Spatial separation of cavitating bubble populations: the nanodroplet injection model

About the sonoluminescent spectral portrait of gasoline water pollution

Single-bubble sonoluminescence spectra of the following samples were recorded in the modes of standing and moving bubble in liquid near the center of its levitation under the action of ultrasound: water contaminated with additives of commercial gasoline (1.5 - 38 mg·L-1), water with additives of individual gasoline components (hexane, benzene, toluene, p-xylene, naphthalene, anthracene, and p-terphenyl), and solutions of these gasoline components in hexane. Characteristic bands λmax of gasoline component emitters are recorded in the sonoluminescence spectra of a moving bubble for water samples contaminated with additives of commercial gasoline: 290 (p-xylene), 340 (p-terphenyl), 381, 399, 424, 449 (anthracene), and 438, 474, 516, 564 nm (C2, a hydrocarbon decomposition product during sonolysis).These bands are as a spectral portrait of gasoline contamination of water: they make it possible to identify gasoline in water in the above mentioned range of its content and to find a quantitative content of individual gasoline components.

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.

Complete defluorination of perfluorooctanoic acid (PFOA) by ultrasonic pyrolysis towards zero fluoro-pollution

Advanced oxidation/reduction of PFAS is challenged and concerned by the formation of toxic, short-chain intermediates during water treatments. In this study, we investigated the complete defluorination of PFOA by ultrasound/persulfate (US/PS) with harmless end-products of CO₂, H₂O, and F^‒ ions. We observed 100% defluorination after 4 h of US treatment alone with a power input of 900 W. PS addition, however, suppressed defluorination. We demonstrated by kinetics-fitted Langmuir-type adsorption modeling, the added PS increased competition with PFOA for adsorption sites on the bubble-water interface where radical oxidation and pyrolysis may occur. Providing sulfate (SO₄^•-) and hydroxyl (^•OH) radicals by means other than US did not defluorinate PFOA, indicating that pyrolysis likely contributes to the high defluorination performance. Bond dissociation energies for CC and CF were independent of pressure but decreased at elevated temperatures within cavitation bubbles (i.e., 5000 K) favoring the pyrolysis reactions. Furthermore, bond length calculations indicated that PFOA cleavage only begins to occur at temperatures in excess of those generated at the bubble interface (i.e., >1500 K) at the femtosecond level. This suggests that PFOA vaporizes or injects by nanodrops upon attachment to the cavitation bubble, enters the bubble, and is then cleaved within the bubble by pyrolysis. Our research in low-frequency ultrasonic horn system challenges the previous founding that defluorination of PFOA initiates and occurs at the bubble-water interface. We describe here that supplementing US-based processes with complementary treatments may have undesired effects on the efficacy of US. The mechanistic insights will further promote the implementation of US technology for PFAS treatment in achieving the zero fluoro-pollution goal.

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.

Single-Bubble Sonoluminescence of Colloidal Suspensions as a New Technique for Sonoluminescent Spectroscopic Analysis

This is a brief research review on the new method of development for element luminescence determination, namely, sonoluminescent spectroscopy. The advantages and disadvantages of the technique of multibubble sonoluminescence (MBSL) in solutions used to apply this method are discussed. It has been shown that the use of a new technique moving single-bubble sonoluminescence (m-SBSL) in colloidal suspensions of nanoparticles (<50 nm) containing the elements analyzed seems preferable for this purpose. This makes it possible to determine elements not only at lower concentrations than when using MBSL in solutions but also to find elements that are unavailable for determination through previous techniques. Thus, this new technique expands the range of elements that can be determined using sonoluminescent spectroscopy. The article provides a detailed description of the standard procedure for the preparation and recording of m-SBSL in colloidal suspensions, as well as examples of characteristic spectra of some elements obtained and recorded for the first time according to this new technique (Al, K, Mn, Cd, Pt, Ni, and Ti), including those not previously found using the MBSL in solutions (Al, Cd, Pt, Ni, and Ti). An example of the analytical line at 396 nm in the Al spectrum obtained through this new technique on the basis of an AlCl₃ initial aqueous solution, the region of the linear dependence of the intensity on the AlCl₃ concentration was registered, and the lower limit of the spectroscopic determination of the Al content in this solution was estimated as 8.3·10^-3 M. Using the analysis of the obtained Cd spectrum as an example, we carried out a spectroscopic measurement of the electronic temperature achieved at m-SBSL in bubble plasma at the moment of greatest compression of a bubble with light emission during its acoustic oscillations in dodecane, T_e = 7900 ± 500 K.

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.

In Situ EPR Spin Trapping and Competition Kinetics Demonstrate Temperature-Dependent Mechanisms of Synergistic Radical Production by Ultrasonically Activated Persulfate

Ultrasound coupled with activated persulfate can synergistically degrade aqueous organic contaminants. Here, in situ electron paramagnetic resonance spin trapping was used to compare radicals produced by ultrasonically activated persulfate (US-PS) and its individual technologies, ultrasound alone (US) and heat-activated persulfate (PS), with respect to temperature. Radicals were trapped using 5,5-dimethyl-1-pyrroline-N-oxide, DMPO, to form detectable nitroxide adducts. Using initial rates of radical adduct formation, and compared to US and PS, US-PS at 40 and 50 °C resulted in the largest synergistic production of radicals. Radicals generated from US were reasonably consistent from 40 to 70 °C, indicating that temperature had little effect on cavitational bubble collapse over this range. However, synergy indexes calculated from initial rates showed that ultrasonic activation of persulfate at the bubble interface changes with temperature. From these results, we speculate that higher temperatures enhance persulfate uptake into cavitation bubbles via nanodroplet injection. DMPO-OH was the predominant adduct detected for all conditions. However, competition modeling and spin trapping in the presence of nitrobenzene and atrazine probes showed that SO₄^•- predominated. Therefore, the DMPO-OH signal is derived from SO₄^•- trapping with subsequent DMPO-SO₄^- hydrolysis to DMPO-OH. Spin trapping is effective in quantifying total radical adduct formation but limited in measuring primary radical speciation in this case.

New sonochemiluminescence involving solvated electron in Ce(III)/Ce(IV) solutions

The moving single-bubble sonoluminescence of Ce³⁺ in water and ethylene glycol solutions of CeCl₃ and (NH₄)₂Ce(NO₃)₆ was studied. As found, a significant part of intensity of the luminescence (100% with cerium concentration less than 10^-4 M) is due to the sonochemiluminescence. A key reaction of sonochemiluminescence is the Ce⁴⁺ reduction by a solvated (or hydrated in water) electron: Ce⁴⁺ + e_s (e_aq) → *Ce³⁺. Solvated electrons are formed in a solution via electrons ejection from a low-temperature plasma periodically generated in deformable moving bubble at acoustic vibrations. Reactions of heterolytic dissociation of solvents make up the source of electrons in the plasma. In aqueous CeCl₃ solutions, the Ce⁴⁺ ion is formed at the oxidation of Ce³⁺ by OH radical. The latter species originates from homolytic dissociation of water in the plasma of the bubble, also penetrating from the moving bubble into the solution. The sonochemiluminescence in cerium trichloride solutions are quenched by the Br^- (acceptor of OH) and H⁺ ions (acceptor of e_aq). In water and ethylene glycol solutions of (NH₄)₂Ce(NO₃)₆, the sonochemiluminescence also quenched by the H⁺ ion. The sonochemiluminescence in CeCl₃ solutions is registered at [Ce³⁺] ≥ 10^-5 M. Then the sonochemiluminescence intensity increases with the cerium ion concentration and reaches the saturation plateau at 10^-2 M. It was shown that sonophotoluminescence (re-emission of light of bubble plasma emitters by cerium ions) also contributes to the luminescence of Ce³⁺ in solutions with [Ce³⁺] ≥ 10^-4 M. If the cerium concentration is more than 10^-2 M, a third source contributes to luminescence, viz., the collisional excitation of Ce³⁺ ions penetrating into the moving bubble.

Ultrasonic degradation of perfluorooctane sulfonic acid (PFOS) correlated with sonochemical and sonoluminescence characterisation

Sonolysis has been proposed as a promising treatment technology to remove per- and polyfluoroalkyl substances (PFASs) from contaminated water. The mechanism of degradation is generally accepted to be high temperature pyrolysis at the bubble surface with dependency upon surface reaction site availability. However, the parametric effects of the ultrasonic system on PFAS degradation are poorly understood, making upscale challenging and leading to less than optimal use of ultrasonic energy. Hence, a thorough understanding of these parametric effects could lead to improved efficiency and commercial viability. Here, reactor characterisation was performed at 44, 400, 500, and 1000 kHz using potassium iodide (KI) dosimetry, sonochemiluminescence (SCL), and sonoluminescence (SL) in water and a solution of potassium salt of PFOS (hereafter, K-PFOS). Then the degradation of K-PFOS (10 mg L^-1 in 200 mL solution) was investigated at these four frequencies. At 44 kHz, no PFOS degradation was observed. At 400, 500, and 1000 kHz the amount of degradation was 96.9, 93.8, and 91.2%, respectively, over four hours and was accompanied by stoichiometric fluoride release, indicating mineralisation of the PFOS molecule. Close correlation of PFOS degradation trends with KI dosimetry and SCL intensity was observed, which suggested degradation occurred under similar conditions to these sonochemical processes. At 1000 kHz, where the overall intensity of collapse was significantly reduced (measured by SL), PFOS degradation was not similarly decreased. Discussion is presented that suggests a hydrated electron degradation mechanism for PFOS may occur in ultrasonic conditions. This mechanism is a novel hypothesis in the field of PFAS sonolysis.

Flow effects on phenol degradation and sonoluminescence at different ultrasonic frequencies

Current literature shows a direct correlation between the sonochemical (SC) process of iodide oxidation and the degradation of phenol solution. This implies phenol degradation occurs primarily via oxidisation at the bubble surface. There is no work at present which considers the effect of fluid flow on the degradation process. In this work, parametric analysis of the degradation of 0.1 mM phenol solution and iodide dosimetry under flow conditions was undertaken to determine the effect of flow. Frequencies of 44, 300 and 1000 kHz and flow rates of 0, 24, 228 and 626 mL/min were applied with variation of power input, air concentration, and surface stabilisation. Phenol degradation was analysed using the 4-aminoantipyrine (4-AAP) method, and sonoluminescence (SL) images were evaluated for 0.1, 20 and 60 mM phenol solutions. Flow, at all frequencies under certain conditions, could augment phenol degradation. At 300 kHz there was excellent correlation between phenol degradation and dosimetry indicating a SC process, here flow acted to increase bubble transience, fragmentation and radical transfer to solution. At 300 kHz, although oxidation is the primary phenol degradation mechanism, it is limited, attributed to degradation intermediates which reduce OH radical availability and bubble collapse intensity. For 44 and 1000 kHz there was poor correlation between the two SC processes. At 44 kHz (0.01 mM), there was little to suggest high levels of intermediate production, therefore it was theorised that under more transient bubble conditions additional pyrolytic degradation occurs inside the bubbles via diffusion/nanodroplet injection mechanisms. At 1000 kHz, phenol degradation was maximised above all other systems attributed to increased numbers of active bubbles combined with the nature of the ultrasonic field. SL quenching, by phenol, was reduced in flow systems for the 20 and 60 mM phenol solutions. Here, where the standing wave field was reinforced, and bubble localisation increased, flow and the intrinsic properties of phenol acted to reduce coalescence/clustering. Further, at these higher concentrations, and in flow conditions, the accumulation of volatile phenol degradation products inside the bubbles are likely reduced leading to an increase SL.

Porous SiO2 nanoparticles containing ruthenium or sulfur compounds: Sonochemical producing and sonoluminescence in aqueous suspensions

Aqueous suspensions of silicon dioxide porous nanoparticles (average size 10-30 nm, average pore size 5.8 nm) were obtained via ultrasonic dispersing. As was shown through recording SiO molecular lines in a moving single-bubble sonoluminescence spectrum, these nanoparticles penetrate into the bubble and then undergo decay. Similarly, suspensions of SiO₂ nanoparticles, the pores of which were saturated with ruthenium dodecacarbonyl or elemental sulfur, were obtained by impregnation of the initial powder with solutions of these reagents in chloroform followed by evaporation of the solvent. Single-bubble sonoluminescence spectra of these suspensions contain more intense lines of Ru or S and Sⁿ⁺ as compared with the SiO lines. This also proves the involvement of water insoluble ruthenium and sulfur compounds into bubble sonoluminescent reactions in the heterogenic aqueous medium. Using the method of comparing the experimentally obtained and computer simulated luminescent spectra, we determined the effective electronic temperature T_e^Ru, which was 9000 ± 500 K, in non-equilibrium plasma of a bubble levitating in the ultrasonic field.

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.

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.

Spectroscopic measurement of electronic temperature in the bubbles during single- and multibubble sonoluminescence of metal carbonyl solutions and nanodispersed suspensions

It is for the first time that we have obtained emission-line spectra of metal atoms during single-bubble sonoluminescence of metal carbonyl W(CO)₆, Mo(CO)₆, Cr(CO)₆ solutions in dodecane and similar spectra during single- and multibubble sonoluminescence of nanodispersed Cr(CO)₆ suspensions in water, 83% H₂SO₄ and 74% H₃PO₄. Nanodispersed suspensions with an average particle size of 15-20 nm were obtained through sonodispersing and filtering Cr(CO)₆ crystals in water. The method for comparing the intensities of two atomic metal lines made it possible to measure the electronic temperatures achieved in cavitation bubbles under different modes of sonolysis that vary in the range of (4.5-16)·10³ K depending on the mode of sonolysis, the type of liquid and saturating gas (He, Ar, Kr) and the acoustic energy entering the solution or suspension. The electronic temperature for multibubble sonoluminescence is, on the average, by (2-2.5)·10³ K less than for single-bubble sonoluminescence for all liquids in question.

The Chemical History of a Bubble

Acoustic cavitation (the growth, oscillation, and rapid collapse of bubbles in a liquid) occurs in all liquids irradiated with sufficient intensity of sound or ultrasound. The collapse of such bubbles creates local heating and provides a unique source of energy for driving chemical reactions. In addition to sonochemical bond scission and formation, cavitation also induces light emission in many liquids. This phenomenon of sonoluminescence (SL) has captured the imagination of many researchers since it was first observed 85 years ago. SL provides a direct probe of cavitation events and has provided most of our understanding of the conditions created inside collapsing bubbles. Spectroscopic analyses of SL from single acoustically levitated bubbles as well as from clouds of bubbles have revealed molecular, atomic, and ionic line and band emission riding atop an underlying continuum arising from radiative plasma processes. Application of spectrometric methods of pyrometry and plasma diagnostics to these spectra has permitted quantitative measurement of the intracavity conditions: relative peak intensities for temperature measurements, peak shifts and broadening for pressures, and peak asymmetries for plasma electron densities. The studies discussed herein have revealed that extraordinary conditions are generated inside the collapsing bubbles in ordinary room-temperature liquids: observable temperatures exceeding 15 000 K (i.e., three times the surface temperature of our sun), pressures of well over 1000 bar (more than the pressure at the bottom of the Mariana Trench), and heating and cooling rates in excess of 10¹² K·s^-1. Scientists from many disciplines, and even nonscientists, have been and continue to be intrigued by the consequences of dynamic bubbles in liquids. As chemists, we are fascinated by the high energy reactions and processes that occur during acoustic cavitation and by the use of SL as a spectroscopic probe of the events during cavitation. Within the chemical realm of SL and cavitation there are many interesting questions that are now answered but also many that remain to be explored, so we hope that this Account reveals to the reader some of the most fascinating of those curiosities as we explore the chemical history of a bubble. The high energy species produced inside collapsing bubbles also lead to secondary reactions from the high energy species created within the collapsing bubble diffusing into the bulk liquid and expanding the range of sonochemical reactions observed, especially in redox reactions relevant to nanomaterials synthesis. Bubbles near solid surfaces deform upon collapse, which lessens the internal heating within the bubble, as shown by SL studies, but introduces important mechanical consequences in terms of surface damage and increased surface reactivity. Our understanding of the conditions created during cavitation has informed the applications of ultrasound to a wide range of chemical applications, from nanomaterials to synthetically useful organic reactions to biomedical and pharmaceutical uses. Indeed, we echo Michael Faraday's observation concerning a candle flame, "There is not a law under which any part of this universe is governed which does not come into play and is touched upon in these phenomena." ( Faraday , M. The Chemical History of a Candle ; Harper & Brothers : New York , 1861 ).

Water-molecular emission from cavitation bubbles affected by electric fields

Orange emission was observed during multibubble sonoluminescence at 1 MHz in water saturated with noble gas. The emission arose in the vicinity of the peeled ground electrode of a piezoceramic transducer exposed to water, suggesting that cavitation bubbles were affected by the electric fields that leaked from the transducer. The spectrum of the emission exhibited a broad component whose intensity increased towards the near-infrared region with peaks at 713 and 813 nm. The spectral shape was independent of the saturation gas of He, Ne, or Kr. The broad component was attributed to the superposition of lines due to vibration-rotation transitions of water molecules, each of which was broadened by the high pressure and electric fields at bubble collapse. An emission mechanism based on charge induction by electric fields and the charged droplet model is proposed.

Sonochemiluminescence in an aqueous solution of Ru(bpy)3Cl2

The sonochemiluminescence spectra of electron-excited ions ^*[Ru(bpy)₃]²⁺ was registered for the first time during sonolysis of argon saturated aqueous solutions of Ru(bpy)₃Cl₂ with low concentration. At single-bubble sonolysis, the luminescence band of ruthenium is recorded at a concentration of Ru(bpy)₃Cl₂ from 10^-6 M, and at multibubble from 10^-5 M. Possible mechanisms for the appearance of the band of a tris-bipyridyl ruthenium(II) complex on the background of an structureless continuum of water in the spectra of sonoluminescence are analyzed. Based on the results of the comparison of the sonoluminescence spectra of Ru(bpy)₃Cl₂ aqueous solutions with the sonoluminescence spectra of aqueous solutions of rhodamine B (which has a high quantum yield of photoluminescence) it was established that a possible mechanism of sonophotoluminescence does not play a decisive role in ruthenium sonoluminescence. The effect of radical acceptors (O₂, C₂H₅OH, Cd²⁺, I^-) on ruthenium sonoluminescence is analyzed. The most significant mechanism for the formation of electron-excited ions ^*[Ru(bpy)₃]²⁺ during sonolysis is the sonochemiluminescence in oxidation-reduction reactions involving [Ru(bpy)₃]²⁺ ions and radical products of sonolysis of water (OH, H, e^-_aq) in the solution volume.

Computational investigation of Tb(III) ion line intensities in single-bubble sonoluminescence

We perform a computational fluid dynamics simulation of trivalent terbium [Tb(III)] ion line emissions from single-bubble sonoluminescence (SBSL). Our simulation includes dynamic boundary conditions as well as the effects of gas properties and quenching by species, such as nitrite ion (NO_{2}^{-}). Simulation results demonstrate that when the maximum temperature inside a dimly luminescing bubble is relatively low, emission peaks from excited Tb(III) ions are prominent within the emission spectra. As the maximum temperature of the bubble increases, emission peaks of Tb(III) ions fade away relative to the continuum background emission. These calculations match observations of Tb(III) line emissions from SBSL occurring in aqueous solutions of terbium nitrate [Tb(NO_{3})_{3}] under an argon gas atmosphere. The evolution of the radiation energy spectrum over time for sonoluminescing bubbles provides a clear mechanism explaining Tb(III) emission peaks gradually merging into the continuous background emission as the radiation power increases.

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.

Could the study of cavitation luminescence be useful in high dilution research?

Cavitation in agitated liquids has been discussed for over five decades as a phenomenon that could play a role in the appearance of structural changes in the solvent of potentised dilutions. However, its lack of specificity as well as the absence of experimental confirmation have so far confined the idea to theory. The light emission associated with cavitational bubble collapse can be used to detect and study cavitation in fluids. The phenomenon has been extensively studied when driven by ultrasound, where it is called sonoluminescence. Sonoluminescence spectra reflect extremely high temperature and pressure in the collapsing bubbles and are parameter sensitive. This article tries to examine whether, despite objections and difficulties, the detection or the study of cavitational luminescence in solutions during potentisation could be useful as a physical tool in high dilution research.

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.

Nanostructured Materials Synthesis Using Ultrasound

Recent applications of ultrasound to the production of nanostructured materials are reviewed. Sonochemistry permits the production of novel materials or provides a route to known materials without the need for high bulk temperatures, pressures, or long reaction times. Both chemical and physical phenomena associated with high-intensity ultrasound are responsible for the production or modification of nanomaterials. Most notable are the consequences of acoustic cavitation: the formation, growth, and implosive collapse of bubbles, and can be categorized as primary sonochemistry (gas-phase chemistry occurring inside collapsing bubbles), secondary sonochemistry (solution-phase chemistry occurring outside the bubbles), and physical modifications (caused by high-speed jets, shockwaves, or inter-particle collisions in slurries).

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.

Sodium Atom Emission from Aqueous Surfactant Solutions Exposed to Ultrasound

Emission from electronically excited sodium atoms (Na*) was observed when argon saturated aqueous solutions of the anionic surfactants, sodium dodecyl sulfate, sodium octyl sulfate, sodium 1-pentanesulfonate, and sodium 1-octanesulfonate were sonicated using 358 kHz ultrasound. The same emission band, centered at about 590 nm, was also obtained in aqueous NaCl solutions, although a ∼100-fold higher concentration than that used for the surfactant solutions was required to obtain an emission of comparable intensity. The results have been interpreted in terms of the surfactant adsorbing at the gas-solution interface of the bubbles generated by the ultrasound, generating an electrostatic surface potential, and attracting Na⁺ counterions to the bubble surface. It is reasoned that Na⁺ ions are simultaneously reduced and electronically excited at the bubble-solution interface during the final stages of the collapse phase of the acoustically driven bubble. It is proposed that sodium ion bound water molecules reduce interfacial Na⁺ under the extreme, perhaps supercritical, conditions the interface experiences on bubble implosion.

The role of ultrasound in pharmaceutical production: sonocrystallization

Objectives

The main aim of this review was to develop a critical discussion of the key role ultrasound (US) can play on the production of active pharmaceutical ingredients (APIs) by discussing the versatile effect this type of energy produces.

Methods

The different crystallization techniques that can be assisted and improved by US are discussed in the light of the available US devices and the effect pursued by application of US energy. Simple and complex analytical methods to monitor API changes are also discussed.

Key findings

The countless achievements of API US-assisted production are summarized in a table, and outstanding effects such as narrower particle size distribution; decreased particle size, induction time, metastable zone and supersaturation levels; or a solubility increase are critically discussed.

Conclusions

The indisputable advantages of sonocrystallization over other ways of API production have been supported on multiple examples, and pending goals in this field (clarify the effect of US frequency on crystallization, know the mechanism of sonocrystallization, determine potential degradation owing to US energy, avoid calculation of the process yield by determining the concentration of the target drug remaining in the solution, etc.) should be achieved.

Ultrasonically treated liquid interfaces for progress in cleaning and separation processes

Cleaning and separation processes of liquids can be advanced by acoustic cavitation through bubbles with unique physico-chemical properties.

Two components of Na emission in sonoluminescence spectrum from surfactant aqueous solutions

Sonoluminescence from sodium dodecyl sulfate (SDS) aqueous solutions exhibits Na emission. The spectrum of Na emission was measured as a function of sonication time for a total of 30 min at an ultrasonic frequency of 148 kHz. The spectral line profiles changed with the sonication time, suggesting that the Na emission consists of two components: broadened lines, which are shifted from the original D lines, and unshifted narrow lines. The intensity of the unshifted narrow lines decreased at a greater rate than that of the broadened lines with increasing sonication time. This effect was enhanced at a higher acoustic power. The shifted broadened lines remained after sonication for 30 min. We propose that these quenching effects are caused by the accumulation of gases decomposed from SDS molecules inside bubbles. The CO₂ gas dependence of Na emission in NaCl aqueous solutions showed a similar change in the line profiles to that in SDS aqueous solutions, which supported this proposition. The unshifted narrow lines are easily affected by foreign gases. The results suggest that the two components originate from different environments around the emitting species, although both of them originate from the gas phase inside bubbles. The generation mechanisms of the two components are discussed.

Influence of the liquid viscosity on the formation of bubble structures in a 20kHz field

The cavitation field in a cylindrical vessel bottom-insonified by a 19.7kHz large area transducer is studied experimentally. By adding controlled amounts of Poly-Ethylene Glycol (PEG) to water, the viscosity of the liquid is varied between one- and nine-fold the viscosity of pure water. For each liquid, and for various displacement amplitudes of the transducer, the liquid is imaged by a high-speed camera and the acoustic field is measured along the symmetry axis. For low driving amplitudes, only a spherical cap bubble structure appears on the transducer, growing with amplitude, and the axial acoustic pressure field displays a standing-wave shape. Above some threshold amplitude of the transducer, a flare-like structure starts to build up, involving bubbles strongly expelled from the transducer surface, and the axial pressure profile becomes almost monotonic. Increasing more the driving amplitude, the structure extends in height, and the pressure profile remains monotonic but decreases its global amplitude. This behavior is similar for all the water-PEG mixtures used, but the threshold for structure formation increases with the viscosity of the liquid. The images of the bubble structures are interpreted and correlated to the measured acoustic pressure profiles. The appearance of traveling waves near the transducer, produced by the strong energy dissipated by inertial bubbles, is conjectured to be a key mechanism accompanying the sudden appearance of the flare-like structure.

Effects of argon sparging rate, ultrasonic power, and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions

The sonoluminescence spectra from acoustic cavitation in aqueous NaCl solutions are systematically studied in a large range of ultrasonic frequencies under variation of electrical power and argon sparging. At the same time, bubble dynamics are analysed by high-speed imaging. Sodium line and continuum emission are evaluated for acoustic driving at 34.5, 90, 150, 365, and 945kHz in the same reactor vessel. The results show that the ratio of sodium line to continuum emission can be shifted by the experimental parameters: an increase in the argon flow increases the ratio, while an increase in power leads to a decrease. At 945kHz, the sodium line is drastically reduced, while the continuum stays at elevated level. Bubble observations reveal a remarkable effect of argon in terms of bubble distribution and stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. As a consequence, the bubble interactions (splitting, merging) appear enhanced which supports a link between non-spherical bubble dynamics and sodium line emission.

Sonocrystallization and sonofragmentation

The application of ultrasound to crystallization (i.e., sonocrystallization) can dramatically affect the properties of the crystalline products. Sonocrystallization induces rapid nucleation that generally yields smaller crystals of a more narrow size distribution compared to quiescent crystallizations. The mechanism by which ultrasound induces nucleation remains unclear although reports show the potential contributions of shockwaves and increases in heterogeneous nucleation. In addition, the fragmentation of molecular crystals during ultrasonic irradiation is an emerging aspect of sonocrystallization and nucleation. Decoupling experiments were performed to confirm that interactions between shockwaves and crystals are the main contributors to crystal breakage. In this review, we build upon previous studies and emphasize the effects of ultrasound on the crystallization of organic molecules. Recent work on the applications of sonocrystallized materials in pharmaceutics and materials science are also discussed.

Unsteady translation and repetitive jetting of acoustic cavitation bubbles

High-speed recordings reveal peculiar details of the oscillation and translation behavior of cavitation bubbles in the vicinity of an ultrasonic horn tip driven at 20 kHz. In particular, a forward jump during collapse that is due to the rapid reduction of virtual mass is observed. Furthermore, frequently a jetting in the translation direction during the collapse phase is resolved. In spite of strong aspherical deformations and frequent splitting, these bubbles survive the jetting collapse, and they rebound recollecting fragments. Because of incomplete restoration of the spherical shape within the following driving period, higher periodic volume oscillations can occur. This is recognized as a yet unknown source of subharmonic acoustic emission by cavitation bubbles. Numerical modeling can capture the essentials of the unsteady translation.

Sonoluminescence quenching and cavitation bubble temperature measurements in an ionic liquid

A comparison between the temperatures within imploding acoustic cavitation bubbles and the extent of sonoluminescence (SL) quenching by C(1)-C(5) aliphatic alcohols in 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO(4)], a well known imidazolium based room temperature ionic liquid (RTIL)), has been made at an ultrasound frequency of 213 kHz. The temperatures obtained ranged from 3500 ± 200K, in neat [EMIM][EtSO(4)], to about 3200 ± 200K in RTIL-alcohol containing solutions. It was also found that the SL intensity decreased with increasing concentration (up to 1M) of the alcohols to a greater extent compared with the relative changes to the bubble temperatures. Both the extent of the reduction in the bubble temperatures and the SL quenching were much smaller than those obtained in comparable aqueous solutions containing aliphatic alcohols. Possible reasons for the differences in the observed trends between water/alcohol and [EMIM][EtSO(4)]/alcohol systems under sonication at 213 kHz are discussed.

Temperature Nonequilibration during Single-Bubble Sonoluminescence

Single-bubble sonoluminescence (SBSL) spectra from liquids having low vapor pressures, especially mineral acids, are exceptionally rich. During SBSL from aqueous sulfuric acid containing dissolved neon, rovibronic emission spectra reveal vibrationally hot sulfur monoxide (SO; Tv = 2100 K) that is also rotationally cold (Tr = 290 K). In addition to SO, excited neon atom emission gives an estimated temperature, for neon, of several thousand Kelvin. This nonequilibrated temperature is consistent with dynamically constrained SO formation at the liquid-vapor interface of the collapsing bubble. Formation occurs via collisions of fast neon atoms (generated within the collapsing bubble) with liquid-phase molecular species in the interfacial region, thus allowing for a mechanistic understanding of the processes leading to light emission.

Sonochemical degradation of cyclic nitroxides in aqueous solution

The sonochemical degradation of eight five- and six-membered nitroxides has been studied by EPR spectroscopy after exposure to ultrasound at a frequency of 354 kHz in argon-saturated aqueous solution. Concentration vs. time profiles do not follow a simple rate law. Octanol/water partition functions have been determined for all eight nitroxides, and an excellent linear correlation has been found between initial decomposition rates and hydrophobicity (log K(octanol/water)). Variation of initial rate with concentration was investigated for one compound (TEMPONE) and is largely consistent with an equilibrium distribution of substrate between bulk solution and the gas/liquid interface.

Spectroscopic and thermodynamic features of conical bubble luminescence

The influence on luminescence from conical bubble collapse (CBL) with varying Ar gas content while perturbing the liquid 1,2-Propanediol (PD) has been investigated. The temporal, spatial, and spectral features were analysed with regards to the dynamics of collapse and liquid degradation. Sulphuric acid and sodium chloride were added to disturb the liquid. The following three cases were studied: PD/Ar, (I), (PD + H(2)SO(4))/Ar, (II), and (PD + H(2)SO(4) + NaCl)/Ar, (III). The intensities of those cases decrease as III > II > I. Temporally, single and multiple light emissions were found to occur. The pulse shape exhibited a large variety of profiles with a main maximum and up to two local maxima around the main maximum. These local maxima resembled those generated by laser cavitation. Spatially, no radial symmetry was detected in the light emissions. Spectrally, the Swan, CH and CN lines were observed at low volumes of gas and driving pressure. The ·OH radical and OH-Ar bands, as well as the Na and K lines, consistently appeared superimposed on an underlying continuum that almost disappeared in (III). The Na line was observed with two satellite diffuse bands representing Na-Ar complexes in (I) and (II), whereas in (III), only the line of sodium could be seen. Weak and diffuse emission lines from the Ar atom in the near-IR region were observed in (I) and (II). The proposed mechanism of bright CBL was based on the energy transfer from electron-excited homolytic cleavage products to the chromophore molecules generated during the collapse-rebound time line (~8200 K and ~1 ms of collapse time from model), which had accumulated inside the liquid and remained on the walls of cavity during the repetition of the collapse. A general mechanism for the bright CBL is broached.

Multibubble sonoluminescence pulses from Na atoms in viscous liquid

Multibubble sonoluminescence pulses of Na and continuum emissions were measured from NaCl-ethylene glycol solution saturated with Xe at 28 kHz. The Na emission consisted of multiple-peak pulses and single pulses. The intrinsic pulse width estimated from single pulses was 0.37 ns, which differs from 10-165 ns obtained by previous work. High-speed shadowgraphs of bubble dynamics and high-speed movies (32000 fps) of sonoluminescence were observed. The observations suggest that the multiple-peak pulse is due to the superposition of single peaks resulting from bubbles fragmented from a characteristic bubble which repeats the fragmentation and coalescence. This phenomenon may be specific to viscous liquids.

Nonlinear bubble dynamics of cavitation

For cavitation clouds generated in a standing sound wave driven by an ultrasonic horn, the nonlinear acoustic wave equation governing cavitation dynamics is numerically solved together with the bubble motion equation under an approximation. This conceptual calculation can qualitatively reproduce the observed characteristics of cavitation.

The role of vapour pressure in multibubble sonoluminescence from organic solvents

The action of high intensity cavitation on several liquid halocarbons (C(2)Cl(4) CCl(4), CHCl(3), C(2)H(2)Br(4)) and other organic solvents (acetone, benzene and their mixtures) was investigated by recording multibubble sonoluminescence UV-Vis spectra over the temperature range between 246 and 298 K. The temperature induced variation of some thermophysical properties of the solvents Favours the interpretations of their role in determining the salient characteristics of the recorded spectra. We observed that high volatility does not necessarily quench sonoluminescence emission and that argon flow plays a key role in the appearance of radical emission lines. While for each investigated substance the intensity of C*(2) emission lines was clearly correlated to temperature, a comparative test between different halocarbons did not show a clear correlation with vapour pressure. Following recently reported results which evidenced the formation of dynamically differentiated populations of emitting bubbles in sulphuric acid, we performed MBSL experiments in liquid mixtures of halocarbons and sulphuric acid to investigate the correlation between the production of emitting species and the halocarbon volatility.