Sonoluminescence as a probe of conditions during cavitation

Abstract

Acoustic cavitation occurs in all liquids irradiated with sufficient intensity of sound or ultrasound. The collapse of such bubbles creates local heating and provides a unique source of energy for driving chemical reactions. In addition to sonochemical bond scission and formation, cavitation also induces light emission in many liquids. This phenomenon of sonoluminescence (SL) and has captured the imagination of many researchers since it was first observed 85 years ago. SL provides a direct probe of cavitation events and has provided most of our understanding of the conditions created inside collapsing bubbles. Spectroscopic analyses of SL from single acoustically levitated bubbles as well as from clouds of bubbles have revealed molecular, atomic, and ionic line and band emission riding atop an underlying continuum arising from radiative plasma processes. These studies permit quantitative measurement of the intracavity conditions: relative peak intensities for temperature measurements, peak shifts and broadening for pressures, and peak asymmetries for plasma electron densities. Extraordinary conditions are generated inside the collapsing bubbles in ordinary room-temperature liquids – observable temperatures exceeding 15 000 K, pressures >1000 bar, and heating and cooling rates in excess of 1012 K/s. For clouds of cavitating bubbles, multibubble sonoluminescence (MBSL) reveal slightly more moderate local conditions inside bubble clouds of ∼5000 K, ∼300 atm. Biomedical applications of SL will be discussed.