Critical Analysis of Hydrogen Production by Aqueous Methanol Sonolysis

Recently, several experimental and theoretical studies have demonstrated the feasibility of enhancing the sonochemical production of hydrogen via methanol pyrolysis within acoustic cavitation bubbles (i.e. sonolysis of aqueous methanol solution). This review includes both the experimental and theoretical achievements in the field of hydrogen production by methanol sonolysis. Additionally, the limits of the process's applicability and plausible solutions are highlighted. The impact of different parameters influencing the process performance is discussed. Finally, the effects of methanol concentration on the size distribution of active cavitation bubbles are analyzed.

Effect of additives on degradation of poly vinyl alcohol (PVA) using ultrasound and microwave irradiation

The degradation of polyvinyl alcohol (PVA) has been investigated using ultrasonic (US) as well as microwave (MW) irradiation techniques with the approach of process intensification based on different additives, such as Titanium Dioxide (TiO2), Sodium Lauryl Sulphate (SLS), Zinc Oxide (ZnO) and air. The effects of sonication time, initial polymer concentration, and temperature on the extent of reduction in viscosity have been thoroughly investigated using US as well as MW irradiation approaches. Basically, the degradation process has been optimized by utilizing two different ultrasonic reactors in a combined approach of ultrasonic horn and bath. The maximum extent of degradation of PVA was found to be 69.33% using MW irradiation with a required energy of 0.321 g/JL, and 62.47% using US horn with a required energy of 0.054 g/JL when operated at 0.1 g/L of TiO2 catalyst. The combination of US horn and US bath results in same degradation as 0.1 g/L of TiO2 catalyst with US horn. It has also been observed that the maximum degradation of PVA was obtained with a minimum treatment time of 3 min using MW irradiation, whereas the US horn required 40 min. Moreover, a lower extent of PVA degradation was obtained when additives were used, such as surfactants (SLS) and air. As a result, it can be inferred that the MW-assisted approach in the presence of process-intensifying additives/catalysts is the best approach for the degradation of PVA with a minimum energy consumption.

Influence of Surface Tension on Dynamic Characteristics of Single Bubble in Free-Field Exposed to Ultrasound

The motion of bubbles in an ultrasonic field is a fundamental physical mechanism in most applications of acoustic cavitation. In these applications, surface-active solutes, which could lower the surface tension of the liquid, are always utilized to improve efficiency by reducing the cavitation threshold. This paper examines the influence of liquids’ surface tension on single micro-bubbles motion in an ultrasonic field. A novel experimental system based on high-speed photography has been designed to investigate the temporary evolution of a single bubble in the free-field exposed to a 20.43 kHz ultrasound in liquids with different surface tensions. In addition, the R-P equations in the liquid with different surface tension are solved. It is found that the influences of the surface tension on the bubble dynamics are obvious, which reflect on the changes in the maximum size and speed of the bubble margin during bubble oscillating, as well as the weaker stability of the bubble in the liquid with low surface tension, especially for the oscillating bubble with higher speed. These effects of the surface tension on the bubble dynamics can explain the mechanism of surfactants for promoting acoustic cavitation in numerous application fields.

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.

Effects of surface tension on the dynamics of a single micro bubble near a rigid wall in an ultrasonic field

Acoustic cavitation is a very important hydrodynamic phenomenon, and is often implicated in a myriad of industrial, medical, and daily living applications. In these applications, the effect mechanism of liquid surface tension on improving the efficiency of acoustic cavitation is a crucial concern for researchers. In this study, the effects of liquid surface tension on the dynamics of an ultrasonic driven bubble near a rigid wall, which could be the main mechanism of efficiency improvement in the applications of acoustic cavitation, were investigated at the microscale level. A synchronous high-speed microscopic imaging method was used to clearly record the temporary evolution of single acoustic cavitation bubble in the liquids with different surface tension. Meanwhile, the bubble dynamic characteristics, such as the position and time of bubble collapse, the size and stability of the bubbles, the speed of bubble boundaries and the micro-jets, were analyzed and compared. In the case of the single bubbles near a rigid wall, it was found that low surface tension reduces the stability of the bubbles in the liquid medium. Meanwhile, the bubbles collapse earlier and farther from the rigid wall in the liquids with lower surface tension. In addition, the surface tension has no significant influence on the speed of the first micro-jet, but it can substantially increase the speed of second and the third micro-jets after the first collapse of the bubble. These effects of liquid surface tension on the bubble dynamics can explain the mechanism of surfactants in numerous fields of acoustic cavitation for facilitating its optimization and application.

Investigation of the Synergistic Effect of Sonolysis and Photocatalysis of Titanium Dioxide for Organic Dye Degradation

Herein, we report the effect of sonoluminescence and an initial dye concentration on the sonophotocatalysis of TiO2 for the degradation of eosin B, a textile dye. We first investigated the light illuminated during ultrasound irradiation (sonoluminescence) by photographic images, a radical indicator (luminol), and photoluminescence spectra of the detection range of 300–1050 nm. Next, we examined the synergistic effect of sonolysis on photocatalysis by comparing the dye degradation of sonophotocatalysis to that of individual contributions of sonolysis and photocatalysis. Since it was found that the synergist effect is highly engaged with a dye concentration and sonication power, we conducted the comparison test in different concentrations of eosin B (5 and 20 mg/L) and ultrasound powers (35.4, 106.1, and 176.8 W/cm2). When the concentration of dyes was low, negative synergistic effects were found at all ultrasound powers, whereas at the high concentration, positive synergistic effects were observed at high ultrasound power. This difference in synergistic effects was explained by the influence of ultrasound on dynamics of dye adsorption on the TiO2 surface.

Oxygen-argon acoustic cavitation bubble in a water-methanol mixture: Effects of medium composition on sonochemical activity

The objective of the present numerical study is to examine the sonochemical production within a single acoustic bubble that oscillates in an aqueous methanol solution under an oxygen-argon mixture. The produced molar yield during the strong collapse was analyzed in accordance with the system composition, i.e. the molar fraction of argon and the volume fraction of methanol. The simulation results based on 180 cases demonstrated the reproducibility of pyrolysis and combustion conditions within the bubble volume. Pure water sonolysis resulted in an optimal production at 90% molar of argon, with O, HO^· and HO₂^· as predominant species at low argon concentrations and O, HO^· et H^· at high concentrations. The addition of methanol changed the whole chemical schema evolving inside the bubble that gave rise to specific species such as CH₂OH,CH₃O,CH₂O,HCO,CO₂ and CO. A common optimum appeared at 40% molar of argon for solutions of 20% (v/v) and up of methanol. An absolute maximum was observed at 40% (v/v) of methanol, in spite of argon concentration. In addition, a significant selectivity of products was observed according to the composition of the medium.

Which Parameters Affect Biofilm Removal with Acoustic Cavitation? A Review

Bacterial biofilms are a cause of contamination in a wide range of medical and biological areas. Ultrasound is a mechanical energy that can remove these biofilms using cavitation and acoustic streaming, which generate shear forces to disrupt biofilm from a surface. The aim of this narrative review is to investigate the literature on the mechanical removal of biofilm using acoustic cavitation to identify the different operating parameters affecting its removal using this method. The properties of the liquid and the properties of the ultrasound have a large impact on the type of cavitation generated. These include gas content, temperature, surface tension, frequency of ultrasound and acoustic pressure. For many of these parameters, more research is required to understand their mechanisms in the area of ultrasonic biofilm removal, and further research will help to optimise this method for effective removal of biofilms from different surfaces.

Theoretical and experimental investigations of ultrasonic sound fields in thin bubbly liquid layers for ultrasonic cavitation peening

Ultrasonic cavitation peening is a potential surface enhancement process. During this process a high input power is necessary to obtain an effective process result. A small gap, usually less than 1 mm, between the sonotrode tip and the treated surface is also required to avoid substantial energy loss. Due to the high vibration of the sonotrode, many cavitation bubbles are generated, forming a thin bubbly liquid layer in the small gap. The cavitation bubbles in the layer seriously disturb the sound wave propagation and interact with each other. The disturbances and interactions change the intensity and the spatial distribution of cavitation bubbles, resulting in the different interactions between cavitation bubbles and workpiece surfaces. The variations of the interactions cause different surface properties of the workpieces after ultrasonic cavitation peening. Therefore, quantifying the ultrasound field in different conditions is of great important to improve the ultrasonic cavitation peening process. A current model of the sound propagation in the bubbly liquid was already developed but did not include the bubble interactions. In this work, the bubble interactions are taken into account to improve the current model. The calculated results of the sound field with the improved model are validated by sonochemiluminescence experiments in various standoff distances and vibration amplitudes. Both of the experimental and the calculated results show that the highest sound pressure is generated when the vibration amplitude is around 25 µm. The strongest cavitation intensity occurs at the gap width of 0.5-0.7 mm.

Effect of carbon tetrachloride on the luminol sonochemiluminescence reaction kinetics during multibubble cavitation

The sonochemiluminescence (SCL) of luminol reaction was studied in alkaline medium using a dissolution of luminol, sodium carbonate, hydrogen peroxide and carbon tetrachloride. The presence of carbon tetrachloride enhances the SCL reaction up to allow the study of the reaction in real time using a cell phone video camera. This experimental setup allows the study of the cavitation dynamics in real time and through all the reactor, including homogeneous and heterogeneous cavitation zones. Finally, it was tested the effect of ethanol, the ionic strength and pH on the SCL.

Ultrasound-assisted liquid–liquid spray extraction for the determination of multi-class trace organic compounds in high-volume water samples

A premier analytical method for the determination of multi-class trace organic compounds in high-volume water matrices.

Effect of ultrasound on the function and structure of a membrane protein: The case study of photosynthetic Reaction Center from Rhodobacter sphaeroides

Ultrasounds are used in many industrial, medical and research applications. Properties and function of proteins are strongly influenced by the interaction with the ultrasonic waves and their bioactivity can be lost because of alteration of protein structure. Surprisingly, to the best of our knowledge no study was carried out on Integral Membrane Proteins (IMPs), which are responsible for a variety of fundamental biological functions. In this work, the photosynthetic Reaction Center (RC) of the bacterium Rhodobacter sphaeroides has been used as a model for the study of the ultrasound-induced IMP denaturation. Purified RCs were suspended in i) detergent micelles, in ii) detergent-free buffer and iii) reconstituted in liposomes, and then treated with ultrasound at 30W and 20kHz at increasing times. The optical absorption spectra showed a progressive and irreversible denaturation in all cases, resulting from the perturbation of the protein scaffold structure, as confirmed by circular dichroism spectra that showed progressive alterations of the RC secondary structure. Charge recombination kinetics were studied to assess the protein photoactivity. The lifetime for the loss of RC photoactivity was 32min in detergent micelles, ranged from 3.8 to 6.5min in the different proteoliposomes formulations, and 5.5min in detergent-free buffer. Atomic force microscopy revealed the formation of large RC aggregates related to the sonication-induced denaturation, in agreement with the scattering increase observed in solution.

Investigations on dynamics of interacting cavitation bubbles in strong acoustic fields

Given its importance to the dynamics of cavitation bubbles, the mutual interaction between bubbles was carefully investigated in this work. The cavitation noises emitted in different sonication conditions were recorded to study the dynamical behavior of the bubbles. The frequency spectra of the noises suggest that the dispersing state of the bubbles severely influence the oscillations of bubbles, and that the nonlinear feature of the dynamics of cavitation bubbles, imposed by the mutual bubble-bubble interaction, gradually develops with the decrease of the dispersing height. Theoretical analysis shows that the size difference between the interacting bubbles should be responsible for the increase of nonlinearity of the oscillation, and that the decrease of the distance between them could effectively enhance the nonlinear feature of the oscillation of the bubble, both of which agree well with the experimental observation.

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.

Investigation in mechanistic issues of sonocatalysis and sonophotocatalysis using pure and doped photocatalysts

This paper attempts to investigate the mechanistic issues of two hybrid advanced oxidation processes (HAOPs), viz. sonocatalysis and sonophotocatalysis, in which the two individual AOPs, viz. sonolysis and photocatalysis, are combined. Three photocatalysts, viz. pure ZnO and Fe-doped ZnO (with two protocols) have been employed. Fe-doped ZnO catalyst has been characterized using standard techniques. Decolorization of two textile dyes has been used as the model reaction. With experiments that alter the characteristics of ultrasound and cavitation phenomena in the medium, the exact synergy between the two AOPs has been determined using a quantitative yard stick. The results revealed a negative synergy between the two AOPs, which is an almost consistent result for decolorization of both dyes using all three photocatalysts. Fe-doping of ZnO catalyst helps in generation of more OH radicals that could augment decolorization. However, these radical mainly react with dye molecules adsorbed on catalyst surface. Intense shock waves generated by cavitation bubbles cause desorption of dye molecules from catalyst surface and reduce the probability of dye-radical interaction, thus reducing the net utility of photochemically generated OH radicals towards dye decolorization. This is rationale underlying the negative synergy between sonolysis and photocatalysis. Fe-doped ZnO catalyst increases the extent of decolorization, but the synergy between the two individual AOPs remains unaltered with doping.

Lipid oxidation volatiles absent in milk after selected ultrasound processing

Ultrasonic processing can suit a number of potential applications in the dairy industry. However, the impact of ultrasound treatment on milk stability during storage has not been fully explored under wider ranges of frequencies, specific energies and temperature applications. The effect of ultrasonication on lipid oxidation was investigated in various types of milk. Four batches of raw milk (up to 2L) were sonicated at various frequencies (20, 400, 1000, 1600 and 2000kHz), using different temperatures (4, 20, 45 and 63°C), sonication times and ultrasound energy inputs up to 409kJ/kg. Pasteurized skim milk was also sonicated at low and high frequency for comparison. In selected experiments, non-sonicated and sonicated samples were stored at 4°C and were drawn periodically up to 14days for SPME-GCMS analysis. The cavitational yield, characterized in all systems in water, was highest between 400kHz and 1000kHz. Volatile compounds from milk lipid oxidation were detected and exceeded their odor threshold values at 400kHz and 1000kHz at specific energies greater than 271kJ/kg in raw milk. However, no oxidative volatile compounds were detected below 230kJ/kg in batch systems at the tested frequencies under refrigerated conditions. Skim milk showed a lower energy threshold for oxidative volatile formation. The same oxidative volatiles were detected after various passes of milk through a 0.3L flow cell enclosing a 20kHz horn and operating above 90kJ/kg. This study showed that lipid oxidation in milk can be controlled by decreasing the sonication time and the temperature in the system depending on the fat content in the sample among other factors.

Ultrasound-based treatment approaches for intrinsic viscosity reduction of polyvinyl pyrrolidone (PVP)

The present work deals with achieving viscosity reduction in polymer solutions using ultrasound-based treatment approaches. Use of simple additives such as salts, or surfactants and introduction of air at varying flow rates as process intensifying parameters have been investigated for enhancing the degradation of polyvinyl pyrrolidone (PVP) using ultrasonic irradiation. Sonication is carried out using an ultrasonic horn at 36 kHz frequency at an optimized concentration (1%) of the polymer. The degradation behavior has been characterized in terms of the change in the viscosity of the aqueous solution of PVP. The intrinsic viscosity of the polymer has been shown to decrease to a limiting value, which is dependent on the operating conditions and use of different additives. Similar extent of viscosity reduction has been observed with 1% NaCl or 0.1% TiO2 at optimized depth of horn and 27°C, indicating the superiority of titanium dioxide as an additive. The combination of ultrasound and ultraviolet (UV) irradiation results in a significantly faster viscosity reduction as compared to the individual operations. A kinetic analysis for the degradation of PVP has also been carried out. The work provides a detailed understanding of the role of the operating parameters and additives in deciding the extent of reduction in the intrinsic viscosity of PVP solutions.

Effect of non-ionic surfactants on transient cavitation in a megasonic field

High resolution chronoamperometry has been used to characterize the effect of two non-ionic surfactants, Triton® X-100 and NCW®-1002, on cavitation in aqueous solutions exposed to ~1 MHz sound field. Specifically, using ferricyanide as the electroactive species, temporal variation of current during its reduction on a 25 μm Pt microelectrode has been measured and is used to elucidate transient cavity behavior. The chronoamperograms for solutions exposed to megasonic field show current 'peaks' riding on the baseline current. These current 'peaks' have been attributed to the diffusion of ferricyanide species concentrated at the liquid-vapor interface of a transient cavity at the end of its collapse. In the presence of surfactants, the frequency of occurrence of current 'peaks' with magnitude ≥ 0.3 μA is found to increase indicating a higher number of transient cavity collapses. A simple mathematical model based on diffusion developed previously by the authors has been used to extract the maximum cavity size and range of distances between the center of the collapsing cavity and the electrode surface in the surfactant solutions.

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.

Multiple observations of cavitation cluster dynamics close to an ultrasonic horn tip

Bubble dynamics in water close to the tip of an ultrasonic horn (∼23 kHz, 3 mm diameter) have been studied using electrochemistry, luminescence, acoustics, light scattering, and high-speed imaging. It is found that, under the conditions employed, a large bubble cluster (∼1.5 mm radius) exists at the tip of the horn. This cluster collapses periodically every three to four cycles of the fundamental frequency of the horn. Following the collapse of the cluster, a short-lived cloud of small bubbles (each tens of microns in diameter) was observed in the solution. Large amplitude pressure emissions are also recorded, which correlate temporally with the cluster collapse. Bursts of surface erosion (measured in real time using an electrochemical technique) and multibubble sonoluminescence emission both also occur at a subharmonic of the fundamental frequency of the horn and are temporally correlated with the bubble cluster collapse and the associated pressure wave emission.

The characterization of acoustic cavitation bubbles - an overview

Acoustic cavitation, in simple terms, is the growth and collapse of preexisting microbubbles under the influence of an ultrasonic field in liquids. The cavitation bubbles can be characterized by the dynamics of oscillations and the maximum temperatures and pressures reached when they collapse. These aspects can be studied both experimentally and theoretically for a single bubble system. However, in a multibubble system, the formation of bubble streamers and clusters makes it difficult to characterize the cumulative properties of these bubbles. In this overview, some recently developed experimental procedures for the characterization of acoustic cavitation bubbles have been discussed.

Ultrasonic cavitation at solid surfaces

In spite of the great potential of applying high-intensity ultrasound, which enables high-temperature and high-pressure chemistry with a reactor near room temperature and ambient pressure, sonochemistry at solid surfaces is at a weak stage of understanding with regards to the development of new materials and composite nanostructures. The science towards a quantitative understanding is only now emerging. On the other hand, in many applications an ultrasonic bath is used without thinking of the mechanism. Often surfaces are exposed to ultrasound for cleaning. Since ultrasonic treatment is not an exotic process and applicable even on large scale in industrial manufacturing, controlling the process may lead to new applications making use of the specially designed surface. This review is intended to summarize recent progress in this field and to point out most promising directions of ultrasound application for the development of new materials with functional surfaces.

Exploring the effects of pulsed ultrasound at 205 and 616 kHz on the sonochemical degradation of octylbenzene sulfonate

Compared to continuous wave (CW) ultrasound, pulsed wave (PW) ultrasound has been shown to result in enhanced sonochemical degradation of octylbenzene sulfonate (OBS). However, pulsed ultrasound was investigated under limited pulsing conditions. In this study, pulse-enhanced degradation of OBS was investigated over a broad range of pulsing conditions and at two ultrasonic frequencies (616 and 205 kHz). The rate of OBS degradation was compared to the rate of formation of 2-hydroxyterephthalic acid (HTA) following sonolysis of aqueous terephthalic acid (TA) solutions. This study shows that sonication mode and ultrasound frequency affect both OBS degradation and HTA formation rates, but not necessarily in the same way. Unlike TA, OBS, being a surface active solute, alters the cavitation bubble field by adsorbing to the gas/solution interface of cavitation bubbles. Enhanced OBS degradation rates during pulsing are attributed to this adsorption process. However, negative or smaller pulse enhancements compared to enhanced HTA formation rates are attributed to a decrease in the high-energy stable bubble population and a corresponding increase in the transient bubble population. Therefore, sonochemical activity as determined from TA sonolysis cannot be used as a measure of the effect of pulsing on the rate of degradation of surfactants in water. Over relatively long sonolysis times, a decrease in the rate of OBS degradation was observed under CW, but not under PW conditions. We propose that the generation and accumulation of surface active and volatile byproducts on the surface and inside of cavitation bubbles, respectively, during CW sonolysis is a contributing factor to this effect. This result suggests that there are practical applications to the use of pulsed ultrasound as a method to degrade surface active contaminants in water.

Ultrasonic degradation of polymers: effect of operating parameters and intensification using additives for carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA)

Use of ultrasound can yield polymer degradation as reflected by a significant reduction in the intrinsic viscosity or the molecular weight. The ultrasonic degradation of two water soluble polymers viz. carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) has been studied in the present work. The effect of different operating parameters such as time of irradiation, immersion depth of horn and solution concentration has been investigated initially using laboratory scale operation followed by intensification studies using different additives such as air, sodium chloride and surfactant. Effect of scale of operation has been investigated with experiments in the available different capacity reactors with an objective of recommending a suitable type of configuration for large scale operation. The experimental results show that the viscosity of polymer solution decreased with an increase in the ultrasonic irradiation time and approached a limiting value. Use of additives such as air, sodium chloride and surfactant helps in increasing the extent of viscosity reduction. At higher frequency operation the viscosity reduction has been found to be negligible possibly attributed to less contribution of the physical effects. The viscosity reduction in the case of ultrasonic horn has been observed to be more as compared to other large capacity reactors. Kinetic analysis of the polymer degradation process has also been performed. The present work has enabled us to understand the role of the different operating parameters in deciding the extent of viscosity reduction in polymer systems and also the controlling effects of low frequency high power ultrasound with experiments on different scales of operation.

Frequency effects during acoustic cavitation in surfactant solutions

The acoustic cavitation-induced events, multibubble sonoluminescence (MBSL) and initial growth of MBSL have been studied in surfactant solutions and correlated with bubble coalescence data at three different ultrasound frequencies. For an ionic surfactant, both the number of ultrasonic pulses required to reach a steady state MBSL intensity (N(crit)) and the magnitude of this intensity increases to a maximum as the surfactant concentration increases and then falls again. The total bubble volume generated for a fixed sonication time, which is indirectly related to bubble coalescence, similarly falls as surfactant concentration increases and then rises again. These effects are caused by a combination of electrostatic and coalescence factors at relatively low surfactant concentrations and the screening of the electrostatic factor as surfactant concentration increases further. The peak in coalescence inhibition occurs almost at the same surfactant concentrations as the acoustic frequency is increased; however, the concentrations at which peaks in MBSL and N(crit) occur vary at different frequencies. These results have been discussed in terms of coalescence, electrostatic interactions, rectified diffusion growth, and the adsorption kinetics of the surfactants.

Bubble population phenomena in sonochemical reactor: I estimation of bubble size distribution and its number density with pulsed sonication - laser diffraction method

To characterize the bubble populations (size and its number distribution) in a sonochemical reactor, a simple but powerful technique based on the Fraunhofer laser diffraction (LD) has been proposed. In this method, the acoustic wave disturbance to the laser probe in the sonochemical reaction field was eliminated by the temporal separation using pulsed sonication (pulsed LD). With this relatively simple strategy, the temporal development of the bubble size distribution could be evaluated by pulsed LD. A number density of bubbles was estimated by using a calibration data obtained with monosized standard particles. In addition, the effect of pulse length and a surfactant on the bubble population phenomena in a multibubble system are discussed.

Direct observation of cavitation fields at 23 and 515 kHz

Direct observation of cavitation fields using photography, sonoluminescence and luminol "mapping" is reported for a 23 kHz horn sonicator and a 515 kHz plate transducer system. The effect of sound intensity and added surfactant on the cavitation fields is described. The observations support previously reported results suggesting significant differences in the cavitation fields between the two sonication systems.

The detection and control of stable and transient acoustic cavitation bubbles

Acoustic cavitation plays a significant role in many applications that include therapeutic medicine, sonochemsitry and materials processing. The type of cavitation bubbles generated, namely transient or stable (repetitive transient), is important for optimizing the efficiency of these applications. We report on a multibubble sonoluminescence (MBSL)-based experimental technique for the detection and the control of type of cavitation at low and high ultrasound frequencies. It is observed that the use of a horn-type sonicator operating at 20 kHz primarily generates transient cavitation bubbles, as reported earlier. However, the use of plate type transducers at low frequencies (25 and 37 kHz) generates a significant amount of stable cavitation, as evidenced from the quenching of sonoluminescence by volatile solutes. Evidence for the existence of transient cavitation at a higher ultrasound frequency (440 kHz) is also provided.