High-amplitude radial pulsations of a single gas bubble in several glycerine and water mixtures have been observed in an acoustic stationary wave system at acoustic pressure amplitudes on the order of 150 kPa ( 1.5 atm) at 21-25 kHz. Sonoluminescence (SL), a phenomenon generally attributed to the high temperatures generated during the collapse of cavitation bubbles, was observed as short light pulses occurring once every acoustic period. These emissions can be seen to originate at the geometric center of the bubble when observed through a microscope. It was observed that the light emissions occurred simultaneously with the bubble collapse. Using a laser scattering technique, experimental radius-time curves have been obtained which confirm the absence of surface waves, which are expected at pressure amplitudes above 100 kPa. [S. Horsburgh, Ph.D. dissertation, University of Mississippi (1990) ]. Also from these radius-time curves, measurements of the pulsation amplitude, the timing of the major bubble collapse, and the number of rebounds were made and compared with several theories. The implications of this research on the current understanding of cavitation related phenomena such as rectified diffusion, surface wave excitation, and sonoluminescence are discussed.

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Biomedical acoustics is rapidly evolving from a diagnostic modality into a therapeutic tool and acoustic cavitation is often found to be the common denominator in a wide range of new therapeutic applications. High-intensity focussed ultrasound (HIFU) waves generated outside the body can be used to deposit heat deep within the body. Through a quantitative analysis of heat deposition by ultrasound, it is shown that inertial cavitation can help address some of the major challenges of HIFU therapy by providing a means of enhancing and monitoring treatment non-invasively. In the context of drug delivery, both inertial and stable cavitation are found to play a role in enhancing drug activity and uptake. In particular, shape oscillations arising during stable cavitation are shown to provide an effective micro-pumping mechanism for enhanced mass transport across inaccessible interfaces.

Acoustic cavitation has been shown to play a key role in a wide array of novel therapeutic ultrasound applications. This paper presents a brief discussion of the physics of thermally relevant acoustic cavitation in the context of high-intensity focussed ultrasound (HIFU). Models for how different types of cavitation activity can serve to accelerate tissue heating are presented, and results suggest that the bulk of the enhanced heating effect can be attributed to the absorption of broadband acoustic emissions generated by inertial cavitation. Such emissions can be readily monitored using a passive cavitation detection (PCD) scheme and could provide a means for real-time treatment monitoring. It is also shown that the appearance of hyperechoic regions (or bright-ups) on B-mode ultrasound images constitutes neither a necessary nor a sufficient condition for inertial cavitation activity to have occurred during HIFU exposure. Once instigated at relatively large HIFU excitation amplitudes, bubble activity tends to grow unstable and to migrate toward the source transducer, causing potentially undesirable pre-focal damage. Potential means of controlling inertial cavitation activity using pulsed excitation so as to confine it to the focal region are presented, with the intention of harnessing cavitation-enhanced heating for optimal HIFU treatment delivery. The role of temperature elevation in mitigating bubble-enhanced heating effects is also discussed, along with other bubble-field effects such as multiple scattering and shielding.

Comparisons of the spectral characteristics of sonoluminescence from cavitation in bubble fields (MBSL) versus cavitation of single bubbles (SBSL) have been made for aqueous solutions under similar experimental conditions. In particular, alkali metal chloride solutions exhibit sonoluminescence emission from excited state Na or K atoms in MBSL, while SBSL exhibits no such emission. Since the metal ions are not volatile, participation of the initially liquid phase must occur in MBSL. Surface wave and microjet formation in cavitating bubble fields versus the high spherical symmetry of collapse of an isolated bubble may account for the observed differences.PACS numbers: 78.60. Mq, 43.25.+y, 47.40.Nm It has long been known that under certain conditions acoustic irradiation of a liquid can result in light emission, a phenomenon called sonoluminescence (SL) [1,2].The process typically involves the application of high intensity ultrasound to a liquid by an immersed acoustic horn driven with a piezoelectric transducer. The resulting cavitation-bubble field is made up of a complex distribution of gas and vapor-filled bubbles of various equilibrium sizes that pulsate at various phases relative to the driving acoustic pressure field. The bubble dynamics is further complicated by interactions with neighboring bubbles [3] as well as with the vessel walls. Depending on the location within the pressure field and these other infIuences, some of the bubbles may grow dramatically during the negative portion of the sound field, followed by a quasiadiabatic collapse that results in the heating of the bubble interior and the subsequent emission of light [4].In spite of the complexity of cavitating bubble fields, many studies have been made of multibubble sonoluminescence (MBSL) and the influences of fluid and gas properties. The optical spectra of MBSL typically contains distinct, pressure broadened molecular or atomic emission bands. Of particular significance here is the identification of individual transitions from excited states of diatomic carbon (Cq) that contribute to the optical spectrum of MBSL in nonaqueous liquids. The fitting of the measured spectrum of C2 permitted the measurement of an effective rotational and vibrational temperature of the excited states of Cq of 5100 K [5].Recent experimental advances [6] have also made it possible to examine both the temporal and spectral nature of sonoluminescence from a single bubble (SBSL). Here a single bubble is acoustically levitated in an aqueous solution that has been partially degassed. The bubble can be made to undergo large-amplitude, nonlinear, presumably radial pulsations during which light emission can occur. Some properties of SBSLinclude [7] the synchronous emission of light with each and every acoustic cycle, tem-poral flash widths of less than 50 ps, and a continuous spectral energy density that increases from the visible into the UV, with eventual fall off due to UV absorption by the surrounding water. In addition, unlike in MBSL, there are little or no electroni...