Acoustic Bubble Research Articles

Insight into acoustic wave-driven gas bubble dynamics in Williamson's fluid

This study's motivation on the behavior of a gas bubble and surrounding fluid during the collapse phase of an acoustic wave leads to the emission of light, which can be modelled using the Rayleigh-Plesset equation. The significant contribution to the major addition to the understanding of gas bubble dynamics in viscoelastic fluids. The results of the study have the potential to impact a wide range of applications, from sonar and underwater acoustic propagation to biomedical applications and industrial operations. Understanding the dynamics of acoustic wave-driven gas bubble oscillations in non-Newtonian fluids is crucial for various applications in medical imaging, therapy, and microfluidics. The aim of this study is to discover the conditions under which these bubbles can undergo sonoluminescence, the emission of light due to the collapse of bubbles under severe sonic pressure. The complex ordinary differential equation is solved numerically, and graphs are generated to evaluates various parameters such as velocity, radius, and pressure. A comparison with the viscous case is also presented to assess the influence of Newtonian and non-Newtonian fluids. A program designed by Mathematica software (13.0) “parametric NDSolve package” is used to solve the equations. The numerical solution supports the boundary conditions, indicating stability and reliability. Notably, the study identifies distinct fluctuations in the phase and stable characteristics associated with non-Newtonian effects. Interestingly, certain fluid parameters lead to discrete group modulation of radial excursions, revealing a novel and previously unknown phenomenon. It is concluded that Higher Reynolds numbers result in turbulent flow, which causes the bubble to enlarge and increase its volume. Furthermore the pressure gradient is affected by magnetic drag force, and as size of bubble increases, required pressure gradient to counteract the drag force also increases. As the magnetic field intensity increases, the velocity of the bubble decreases. Also, considering the implications of these results, the study explores their relevance to medical ultrasonography applications.

Acoustic Bubble and Magnetic Actuation-Based Microrobot for Enhanced Multiphase Drug Delivery Efficiency.

This paper proposes an acoustic bubble and magnetic actuation-based microrobot for enhancing multiphase drug delivery efficiency. The proposed device can encapsulate multiphase drugs, including liquids, using the two bubbles embedded within the microtube. Additionally, using the magnetic actuation of the loaded magnetic liquid metal, it can deliver drugs to target cells. This study visualized the flow patterns generated by the oscillating bubble within the tube to validate the drug release principle. In addition, to investigate the effect of the oscillation properties of the inner bubble on drug release, the oscillation amplitude of the inner bubble was measured under various experimental variables using a high-speed camera. Subsequently, we designed a microrobot capable of encapsulating bubbles, drugs, and magnetic liquid metal and fabricated it using microfabrication technology based on ultra-precision 3D printing. As a proof of concept, we demonstrated the transport and drug release of the microrobot encapsulating the drug in a Y-shaped channel simulating a blood vessel. The proposed device is anticipated to enhance the efficiency of drug therapy by minimizing drug side effects, reducing drug administration frequency, and improving the stability of the drug within the body. This paper is expected to be applicable not only to targeted drug delivery but also to various biomedical fields, such as minimally invasive surgery and cell manipulation, by effectively delivering multiphase drugs using the simple structure of a microrobot.

Bubble cloud characteristics and ablation efficiency in dual-frequency intrinsic threshold histotripsy

Histotripsy is a non-thermal focused ultrasound ablation method that destroys tissue through the generation and activity of acoustic cavitation bubble clouds. Intrinsic threshold histotripsy uses single-cycle pulses to generate bubble clouds when the dominant negative pressure phase exceeds an intrinsic threshold of ∼25–30 MPa. The ablation efficiency is dependent upon the size and density of bubbles within the bubble cloud. This work investigates the effects of dual-frequency pulsing schemes on the bubble cloud behavior and ablation efficiency in intrinsic threshold histotripsy. A modular 500 kHz:3 MHz histotripsy transducer treated agarose phantoms using dual-frequency histotripsy pulses with a 1:1 pressure ratio from 500 kHz and 3 MHz frequency elements and varying arrival times for the 3 MHz pulse relative to the arrival of the 500 kHz pulse (−100 ns, 0 ns, and +100 ns). High-speed optical imaging captured cavitation effects to characterize bubble cloud and individual bubble dynamics. The effects of dual-frequency pulsing on lesion formation and ablation efficiency were also investigated in red blood cell (RBC) phantoms. Results showed that the single bubble and bubble cloud size for dual-frequency cases were intermediate to published results for the component single-frequencies of 500 kHz and 3 MHz. Additionally, bubble cloud size and dynamics were shown to be altered by the arrival time of the 3 MHz pulse with respect to the 500 kHz pulse, with more uniform cloud expansion and collapse observed for early (−100 ns) arrival. Finally, RBC phantom experiments showed that dual-frequency exposures were capable of generating precise lesions with smaller areas and higher ablation efficiencies than previously published results for 500 kHz or 3 MHz. Overall, results demonstrate dual-frequency histotripsy’s ability to modulate bubble cloud size and dynamics can be leveraged to produce precise lesions at higher ablation efficiencies than previously observed for single-frequency pulsing.

Ultrasound-enhanced adsorption of amino acids from aqueous solution onto macroporous resins for green separation: Mass transfer mechanism and acoustic cavitation properties

The utilization of green technology for the recovery of renewable components has garnered significant attention. In this study, efficient and rapid separation of amino acids from aqueous solutions was achieved through ultrasound-assisted macroporous resin adsorption. The numerical simulation revealed the underlying mechanism of mass transfer during ultrasound-assisted adsorption and clarified the impact of acoustic properties on the adsorption process. Low-frequency (LFUS, 20 kHz) caused the damage of resin structure and the increase of adsorption capacity whereas these were less influenced by high-frequency ultrasound (HFUS, 1056 kHz). The external mass transfer coefficient was increased by 161% and 45% during LFUS- and HFUS-assisted adsorption than agitation-assisted adsorption. In addition, both US modes contributed to an enhancement of surface diffusion coefficient by over 100%. The predominant factor governing the intraparticle diffusion for amino acid adsorption was found to be pore volume diffusion, accounting for more than 70% of the overall process. Both LFUS and HFUS were observed to augment the influence of surface diffusion within the overarching diffusion phenomenon, aligning with the concurrent elevation of the surface diffusion coefficient. Furthermore, the mechanism of ultrasound-assisted adsorption was illustrated from the aspect of physical and sonochemical effects generated by the acoustic cavitation. The accumulated acoustic pressure of LFUS exceeded that of HFUS by a factor of 105, reflecting its strong physical effects. The connection between the mass transfer behaviour and acoustic bubble properties during ultrasound-assisted adsorption can provide guidance for the industry to apply ultrasound as a green technology to separate and recover renewable biomaterials from wastewater.

The relevance of bubble dynamics in ultrasonic/sonochemical processes

Acoustic cavitation bubble dynamics has been extensively studied by physicists and mathematicians. Process intensification is primarily studied by chemical engineers. Chemists tend to focus on bubble dynamics in a multibubble field to optimize the physical and chemical forces generated during acoustic cavitation with an intention to maximise/intensify chemical processes. To achieve process intensification successfully a multidisciplinary approach is required. Our early work on multibubble cavitation unveiled various factors that contribute to process optimization. For example, while single bubble dynamics can estimate the maximum bubble temperatures, they may not be relevant to achieving higher chemical yield, which in turn depends on varus other factors such as average bubble temperatures, number of active cavitation bubbles, etc. Other factors critical for process intensification include the generation of a homogeneous cavitation field, control over mass transfer effects caused by bubble oscillations, reactor design, etc. The presentation will provide an overview on the relevance of single and multibubble bubble dynamics in ultrasonic/sonochemical processes.

Evaluation of passive acoustic methods for ambient noise baseline and gas flow rate quantification at a proposed nearshore carbon capture and storage site in Australia

Measurement, monitoring, and verification (MMV) is an integral component of carbon capture and storage (CCS) projects. Within an operational MMV equipment, hydrophone-based passive acoustic techniques are used to establish ambient noise baseline and flow rate quantification at short range, specifically to facilitate “detect-attribute-quantify” sequence of an MMV program. However, nearshore environments are acoustically complex with different soundscape components that can disproportionately dominate ambient noise levels, potentially masking acoustic signatures of bubbles used to quantify seabed gas seeps. Therefore, a robust baseline describing ambient noise variability across the range of frequencies associated with acoustic emissions of gas seeps is required, from which changes can be detected and monitored. In this context, hydrophone measurements from a proposed nearshore CCS site in Australia are analyzed to establish a temporally resolved baseline, identifying key drivers causing overall ambient noise variability. These results are compared with acoustic bubble spectrum features and flow rate estimates from a controlled in situgas release experiment to understand the likelihood of detecting bubbles and quantifying flow rate at the proposed CCS site. Despite the complexities of nearshore environment, the evaluation highlights that passive acoustic methods can provide a practical solution to complement quantification component of operational MMV programs.

Insight into the role of dissolution mechanism in the sonochemistry of acoustic cavitation bubble

AbstractUsing a detailed numerical model, in the present work, the dissolution process of the different species generated by the acoustic cavitation bubble was investigated through the analysis of bubble chemistry over a range of wave frequencies from 140 to 515 kHz. It has been observed that during the first bubble collapse, at 140 and 213 kHz, significant amounts of ●OH, O, H●, H2, and O2 molecules (from ~ 2.1×10‐20 to 4.86×10‐18 mol) are dissolved into the bulk liquid (2.84‐0.067%). However, with the rise of ultrasound frequency (>213 kHz), the number and the quantity of the dissolved substances are decreased (<1.16×10‐20 mol) until to be completely suppressed at 515 kHz. Over the first compression period, at 140 and 213 kHz, the dissolution tendency is in the order: H2 (4.86×10‐18‐9.44×10‐19 mol) > H● (4.41×10‐18‐7.76×10‐19 mol) > O (1.8×10‐18‐2.75×10‐19 mol) > ●OH (4.68×10‐19‐1.91×10‐19 mol) > O2 (8.43×10‐20‐2.1×10‐20 mol). Nevertheless, at 355 kHz, the dissolution of the main substances is in the order: H● (1.16×10‐20 mol)>●OH (5.13×10‐21 mol)>H2 (3.59×10‐21 mol). Despite the low dissolution percentages of the different species (compared to the total yield) during the first bubble collapse (<3%), it has been observed that the corresponding molar amounts (depending on the applied frequency) are of great importance (≤ 4.86×10‐18 mol). On the other hand, independently of the number of acoustic cycles (1, 2 or 3), the dissolution tendency of the different species, at 140 and 213 kHz, is in the order: H2 > H● > O > O2 > ●OH > O3 > HO2● > H2O2. Nevertheless, above 213 kHz, this ranking starts to be disturbed with the dominance of the main species, i.e. H2, H●, O, ●OH, and O2 molecules. According to the obtained findings in the present paper, the importance of the dissolution mechanism (into the bubble chemistry) is clearly evidenced; therefore, for an accurate simulation of the chemistry of an acoustic cavitation bubble, the consideration of the dissolution process should be taken into account throughout the bubble's oscillation.

Enhancing medical ultrasound imaging through fractional mathematical modeling of ultrasound bubble dynamics

The classical mathematical modeling of ultrasound acoustic bubble is so far using to improve the medical imaging quality. A clear and visible medical ultrasound image relies on bubble’s diameter, wavelength and intensity of the scattered sound. A bubble with diameter much smaller than the sound wavelength is regarded as highly efficient source of sound scattering. The dynamical equation for a medical ultrasound bubble is primarily modeled in classical integer-order differential equation. Then a reduction of order technique is used to convert the modeled dynamic equation for the bubble surface into a system of incommensurate fractional-orders. The incommensurate fractional-order values are calculated directly, by using Riemann stability region. On the basis of stability the convergence and accuracy of the numerical scheme is also discussed in detail. It has been found that the system will remain stable and chaotic for the incommensurate values α1<0.737 and α2<2.80, respectively.

Evaluation of passive acoustic methods for ambient noise baseline and gas flow rate quantification at a proposed nearshore carbon capture and storage site in Australia

Measurement, monitoring, and verification (MMV) is an integral component of carbon capture and storage (CCS) projects, providing valuable feedback for management practices and assuring integrity. Within an operational MMV equipment, hydrophone-based passive acoustic techniques are used to establish ambient noise baseline and flow rate quantification at short range, specifically to facilitate ‘detect-attribute-quantify’ sequence of an MMV program. But nearshore environments are acoustically complex with overlapping components of biophony, geophony, anthropophony, and deployment noise resulting from multiple instruments used for MMV. These soundscape components can disproportionately dominate ambient noise levels, potentially masking acoustic signatures of bubbles used to quantify seabed gas seeps. Therefore, a robust baseline describing ambient noise variability across the range of frequencies associated with acoustic emissions of gas seeps is required, from which changes can be detected and monitored. In this context, multi-year hydrophone measurements (20 Hz to 24 kHz) from a proposed nearshore CCS site in Australia are analysed to establish a temporally resolved baseline, identifying key drivers causing overall ambient noise variability. These results are compared with acoustic bubble spectrum features and flow rate estimates from a controlled in situ gas release experiment. The experiment was performed using a custom-made bubble generator at 1, 10, and 25 m range from the hydrophone to understand the likelihood of detecting bubbles and quantifying flow rate at the proposed CCS site. Despite the complexities of nearshore environment, the evaluation highlights that low-cost hydrophone-based passive acoustic methods can provide a practical solution to complement quantification component of operational MMV programs.

Sonochemical reactor characterization in the presence of cylindrical and conical reflectors

Ultrasonic systems must be able to produce an acoustic field with the highest possible energy concentration in sonochemical reactors to accomplish maximum efficacy in the sonolytic degradation of water contaminants. In the present study, the impact of cylindrical and conical stainless-steel reflectors placed on the liquid surface on the sonochemical oxidation activity of ultrasonication reactors was investigated. The amount of effective acoustic power transferred to the ultrasonicated medium without and with reflectors was measured by calorimetric characterization of the sono-reactors at diverse ultrasonication frequencies in the interval of 300–800 kHz and different electrical powers in the range of 40–120 W. Iodide dosimetry without and with reflectors at diverse ultrasonication conditions (300–800 kHz and 40–120 W) and various aqueous solution volumes in the range of 300–500 mL was used to assess the sonochemical oxidation activity, i.e., the generation of oxidative species (mainly hydroxyl radicals). Sonochemiluminescence (SCL) imaging was used to study the active acoustic cavitation bubbles distribution in the sono-reactors without and with reflectors. Significant impacts of the position and shape of the reflectors on the active acoustic cavitation bubble distribution and the sonochemical oxidation activity were observed due to remarkable modifications of the ultrasonic field by directing and focusing of the ultrasonic waves. A significant augmentation in the triiodide formation rate was obtained in the presence of the conical reflector, especially at 630 kHz and 120 W (60.5% improvement), while iodide oxidation was quenched in the presence of the cylindrical reflector at all ultrasonication frequencies and powers. The SCL images show a noteworthy modification in the ultrasonic field and the acoustic cavitation bubble population when reflectors were used. The sonochemical oxidation activity was improved by the conical reflector when placed in the Fresnel zone (near field region).

Bubble rupture &amp; viability of red blood cells under resonant acoustic standing waves

Objective: The presentation of a novel prospective treatment for scenarios where bubble presence in the bloodstream poses a clinical risk. The method relies on generating resonant acoustic standing waves within a limb to non-invasively accelerate dissolution of bubbles present in the bloodstream via bubble rupture. Additionally, a preliminary assessment of the effects of the resonant acoustic waves and bubble rupture events on red blood cell viability is provided. Methods: Two semicircular piezoelectric (PZT) transducers electrically connected to each other were assembled around a small-girth segment of a rear thigh removed from a swine specimen. When driven at the frequency of electric resonance, this swine thigh and PZT transducer arrangement generates resonant acoustic standing waves within the swine thigh. Consequently, mechanical resonance of the system was non-invasively established by monitoring the electric response of the PZT to the applied frequency. The resonant acoustic field generated was used for the detection and rupture of bubbles that travel through a simulated blood vessel installed across the swine thigh. Two sets of experiments were carried out using this methodology, one with the artificial blood vessel filled with saline solution and one with defibrinated sheep blood. For the latter case, a preliminary hematologic assessment was done with red blood cell counts. Conclusion: Resonant acoustic standing waves effectively rupture bubbles of 300µm to 900µm within a simplified swine thigh model. The average dissolved gas content was 44% due to resonant acoustic waves at powers above 20W. No significant effect on red blood cell counts was observed.

Numerical investigation on the hydrodynamic performance with special emphasis on the cavitation intensity detection in a Venturi cavitator

Venturi is one of the structures that have been frequently adopted for the generation of hydrodynamic cavitation, which is now a promising technology and method in wastewater treatment. Despite the wide spread application, insight into the characteristics of cavity dynamics and cavitation intensity to optimize cavitation performance remains lacking. This paper presents computational investigation of the cavitation performance with emphasis on cavitation intensity prediction inside a Venturi cavitator, through a robust modeling approach. Particularly, a new physical model as the criterion to account for the evaluation of cavitation intensity was developed. The predictions gave satisfactory agreement with published data for acoustic bubble dynamics, and experimental results for typical cavitation characteristics of the Venturi. Further investigations analyzed the size oscillations, collapse pressure and collapse temperature of bubbles, as well as the cavitation intensity during its dynamic behavior of the hydrodynamic cavitation in this cavitator. The results showed that the bubbles originally generated near the wall of the Venturi had the strongest oscillations for cavity dynamics. The normalized average bubble collapse pressure was consistent with the distributions of the cavitation intensity within the whole flow domain. Besides, the rear part of the attached cavity at the stage of developing cavitation showed the strongest intensity of cavitation, while the chocking cavitation had no advantage for this issue. This research provides an available and useful criterion to effectively compare and evaluate the bubble dynamics and cavitation intensity in the multiphase flow of cavitational reactors.

Water Skating Miniature Robot Propelled by Acoustic Bubbles.

This paper presents a miniature robot designed for monitoring its surroundings and exploring small and complex environments by skating on the surface of water. The robot is mainly made of extruded polystyrene insulation (XPS) and Teflon tubes and is propelled by acoustic bubble-induced microstreaming flows generated by gaseous bubbles trapped in the Teflon tubes. The robot's linear motion, velocity, and rotational motion are tested and measured at different frequencies and voltages. The results show that the propulsion velocity is proportional to the applied voltage but highly depends on the applied frequency. The maximum velocity occurs between the resonant frequencies for two bubbles trapped in Teflon tubes of different lengths. The robot's maneuvering capability is demonstrated by selective bubble excitation based on the concept of different resonant frequencies for bubbles of different volumes. The proposed water skating robot can perform linear propulsion, rotation, and 2D navigation on the water surface, making it suitable for exploring small and complex water environments.

Sono-electrolysis performance based on indirect continuous sonication and membraneless alkaline electrolysis: Experiment, modelling and analysis

In the present study, experiments of membraneless alkaline sono-electrolysis are combined to a mathematical model describing the performance of a sono-electrolyzer based on the electrochemical resistances and overpotentials (activation, Ohmic and concentration) and the oscillation of the acoustic cavitation bubble, and its related sono-physical and sonochemical effects, as a single unit and within population. The study aims to elucidate the mechanism of action of acoustic cavitation when coupled to alkaline electrolysis, using a membraneless H-cell configuration and indirect continuous sonication (40 kHz, 60 We). The calorimetric characterization constituted the bridge between experimental results and the numerical and simulation approach, while the quantification of the rate of produced hydrogen both experimentally and numerically highlighted the absence of the contribution of sonochemistry, and explained the role of ultrasounds by the action of shockwaves and microjets. Finally, the energetic sono-physical approach allowed an estimation of the predominance of the shockwaves and microjets effects according to the bubble size distribution within the population corresponding to the acoustic conditions of the study. The resulting macroscopic effect in sono-electrolysis process has been assessed considering the induced degassing. A reduction in the fraction of electrodes’ coverage by bubbles from 76% to 42% has been recorded, corresponding to a decrease of 7.2% in Ohmic resistance and 62.35% in bubble resistance.

Particle propulsion from attached acoustic cavitation bubble under strong ultrasonic wave excitation

Particle propulsion by an attached acoustic cavitation bubble under strong ultrasonic wave excitation occupies the core of many applications, including ultrasonic cleaning, ultrasonography, targeted therapy, and microbubble motors. However, the driving capacity and mode of bubbles in the field of ultrasonics are far from being well understood, which severely limits its applicability in a variety of fields. In this study, a fluid–structure interaction model based on the boundary integral method is proposed to simulate complex interactions between a suspended spherical particle and an attached cavitation bubble. A one-to-one comparison between the numerical results and experimental data demonstrates the distinct advantage of our model over conventional approaches. Thereafter, we systematically investigate the dependence of bubble–particle interactions on the governing parameters, including the amplitude and phase of the ultrasonic wave, particle density, and particle-to-bubble size ratio. We also document different types of bubble dynamic behaviors under various governing parameters. Finally, we obtain scaling laws for the maximum displacement of the particle with respect to the governing parameters.

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.

Dynamics of two interacting acoustic bubbles at short separation distances

We investigate experimentally the attraction between two closely spaced, oscillating microbubbles. Above a certain value of the applied acoustic field, the bubbles jump to a new equilibrium location, where they are separated by a thin fluid layer whose the thickness is much smaller than the bubble radii. We demonstrate that this new equilibrium is caused by the sign reversal of the radiation interaction force acting between the two bubbles, attributed to the multiple rescattering effects of the waves emitted by the bubbles. Theoretical investigation reveals that a new stable equilibrium appears at short distances, resulting in a quasi-contacting bubble pair.

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 cavitationbubbles are analyzed.

Recent study progress of underwater sound absorption coating

AbstractBased on the recent development of underwater sound absorption coatings, this paper analyzes the evolution of underwater sound absorption coatings from two aspects: traditional structures and metamaterials. The traditional structures are divided into three types: cavity structures, composite structures, and impedance‐matching structures. First, the development history of traditional structures, from cavities to hybrid structures, is presented, and the optimization and recent improvements in sound absorption fundamentals are explained. For different types of structures, the sound absorption effects of their respective structures are presented. At the same time, recent development progress of underwater coating metamaterials is summarized, and the sound absorption fundamentals and effects of local and nonlocal resonance types are compared. In addition, typical underwater sound absorption metamaterials are described in detail, and the application prospect of the acoustic bubble structure in practice is summarized. Moreover, this paper compares and discusses various structural types of underwater sound absorption coatings and points out the shortcomings of recent related research. Finally, it points out current open issues in the field and suggests future development directions of underwater sound absorption coatings, defining the possible development focus of underwater sound absorption coatings.

A hydrogen-based technique for determining the number density of acoustic microreactors (actives bubbles) in sonicated solutions

In sonochemistry, acoustic bubbles are a population of microreactors where hydrogen and oxidants are produced. Optimizing the effectiveness of sonochemical processes and, as a result, designing ultrasonic reactors for diverse uses, including hydrogen generation, requires determining the number density of acoustic microreactors.The number density of micro-bubbles during water sonolysis was determined in this study using a novel semi-empirical method developed (for the first time) using hydrogen sono-production. The technique is based on relying on the overall molar production rate of hydrogen (i.e. resulted from the sonicated solution) to the amount of hydrogen produced per a single collapsing bubble, either from its internal gas phase reaction (pyrolysis) or from both the bubble inside and its liquid shell (via H•+H•→H2). The retrieved number density of bubbles varied between ∼108 to ∼1013 L−1 s−1 (depending on empirical conditions), showing an excellent order with that reported in the literature. As the frequency increased, the number of active bubbles increased, regardless of whether the number density is calculated through the amount of hydrogen formed inside the bubble or the total single-bubble yield (gas phase + liquid shell). However, a reduced number density was obtained as it was calculated via the total single-bubble yield, where this decrease goes up with the rise of ultrasound frequency (from 210 to 724 kHz) and the decrease of the liquid temperature. It has been deduced that hydrogen is mainly formed at the bubble's liquid shell (via H•+H•→H2), particularly at higher frequency and cold liquid.