Influence of frequency sweep on sonochemiluminescence and sonoluminescence

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

Spatial distribution of sonoluminescence and sonochemiluminescence generated by cavitation bubbles in 1.2 MHz focused ultrasound field

An intensified charge coupled device (ICCD) camera was used to observe the spatial distribution of sonoluminescence (SL) and sonochemiluminescence (SCL) generated by cavitation bubbles in a 1.2 MHz focused ultrasound (FU) field in order to investigate the mechanisms of acoustic cavitation under different sonication conditions for FU therapeutic applications.It was found that SL emissions were located in the post-focal region. When the intensity of SL and SCL increased as the power rose, the growth of SCL was much higher than that of SL. In the post-focal region, the SCL emissions moved along specific paths and formed branch-like streamers. At the beginning of the ultrasound irradiation, cavitation bubbles generated SCL in both the pre-focal and the post-focal region. When the electrical power or the sonication time increased, the SCL in the post-focal region increased and became higher than that in the pre-focal region. The intensity of SCL in the focal region is usually the weakest because of “oversaturation”.The spatial distribution of SCL near a tissue boundary differed from that obtained in free fields. It organized into special structures under different acoustic amplitudes. When the electrical power was relatively low, the SCL emission was conical shape which suggested a standing wave formation at the tissue-fluid boundary. When the electrical power exceeded a certain threshold, only a bright spot could be captured in the focus. The cavitation bubbles which centralized in the focus concentrated energy and hindered the formation of standing waves. With rising electrical power at high levels, besides a bright spot in the focus, there were some irregular light spots in pre-focal region, which indicated some cavitation bubbles or small bubble clusters achieved the threshold of SCL and induced the reaction with the luminol solution.     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

Ultrasonic cavitation at liquid/solid interface in a thin Ga–In liquid layer with free surface

Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga–In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.     

Sonoluminescence and sonochemiluminescence from a microreactor

Micromachined pits on a substrate can be used to nucleate and stabilize microbubbles in a liquid exposed to an ultrasonic field. Under suitable conditions, the collapse of these bubbles can result in light emission (sonoluminescence, SL). Hydroxyl radicals (OH()) generated during bubble collapse can react with luminol to produce light (sonochemiluminescence, SCL). SL and SCL intensities were recorded for several regimes related to the pressure amplitude (low and high acoustic power levels) at a given ultrasonic frequency (200kHz) for pure water, and aqueous luminol and propanol solutions. Various arrangements of pits were studied, with the number of pits ranging from no pits (comparable to a classic ultrasound reactor), to three-pits. Where there was more than one pit present, in the high pressure regime the ejected microbubbles combined into linear (two-pits) or triangular (three-pits) bubble clouds (streamers). In all situations where a pit was present on the substrate, the SL was intensified and increased with the number of pits at both low and high power levels. For imaging SL emitting regions, Argon (Ar) saturated water was used under similar conditions. SL emission from aqueous propanol solution (50mM) provided evidence of transient bubble cavitation. Solutions containing 0.1mM luminol were also used to demonstrate the radical production by attaining the SCL emission regions.     

Effect of dissolved gases in water on acoustic cavitation and bubble growth rate in 0.83 MHz megasonic of interest to wafer cleaning

Changes in the cavitation intensity of gases dissolved in water, including H₂, N₂, and Ar, have been established in studies of acoustic bubble growth rates under ultrasonic fields. Variations in the acoustic properties of dissolved gases in water affect the cavitation intensity at a high frequency (0.83 MHz) due to changes in the rectified diffusion and bubble coalescence rate. It has been proposed that acoustic bubble growth rates rapidly increase when water contains a gas, such as hydrogen faster single bubble growth due to rectified diffusion, and a higher rate of coalescence under Bjerknes forces. The change of acoustic bubble growth rate in rectified diffusion has an effect on the damping constant and diffusivity of gas at the acoustic bubble and liquid interface. It has been suggested that the coalescence reaction of bubbles under Bjerknes forces is a reaction determined by the compressibility and density of dissolved gas in water associated with sound velocity and density in acoustic bubbles. High acoustic bubble growth rates also contribute to enhanced cavitation effects in terms of dissolved gas in water. On the other hand, when Ar gas dissolves into water under ultrasound field, cavitation behavior was reduced remarkably due to its lower acoustic bubble growth rate. It is shown that change of cavitation intensity in various dissolved gases were verified through cleaning experiments in the single type of cleaning tool such as particle removal and pattern damage based on numerically calculated acoustic bubble growth rates.     

Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this “bubble streamers” is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.     

The impact of methanol mass transport on its conversion for the production of hydrogen and oxygenated reactive species in sono-irradiated aqueous solution

This study aims principally to assess numerically the impact of methanol mass transport (i.e., evaporation/condensation across the acoustic bubble wall) on the thermodynamics and chemical effects (methanol conversion, hydrogen and oxygenated reactive species production) of acoustic cavitation in sono-irradiated aqueous solution. This effect was revealed at various ultrasound frequencies (from 213 to 1000 kHz) and acoustic intensities (1 and 2 W/cm²) over a range of methanol concentrations (from 0 to 100%, v/v). It was found that the impact of methanol concentration on the expansion and compression ratios, bubble temperature, CH₃OH conversion and the molar productions inside the bubble is frequency dependent (either with or without consideration of methanol mass transport), where this effect is more pronounced when the ultrasound frequency is decreased. Alternatively, the decrease in acoustic intensity decreases clearly the effect of methanol mass transport on the bubble sono-activity. When methanol mass transfer is eliminated, the decrease of the bubble temperature, CH₃OH conversion and the molar yield of the bubble with the rise of methanol concentration was found to be more amortized as the wave frequency is reduced from 1 MHz to 213 kHz, compared to the case when the mass transport of methanol is taken into account. Our findings indicate clearly the importance of incorporating the evaporation and condensation mechanisms of methanol throughout the numerical simulations of a single bubble dynamics and chemical activity.     

Monitoring of transient cavitation induced by ultrasound and intense pulsed light in presence of gold nanoparticles

One of the most important challenges in medical treatment is invention of a minimally invasive approach in order to induce lethal damages to cancer cells. Application of high intensity focused ultrasound can be beneficial to achieve this goal via the cavitation process. Existence of the particles and vapor in a liquid decreases the ultrasonic intensity threshold required for cavitation onset. In this study, synergism of intense pulsed light (IPL) and gold nanoparticles (GNPs) has been investigated as a means of providing nucleation sites for acoustic cavitation. Several approaches have been reported with the aim of cavitation monitoring. We conducted the experiments on the basis of sonochemiluminescence (SCL) and chemical dosimetric methods. The acoustic cavitation activity was investigated by determining the integrated SCL signal acquired over polyacrylamide gel phantoms containing luminol in the presence and absence of GNPs in the wavelength range of 400–500 nm using a spectrometer equipped with cooled charged coupled devices (CCD) during irradiation by different intensities of 1 MHz ultrasound and IPL pulses. In order to confirm these results, the terephthalic acid chemical dosimeter was utilized as well. The SCL signal recorded in the gel phantoms containing GNPs at different intensities of ultrasound in the presence of intense pulsed light was higher than the gel phantoms without GNPs. These results have been confirmed by the obtained data from the chemical dosimetry method. Acoustic cavitation in the presence of GNPs and intense pulsed light has been suggested as a new approach designed for decreasing threshold intensity of acoustic cavitation and improving targeted therapeutic effects.     

Spatial-temporal dynamics of cavitation bubble clouds in 1.2 MHz focused ultrasound field

Cavitation bubbles have been recognized as being essential to many applications of ultrasound. Temporal evolution and spatial distribution of cavitation bubble clouds induced by a focused ultrasound transducer of 1.2 MHz center frequency are investigated by high-speed photography. It is revealed that at a total acoustic power of 72 W the cavitation bubble cloud first emerges in the focal region where cavitation bubbles are observed to generate, grow, merge and collapse during the initial 600 μs. The bubble cloud then grows upward to the post-focal region, and finally becomes visible in the pre-focal region. The structure of the final bubble cloud is characterized by regional distribution of cavitation bubbles in the ultrasound field. The cavitation bubble cloud structure remains stable when the acoustic power is increased from 25 W to 107 W, but it changes to a more violent form when the acoustic power is further increased to 175 W.     

脉冲超声激励下SonoVue微泡的瞬态空化特性

将SonoVue微泡从临床疾病诊断拓展至治疗引起了诸多研究人员的兴趣。为了平衡治疗效率和生物安全性,深入理解声学参数和SonoVue微泡瞬态空化的关系至关重要。本研究自行制备仿体容器放置SonoVue微泡,使用1 MHz发射换能器激励其产生空化效应,另一个7.5 MHz的聚焦换能器接收声信号,经放大及高速数据采集后送上位机处理。通过深入分析信号的时频域特征,我们提出以宽带信号的能量及其随时间变化曲线的半高宽来表征瞬态空化的剂量(ICD)和相对持续时间(ICP),并确定:瞬态空化的发生和ICD依赖于峰值负声压,但ICP随峰值负声压的增加而减小;脉冲重复频率和脉冲持续时间都和ICD及ICP正相关;且脉冲持续时间的影响较大。这些结果有望为SonoVue微泡的治疗应用提供理论支持。      

Studies of a novel sensor for assessing the spatial distribution of cavitation activity within ultrasonic cleaning vessels

This paper describes investigations of the spatial distribution of cavitation activity generated within an ultrasonic cleaning vessel, undertaken using a novel cavitation sensor concept. The new sensor monitors high frequency acoustic emissions (>1 MHz) generated by micron-sized bubbles driven into acoustic cavitation by the applied acoustic field. Novel design features of the sensor, including its hollow, cylindrical shape, provide the sensor with spatial resolution, enabling it to associate the megahertz acoustic emissions produced by the cavitating bubbles with specific regions of space within the vessel. The performance of the new sensor has been tested using a 40 kHz ultrasonic cleaner employing four transducers and operating at a nominal electrical power of 140 W under controlled conditions. The results demonstrate the ability of the sensors to identify 'hot-spots' and 'cold-spots' in cavitation activity within the vessel, and show good qualitative agreement with an assessment of the spatial distribution of cavitation determined through erosion monitoring of thin sheets of aluminium foil. The implications of the studies for the development of reliable methods of quantifying the performance of cleaning vessels are discussed in detail.     

Influence of dissolved gases on sonochemistry and sonoluminescence in a flow reactor

In the present work, the influence of gas addition is investigated on both sonoluminescence (SL) and radical formation at 47 and 248 kHz. The frequencies chosen in this study generate two distinct bubble types, allowing to generalize the conclusions for other ultrasonic reactors. In this case, 47 kHz provides transient bubbles, while stable ones dominate at 248 kHz. For both bubble types, the hydroxyl radical and SL yield under gas addition followed the sequence: Ar > Air > N₂ >> CO₂. A comprehensive interpretation is given for these results, based on a combination of thermal gas properties, chemical reactions occurring within the cavitation bubble, and the amount of bubbles. Furthermore, in the cases where argon, air and nitrogen were bubbled, a reasonable correlation existed between the OH-radical yield and the SL signal, being most pronounced under stable cavitation at 248 kHz. Presuming that SL and OH originate from different bubble populations, the results indicate that both populations respond similarly to a change in acoustic power and dissolved gas. Consequently, in the presence of non-volatile pollutants that do not quench SL, sonoluminescence can be used as an online tool to qualitatively monitor radical formation.     

Bubble size distribution in acoustic droplet vaporization via dissolution using an ultrasound wide-beam method

Performance and efficiency of numerous cavitation enhanced applications in a wide range of areas depend on the cavitation bubble size distribution. Therefore, cavitation bubble size estimation would be beneficial for biological and industrial applications that rely on cavitation. In this study, an acoustic method using a wide beam with low pressure is proposed to acquire the time intensity curve of the dissolution process for the cavitation bubble population and then determine the bubble size distribution. Dissolution of the cavitation bubbles in saline and in phase-shift nanodroplet emulsion diluted with undegassed or degassed saline was obtained to quantify the effects of pulse duration (PD) and acoustic power (AP) or peak negative pressure (PNP) of focused ultrasound on the size distribution of induced cavitation bubbles. It was found that an increase of PD will induce large bubbles while AP had only a little effect on the mean bubble size in saline. It was also recognized that longer PD and higher PNP increases the proportions of large and small bubbles, respectively, in suspensions of phase-shift nanodroplet emulsions. Moreover, degassing of the suspension tended to bring about smaller mean bubble size than the undegassed suspension. In addition, condensation of cavitation bubble produced in diluted suspension of phase-shift nanodroplet emulsion was involved in the calculation to discuss the effect of bubble condensation in the bubble size estimation in acoustic droplet vaporization. It was shown that calculation without considering the condensation might underestimate the mean bubble size and the calculation with considering the condensation might have more influence over the size distribution of small bubbles, but less effect on that of large bubbles. Without or with considering bubble condensation, the accessible minimum bubble radius was 0.4 or 1.7 μm and the step size was 0.3 μm. This acoustic technique provides an approach to estimate the size distribution of cavitation bubble population in opaque media and might be a promising tool for applications where it is desirable to tune the ultrasound parameters to control the size distribution of cavitation bubbles.     

Experimental investigation of conical bubble structure and acoustic flow structure in ultrasonic field

The objective of this paper is to investigate the transient conical bubble structure (CBS) and acoustic flow structure in ultrasonic field. In the experiment, the high-speed video and particle image velocimetry (PIV) techniques are used to measure the acoustic cavitation patterns, as well as the flow velocity and vorticity fields. Results are presented for a high power ultrasound with a frequency of 18 kHz, and the range of the input power is from 50 W to 250 W. The results of the experiment show the input power significantly affects the structures of CBS, with the increase of input power, the cavity region of CBS and the velocity of bubbles increase evidently. For the transient motion of bubbles on radiating surface, two different types could be classified, namely the formation, aggregation and coalescence of cavitation bubbles, and the aggregation, shrink, expansion and collapse of bubble cluster. Furthermore, the thickness of turbulent boundary layer near the sonotrode region is found to be much thicker, and the turbulent intensities are much higher for relatively higher input power. The vorticity distribution is prominently affected by the spatial position and input power.     

Sonoluminescence

Sonoluminescence (SL) is the name given to the light emitted when a liquid is cavitated in a particular (rather violent) manner. The appropriate cavitation conditions can be realized by using high intensity ultrasound, a spark discharge, a laser pulse, or by flowing the liquid through a Venturi tube. SL occurs in a wide variety of liquids, its intensity and spectrum depending on the nature of the solvent and the solute (including dissolved gas). The intensity, but apparently not the spectrum, also depends on the frequency of the sound and on the temperature and hydrostatic pressure of the liquid. In a standing wave sound field the SL originates from bubbles attracted to the pressure antinodes and has its maximum intensity when the bubble volume is a minimum. The phase of the sound cycle at which this occurs depends on the amplitude and frequency of the sound field. Spectral measurements show that SL originates mainly from the recombination of free radicals created within the high temperature and high pressure environment of a bubble undergoing an adiabatic compression, as may happen either during transient cavitation or during highly non-linear, but stable, cavitation. In discussing these, and other, attributes of SL this review emphasizes developments over the past 20 years. Because of the importance of the dynamical theory of bubbles to a full understanding of SL, it includes an account of bubble dynamics. In addition, it describes the various experimental techniques employed in the creation and analysis of SL. Although the review lays particular stress on the SL produced via acoustic cavitation, it also examines the characteristics of the SL produced using other methods of cavitation.     

Nonlinear dynamics and acoustic emissions of interacting cavitation bubbles in viscoelastic tissues

The cavitation-mediated bioeffects are primarily associated with the dynamic behaviors of bubbles in viscoelastic tissues, which involves complex interactions of cavitation bubbles with surrounding bubbles and tissues. The radial and translational motions, as well as the resultant acoustic emissions of two interacting cavitation bubbles in viscoelastic tissues were numerically investigated. Due to the bubble–bubble interactions, a remarkable suppression effect on the small bubble, whereas a slight enhancement effect on the large one were observed within the acoustic exposure parameters and the initial radii of the bubbles examined in this paper. Moreover, as the initial distance between bubbles increases, the strong suppression effect is reduced gradually and it could effectively enhance the nonlinear dynamics of bubbles, exactly as the bifurcation diagrams exhibit a similar mode of successive period doubling to chaos. Correspondingly, the resultant acoustic emissions present a progressive evolution of harmonics, subharmonics, ultraharmonics and broadband components in the frequency spectra. In addition, with the elasticity and/or viscosity of the surrounding medium increasing, both the nonlinear dynamics and translational motions of bubbles were reduced prominently. This study provides a comprehensive insight into the nonlinear behaviors and acoustic emissions of two interacting cavitation bubbles in viscoelastic media, it may contribute to optimizing and monitoring the cavitation-mediated biomedical applications.     

Acoustic power dependences of sonoluminescence and bubble dynamics

The decreasing effect of sonoluminescence (SL) in water at high acoustic powers was investigated in relation to bubble dynamics and acoustic emission spectra. The intensity of SL was measured in the power range of 1–18 W at 83.8 kHz for open-end (free liquid surface and film-covered surface) and fixed-end boundaries of sound fields. The power dependence of the SL intensity showed a maximum and then decrease to zero for all the boundaries. Similar results were obtained for sonochemiluminescence in luminol solution. The power dependence of the SL intensity was strongly correlated with the bubble dynamics captured by high-speed photography at 64 k fps. In the low-power range where the SL intensity increases, bubble streamers were observed and the population of streaming bubbles increased with the power. At powers after SL maximum occurred, bubble clusters came into existence. Upon complete SL reduction, only bubble clusters were observed. The subharmonic in the acoustic emission spectra increased markedly in the region where bubble clusters were observed. Nonspherical oscillations of clustering bubbles may make a major contribution to the subharmonic.     

Acoustic cavitation in 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide based ionic liquid

In this work, a comparison between the temperatures/pressures within acoustic cavitation bubble in an imidazolium-based room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide ([BMIM][NTf₂]), and in water has been made for a wide range of cavitation parameters including frequency (140–1000 kHz), acoustic intensity (0.5–1 W cm⁻²), liquid temperature (20–50 °C) and external static pressure (0.7–1.5 atm). The used cavitation model takes into account the liquid compressibility as well as the surface tension and the viscosity of the medium. It was found that the bubble temperatures and pressures were always much higher in the ionic liquid compared to those predicted in water. The valuable effect of [BMIM][NTf₂] on the bubble temperature was more pronounced at higher acoustic intensity and liquid temperature and lower frequency and external static pressure. However, confrontation between the predicted and the experimental estimated temperatures in ionic liquids showed an opposite trend as the temperatures measured in some pure ionic liquids are of the same order as those observed in water. The injection of liquid droplets into cavitation bubbles, the pyrolysis of ionic liquids at the bubble-solution interface as well as the lower number of collapsing bubbles in the ionic liquid may be the responsible for the lower measured bubble temperatures in ionic liquids, as compared with water.     

Acoustic noise spectra under hydrothermal conditions

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