have been calculated after deconvolution of the SL spectra. The results of this study provide evidence for nonequilibrium plasma formation during sonolysis of water in the presence of argon. At low ultrasonic frequency (20 kHz), a weakly excited plasma with Brau vibrational distribution is formed (T(e) ~ 0.7 eV and T(v) ~ 5000 K). By contrast, at high-frequency ultrasound, the plasma inside the collapsing bubbles exhibits Treanor behavior typical for strong vibrational excitation. The T(e) and T(v) values increase with ultrasonic frequency, reaching T(e) ~ 1 eV and T(v) ~ 9800 K at 1057 kHz.

Simultaneous H/D and 13C/12C Anomalous Kinetic Isotope Effects during the Sonolysis of Water in the Presence of Carbon Monoxide.

Splitting of water molecules driven by ultrasound plays a central role in sonochemistry. While studies of sonoluminescence revealed the formation of a plasma inside the cavitation bubble, much less is known about the contribution of plasma chemical processes to the sonochemical mechanisms. Herein, we report for the first time sonochemical processes in water saturated with pure CO. The presence of CO causes a large increase in the H/D kinetic isotope effect (KIE) to αH = 14.6 ± 1.8 in a 10% H2O/D2O mixture under 20 kHz ultrasound. The anomalous H/D KIE is attributed to electron quantum tunneling in the plasma produced by cavitation. In addition, CO2 formed simultaneously with hydrogen during the sonochemical process is enriched with the 13C isotope, which indicates a V-V pumping mechanism typical for non-equilibrium plasma. Both observed KIEs unambiguously point to the contribution of quantum effects in sonochemical mechanisms.

Desorption of Cs from vermiculite by ultrasound assisted ion exchange.

Nuclear power plant accidents typically lead to the contamination of large volumes of soils with radioactive cesium. This element is hard to desorb from soil, especially when it is bound to mica minerals, and aggressive and energy-consuming techniques are often required. In this study, we investigated the use of ultrasound with Mg2+ cation exchange for the removal of a133Cs-contaminated vermiculite over a wide range of temperatures (20-200 °C). At room temperature, ultrasound was found to significantly accelerate Cs desorption but only reversibly adsorbed Cs species were removed. Under hydrothermal conditions and ultrasonic irradiation in contrast, the removal efficiency after 1 h was 50% at 100 °C and more than 95% at 200 °C, compared with only 50% without ultrasonication at 200 °C. Cs contamination can therefore be nearly totally removed, even from collapsed vermiculite sites where sorption is considered irreversible. Ultrasound waves and high temperatures both make trapped Cs more accessible by spreading the sheets and improving mass transfer. Acoustic noise spectra show that even at high pressure and temperature, cavitation bubbles form, oscillate and collapse, with the desired physical effects. These results demonstrate the potential of synergistic ultrasound and hydrothermal treatment for soil remediation.

Sonoluminescence emission spectra of a 3.6 MHz HIFU in sweeping mode.

Use of sweeping mode with a 3.6 MHz High Intensity Focused Ultrasound (HIFU) allows cavitation activity to be controlled. This is especially true in the pre-focal zone where the high concentration of bubbles acts as an acoustic reflector and quenches cavitation above this area. Previous studies attributed the enhancement of cavitation activity under negative sweep to the activation of more bubble nuclei, requiring deeper investigations. After mapping this activity with SCL measurements, cavitation noise spectra were recorded. The behavior of the acoustic broadband noise follows the sonochemical one i.e., showing the same attenuation (positive scan) or intensification (negative scan) of cavitational activity. In 1 M NaCl 3.7 mM 2-propanol solution saturated by a mixture of Ar-15.5%O2-2.2%N2, intensities of SL spectra are high enough to allow detection of several molecular emissions (OH, NH, C2, Na) under negative frequency sweeps. This is the first report of molecular emissions at such high frequency. Their intensities are low, and they are very broad, following the trend obtained at fixed frequency up to 1 MHz. Under optimized conditions, CN emission chosen as a spectroscopic probe is strong enough to be simulated, which is reported for the first time at such high frequency. The resulting characteristics of the plasma do not show any spectral difference, so bubble nature is the same in the pre-and post-focal zone under different sweeping parameters. Consequently, SL and SCL intensification was not related to a change in plasma nature inside the bubbles but to the number of cavitation bubbles.

Autocatalytic sonolysis of iron pentacarbonyl in room temperature ionic liquid [BuMeIm][Tf2N].

The autocatalytic sonochemical reaction of Fe(CO)(5) decomposition in [BuMeIm][Tf(2)N] provides iron nanoparticles in higher yields than in tetralin. Such a difference is explained by the higher decomposition of the intermediate Fe(3)(CO)(12) according to the two-sites model of the sonochemical reactions and the specific properties of the ionic liquid.

Line emission of sodium and hydroxyl radicals in single-bubble sonoluminescence.

Spectroscopic studies of single-bubble sonoluminescence (SBSL) in water and aqueous sodium chloride solutions with a defined concentration of argon were performed as a function of the driving acoustic pressure. The broad-band continuum ranging from 200 to 700 nm is characterized by fits using Planck's law of blackbody radiation. The obtained blackbody temperatures are in the range of 10(4) K and are revealed to be independent of the presence of a salt and the acoustic pressure, whereas the SL intensity increases by a factor of more than 10 within the studied acoustic pressure range. The different trends followed by SL intensity and blackbody temperatures question the blackbody model. In solutions with 70 mbar of argon, line emissions of OH(•) radicals and Na* are observed. The shape of the OH(•) radical emission spectrum is very similar to that in MBSL spectra, indicating the strong similarity of intrabubble conditions. An increase of the acoustic pressure causes the continuum to overlap the lines until they become indistinguishable. The emission line of Na* in NaCl is observed only at high NaCl concentrations. When sodium dodecylsulfate is used a pronounced Na* line is already observed in a 1 mM solution thanks to enrichment of sodium ions at the interface. The results presented in this work reveal the strong similarity of SBSL and MBSL under certain experimental conditions.

Sonochemical decontamination of magnesium and magnesium-zirconium alloys in mild conditions.

UNGG cladding nuclear wastes constitute a huge volume of Mg-based materials that raises economic and safety concerns, particularly due to their radioactivity coupled to the potential generation of H2 gas under deep underground disposal. Their significant decontamination would result in more secure and less expensive storage, with a better containment of the separated long-lived radioisotopes that could enter in a classical channel. Sonication of genuine UNGG cladding materials and simulants at 345 kHz in 0.01 M oxalic acid solution (20 °C) allowed the structuring of their surfaces with the observation of homogeneously distributed craters of 20-40 µm in diameter. After a thorough characterization and comparison of the ultrasound effects generated at the surface, the various samples were artificially contaminated and characterized before sonication. The complete and rapid sonochemical decontamination of Mg-based materials was then observed, in addition to the removal of the carbon layer promoting corrosion on the inner UNGG cladding. The extension of sonication allows the neo-formed brucite (Mg(OH)2) and zirconium-based phases to accumulate on the surface, thus contributing in a slight but continuous surface recontamination process. This phenomenon results from the re-adsorption of uranyl cations from the solution which can be avoided by optimizing the duration of treatment.

Impact of bubble coalescence in the determination of bubble sizes using a pulsed US technique: Part 2 - Effect of the nature of saturating gas.

Knowledge on cavitation bubble size distribution, ambient radius of bubbles is of interest for many applications that include therapeutic and diagnostic medicine. It however becomes a hard quest when increasing the ultrasonic frequency, when direct observation of bubble dynamics is no longer possible. An indirect method based on the estimation of the bubble dissolution time under pulsed ultrasound (362 kHz) is used here under optimized conditions to derive ambient radii of cavitation bubbles in water saturated with He, Ar, Xe, O2, N2 and air: 3.0 µm for Ar, 1.2 µm for He, 3.1 µm for Xe, 2.8 µm for O2, around 1 µm for N2 and air. If the pulse on-time is increased, bubble coalescence occurs, the extent of which is rather limited for Ar but extremely high for He or N2.

Impact of bubble coalescence in the determination of bubble sizes using a pulsed US technique: Part 1 - Argon bubbles in water.

A powerful experimental approach to measure the size distribution of bubbles active in sonoluminescence and/or sonochemistry is a technique based on pulsed ultrasound and sonoluminescence emission. While it is an accepted technique, it is still lacking an understanding of the effect of various experimental parameters, including the duration of the pulse on-time, the nature of the dissolved gas, the presence of a gas flow rate, etc. The present work, focusing on Ar-saturated water sonicated at 362 kHz, shows that increasing the pulse on-time leads to the measurement of coalesced bubbles. Reducing the on-time to a minimum and/or adding sodium dodecyl sulfate to water allows to reducing coalescence so that natural active cavitation bubble sizes can be measured. A radius of 2.9-3.0 µm is obtained in Ar-saturated water at 362 kHz. The effects of acoustic power and possible formation of a standing-wave on coalescence and measured bubble sizes are discussed.

Acoustic noise spectra under hydrothermal conditions.

Acoustic noise spectra were studied for the first time in overheated water using sonohydrothermal reactor operating at 20 kHz ultrasound in the temperature range from 25 to 200 °C at the autogenic pressure of 1-14 bar. The obtained results highlighted a dominating role of stable cavitation during ultrasonic treatment of hot water. Heating of sonicated water results in the formation of large number of nonlinearly oscillating bubbles synchronous with the driving frequency. At 200 °C, the acoustic spectra also display strong subharmonic and multiple ultraharmonic bands. Moreover, cavitation bubbles formed at 200 °C exhibit chaotic and random motions. It has been shown that the addition of TiO2 nanoparticles to hydrothermal water heated at 200 °C allows to eliminate subharmonic/ultraharmonic bands and stochastic oscillations as well. This effect was assigned to Pickering-like bubble stabilization due to the particle accumulation at the bubble surface.

Hypothesis about electron quantum tunneling during sonochemical splitting of water molecule.

Quantum tunneling in chemistry is often attributed to the processes at low or near room temperatures when the rate of thermal reactions becomes far less than the rate of quantum tunneling. However, in some rapid processes, quantum tunneling can be observed even at high temperatures. Herein, we report the experimental evidence for anomalous H/D kinetic isotope effect (KIE) during sonochemical dissociation of water molecule driven by 20 kHz power ultrasound measured in H2O/D2O mixtures saturated with Ar or Xe. Hydrogen released during ultrasonic treatment is enriched by light isotope. The observed H/D KIE (α = 2.15-1.50) is much larger than what is calculated assuming a classical KIE for Tg = 5000 K (α = 1.15) obtained from the sonoluminescence spectra in H2O and D2O. Furthermore, the α values sharply decrease with increasing of H2O content in H2O/D2O mixtures reaching a steady-state value close to α = 1.50, which also cannot be explained by O-H/O-D zero-point energy difference. We suggest that these results can be understood in terms of quantum electron tunneling occurring in nonequilibrium picosecond plasma produced at the last stage of cavitation bubble collapse. Thermal homolytic splitting of water molecule is inhibited by extremely short lifetime of such plasma. On the contrary, immensely short traversal time for electron tunneling in water allows H2O dissociation by quantum tunneling mechanism.

Effect of NaCl salt on sonochemistry and sonoluminescence in aqueous solutions.

The presence of salts in a solution is known to affect sonochemistry, but until now no consensus has been reached in the literature on how and why a salt influences sonochemistry. The present study focuses on the effect of NaCl on sonochemical activity and sonoluminescence at 362-kHz frequency in aqueous solutions saturated with He and Ar. It is shown that the presence of salt has a multiple impact: the global population of active bubbles decreases due to the decreasing gas solubility, new chemical reactions involving Na and Cl atoms occur that influence hydrogen and hydrogen peroxide yields and the standing wave component of the US wave is enhanced, favoring sonoluminescence emission. Interestingly, the effect of salt greatly depends on the nature of the saturating gas: for instance, strong acidification occurs under He, while it is limited under Ar.

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.