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

Multibubble sonoluminescence in supercooled water

Cavitation in supercooled water has been induced by the short ultrasound pulses of an ultrasonic horn driven at 20 kHz. The cavitation during the ultrasonic pulses and occasionally the crystallization events thereafter have been imaged by a high-speed camera. The probability of ice crystallization in dependence on the pulse duration and temperature showed a high chance for the water to remain liquid if sufficiently short bursts of moderate acoustic power were applied. This regime has been used for the assessment of sonoluminescence (SL) from the generated cavitation bubbles in the supercooled liquid state. To this end, light emitting events were summed up over a number of ultrasonic pulses by an image intensifier. SL appeared mostly directly under the tip of the ultrasonic horn and sometimes also a few millimeters below the tip. The intensity of SL events showed a slight rise for a decrease in temperature, i.e., for an increase in supercooling. This behavior is in accord with the SL dependence on temperature above the freezing point and might be attributed to a further lowering of vapor pressure. An increase in the bubble collapse peak temperature for increased supercooling is calculated on the basis of spherical bubble model calculations, which supports the findings. The simulations predict further that the peak temperature will fall off again beyond a certain supercooling level.

Quantifying the chemical activity of cavitation bubbles in a cluster

Acoustic cavitation bubbles drive chemical processes through their dynamic lifecycle in liquids. These bubbles are abundant within sonoreactors, where their behavior becomes complex within clusters. This study quantifies their chemical effects within well-defined clusters using a new laser-based method. We focus a laser beam into water, inducing a breakdown that generates a single cavitation bubble. This bubble undergoes multiple collapses, releasing several shockwaves. These shockwaves propagate into the surrounding medium, leading to the formation of secondary bubbles near a reflector, separated from the input laser beam. We evaluate the chemical activity of these bubble clusters of various sizes by KI dosimetry, and to gain insights into their dynamics, we employ high-speed imaging. Hydrophone measurements show that conversion from focused shockwave energy to chemical reactions increases to a maximum of 16.5%. Additional increases in shockwave energy result in denser bubble clusters and a slightly decreased conversion rate, falling to 14.9%, highlighting the key role of bubble dynamics in the transformation of mechanical to chemical energy and as a result in the efficiency of the sonoreactors. The size and frequency of bubble collapses influence the cluster's chemical reactivity. We introduce a correlation for predicting the conversion rate of cluster energy to chemical energy, based on the cluster's energy density. The maximum conversion rate occurs at a cluster energy density of 2500 J/L, linked to a cluster with an average bubble diameter of 91 μ m, a bubble density of 3500 bubbles/ml, and a bubble-to-bubble distance ratio of 8.

The violent collapse of vapor bubbles in cryogenic liquids

Vapor bubbles in cryogenic fluids may collapse violently under subcooled and pressurized conditions. Despite important implications for engineering applications such as cavitation erosion in liquid propellant rocket engines, these intense phenomena are still largely unexplored. In this paper, we systematically investigate the ambient conditions leading to the occurrence of violent collapses in liquid nitrogen and analyze their thermodynamic characteristics. Using Brenner's time ratio χ, the regime of violent collapse is identified in the ambient pressure-temperature parameter space. Complete numerical simulations further refine the prediction and illustrate two classes of collapses. At 1 < χ < 10, the collapse is impacted by significant thermal effects and attains only moderate wall velocity. Only when χ > 10 does the collapse show more inertial features. A mechanism analysis pinpoints a critical time when the surrounding liquid enters supercritical state. The ultimate collapse intensity is shown to be closely associated with the dynamics at this moment. Our study provides a fresh perspective to the treatment of cavitation in cryogenic fluids. The findings can be instrumental in engineering design to mitigate adverse effects arising from intense cavitational activities.

Extensive investigation of geometric effects in sonoreactors: Analysis by luminol mapping and comparison with numerical predictions

This investigation focuses on the influence of geometric factors on cavitational activity within a 20kHz sonoreactor containing water. Three vessels with different shapes were used, and the transducer immersion depth and liquid height were varied, resulting in a total of 126 experiments conducted under constant driving current. For each one, the dissipated power was quantified using calorimetry, while luminol mapping was employed to identify the shape and location of cavitation zones. The raw images of blueish light emission were transformed into false colors and corrected to compensate for refraction by the water-glass and glass-air interfaces. Additionally, all configurations were simulated using a sonoreactor model that incorporates a nonlinear propagation of acoustic waves in cavitating liquids. A systematic visual comparison between luminol maps and color-plots displaying the computed bubble collapse temperature in bubbly regions was conducted. The calorimetric power exhibited a nearly constant yield of approximately 70% across all experiments, thus validating the transducer command strategy. However, the numerical predictions consistently overestimated the electrical and calorimetric powers by a factor of roughly 2, indicating an overestimation of dissipation in the cavitating liquid model. Geometric variations revealed non-monotonic relationships between transducer immersion depth and dissipated power, emphasizing the importance of geometric effects in sonoreactor. Complex features were revealed by luminol maps, exhibiting appearance, disappearance, and merging of different luminol zones. In certain parametric regions, the luminol bright regions are reminiscent of linear eigenmodes of the water/vessel system. In the complementary parametric space, these structures either combine with, or are obliterated by typical elongated axial structures. The latter were found to coincide with an increased calorimetric power, and are conjectured to result from a strong cavitation field beneath the transducer producing acoustic streaming. Similar methods were applied to an additional set of 57 experiments conducted under constant geometry but with varying current, and suggested that the transition to elongated structures occurs above some amplitude threshold. While the model partially reproduced some experimental observations, further refinement is required to accurately account for the intricate acoustic phenomena involved.

Impact of sonication power on the degradation of paracetamol under single- and dual-frequency ultrasound

The effects of sonication power on the ultrasonic cavitation and sonochemistry as well as the degradation of paracetamol were studied and compared for single- and dual-frequency sonoreactors. For the single-frequency sonication, a 500 kHz plate transducer was employed, with three different calorimetric powers of 8.4, 16.7 and 27.9±3.9 W. For the dual-frequency sonication, the plate transducer was perpendicularly coupled with a low-frequency 20 kHz ultrasonic horn, and three calorimetric powers of 27.9, 33.4, 44.6±3.9 W were studied. At all the studied powers, dual-frequency sonication led to a synergistic effect in the degradation of paracetamol, though varying the power of the horn did not affect the degradation rate. A comparison of the degradation data versus the yield of oxidants as well as the overall intensities of sonoluminescence and sonochemiluminescence suggested the degradation is by the action of oxidants near the surface of the bubbles as the major reaction mechanism. Despite the enhancement observed for the degradation, dual-frequency sonication had no significant effect on the yield of either of the oxidants, regardless of the applied power to the horn. In contrast, dual-frequency sonication decreased the overall sonoluminescence and sonochemiluminescence intensities at all powers studied, suggesting that the application of dual-frequency sonication reduces the size of cavitation bubbles. Normal distribution function analysis confirmed dual-frequency sonication resulted in smaller sonoluminescing bubbles, hence the reduction in the sonoluminescence intensity. The increase in degradation rate under DFUS is attributed to the increase in the transfer of paracetamol from the bulk towards the bubbles. As a result, the availability of the pollutant molecules in the vicinity of the bubbles to react with HO• would increase and consequently, the degradation rate would enhance under DFUS.

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.

Penetration of hydroxyl radicals in the aqueous phase surrounding a cavitation bubble

In the sonochemical degradation of nonvolatile compounds, the free radicals must be delivered into the aqueous solution from the cavitation bubble to initiate reduction-oxidation reactions. The penetration depth in the liquid becomes an important parameter that influences the radical delivery efficiency and eventual treatment performance. However, the transport of radicals in the liquid phase is not well understood yet. In this paper, we focus on the most reactive OH radical and numerically simulate its penetration behavior. This is realized by solving the coupled equations of bubble dynamics, intracavity chemistry, and radical dispersion in the aqueous phase. The results present both the local and global penetration patterns for the OH radicals. By performing simulations over a wide range of acoustic parameters, we find an undesirable phenomenon that the penetration can be adversely suppressed when strengthening the radical production. A mechanistic analysis attributes this to the excessively vigorous recombination reactions associated with high radical concentrations near the bubble interface. In this circumstance, the radicals are massively consumed and converted into molecular species before they can appreciably diffuse away. Our study sheds light on the interplay between radical production inside the bubble and dispersion in the outside liquid. The derived conclusions provide guides for sonochemical applications from a new perspective.

A correlation between cavitation bubble temperature, sonoluminescence and interfacial chemistry - A minireview

Ultrasound induced cavitation (acoustic cavitation) process is found useful in various applications. Scientists from various disciplines have been exploring the fundamental aspects of acoustic cavitation processes over several decades. It is well documented that extreme localised temperature and pressure conditions are generated when a cavitation bubble collapses. Several experimental techniques have also been developed to estimate cavitation bubble temperatures. Depending upon specific experimental conditions, light emission from cavitation bubbles is observed, referred to as sonoluminescence. Sonoluminescence studies have been used to develop a fundamental understanding of cavitation processes in single and multibubble systems. This minireview aims to provide some highlights on the development of basic understandings of acoustic cavitation processes using cavitation bubble temperature, sonoluminescence and interfacial chemistry over the past 2-3 decades.

Acoustic characterization of cavitation intensity: A review

Cavitation intensity is used to describe the activity of cavitation, and several methods are developed to identify the intensity of cavitation. This work aimed to provide an overview and discussion of the several existing characterization methods for cavitation intensity, three acoustic approaches for charactering cavitation were discussed in detail. It was showed that cavitation noise spectrum is too complex and there are some differences and disputes on the characterization of cavitation intensity by cavitation noise. In this review, we recommended a total cavitation noise intensity estimated via the integration of real cavitation noise spectrum over full frequency domain instead of artificially adding inaccurate filtering processing.

Multibubble Sonoluminescence from a Theoretical Perspective

In the present review, complexity in multibubble sonoluminescence (MBSL) is discussed. At relatively low ultrasonic frequency, a cavitation bubble is filled mostly with water vapor at relatively high acoustic amplitude which results in OH-line emission by chemiluminescence as well as emissions from weakly ionized plasma formed inside a bubble at the end of the violent bubble collapse. At relatively high ultrasonic frequency or at relatively low acoustic amplitude at relatively low ultrasonic frequency, a cavitation bubble is mostly filled with noncondensable gases such as air or argon at the end of the bubble collapse, which results in relatively high bubble temperature and light emissions from plasma formed inside a bubble. Ionization potential lowering for atoms and molecules occurs due to the extremely high density inside a bubble at the end of the violent bubble collapse, which is one of the main reasons for the plasma formation inside a bubble in addition to the high bubble temperature due to quasi-adiabatic compression of a bubble, where "quasi" means that appreciable thermal conduction takes place between the heated interior of a bubble and the surrounding liquid. Due to bubble-bubble interaction, liquid droplets enter bubbles at the bubble collapse, which results in sodium-line emission.

Pump-probe X-ray holographic imaging of laser-induced cavitation bubbles with femtosecond FEL pulses

Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water. The fast dynamics are associated with extreme states of gas and liquid, especially in the nascent state. This offers a unique setting to probe water and water vapor far-from equilibrium. However, current optical techniques cannot quantify these early states due to contrast and resolution limitations. X-ray holography with single X-ray free-electron laser pulses has now enabled a quasi-instantaneous high resolution structural probe with contrast proportional to the electron density of the object. In this work, we demonstrate cone-beam holographic flash imaging of laser-induced cavitation bubbles in water with nanofocused X-ray free-electron laser pulses. We quantify the spatial and temporal pressure distribution of the shockwave surrounding the expanding cavitation bubble at time delays shortly after seeding and compare the results to numerical simulations.

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.

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.

Molecular emissions in sonoluminescence spectra of water sonicated under Ar-based gas mixtures

Sonoluminescence (SL) spectroscopy is one of the very few ways to study the plasma formed in solutions submitted to ultrasound. Unfortunately, up to now only very limited emission bands were reported in SL spectra of aqueous solutions, moreover broad and badly resolved. It is shown here that by adding some N₂ and/or CO₂ in Ar, new molecular emissions (CN, N₂ and CO) can be observed and that for some of them rovibronic temperatures can be derived. The paramount importance of Stark broadening in these emissions is underlined, together with the need for data on Stark parameters for molecular emissions.

Tb(III) line intensities in multibubble sonoluminescence

We observed the line emissions of trivalent terbium [Tb(III)] ions during multibubble sonoluminescence (MBSL) in the aqueous solutions of terbium chloride (TbCl₃) under argon gas atmosphere. The line intensities of excited Tb(III) ions increased with TbCl₃ concentration(mass percentage) in aqueous solutions. This phenomenon was interpreted qualitatively by numerically computing the Tb(III) line intensities in one sonoluminescing bubble among the cavitation bubbles in a liquid. The driving pressure for this sonoluminescing bubble was obtained by numerically solving the cavitation dynamic equation and bubble-pulsation equation. The Tb(III) ion line intensities obtained from the sonoluminescing bubble were attained by solving computing fluid dynamics equations and the spectral radiation formula.

Asymmetricity and sign reversal of secondary Bjerknes force from strong nonlinear coupling in cavitation bubble pairs

Most of the current applications of acoustic cavitation use bubble clusters that exhibit multibubble dynamics. This necessitates a complete understanding of the mutual nonlinear coupling between individual bubbles. In this study, strong nonlinear coupling is investigated in bubble pairs which is the simplest case of a bubble-cluster. This leads to the derivation of a more comprehensive set of coupled Keller-Miksis equations (KMEs) that contain nonlinear coupling terms of higher order. The governing KMEs take into account the convective contribution that stems from the Navier-Stokes equation. The system of KMEs is numerically solved for acoustically excited bubble pairs. It is shown that the higher-order corrections are important in the estimation of secondary Bjerknes force for closely spaced bubbles. Further, asymmetricity is witnessed in both magnitude and sign reversal of the secondary Bjerknes force in weak, regular, and strong acoustic fields. The obtained results are examined in the light of published scientific literature. It is expected that the findings reported in this paper may have implications in industries where there is a requirement to have a control on cavitation and its effects.

High-speed photographic observation of the sonication of a rabbit carotid artery filled with microbubbles by 20-kHz low frequency ultrasound

The aim of this study is to assess the physical damage of cavitation effects induced by low frequency ultrasound and microbubbles (MBs) to an in vitro vessel. A rabbit carotid artery filled with SonoVue MBs and methylene blue was irradiated with 20-kHz ultrasound, and the results were recorded by high-speed photography at 3000 frames per second. The carotid artery filled with MBs experienced a slight tremor during ultrasonication. Six intermittent blue flow events occurred in two places on the artery wall during the 5-s process. The duration of each leakage event was 90-360ms with an average of 200ms. Hematoxylin-eosin (H-E) staining demonstrated the separation of the carotid artery elastic membrane, local blood vessel wall defects and hole formation, and the surface of the ruptured area was rough and irregular. Another carotid artery was filled with a 0.9% NaCl solution and methylene blue as a control and irradiated with 20-kHz ultrasound. No blue liquid flow was seen, and no holes in the vessel were observed. H-E staining revealed intact vascular endothelial cells and smooth muscles with no vascular wall defects. Low-frequency ultrasound combined with MBs can cause a vessel to rupture and holes to form in a short time. High-speed photography is useful for observing transient changes caused by the effects of ultrasound cavitation on an in vitro vessel.

Pressure and tension waves from bubble collapse near a solid boundary: A numerical approach

The acoustic waves being generated during the motion of a bubble in water near a solid boundary are calculated numerically. The open source package OpenFOAM is used for solving the Navier-Stokes equation and extended to include nonlinear acoustic wave effects via the Tait equation for water. A bubble model with a small amount of gas is chosen, the gas obeying an adiabatic law. A bubble starting from a small size with high internal pressure near a flat, solid boundary is studied. The sequence of events from bubble growth via axial microjet formation, jet impact, annular nanojet formation, torus-bubble collapse, and bubble rebound to second collapse is described. The different pressure and tension waves with their propagation properties are demonstrated.

Luminescence from cavitation bubbles deformed in uniform pressure gradients

Presented here are observations that demonstrate how the deformation of millimetric cavitation bubbles by a uniform pressure gradient quenches single-collapse luminescence. Our innovative measurement system captures a broad luminescence spectrum (wavelength range, 300-900 nm) from the individual collapses of laser-induced bubbles in water. By varying the bubble size, driving pressure, and perceived gravity level aboard parabolic flights, we probed the limit from aspherical to highly spherical bubble collapses. Luminescence was detected for bubbles of maximum radii within the previously uncovered range, R_{0}=1.5-6 mm, for laser-induced bubbles. The relative luminescence energy was found to rapidly decrease as a function of the bubble asymmetry quantified by the anisotropy parameter ζ, which is the dimensionless equivalent of the Kelvin impulse. As established previously, ζ also dictates the characteristic parameters of bubble-driven microjets. The threshold of ζ beyond which no luminescence is observed in our experiment closely coincides with the threshold where the microjets visibly pierce the bubble and drive a vapor jet during the rebound. The individual fitted blackbody temperatures range between T_{lum}=7000 and T_{lum}=11500 K but do not show any clear trend as a function of ζ. Time-resolved measurements using a high-speed photodetector disclose multiple luminescence events at each bubble collapse. The averaged full width at half-maximum of the pulse is found to scale with R_{0} and to range between 10 and 20 ns.