Subharmonic and Other Low-Frequency Emission from Bubbles in Sound-Irradiated Liquids

Abstract

This paper presents measurements of the acoustic emission from gas bubbles of controlled sizes “seeded” into water and glycerol-water mixtures and subjected to sound fields over a wide range of frequencies and intensities up to, and beyond, the transient cavitation threshold. The emission from unprepared liquids was also recorded for comparison. A mechanical device has been developed for generating small bubbles of uniform sizes at controlled rates. The results suggest that there are at least two mechanisms for generating signals below the excitation frequency (f0): (1) Subharmonics of f0 may be produced by forced oscillations of bubbles whose radial resonance frequencies are submultiples of f0; (2) bubbles of any size may be shock excited to emit signals at their radial resonance frequencies. Large bubbles of certain preferred sizes are normally present, and their resonances probably account for many low-frequency signals that are not integral submultiples of f0. It also seems certain that some other mechanism unconnected with resonant bubbles contributes to the relatively strong signals observed at the first subharmonic (f0/2).

The role of cavitation in acoustically activated drug delivery

“…With respect to the relationship between collapse cavitation and the subharmonic, there are reports that correlate subharmonic emission with positive indicators of collapse cavitation, such as iodine release, sonoluminescence and acoustic white noise [15][16][17][18]. Leighton maintains that cavitating bubbles produce a signal at f/2 because of a prolonged expansion phase immediately preceding a delayed collapse phase of the inertial cavitation event [11].…”

“…Stable cavitation has been associated with the observation of the harmonic and subharmonic frequencies [9,11,14]. However, inertial cavitation as evidenced by sonoluminescence, generation of free radicals, and the intensity of the non-harmonic background noise also has been correlated with subharmonic acoustic emission [15][16][17][18]. A positive correlation between Dox release and harmonic or subharmonic acoustic emissions in the absence of non-harmonic background noise would show that stable (not inertial) cavitation causes drug release.…”

Further investigation of the mechanism of Doxorubicin release from P105 micelles using kinetic models

“…Two sources for the subharmonic peak at half the driving frequency (f/2) have been proposed in the literature. One possible source is bubbles undergoing stable cavitation [14,15], where the bubbles oscillate in the acoustic field near, but not quite at the points of implosion. Stable cavitation with a subharmonic component has been previously reported [15][16][17][18][19][20].…”

“…Collapse cavitation is characterized acoustically by a shock wave that produces broad band white noise [14,15]. An f/2 signal that is always accompanied by an increase in background noise is usually attributed to collapse cavitation.…”

“…The amount of Dox which a micelle releases into solution upon destruction is assumed to be the average amount of Dox contained in micelles within its group, E j /M j . (15) Substituting Eq. (4) into the above equation yields the following relation.…”