Bubble Cloud Research Articles

Synchrotron X-ray imaging of the onset of ultrasonic horn cavitation.

High-power ultrasonic horns operating at low frequency are known to generate a cone-shaped cavitation bubble cloud beneath them. The exact physical processes resulting in the conical structure are still unclear mainly due to challenges associated with their visualization. Herein, we address the onset of the cavitation cloud by exploiting high-speed X-ray phase contrast imaging. It reveals that the cone formation is not immediate but results from a three-step phenomenology: (i) inception and oscillation of single bubbles, (ii) individual cloud formation under splitting or lens effects, and (iii) cloud merging leading to the formation of a bubble layer and, eventually, to the cone structure due to the radial pressure gradient on the horn tip.

A phenomenological analysis of droplet shock-induced cavitation using a multiphase modeling approach

Investigations of shock-induced cavitation within a droplet are highly challenged by the multiphase nature of the mechanisms involved. Within the context of heterogeneous nucleation, we introduce a thermodynamically well-posed multiphase numerical model accounting for phase compression and expansion, which relies on a finite pressure-relaxation rate formulation. We simulate (i) the spherical collapse of a bubble in a free field, (ii) the interaction of a cylindrical water droplet with a planar shock wave, and (iii) the high-speed impact of a gelatin droplet onto a solid surface. The determination of the finite pressure-relaxation rate is done by comparing the numerical results with the Keller–Miksis model, and the corresponding experiments of Sembian et al. and Field et al., respectively. For the latter two, the pressure-relaxation rate is found to be μ=3.5 and μ=0.5, respectively. Upon the validation of the determined pressure-relaxation rate, we run parametric simulations to elucidate the critical Mach number from which cavitation is likely to occur. Complementing simulations with a geometrical acoustic model, we provide a phenomenological description of the shock-induced cavitation within a droplet, as well as a discussion on the bubble-cloud growth effect on the droplet flow field. The usual prediction of the bubble cloud center, given in the literature, is eventually modified to account for the expansion wave magnitude.

Numerical simulation of the collapse of a bubble cloud near a wall

Numerical simulation of the collapse of a bubble cloud near a wall

DNA release from plant tissue using focused ultrasound extraction (FUSE).

Sample preparation in genomics is a critical step that is often overlooked in molecular workflows and impacts the success of downstream genetic applications. This study explores the use of a recently developed focused ultrasound extraction (FUSE) technique to enable the rapid release of DNA from plant tissues for genetic analysis. FUSE generates a dense acoustic cavitation bubble cloud that pulverizes targeted tissue into acellular debris. This technique was applied to leaf samples of American chestnut (Castanea dentata), tulip poplar (Liriodendron tulipifera), red maple (Acer rubrum), and chestnut oak (Quercus montana). We observed that FUSE can extract high quantities of DNA in 9-15 min, compared to the 30 min required for control DNA extraction methods. FUSE extracted DNA quantities of 24.33 ± 6.51 ng/mg and 35.32 ± 9.21 ng/mg from American chestnut and red maple, respectively, while control methods yielded 6.22 ± 0.87 ng/mg and 11.51 ± 1.95 ng/mg, respectively. The quality of the DNA released by FUSE allowed for successful amplification and next-generation sequencing. These results indicate that FUSE can improve DNA extraction efficiency for leaf tissues. Continued development of this technology aims to adapt to field-deployable systems to increase the cataloging of genetic biodiversity, particularly in low-resource biodiversity hotspots.

Thermodynamics of the inception and interactions of multiple laser-produced cavitation bubbles using the lattice Boltzmann method

Understanding the thermodynamics associated with the inception and interactions of multiple cavitation bubbles is vital in developing deeper insights into the flow field and temperature properties of cavitation bubble clouds. This paper describes a double-distribution-function thermal lattice Boltzmann method for investigating the thermodynamics of multiple-bubble interactions, and proposes a new two-dimensional method for cavitation bubble inception without the initialization of gas nuclei. This approach is validated by simulating the evolution of a laser-produced bubble. The interaction between two vapor bubbles is simulated, and the hydrodynamics and thermodynamics of cavitation bubbles are systematically investigated. Both weak and strong interactions among bubbles of equal size are accurately reproduced. The coalescence mode is further categorized according to differences in the collapse behaviors: the remaining parts collapse away from each other for coalescence in the collapse stage, but collapse toward each other for coalescence in the growth stage. Interactions among bubbles of different sizes are also explored and the different morphologies for various interaction modes are reported. In the case of small distances between bubbles under the weak interaction regime, the smaller bubble is pushed away from the symmetry axis, leading to an initial decrease in collapse intensity and then an increase as the distance between the two bubbles increases. Finally, the thermodynamics of a bubble cluster are investigated. The hydrodynamic and thermodynamic processes in different regions of the cavitation bubble cluster are determined, and the toroidal shape in the final stage is reproduced. The results demonstrate that the proposed model accurately simulates the complex interactions among multiple cavitation bubbles.

Design of a Low-Frequency Active Sonar Underwater Target Simulator Based on Bubble Clouds

According to the theory that bubble clouds have strong scattering properties in the water, effective medium theory (EMT) based on bubble clouds has been developed for a new kind of target simulator available for multi-static sonar experiments. According to the theory, the effects of the porosity of the bubble clouds and the size of the simulator on the backscattering strength of the target simulator are investigated. The optimal parameters for designing the target simulator are given by simulation analysis. Finally, by comparing with the traditional target simulator design scheme, it is shown that the target simulator designed based on EMT theory has the characteristics of low-frequency bandwidth, high target strength, small size, low cost, and application flexibility.

Assessment of bubble activity generated by histotripsy combined with echogenic liposomes

Objective. Histotripsy is a form of focused ultrasound therapy that uses the mechanical activity of bubbles to ablate tissue. While histotripsy alone degrades the cellular content of tissue, recent studies have demonstrated it effectively disrupts the extracellular structure of pathologic conditions such as venous thrombosis when combined with a thrombolytic drug. Rather than relying on standard administration methods, associating thrombolytic drugs with an ultrasound-triggered echogenic liposome vesicle will enable targeted, systemic drug delivery. To date, histotripsy has primarily relied on nano-nuclei inherent to the medium for bubble cloud generation, and microbubbles associated with echogenic liposomes may alter the histotripsy bubble dynamics. The objective of this work was to investigate the interaction of histotripsy pulse with echogenic liposomes. Approach. Bubble clouds were generated using a focused source in an in vitro model of venous flow. Acoustic emissions generated during the insonation were passively acquired to assess the mechanical activity of the bubble cloud. High frame rate, pulse inversion imaging was used to track the change in echogenicity of the liposomes following histotripsy exposure. Main results. For peak negative pressures less than 20 MPa, acoustic emissions indicative of stable and inertial bubble activity were observed. As the peak negative pressure of the histotripsy excitation increased, harmonics of the excitation were observed in OFP t-ELIP solutions and plasma alone. Additional observations with high frame rate imaging indicated a transition of bubble behavior as the pulse pressure transitioned to shock wave formation. Significance. These observations suggest that a complex interaction between histotripsy pulses and echogenic liposomes that may be exploited for combination treatment approaches.

A clouded bubble-based mass transfer model for the simulation of gas–solid bubbling fluidization

Bubble mesoscale structures play a critical role in the transport process in bubbling fluidization. A bubble-based multiscale method has been developed to modify the conventional model. Whereas the effect of the circulating gas flow features around bubble, called “bubble cloud”, is usually neglected. The bubble cloud is an essential way to transport the reactant species from the bubble phase to the bed. In this work, the impact of the bubble cloud in the mass transfer process is considered. The bubble cloud is taken as the inter-phase based on the multi-scale resolution of the bubbling fluidized bed system and a clouded bubble-based mass transfer model is established. The proposed model is incorporated in the two-fluid model to simulate the ozone decomposition process in 3D bubbling fluidized beds. It is proven that the results using the proposed mass transfer model can give a better agreement with the experimental data than the homogeneous model. The role of the bubble cloud is significant in the mass exchange process.

Effects of pulse repetition frequency on bubble cloud characteristics and ablation for single-cycle histotripsy

Histotripsy is a cavitation-based focused ultrasound ablation method in development for multiple clinical applications. This work investigates the effects of pulse repetition frequency (PRF) on histotripsy bubble cloud characteristics and ablative capabilities for single cycle histotripsy. Bubble clouds produced by a 500 kHz histotripsy system at PRF’s from 0.1 to 1000 Hz were visualized using high speed imaging in 1% agarose phantoms. Cloud images were analyzed to determine bubble cloud density (bubbles/mm2) and pulse-to-pulse bubble correlation. Ablation was assessed through lesion analysis in red blood cell (RBC) phantoms. Results showed cavitation clouds generated at low PRF were characterized by consistently dense bubble clouds (49.6 ± 7.7 bubbles/mm2, 1 Hz) that closely matched regions above the histotripsy intrinsic threshold. Bubble clouds formed at high PRF had significantly lower cloud density (20.0 ± 8.3 bubbles/mm2, 1000 Hz). In addition, bubbles in higher PRF clouds had significantly increased pulse-to-pulse correlation, characteristic of what has been reported as the cavitation memory effect. Results from RBC ablation showed that higher PRF generated lesions had lower adherence to the focal region and less repeatability compared to low PRF ablations. This study demonstrates essential differences in bubble cloud characteristics that will help guide future histotripsy pulsing strategies.

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

Intrinsic threshold histotripsy has been shown to nucleate bubble clouds and generate ablation closely matching the dimensions of the focal region above the intrinsic threshold (∼25–28MPa) with the ablation efficiency dependent upon the size and density of bubbles within the cloud. This work investigates the effects of dual-frequency histotripsy pulsing on bubble-cloud characteristics and ablation efficiency. Histotripsy was applied to agarose tissue phantoms (1%) using a dual-frequency 500kHz–3MHz array transducer with equal pressure delivered by each frequency. The arrival time of the 3 MHz pulse was varied relative to the 500 kHz pulse, and high-speed imaging was used to characterize the bubble-cloud dimensions, bubble density, and individual bubble size. Ablation was determined using red blood cell phantoms. Results indicated that dual-frequency pulsing generated bubble clouds with intermediate cloud size, bubble size, and bubble density compared to single-frequency (500 kHz and 3 MHz) pulsing, with these characteristics further modulated by the respective pulse arrival times. Additionally, dual-frequency pulsing increased ablation efficiency compared to previously published single frequencies (500 kHz and 3 MHz) with the most efficient ablation observed for cases where the 3 MHz pulse arrived at the leading edge of the 500 kHz pulse. Overall, this investigation demonstrates possible benefits of dual-frequency histotripsy pulsing, warranting further investigation.

Extension of boiling histotripsy lesions by axial focus steering during pulse delivery

Boiling histotripsy (BH) is a pulsed high intensity focused ultrasound (HIFU) method relying on the generation of high amplitude shocks and bubble activity to induce tissues liquefaction. A sequence of pulses, 1–20 ms long, generates boiling bubbles at the focus of the HIFU transducer within each pulse, and the remainder of the pulse then interacts with those bubbles. One effect is the creation of a prefocal bubble cloud due to shock scattering: the shock is inverted when reflected from the bubble wall resulting in sufficient negative pressure to reach intrinsic cavitation threshold immediately proximally to these bubbles. Here, a methodology is proposed to extend the length of this prefocal bubble cloud by steering the focus toward the transducer during the BH pulse and thus accelerate treatment. A BH system comprising a 1.5 MHz 256-element phased array connected to a Verasonics V1 system was used. High-speed imaging in transparent gels was performed to observe the extension of the bubble cloud resulting from shock scattering. Volumetric BH lesions were generated in ex vivo tissue. Results showed a threefold increase of the volumetric ablation rate with focus steering compared to standard BH. [Work supported by NIH R01EB007643, R01GM122859, and R01EB25187.]

Intrusive temperature measurement of the cavitation bubbles using cold wire

Cavitation is a phenomenon of rupturing a liquid when the liquid is subjected to a decrease in pressure at roughly constant temperature. Cavitation is the fundamental reason for the material erosion when the bubbles are collapsing closer to the surface. These bubbles also produce a very high temperature (thousands of degrees Celsius) for a very short time (in the order of nano or microseconds). Cavitation is also being used in the non-invasive cancer tumor ablation treatment using high intensity focused ultrasound (HIFU) called histotripsy. So, our research in measuring the temperature of the bubble collapse will help the histotripsy treatment to understand its thermal behavior. We have designed a cold wire sensor of the thickness of ∼10 to 50 micrometers. And, the sensing region is 2 millimeter. These cold wires are made with kevlar or nylon fibers, titanium, nickel, silver, and silicon dioxide. The single bubble or histotripsy cloud, is allowed to collapse on the sensor, and we measure the temperature based on its change in resistance. These changes in resistance are corresponded to the temperature using the mathematical function which is calibrated prior to the experiment. We measure it in three different experimental settings like Tube Arrest Method (TAM), LASER induced bubbles, and HIFU induced bubbles. The preliminary results have shown that the temperature of the single spherical bubble collapse (TAM) can rise to thousands of degrees celsius as mentioned in the literature based on the theoretical analysis.

Nanoparticle-mediated histotripsy using dual-frequency histotripsy pulsing: Comparison of bubble-cloud characteristics and ablation efficiency

Nanoparticle-mediated histotripsy (NMH) is a targeted ablation method using perfluorocarbon-filled nanoparticles to generate bubble-clouds at pressure levels (9–12 MPa) significantly below the histotripsy intrinsic threshold (>25 MPa). Prior studies have also shown a significant reduction in ablation efficiency compared to conventional histotripsy, likely from reduced bubble expansion and bubble-cloud density. Here, we investigate the bubble-cloud characteristics and ablation efficiency for NMH using dual-frequency pulsing. We hypothesize this method will increase ablation efficiency by increasing the bubble-cloud density and individual bubble expansion. High-speed optical imaging was used to characterize the cavitation threshold, cloud dimensions, and bubble-density of bubble clouds generated in agarose tissue phantoms, with and without perfluorohexane-filled nanocones, exposed to single-cycle dual-frequency pulses using a 500 kHz–3 MHz array transducer (1:1 pressure ratio). Ablation efficiency was investigated using red blood cell phantoms. Results showed dual-frequency NMH predictably produced smaller, denser, and more well-confined bubble-clouds and increased ablation efficiency compared to previous single-frequency studies, with complete ablation of the focal volume observed within 2000 pulses. This study demonstrates the potential of enhancing NMH ablation efficiency with dual-frequency pulsing and highlights the need for further studies to optimize NMH pulsing parameters for future clinical therapy development.

Numerical Analysis and Experimental Investigation of Cavitating Flows Considering Thermal and Compressibility Effects

This article deals with the numerical simulation of unsteady cavitating flow around hydrofoils, supported by experimental research realized in a cavitation tunnel situated in the Centre of Hydraulic Research. Two straight NACA hydrofoils (NACA0020 and NACA2412) were employed. The comprehensive unsteady CFD analysis was based on scale-resolving simulations (hereinafter SRS) with the aim of capturing correctly the interactions between the cavitation structures and re-entrant flow as well as the compressibility and thermal effects of cavitation. The static pressure fluctuations during the cavity oscillation cycles and the evaporation and condensation processes are discussed in detail. To predict correctly the high-pressure peaks during the bubble cloud collapses and the pressure pulse propagation speed, the real properties of water and the mixture total energy conservation equation were considered. In addition, the estimated content of undissolved air was taken into account. The numerical simulations were validated by means of already published experiments or compared with experiments conducted by the authors, and with good agreement.

A clouded bubble-based drag model for the simulations of bubbling fluidized beds

• A clouded bubble-based drag model is developed for bubbling fluidization. • The non-uniformity of drag force caused by bubble cloud is considered. • The feasibility of the clouded bubble-based drag model is validated. • Application of the modified drag model for Geldart A/B and A particle. A clouded bubble-based (CBB) drag model is developed in view of gas flow patterns around the clouded bubble. The bubble cloud is taken as the inter-phase between the bubble phase and emulsion phase based on the muti-scale resolution of the heterogeneous system. Moreover, the non-uniform distributions of drag force in the bubble crown and wake as well as the bubble sides are considered. The structure parameters are obtained by solving a series of equations including the mass conservation equations and empirical or semi-empirical correlations. The heterogeneous drag model is implemented into the two-fluid model (TFM) for 2D & 3D simulations of bubbling fluidized beds with Geldart A/B and A particles. The results demonstrate that the simulated results using the CBB drag model agree better with the experimental results compared to the homogeneous drag model and the structure-based drag model without the bubble cloud effect.

Histotripsy for the Treatment of Uterine Leiomyomas: A Feasibility Study in Ex Vivo Uterine Fibroids.

Uterine fibroids (leiomyomas), the most common benign tumors in women of reproductive age, are a frequent cause of abnormal vaginal bleeding and other reproductive complaints among women. This study investigates the feasibility of using histotripsy, a non-invasive, non-thermal focused ultrasound ablation method, to ablate uterine fibroids. Human fibroid samples (n=16) were harvested after hysterectomy or myomectomy procedures at Carilion Memorial Hospital. Histotripsy was applied to ex vivo fibroids in two sets of experiments using a 700-kHz clinical transducer to apply multicycle histotripsy pulses and a prototype 500-kHz transducer to apply single-cycle histotripsy pulses. Ultrasound imaging was used for real-time treatment monitoring, and post-treatment ablation was quantified histologically using hematoxylin and eosin and Masson trichrome stains. Results revealed that multicycle histotripsy generated diffuse cavitation in targeted fibroids, with minimal cellular ablative changes after treatment with 2000 pulses/point. Single-cycle pulsing generated well-confined bubble clouds with evidence of early coagulative necrosis on histological assessment in samples treated with 2000 pulses/point, near-complete ablation in samples treated with 4000 pulses/point and complete tissue destruction in samples treated with 10,000 pulses/point. This study illustrates that histotripsy is capable of fibroid ablation under certain pulsing parameters and warrants further investigation as an improved non-invasive ablation method for the treatment of leiomyomas.

Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo

Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo

Fermi bubble clouds originate in both the disk and halo of the Milky Way

Fermi bubble clouds originate in both the disk and halo of the Milky Way

Hunting, Fighting, or Playing with Bubbles: Possible Usage and Acoustic Characteristics of Bubble Burst Sounds Produced by the Amazon River Dolphin (Inia geoffrensis)

Acoustic characteristics of bubble production by an odontocete were documented for the first time. Bubble sounds produced by the Amazon river dolphin (Inia geoffrensis) were recorded incidentally as part of a survey of fish sounds in the Pacaya–Samiria National Reserve of Peru on six dates between 4 and 24 July 2012. Dolphins were observed to periodically produce large clouds of bubbles underneath or near the survey boat (averaging 7/survey or 8/h) as it drifted through areas of actively foraging dolphins. The bubble production was classified as bubble bursts due to their similarity to bubble bursts produced by other cetaceans. Bubble burst sounds had a mean peak frequency of 402 Hz and duration of 8.9 s (n = 51). Bubble bursts were temporally clustered with an average interval of 169 s (0.1 to 1,187 s) between bursts. Bubble bursts were disproportionately more likely to be present when fish sounds were also present, but it is not known if the association was due to predation or other factors. A review of the literature finds similar bubble production has been reported in at least 14 other species of cetaceans (4 mysticetes and 10 odontocetes). Most are commonly associated with play, surprise, agonistic, and foraging behaviors. We discuss each of these possibilities and conclude that Amazon river dolphin bubble burst behavior is most likely related to foraging or aggressive behavior because the behavior occurred in feeding areas and appeared to be directed at the drifting boat. We further propose a novel hypothesis that the bubble bursts are a hunting strategy used to disperse prey associated with floating vegetation mats and other forms of drifting materials used by fishes for shelter. Future research is needed to better understand the behavior associated with bubble production by the Amazon river dolphin.

Numerical model of an ultrasonically induced cavitation reactor and application to heavy oil processing

This study describes a numerical approach to model ultrasonically induced cavitation (UIC) reactors. UIC forms vapour-filled cavities in a liquid medium due to an applied acoustic field and their eventual collapse. UIC reactors are characterized by the presence of a vibrating probe that generates pressure waves by high-frequency oscillations (>20 kHz), which control the formation, dynamics, and eventual collapse of the vapour cavities. Those vapour cavities eventually enhance mixing and favour the occurrence of gas-liquid reactions. The zones of high mixing and reactivity coincide with the presence of the bubble cloud, which depends on the shape of the vessel and sonotrode. The development of advanced computational fluid dynamics (CFD) models is crucial to optimizing UIC processes’ geometry and operation parameters. A new algorithm for modelling UIC has been implemented within the OpenFOAM framework in the present study. The volume-of-fluid (VoF) method employs a diffuse interface approach for the volume fraction transport equation. The bubble dynamics are solved with sub-grid models, and the coupling between the main flow field and the sub-grid scales is performed through source terms in the transport equations. The source terms are de-coupled from convective and diffusive components of the volume fraction equation. The history of the bubbles is considered to consist of nucleation, oscillations, and collapse. The oscillations are resolved via the Rayleigh–Plesset equation. The concluding part of the work demonstrates the application of the algorithm to simulate the operation of an UIC reactor, which was designed to desulfurize fuels using the oxidative (ODS) process.