Chapter 7 - Nonlinear oscillations and resonances of the acoustic bubble and the mechanisms of energy dissipation

Abstract

The oscillations of ultrasonically excited bubbles are complex and chaotic. The bubble oscillator exhibits fundamental, super-harmonic, and subharmonic resonance frequencies that depend on the acoustic pressure amplitude and frequency. Acoustic energy is dissipated upon passing through a bubbly medium by thermal, viscous, and radiation damping. Nonlinear bubble oscillations and their relation to the ultrasound energy dissipation mechanisms need to be understood for optimizing and controlling bubble-related applications, yet due to its complexity this relationship has not been studied comprehensively. In this chapter, the linear and nonlinear resonance frequencies of the bubble oscillator are discussed for a range of exposure regimes. The pressure-dependent nonlinear and chaotic bubble oscillations as a function of applied acoustic pressure are introduced and then classified. The energy dissipation terms are derived in both the linear and nonlinear regimes for free and encapsulated bubbles. The differences between linear and nonlinear regimes are highlighted by analyzing the dissipated energies at bubble resonances. Furthermore, the evolution of the damping mechanisms as a function of pressure is presented and classified. It is shown that at optimized exposure parameters that depend on the bubble resonances, a specific category of energy dissipation can be maximized or minimized. The impact of selecting the appropriate exposure parameters for ultrasound applications is discussed.