The acoustic emissions of cavitation bubbles in stretched vortices

Pairs of unequal strength, counter-rotating vortices were produced in order to examine the inception, dynamics, and acoustic emission of cavitation bubbles in rapidly stretching vortices. The acoustic signatures of these cavitation bubbles were characterized during their inception, growth, and collapse. Growing and collapsing bubbles often produced a sharp, broadband, pop sound. The spectrum of these bubbles, and the peak resonant frequency can generally be related to quiescent flow bubble dynamics and corresponding resonant frequencies. However, some elongated cavitation bubbles produced a short tonal burst, or chirp, with frequencies on the order of a few kilohertz. Theses frequencies are too low to be related to resonant frequencies of a bubble in a quiescent flow. Instead, the frequency content of the acoustic signal during bubble inception and growth is related to the volumetric oscillations of the bubble while it interacted with vortical flow that surrounds the bubble (i.e., the resonant frequency of the vortex-bubble system). A relationship was determined between the observed peak frequency of the oscillations, the highly stretched vortex properties, and the water nuclei content. It was found that different cavitation spectra could relate to different flow and fluid properties and therefore would not scale in the same manner.     

The characterization of acoustic cavitation bubbles - an overview

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

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.     

Investigations on dynamics of interacting cavitation bubbles in strong acoustic fields

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

Near threshold nucleation and growth of cavitation bubbles generated with a picosecond laser

The nucleation and growth of cavitation bubbles few micrometers in size in water generated by a 60 ps 515 nm fiber laser is observed and visualized near nucleation threshold. The study is performed by monitoring the plasma size, the cavitation bubble size and the emitted shock waves. The latter two aspects are supported by the Gilmore model using a Noble-Abel-stiffened-gas (NASG) equations of state. For the first time, two types of cavitation events are identified and visualized that exhibit a difference of more than two orders of magnitude in the excitation energy converted to mechanical effects with minimal change in excitation laser pulse energy. The result is localized cavitation and reduced mechanical stress on water-based media with potentially positive implications for laser treatments of biological tissue.     

Ultrasonic-induced nucleation of ice in water containing air bubbles

Cavitation induced by ultrasonic vibrations can cause nucleation of ice in supercooled water. In this study, the time required for ultrasonic-induced nucleation of ice was measured for water containing two different size distributions of air bubbles. When the water was supersaturated with air bubbles, there was a time lag of about 0.5 s between the onset of ultrasonic irradiation and the onset of ice nucleation, and the probability of ice nucleation was unusually high within 0.5-1.1 s after the onset of ultrasonic irradiation. These results cannot be explained by conventional models alone, in which the collapse of a cavitation bubble triggers the nucleation of ice. Secondary effects appear to also influence ice nucleation.     

Observation of acoustic cavitation bubbles at 2250 frames per second.

Acoustically induced cavitation at 20 kHz is observed by means of high speed CCD recording at a frame-rate of 2250 per second. Using digital image processing the bubbles' trajectories are reconstructed. The experimental data reveal that collision and coalescence of bubbles is a predominant phenomenon that limits their individual lifetime. Measurements of bubble sizes and velocities are in agreement with previous results.     

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.     

A theoretical model for ice primary nucleation induced by acoustic cavitation

The modelling and simulation of ice nucleation triggered by acoustic cavitation was addressed in this study. The objective was to evaluate the number of nuclei generated by a single gas bubble and afterwards by a multi-bubble system as function of the acoustic pressure (ultrasound wave amplitude) and supercooling level (liquid temperature). According to our calculations, the nucleation could be initiated with moderated acoustic pressure amplitude (around one bar) even at low supercooling levels (around few degrees). These results may provide a sound basis for the control of ice crystal size and morphology which is a key issue in industrial freezing and freeze-drying processes.     

Laser-induced cavitation bubbles and shock waves in water near a concave surface

The interplay among the cavitation structures and the shock waves following a nanosecond laser breakdown in water in the vicinity of a concave surface was visualized with high-speed shadowgraphy and schlieren cinematography. Unlike the generation of the main cavitation bubble near a flat or a convex surface, the concave surface refocuses the emitted shock waves and causes secondary cavitation near the acoustic focus which is most pronounced when triggered by the shock wave released during the first main bubble collapse. The shock wave propagation, reflection from the concave surface and its scattering on the dominant cavity is clearly resolvable on the shadowgraphs. The schlieren approach revealed the pressure build up in the last stage of the collapse and the first stage of the rebound. A persistent low-density watermark is left behind the first collapse. The observed effects are important wherever cavities collapse near indented surfaces, such as in cavitation peening, cavitation erosion and ophthalmology.     

Characterization of acoustic cavitation in water and molten aluminum alloy

High-intensive ultrasonic vibrations have been recognized as an attractive tool for refining the grain structure of metals in casting technology. However, the practical application of ultrasonics in this area remains rather limited. One of the reasons is a lack of data needed to optimize the ultrasonic treatment conditions, particularly those concerning characteristics of cavitation zone in molten aluminum.The main aim of the present study was to investigate the intensity and spectral characteristics of cavitation noise generated during radiation of ultrasonic waves into water and molten aluminum alloys, and to establish a measure for evaluating the cavitation intensity. The measurements were performed by using a high temperature cavitometer capable of measuring the level of cavitation noise within five frequency bands from 0.01 to 10 MHz. The effect of cavitation treatment was verified by applying high-intense ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al–17Si alloy. It was found that the level of high frequency noise components is the most adequate parameter for evaluating the cavitation intensity. Based on this finding, it was concluded that implosions of cavitation bubbles play a decisive role in refinement of the alloy structure.     

亚临界水中超声激励空化泡动力学分析

考察亚临界水中压力和温度对超声空化泡动力学的影响。应用非线性Rayleigh-Plesset方程模拟空化泡运动过程,并利用Matlab软件编程求数值解,用碘量法测定超声在亚临界水中的声空化产额。结果表明:当亚临界水的压力相似文献   

Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. “Transient” cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, “stable” cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble–bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both “stable” and “transient” cavitation bubbles which pulsate nonlinearly with the period of ultrasound.     

An alternative technique for determining the number density of acoustic cavitation bubbles in sonochemical reactors

The present paper introduces a novel semi-empirical technique for the determination of active bubbles’ number in sonicated solutions. This method links the chemistry of a single bubble to that taking place over the whole sonochemical reactor (solution). The probe compound is CCl₄, where its eliminated amount within a single bubble (though pyrolysis) is determined via a cavitation model which takes into account the non-equilibrium condensation/evaporation of water vapor and heat exchange across the bubble wall, reactions heats and liquid compressibility and viscosity, all along the bubble oscillation under the temporal perturbation of the ultrasonic wave. The CCl₄ degradation data in aqueous solution (available in literature) are used to determine the number density through dividing the degradation yield of CCl₄ to that predicted by a single bubble model (at the same experimental condition of the aqueous data). The impact of ultrasonic frequency on the number density of bubbles is shown and compared with data from the literature, where a high level of consistency is found.     

Mechanics of collapsing cavitation bubbles

A brief survey is given of the dynamical phenomena accompanying the collapse of cavitation bubbles. The discussion includes shock waves, microjets and the various ways in which collapsing bubbles produce damage.     

A study of cavitation nucleation in pure water using molecular dynamics simulation

To discover the microscopic mechanism responsible for cavitation nucleation in pure water, nucleation processes in pure water are simulated using the molecular dynamics method. Cavitation nucleation is generated by uniformly stretching the system under isothermal conditions, and the formation and development of cavitation nuclei are simulated and discussed at the molecular level. The processes of energy, pressure, and density are analyzed, and the tensile strength of the pure water and the critical volume of the bubble nuclei are investigated. The results show that critical states exist in the process of cavitation nucleation. In the critical state, the energy, density, and pressure of the system change abruptly, and a stable cavitation nucleus is produced if the energy barrier is broken and the critical volume is exceeded. System pressure and water density are the key factors in the generation of cavitation nuclei. When the critical state is surpassed, the liquid is completely ruptured, and the volume of the cavitation nucleus rapidly increases to larger than 100 nm³; at this point, the surface tension of the bubble dominates the cavitation nucleus, instead of intermolecular forces. The negative critical pressure for bubble nucleation is -198.6 MPa, the corresponding critical volume is 13.84 nm³, and the nucleation rate is 2.42×10³² m^-3·^-1 in pure water at 300 K. Temperature has a significant effect on nucleation: as the temperature rises, nucleation thresholds decrease, and cavitation nucleation occurs earlier.     

Electrochemical investigations of stable cavitation from bubbles generated during reduction of water

Megasonic cleaning is traditionally used for removal of particles from wafer surfaces in semiconductor industry. With the advancement of technology node, the major challenge associated with megasonic cleaning is to be able to achieve high cleaning efficiency without causing damage to fragile features. In this paper, a method based on electrochemistry has been developed that allows controlled formation and growth of a hydrogen bubbles close to a solid surface immersed in an aqueous solution irradiated with ∼1 MHz sound field. It has been shown that significant microstreaming from resonating size bubble can be induced by proper choice of transducer duty cycle. This method has the potential to significantly improve the performance of megasonic cleaning technology through generation of local microstreaming, interfacial and pressure gradient forces in close vicinity of conductive surfaces on wafers without affecting the transient cavitation responsible for feature damage.     

Effect of static pressure on acoustic energy radiated by cavitation bubbles in viscous liquids under ultrasound

The effect of static pressure on acoustic emissions including shock-wave emissions from cavitation bubbles in viscous liquids under ultrasound has been studied by numerical simulations in order to investigate the effect of static pressure on dispersion of nano-particles in liquids by ultrasound. The results of the numerical simulations for bubbles of 5 μm in equilibrium radius at 20 kHz have indicated that the optimal static pressure which maximizes the energy of acoustic waves radiated by a bubble per acoustic cycle increases as the acoustic pressure amplitude increases or the viscosity of the solution decreases. It qualitatively agrees with the experimental results by Sauter et al. [Ultrason. Sonochem. 15, 517 (2008)]. In liquids with relatively high viscosity (～200 mPa s), a bubble collapses more violently than in pure water when the acoustic pressure amplitude is relatively large (～20 bar). In a mixture of bubbles of different equilibrium radius (3 and 5 μm), the acoustic energy radiated by a 5 μm bubble is much larger than that by a 3 μm bubble due to the interaction with bubbles of different equilibrium radius. The acoustic energy radiated by a 5 μm bubble is substantially increased by the interaction with 3 μm bubbles.     

Field-dependent quantum nucleation of antiferromagnetic bubbles

The phenomenon of quantum nucleation is studied in a nanometer-scale antiferromagnet with biaxial symmetry in the presence of a magnetic field at an arbitrary angle. Within the instanton approach, we calculate the dependence of the rate of quantum nucleation and the crossover temperature on the orientation and strength of the field for bulk solids and two-dimensional films of antiferromagnets, respectively. Our results show that the rate of quantum nucleation and the crossover temperature from thermal-to-quantum transitions depend on the orientation and strength of the field distinctly, which can be tested with the use of existing experimental techniques. Received 13 June 2000 and Received in final form 24 October 2000