Transient Acoustic Cavitation Research Articles

Stable tridimensional bubble clusters in multi-bubble sonoluminescence (MBSL)

In the present work, stable clusters made of multiple sonoluminescent bubbles are experimentally and theoretically studied. Argon bubbles were acoustically generated and trapped using bi-frequency driving within a cylindrical chamber filled with a sulfuric acid aqueous solution (SA85w/w). The intensity of the acoustic pressure field was strong enough to sustain, during several minutes, a large number of positionally and spatially fixed (without pseudo-orbits) sonoluminescent bubbles over an ellipsoidally-shaped tridimensional array. The dimensions of the ellipsoids were studied as a function of the amplitude of the applied low-frequency acoustic pressure (PAcLF) and the static pressure in the fluid (P0). In order to explain the size and shape of the bubble clusters, we performed a series of numerical simulations of the hydrodynamic forces acting over the bubbles. In both cases the observed experimental behavior was in excellent agreement with the numerical results. The simulations revealed that the positionally stable region, mainly determined by the null primary Bjerknes force (F→Bj), is defined as the outer perimeter of an axisymmetric ellipsoidal cluster centered in the acoustic field antinode. The role of the high-frequency component of the pressure field and the influence of the secondary Bjerknes force are discussed. We also investigate the effect of a change in the concentration of dissolved gas on the positional and spatial instabilities through the cluster dimensions. The experimental and numerical results presented in this paper are potentially useful for further understanding and modeling numerous current research topics regarding multi-bubble phenomena, e.g. forces acting on the bubbles in multi-frequency acoustic fields, transient acoustic cavitation, bubble interactions, structure formation processes, atomic and molecular emissions of equal bubbles and nonlinear or unsteady acoustic pressure fields in bubbly media.

The detection and control of stable and transient acoustic cavitation bubbles

Acoustic cavitation plays a significant role in many applications that include therapeutic medicine, sonochemsitry and materials processing. The type of cavitation bubbles generated, namely transient or stable (repetitive transient), is important for optimizing the efficiency of these applications. We report on a multibubble sonoluminescence (MBSL)-based experimental technique for the detection and the control of type of cavitation at low and high ultrasound frequencies. It is observed that the use of a horn-type sonicator operating at 20 kHz primarily generates transient cavitation bubbles, as reported earlier. However, the use of plate type transducers at low frequencies (25 and 37 kHz) generates a significant amount of stable cavitation, as evidenced from the quenching of sonoluminescence by volatile solutes. Evidence for the existence of transient cavitation at a higher ultrasound frequency (440 kHz) is also provided.

Sonoluminescence from transient cavitation at high pressures in water and acetone

In highly degassed, clean liquids, transient acoustic cavitation can be triggered by fast neutrons, a phenomenon that has been known since ∼1960’s. The kinetic energy acquired by the nuclei allows it to ionize a small volume (∼100 nm dia.) in the liquid, creating a vapor cavity that would normally last a couple of microseconds. If the acoustic amplitude and phase are right, this cavity expands by several orders of magnitude (∼500 to 1500 microns dia.) and then collapses, emitting a short flash of light (1 to 40 nsec). The bubble continues to expand and collapse for several hundred cycles, eventually evolving into a larger (2 to 6 mm dia.) bubble cloud which lasts several milliseconds, depending on the conditions. The time duration of the light pulses is longer and their amplitude larger than those of single bubble sonoluminescense in water (∼100 to 300 psec, 105 photons/flash). The amplitude and time evolution of the light flashes has been analyzed as a function of the driving conditions and compared with computer simulations in an effort to infer the maximum plasma temperatures and densities, and perhaps the presence of shock waves, in the cavities.

Correlation of ultrasound-induced hemolysis with cavitation detector output in vitro

A 20-MHz passive acoustic detector was used to quantify the amount of transient acoustic cavitation occurring in a sample exposed to intense pulsed ultrasound. A dilute suspension of human erythrocytes with and without a microbubble echo-contrast agent was exposed in vitro to 500 W/cm 2 (SPPA) ultrasound of center frequency 1 MHz and tone burst duration 20, 100, 200, 500 and 1000 μs at a pulse repetition frequency of 20 Hz. Inertial cavitation occurring within the sample, as measured by the temporal average of the detector output, correlated well with hemolysis, suggesting that violent bubble collapse is responsible for cell damage. The result also raises the prospect of cavitation monitoring as a possible predictor of adverse bioeffects when echo-contrast agents are used clinically.

Transient acoustic cavitation in gallstone fragmentation: a study of gallstones fragmented in vivo.

The mechanism of gallstone fragmentation by shock waves in vivo is uncertain. We used scanning electron microscopy to study 9 partially fragmented stones obtained from 6 patients who underwent lithotripsy and subsequently had surgery because of incomplete fragmentation. Surface characteristics of the stone were studied using scanning electron microscopy and compared to gallstones fragmented in degassed water in vitro and to control stones obtained from patients with uncomplicated cholelithiasis. Characteristic damage caused by transient acoustic cavitation was visible in all stones fragmented in vivo and in vitro as pits 10-100 microns diameter. In these pits, cholesterol crystals were fractured, and the symmetry of crystal boundaries was damaged. In areas of more severe damage, individual pits coalesced to form craters on the stone surface. High magnification of the pit walls revealed cracks, which in some instances radiated out onto the unpitted surface of the stone. We conclude that transient acoustic cavitation plays a role in gallstone fragmentation in vivo through the mechanism of surface pitting and the formation of cracks that radiate outward from the surface pits. Measures to enhance cavitation may improve the results of gallstone lithotripsy.

Megahertz-frequency acoustic cavitation at high duty cycles and long pulse lengths

The threshold pressure for the onset of transient acoustic cavitation due to high-frequency, pulsed ultrasound has been shown to be dependent on the frequency and initial nucleus size. Therefore, assuming that a range of optimally sized nuclei are present in a fluid, the amount of cavitation one gets depends on the applied acoustic pressure and the frequency. This idea has been incorporated into the concept of the ‘‘cavitation index’’ [Apfel and Holland, Ultrasound Med. Biol. 17, 179–185 (1991)], which specifies the conditions below which cavitation should not occur. However, the influence that time-averaged effects, such as rectified diffusion, have on the cavitation threshold has not been extensively studied yet. A set of laboratory experiments designed to study these effects has been performed. Clean water is seeded with polystyrene spheres (to act as nucleation sites), and the cavitation threshold is measured using a modified version of a passive detection technique [Atchley etal., Ultrasonics 26, 280–285 (1988)]. The dependence of the threshold pressure on duty cycle (0.1%–10%) and pulse length (about 3–100 cycles) is presented at frequencies of 1, 2.25, and 5 MHz. Results are interpreted in light of existing theories for bubble dynamics. [Work supported by the NIH.]

Biological effects of lithotripter fields.

Since the early 1980s, extracorporeal shockwave lithotripsy (ESWL) has grown to be one of the primary techniques used for the clinical treatment of both kidney stone and gallstone disease. The mechanisms of soft-tissue damage and stone breakup in the fields of spark-gap, piezoelectric, and catheter lithotripters, as well as those of diagnostic and therapeutic ultrasound devices have been studied. The results are consistent with the hypothesis that transient acoustic cavitation is the primary mechanism responsible for the observed bioeffects, and plays a role as well in the breaking of the stone. Thresholds have been established for bioeffects in mouse kidney (less than 3–5 MPa peak positive pressure), chick embryos (less than 10 MPa), and Drosophila larvae (less than 1 MPa). The extremely low thresholds for lung hemorrhage in mice for 10 lithotripter pulses (1.5 MPa) and short pulses of diagnostic ultrasound (less than 1 MPa) suggest that gas-filled tissues within the body are especially susceptible to lithotripsy damage. The efficacy of gallstone lithotripsy depends critically upon the mechanical properties of the stones themselves. These properties influence which destruction mechanism−cavitation or stress fracture−is dominant.

The Blake threshold of a cavitation nucleus having a radius-dependent surface tension

The Blake threshold is encompassed in most theoretical models of transient acoustic cavitation. Previously, this threshold has been calculated for a bubble having constant surface tension. However, such a bubble is unstable against dissolution and is not a realistic model for a cavitation nucleus. In this article, the Blake threshold is calculated for a more realistic nucleus—a bubble having a radius-dependent surface tension. An expression is derived for the dependence of the surface tension on radius for a surfactant-stabilized cavitation nucleus. Two cases, appropriate for charged and polar surfactants, are examined. The Blake threshold for the former case exceeds that of the standard (constant surface tension) case by approximately 1% to 20% for radii ranging from 3.0 μm down to 0.01 μm. In the latter case, the Blake threshold exceeds that of the standard case by approximately 2% to 38% over the same range of radii. It is concluded that, for the sizes important for cavitation processes, both unstabilized and surfactant-stabilized bubbles become mechanically unstable under essentially the same acoustic conditions. Thus the use of the simpler, albeit unrealistic, nucleus in theoretical models results in no serious error.

The Blake threshold of a bubble having a radius‐dependent surface tension

The Blake threshold is incorporated in most theoretical models of transient acoustic cavitation. In standard derivations, this threshold is calculated for a bubble having constant surface tension. However, such a bubble is unstable against dissolution and not a realistic model for a cavitation nucleus. In this work, the Blake threshold is calculated for a more realistic nucleus—a bubble having a radius‐dependent surface tension. A relationship is derived for this dependence, based upon the properties of a surfactant‐stabilized cavitation nucleus. Two cases, appropriate for charged and polar surfactants, are examined. The Blake threshold for the former case is identical to that of the standard case by approximately 2%–20% for radii ranging from 3.0 μm down to 0.01 μm. Therefore, it is concluded that both unstabilized and surfactant‐stabilized bubbles become mechanically unstable under essentially the same acoustic conditions. Thus the use of the simpler, unrealistic nucleus in theoretical models results in no serious error. [Work supported by NPS Foundation.]

Possibility of Microcavitation from Diagnostic Ultrasound

Abstmct-There is little or no definitive data in the literature that addresses the question of the possibility of adverse bioeffects in a single biological cell due to diagnostic ultrasound. A formula is derived that predicts the conditions that will produce transient acoustic cavitation on the scale of a single biological cell in one acoustic cycle. It is found that if the viscosity of the cellular medium or its surroundings is sufficiently low (within a factor of two or three of water, as with plasma) and if the frequency is sufficiently low (below 2-3 MHz), then some current diagnostic units may produce microcavitation when a small cavitation nuclei exists in or near a cell.

The influence of the dissolved ion concentration on the acoustic cavitation threshold of water

The variation in the transient acoustic cavitation threshold of water was measured as a function of the dissolved ion concentration of various salts. The measurements were made with a resonant sphere system in which solid particle size, water temperature, and dissolved gas content could be controlled within a closed circulating system. The normal criterion used for a cavitation event was the detection, by a photomultiplier tube, of sonoluminescence produced by a collapsing cavity. Water with a specific conductance of about 1 μS cm−1 was assigned zero concentration for the first data point. Measurements were then made at successively higher concentrations, up to 10 mmoles 1−1. The salts were chosen to check and extend the measurements of a previous researcher [V. A. Akulichev, Sov. Phys. Acoust. 12, 144–149 (1966)], who first detected a dependence of the cavitation threshold on dissolved ion concentration. The results of our measurements show that the threshold increases with ion concentration, reaching saturation at concentrations of 2 mmoles 1−1 or less. This dependence is in the opposite direction to that observed by Akulichev. Our results will be analyzed with regard to the standard models of cavitation nucleation. [Work supported in part by the Office of Naval Research and the National Science Foundation.]

Acoustic cavitation prediction

We have derived an approximate expression for the threshold pressure for transient acoustic cavitation and have used this result with published expressions for the thresholds for gas bubble nucleation and for rectified diffusion in synthesizing cavitation prediction charts for water covering a frequency range from 10 kHz to 2.5 MHz, and for gas saturation percentages 10%, 50%, and 100%. This synthesis, which is applicable to other liquids, may provide guidance about the phenomena associated with acoustic cavitation to those who are either trying to minimize or to optimize its effects.