Acoustic Cavitation Threshold Research Articles

Radiation‐Induced Ultrasonic Cavitation

Working with a cylindrical resonator we find the acoustic cavitation threshold of several liquids to rise as more of the “motes” are removed, with no limit in sight. Exposure to high‐energy neutrons yields a reproducible threshold if the liquid is clean enough. The cavitation rate rises sharply with stress, and at a fixed stress is proportional to neutron flux. The events are random. No appreciable induction or decay times are observed, even in water. It is difficult to clean water (strong) so that it sustains stresses >10 dB above neutron threshold, but easy for isopropanol (weak). “Freon TF” (F2Cl C C Cl2F) has a very low neutron threshold and sustains pressures >18 dB above it (neutrons absent). Isopropanol with a dissolved thorium salt exhibits the same sharp neutron threshold, a well‐defined α‐particle threshold about 5 dB higher, and a poorly defined fast‐fission threshold >10 dB lower. All results were obtained at 20–50 kHz and were largely independent of dissolved gas content. [Work supported in part by the U. S. Office of Naval Research.]

Acoustic Implications of Persistent Microbubble Model

A model for the persistent microbubble was presented previously [J. Acoust. Soc. Am. 36, 1009 (1964)], the hypothesis being given that the bubble is stabilized by minute particles picked up on the bubble surface, which compress upon out-diffusion of gas to form a wall capable of supporting the hydrostatic pressure, permitting the bubble internal pressure to drop to that of the gas dissolved in the liquid, thus equalizing boundary diffusion and supplying the mass necessary to neutralize buoyancy. The additional stiffness and mass of the compressed wall should affect the microbubble resonant frequency and damping constant. The wall's nonlinear elasticity may be a predominant factor affecting the acoustic cavitation threshold. These implications are explored and applied to several examples of recent experimental evidence.

The Properties of Gaseous Solutions as Revealed by Acoustic Cavitation Measurements

That very small gas bubbles can serve as nuclei for the formation of cavities in liquids is well established. That experimental observations on the rupture of liquids can be interpreted only on this basis is perhaps not so obvious but none the less true. In the present series of experiments on acoustic cavitation in water, two distinct types of bubble formation occur. One, the violent formation and collapse of vapor-filled cavities, results from mechanical instability of gas nuclei. The other, relatively quiet gas bubble formation, occurs as a consequence of the slow growth of nuclei by “rectified” diffusion of dissolved gas from the surrounding liquid. Each process has a threshold of excitation by a sound field; which has the lower threshold in any given case depends largely upon the concentration of dissolved gas in the liquid. Measurements of the acoustic cavitation threshold in conventionally “deaerated” water, as a function of temperature and ambient hydrostatic pressure, reveal how the equilibrium size of gas nuclei depends upon these variables. Observations on sonically induced effervescence in saturated solutions provide at least a qualitative explanation for the pulse length and viscosity effects observed elsewhere. Cavitation at the surface of a sound projector apparently is profoundly affected by adsorbed gases. The conclusion that gaseous nuclei exist more or less in equilibrium with solutions not supersaturated with gas is contrary to the conventional theory of gaseous solutions. Stabilization of nuclei in the surface cracks of suspended solid particles is a very plausible but not entirely satisfactory explanation. Revision of the theory is a tempting subject for speculation.