Acoustic Cavitation Threshold Research Articles

Application of Machine Learning to Predict the Acoustic Cavitation Threshold of Fluids

The acoustic cavitation of fluids, as well as related physical and chemical phenomena, causes a variety of effects that are highly important in technological processes and medicine. Therefore, it is important to be able to control the conditions that allow cavitation to begin and progress. However, the accurate prediction of acoustic cavitation is dependent on a complex relationship between external influence parameters and fluid characteristics. The multiparameter problem restricts the development of successful theoretical models. As a result, it is critical to identify the most important parameters influencing the onset of the cavitation process. In this paper, the ultrasonic frequency, hydrostatic pressure, temperature, degassing, density, viscosity, volume, and surface tension of a fluid were investigated using machine learning to determine their significance in predicting acoustic cavitation strength. Three machine learning models based on support vector regression (SVR), ridge regression (RR), and random forest (RF) algorithms with different input parameters were trained. The results showed that the SVM algorithm performed better than the other two algorithms. The parameters affecting the active cavitation nuclei, namely hydrostatic pressure, ultrasound frequency, and outgassing degree, were found to be the most important input parameters influencing the prediction of the cavitation threshold. Other parameters have a minor impact when compared to the first three, and their role can be compensated for by alternative variables. The further development of the obtained results provides a new way to optimize and improve existing theoretical models.

Critical Roles of Impurities and Imperfections in Various Phases of Materials.

In many materials, impurities and imperfections play a critical role on the physical and chemical properties. In the present review, some examples of such materials are discussed. A bulk nanobubble (an ultrafine bubble) is stabilized against dissolution by hydrophobic impurities attached to the bubble surface. An acoustic cavitation threshold in various liquids decreases significantly by the presence of impurities such as solid particles, etc. The strength of brittle ceramics is determined by the size and number of pre-existing microcracks (imperfections) in the specimen. The size effect of a BaTiO3 nanocrystal is influenced by the amount and species of adsorbates (impurities) on its surface as adsorbate-induced charge-screening changes the free energy. The dielectric constant of an assembly of BaTiO3 nanocubes is influenced by a small tilt angle (imperfection) between two attached nanocubes, which induces strain inside a nanocube, and is also influenced by the spatial strain-relaxation due to defects and dislocations (imperfections), resulting in flexoelectric polarization.

Experimental Study of the Acoustic Cavitation Threshold in Sunflower Oil Depending on Different Impact Regime

In engineering problems associated with acoustic wave propagation in a liquid, cavitation onset could be an adverse phenomenon, or, conversely, a required process. In both cases, knowledge of the ultrasonic parameters that lead to cavitation onset under given external conditions is relevant and necessary for solving both fundamental and practical problems. The present work proposes experimental results of studying the threshold of acoustic cavitation, which was implemented at different ultrasound frequencies with a change in external pressure, power of transducer and temperature of the liquid. The experiments were carried out for sunflower oil. The test findings demonstrated how the cavitation threshold changes with varying the power of ultrasound exposure in time. In addition, the effect of external pressure fluctuations on cavitation onset was investigated. The obtained results contribute to the understanding of cavitation processes and could be necessary for verification of theoretical models.

The effect of stiffness and impurities on bubble nucleation in polyacrylamide hydrogels

Tissue mimicking hydrogels can help us understand how viscoelastic properties (stiffness, elastic modulus, etc.) of biological tissues impact bubble nucleation. Adding impurities to hydrogels introduces inhomogeneities and increases their similarity to biological tissues. In this study, we evaluated the effect of stiffness and impurities on bubble nucleation in polyacrylamide (PA) hydrogels. Bubble nuclei were evaluated in 17.5%, 20%, and 22.5% v/v PA hydrogels, after which 0.25% w/v cholesterol crystals (maximum dimension = 0.6 mm) were embedded in the gels (n = 3 each). A 1.5 MHz focused ultrasound transducer was used to induce cavitation using 10-ms pulses with pressures ranging up to p+ = 89 MPa, and p− = 26 MPa and −6 dB focal dimensions of 9.4 × 1.2 mm (p−). Image analysis from high-speed photography showed bubble nucleation increases with increasing peak negative pressure and decreasing hydrogel stiffness. Adding cholesterol crystals largely decreases the acoustic cavitation threshold from p− = 19 MPa for 17.5% v/v hydrogels with no added impurities to p− = 12 MPa for the same concentration hydrogel with added cholesterol crystals. This suggests that hydrophobic cholesterol crystals weaken the gel or trap bubble nuclei, thus lowering the cavitation threshold. Future work includes investigating bubble nuclei in rat hepatocytes. [Work supported by NSF CAREER 1943937 and PSU Riess Fellowship.]

Numerical modelling of acoustic cavitation threshold in water with non-condensable bubble nuclei

Numerical modelling of acoustic cavitation threshold in water is presented taking into account non-condensable bubble nuclei, which are composed of water vapor and non-condensable air. The cavitation bubble growth and collapse dynamics are modeled by solving the Rayleigh-Plesset or Keller-Miksis equation, which is combined with the energy equations for both the bubble and liquid domains, and directly evaluating the phase-change rate from the liquid and bubble side temperature gradients. The present work focuses on elucidating acoustic cavitation in water with a wide range of cavitation thresholds (0.02–30 MPa) reported in the literature. Computations for different nucleus sizes and acoustic frequencies are performed to investigate their effects on bubble growth and cavitation threshold. The numerical predictions are observed to be comparable to the experimental data in the previous works and show that the cavitation threshold in water has a wide range depending on the bubble nucleus size.

Gas-stabilizing solid cavitation nuclei for systemic or transdermal ultrasound-enhanced drug and vaccine delivery and immunomodulation

Originally developed for diagnostic purposes and already approved for clinical use, lipid and protein-shelled microbubbles were a natural choice as initial nucleation agents for cavitation-based therapies entering the clinic. However, several emerging therapeutic ultrasound applications require nuclei that: (i) are significantly smaller in size, in order to overcome a particular biological barrier such as the leaky vasculature of tumours or the stratum corneum; (ii) offer greatly increased cavitation persistence, both during a single extended ultrasound pulse and in terms of extended circulation following intravenous administration; (iii) have better resilience to sudden ambient pressure changes in order to enable direct injection without nuclei destruction into tissue targets via a needle and syringe; and (iv) are made of materials or have an increased payload or surface area that can interact beneficially with the relevant tissue target during or following cavitation. Gas-stabilizing solid particles will be reviewed in this context, providing an overview of their known characteristics in terms of size distribution, associated acoustic emissions, cavitation thresholds, cavitation persistence and circulation. The relationship between this acoustic characterization and associated bioeffects including drug and vaccine delivery, and immunomodulation will subsequently be explored.

An Analysis of Acoustic Cavitation Thresholds of Water Based on the Incubation Time Criterion Approach

Researchers are still working on the development of models that facilitate the accurate estimation of acoustic cavitation threshold. In this paper, we have analyzed the possibility of using the incubation time criterion to calculate the threshold of the onset of acoustic cavitation depending on the ultrasound frequency, hydrostatic pressure, and temperature of a liquid. This criterion has been successfully used by earlier studies to calculate the dynamic strength of solids and has recently been proposed in an adapted version for calculating the cavitation threshold. The analysis is carried out for various experimental data for water presented in the literature. Although the criterion assumes the use of macroparameters of a liquid, we also considered the possibility of taking into account the size of cavitation nuclei and its influence on the calculation result. We compared the results of cavitation threshold calculations done using the incubation time criterion of cavitation and the classical nucleation theory. Our results showed that the incubation time criterion more qualitatively models the results of experiments using only three parameters of the liquid. We then discussed a possible relationship between the parameters of the two approaches. The results of our study showed that the criterion under consideration has a good potential and can be conveniently used for applications where there are special requirements for ultrasound parameters, maximum negative pressure, and liquid temperature.

Acoustic Measurements of Nucleus Size Distribution at the Cavitation Threshold

An understanding of the acoustic cavitation threshold is essential for minimizing cavitation bio-effects in diagnostic ultrasound and for controlling cavitation-mediated tissue ablation in focused ultrasound procedures. The homogeneous cavitation threshold is an intrinsic material property of recognized importance to biomedical ultrasound as well as a variety of other applications requiring cavitation control. However, measurements of the acoustic cavitation threshold in water differ from those predicted by classic nucleation theories. This persistent discrepancy is explained by combining recently developed methods for acoustically nucleating single bubbles at threshold with numerical modeling to obtain a nucleus size distribution consistent with first-principles estimates for ion-stabilized nuclei. We identify acoustic cavitation at threshold as a reproducible subtype of heterogeneous cavitation with a characteristic nucleus size distribution. Knowledge of the nucleus size distribution could inspire new approaches to achieving cavitation control in water, tissue and a variety of other media.

Horses for courses (or something much smaller!): The design, development and acoustic testing of sub-micron cavitation nucleation agents for biomedical applications

Lipid and protein-shelled microbubbles have been the mainstay of cavitation nucleation strategies for therapeutic applications for several decades. However, a growing number of therapeutic ultrasound applications require nuclei that (i) are significantly smaller in size, in order to overcome a particular biological barrier such as the leaky vasculature of tumours or the stratum corneum; (ii) offer greatly increased cavitation persistence, both during a single extended ultrasound pulse and in terms of extended circulation following intravenous administration; and (iii) have better resilience to sudden ambient pressure changes in order to enable direct injection without nuclei destruction into tissue targets via a needle and syringe. We will review a range of novel cavitation agents currently under development, including sub-micron gas-stabilizing solid particles and nanodroplets, and provide an overview of their known characteristics in terms of acoustic emissions, activation and cavitation thresholds, cavitation persistence, and circulation. Where possible, the relevant performance of these cavitation nucleation agents will be compared to microbubbles for applications ranging from drug delivery to nucleated tissue fractionation.

Determination of Acoustic Cavitation Probabilities and Thresholds Using a Single Focusing Transducer to Induce and Detect Acoustic Cavitation Events: II. Systematic Investigation in an Agar Material

In the accompanying article (Part I), a method is described to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) using a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events, and its suitability for different sonication modes like continuous wave, single pulses (with pulse lengths from microseconds to milliseconds) and repeated burst signals is discussed. In Part II, the use of the method for a systematic study of the dependence of the acoustic cavitation thresholds in 3% (by weight) agar phantoms on the temporal sonication parameters is discussed. The values obtained at a frequency of 1.06 MHz, ranging from (0.58 ± 0.12) MPa for a 3-s continuous wave mode sonication to (5.2 ± 1.0) MPa for single shots with a length of 10 wave cycles, are discussed and interpreted on the basis of literature values and their self-consistency.

Determination of Acoustic Cavitation Probabilities and Thresholds Using a Single Focusing Transducer to Induce and Detect Acoustic Cavitation Events: I. Method and Terminology

A method to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) is described that uses a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events. The method was evaluated by studying acoustic cavitation probabilities in agar-based TMMs with and without scatterers and for different sonication modes like continuous wave, single pulses (microseconds to milliseconds) and repeated burst signals. Acoustic cavitation thresholds (defined here as the peak rarefactional in situ pressure at which the acoustic cavitation probability reaches 50%) at a frequency of 1.06 MHz were observed between 1.1 MPa (for 1 s of continuous wave sonication) and 4.6 MPa (for 1 s of a repeated burst signal with 25-cycle burst length and 10-ms burst period) in a 3% (by weight) agar phantom without scatterers. The method and its evaluation are described, and general terminology useful for standardizing the description of insonation conditions and comparing results is provided. In the accompanying second part, the presented method is used to systematically study the acoustic cavitation thresholds in the same material for a range of sonication modes.

Study on separation characteristics of water in oil (W/O) emulsion under ultrasonic standing wave field

The separation characteristics of W/O emulsion were investigated under ultrasonic standing wave field in this paper. The effects of the irradiation time, acoustic intensity, frequency, oil viscosity, and oil-water interfacial tension on the separation characteristics of emulsion were studied in details. It was proved that there existed an optimal irradiation time, acoustic intensity and oil-water interfacial tension to achieve the maximum separation efficiency in ultrasonic treatment process. Excessive long irradiation time had little effect on improving the dehydration rate of emulsion but rather increased unnecessary processing time. Excessive high acoustic intensity caused the occurrence of acoustic cavitation phenomenon. The acoustic cavitation would cause the aggregated droplets to be dispersed by oscillating bubbles or even emulsified again by high speed micro jets. The acoustic cavitation threshold is effectively and accurately obtained by a spectral analysis method. The micro acoustic streaming which resulted from the attenuation of acoustic energy occurred with the increase of frequency. It sheared the aggregated droplets, destroyed the stable droplets banding and reduced the separation efficiency of emulsion. The excess surfactant adsorbed on the oil-water interface resulted in the increase of the interfacial film strength, which hindered the coalescence process of droplets and reduced the separation efficiency.

From underwater thunder to cavitation from pulsed ultrasound: One graduate student’s perspective of the years leading up to the formation of NCPA

The Physical Acoustics Research Group at the University of Mississippi (PARGUM) fostered a vibrant, multidisciplinary research environment in the early 1980s that incubated the concept of the National Center for Physical Acoustics (NCPA). It also provided graduate students, new to physical acoustics, an enticing glimpse into the breadth of the field and an exposure to a range of important contemporary problems. The research programs included terrestrial lightning detection and location, the underwater sound generated by lightning striking the ocean surface, long range sound propagation, propagation through porous materials, sound amplification by selective molecular excitation, acoustic levitation of bubbles and droplets, studies of acoustic cavitation thresholds and sonoluminescence, numerical simulation of bubble dynamics, and others. Beyond the scientific activities, PARGUM introduced a number of new acousticians to the ASA, many of whom have contributed to the leadership of the Society. The purpose of this presentation is to review some of the research results produced by the members of group from the time period as well as their contributions to the ASA.

Nanoparticle dispersion in liquid metals by electromagnetically induced acoustic cavitation

Aim of this study is to investigate experimentally the effect of magnetically induced cavitation applied for the purpose of nanoparticle dispersion in liquid metals. The oscillating magnetic force due to the azimuthal induction currents and the axial magnetic field excites power ultrasound in the sample. If the fields are sufficiently high then it is possible to achieve the acoustic cavitation threshold in liquid metals. Cavitation bubble collapses are known to create microscale jets with a potential to break nanoparticle agglomerates and disperse them. The samples are solidified under the contactless ultrasonic treatment and later analyzed by electron microscopy and energy-dispersive X-ray spectroscopy (EDX). It is observed that SiC nanoparticles are dispersed in an aluminum magnesium alloy, whereas in tin the same particles remain agglomerated in micron-sized clusters despite a more intense cavitation.

Acoustic strength of water and effect of ultrasound on the liquid-vapor phase diagram

The structure-time approach is used to develop an analytical model that makes it possible to predict the dependences of the acoustic cavitation threshold of water on temperature and background pressure. The calculated dependences are compared with the results of experiments carried out in the leading laboratories. It is demonstrated that the proposed approach allows the estimation of the effect of the acoustic field on the phase state of the substance under study. The calculated liquid-vapor phase curves for water in the presence of acoustic fields are presented.

Modeling cavitation nucleation from laser-illuminated nanoparticles subjected to acoustic stress

In an earlier work by Farny et al. [ARLO 6, 138-143 (2005).] it was demonstrated that the acoustic cavitation threshold in a tissue mimicking gel phantom can be lowered from 4.5 to ∼1 MPa by "seeding" the optically transparent phantom with light absorptive gold nanoparticles and irradiating these absorbers with nanosecond pulses of laser light at intensities less than 10 mJ/cm(2). As a follow-up study, a three-stage numerical model was developed to account for prenucleation heating, the nucleation and formation of the vapor cavity, and the resulting vapor bubble dynamics. Through examination of the radius-time evolution of the cavity, the combined thresholds for laser radiant exposure and acoustic peak pressure required to induce inertial cavitation are deduced. It is found that the threshold pressure decreases when laser exposure increases; but the rate depends on exposure levels and the size of the particle. Investigations of the roles of particle size and laser pulse length are performed and optimum choices for these parameters determined in order to obtain inertial cavitation at the lowest possible acoustic pressure and laser intensity.

Cavitation in Heavy Water and Other Liquids

We report on measurements of the cavitation pressure in several liquids subjected to tension in an acoustic wave and compare the results to classical nucleation theory (CNT). This study is motivated by the sizable discrepancy between the acoustic cavitation threshold measured in water and the value predicted by CNT. We find that the same discrepancy is present for heavy water, whereas the agreement is better for ethanol and heptane and intermediate in the case of dimethyl sulfoxide. It is well-known that water is an anomalous liquid, a consequence of its hydrogen-bonded network. The other liquids studied represent very different molecular interactions. Our results indicate that the cavitation threshold approaches the prediction of CNT as the surface tension gets smaller. Conversely, this raises the question of the validity of a simple theory such as CNT to account for high surface tension liquids and suggests that an appropriate microscopic model of such liquids may be necessary to correctly predict the cavitation threshold.

Equation of state of water under negative pressure

We report on the simultaneous measurements of the speed of sound and the density in liquid water under negative pressure. Application of a focused acoustic wave to the bulk liquid is able to generate negative pressures before nucleation of the vapor phase occurs. A method for time-resolved Brillouin scattering is developed to measure the speed of sound during the passage of a 1 MHz ultrasonic wave. This is coupled with a fiber optic probe hydrophone which allows the determination of the density. Together, these methods give an ambient temperature equation of state of metastable liquid water down to the acoustic cavitation threshold. Empirical equations of state of water are based on experimental data at positive pressure; the validity of their extrapolation to negative pressures had been tested only indirectly or with very weakly metastable liquid. We provide thermodynamic data that prove the fidelity of recent equations of state down to -26 MPa. However, this raises questions regarding the nature of the cavitation threshold observed in acoustic experiments, which is far less negative than expected.

Boiling-up of superheated liquid argon in an acoustic field

The method of lifetime measurement has been used to investigate spontaneouscavitation kinetics in superheated liquid argon in weak acoustic fields. It is shownthat acoustic cavitation may proceed both by the mechanism of homogeneousnucleation of the vapor phase and by way of ‘build-up’ of vapor bubbles generatedby high-energy particles or the action of some other external factor. Acousticcavitation thresholds are adequately described by homogeneous nucleation theory.

Simulation of Intracranial Acoustic Fields in Clinical Trials of Sonothrombolysis

Two clinical trials have used ultrasound to improve tPA thrombolysis in patients with acute ischemic stroke. The Combined Lysis of Thrombus in Brain Ischemia Using Transcranial Ultrasound and Systemic tPA (CLOTBUST) trial reported accelerated recanalisation of the middle cerebral artery (MCA) in patients with symptoms of MCA infarction, which were monitored with 2-MHz transcranial Doppler. In CLOTBUST, there was no increased bleeding as evidenced by cranial computed tomography. The Transcranial Low-Frequency Ultrasound-Mediated Thrombolysis in Brain Ischemia (TRUMBI) trial, which employed magnetic resonance imaging (MRI) before and after tPA thrombolysis, was discontinued prematurely because of an increased number of secondary hemorrhages, possibly related to the use of low frequency 300-kHz ultrasound. The purpose of our work is to help identify possible mechanisms of intracerebral hemorrhage resulting from sonothrombolysis by applying a simulation tool that estimates the pressure levels in the human brain that are produced with different sonothrombolysis devices. A simulation software based on a finite difference time domain (FDTD) three-dimensional (3D) scheme was developed to predict acoustic pressures in the brain. This tool numerically models the wave propagation through the skull and reproduces both ultrasound protocols of CLOTBUST and TRUMBI for analysis of the distribution of acoustic pressure in the brain during stroke treatment. For the simulated TRUMBI trial, we analyzed both a “high” and “low” hypothesis according to published parameters (for high and low amplitude excitations). For these hypotheses, the mean peak rarefactional pressures in the brain were 0.26 ± 0.2 MPa (high hypothesis) and 0.06 ± 0.05 MPa (low hypothesis), with maximal local values as high as 1.2 MPa (high hypothesis) and 0.27 MPa (low hypothesis) for configurations modelled in this study. The peak rarefactional pressure was thus higher than the inertial acoustic cavitation threshold in the presence of a standing wave in large areas of the brain, even outside the targeted clot. For the simulated CLOTBUST trial, the maximum peak negative pressure was less than 0.07 MPa. This simulated pressure is below the threshold for both inertial and stable acoustic cavitation but likewise lower than any acoustic pressure that has been reported as sufficient for effective sonothrombolysis. Simulating the pressure field of ultrasound protocols for clinical trials of sonothrombolysis may help explain mechanisms of adverse effects. Such simulations could prove useful in the initial design and optimization of future protocols for this promising therapy of ischemic stroke. (E-mail: norabelic@yahoo.fr)