Ronald A. Roy

Comparisons of sonoluminescence from single-bubbles and cavitation fields: bridging the gap

Sonoluminescence (SL) refers to the generation of light through the energetic pulsations of acoustic cavitation bubbles in a liquid. For years, SL was observed primarily in cavitation fields. These bubbles are believed by many to undergo near-adiabatic compression, resulting in the heating of the bubble contents and the subsequent emission of light. Recently, researchers have discovered a new form of sonoluminescence in which light is observed to emanate from a single bubble undergoing very large volume excursions. The mechanism for light production is unknown, but many believe it is due to a rapid heating of the central core by an imploding shock wave. Based in part on the emission time scales, there is a common belief that the two forms of SL are quite distinct. We address this issue by comparing the two phenomena with regards to their light-flash durations and emission spectra--leading to some surprising differences and similarities.

Sound Scattering from Microbubble Distributions Near the Sea Surface

Backscatter from the sea surface is thought to be governed by the roughness of the surface and subsurface bubble distributions. At low frequencies, due to the paucity of large bubbles, scattering results primarily from coherent and/or collective scatter from bubbles entrained by the subsurface vorticity or carried to depth by the Langmuir circulation and thermal convection. It is shown that scattering from compact regions is a function of the volume fraction of air and to first order can be described by a Minnaert formula modified with the volume fraction. Measurement of sound scattering from a submerged cloud of bubbles produces low-frequency peaks with large low-frequency target strength consistent with this theory.

Physical aspects of sonoluminescence from acoustic cavitation

Abstract The passage of an ultrasonic wave through a medium can sometimes result in the production of acoustic cavitation which is often accompanied by the generation of light; a process known as sonoluminescence or ‘light from sound’. A brief historical overview of early sonoluminescence (SL) research is presented, followed by a phenomenological description of the physics of cavitation bubble fields. Many of the published results describing the features of SL (pulse duration, phasing, spectra, etc.) are inconsistent and sometimes contradictory. However, recent studies of sonoluminescence from individual, radially pulsating bubbles draw a consistent picture. Single-bubble SL flashes occur once every acoustic cycle and coincide precisely with bubble collapse. The duration of the flash is believed to be ⩽50 ps and the emission spectra indicate substantial heating of the bubble interior. These results suggest that the primary mechanism for single-bubble sonoluminescence is the rapid heating of the interior of the collapsing bubble.

BUBBLE-RELATED SOURCES OF OCEAN AMBIENT NOISE

A review of our recent work involving experimental investigations of bubble-related ambient noise mechanisms is presented. Intermediate and high-frequency ambient noise is associated with precipitation and bubble entrainment from capillary-gravity waves. Evidence of intermediate and lowfrequency noise from collective oscillations of bubble clouds is also presented. All results are viewed in light of existing theories for underwater bubble-noise production.