Acoustic Cavitation and Bubble Dynamics

Acoustic cavitation is the formation and subsequent violent collapse of bubbles in liquid irradiated with intense ultrasound. Ultrasound is radiated by a vibrating plate connected to ultrasonic transducers made of piezoelectric materials driven by electrical power. Microscopic mechanism for vibration of piezoelectric materials is briefly described. There are two types of ultrasonic experimental equipment used to generate acoustic cavitation: ultrasonic horn (or probe) and ultrasonic bath. Ultrasonic standing waves and traveling waves are discussed by means of mathematical equations. Acoustic impedance is discussed, and transmission and reflection coefficients are described. Various types of acoustic cavitations are discussed: transient and stable cavitations, vaporous and gaseous cavitations. Fluctuations in degassing and re-gassing cause repeated change between vaporous and gaseous cavitation. Light emission associated with violent bubble collapse as well as chemical reactions inside and outside a bubble is discussed in the sections entitled “sonoluminescence” and “sonochemistry,” respectively. Unsolved problems in sonoluminescence are briefly discussed. Reasons for lesser amount of produced H radicals (H·) than that of OH radicals (OH·) in sonochemical reactions are discussed based on results generated from numerical simulations. In the last section, ultrasonic cleaning, especially for the application to silicon wafers, is discussed.

Bubble pulsation is mathematically described by the Rayleigh–Plesset equation and by Keller equation. Derivation of the equations is fully described herein. Using the Rayleigh–Plesset equation, the violent collapse of a bubble is discussed. A method of numerical simulations of bubble pulsation is also described. In relation to numerical simulations, non-equilibrium evaporation and condensation of water vapor at the bubble wall, the variation in liquid temperature at the bubble wall, the gas diffusion across the bubble wall, and the chemical reactions inside a bubble are discussed. Comparison between numerical results and experimental data for a single-bubble system is shown. The main oxidants created inside a bubble are described based upon numerical simulations data. Linear and nonlinear resonance radius of a bubble is discussed as well as the analytical solution of the linearized equation of bubble pulsation. The mechanism of shock wave emission from a bubble into surrounding liquid is discussed. Inside a collapsing bubble, a shock wave is seldom formed due to lower temperature near the bubble wall. A liquid jet penetrates into a collapsing bubble near the solid surface. The bubble pulsation is influenced by the acoustic emissions from the surrounding bubbles, which is called bubble–bubble interaction. The origin of acoustic cavitation noise is discussed based upon results of numerical simulations. It is shown that surfactants and salts strongly retard bubble–bubble coalescence.

Although acoustic cavitation and bubble dynamics have been studied for more than 100 years, this field is still very active and there are a variety of unsolved problems. The old problem on cavitation nuclei is now in the spotlight because of the mysteries of bulk nanobubbles. With regard to sonochemical products, ammonia (NH₃) and oxygen atom (O) have not yet been fully studied. At the final moment of bubble collapse, solidification of water may take place near the bubble wall by the high pressure. A related unsolved problem is the mechanism of sonocrystallization that crystal nucleation is accelerated by acoustic cavitation. Plasma formation inside a sonoluminescing bubble has been confirmed by the spectroscopic observation. Is there a hot plasma core formed by shock wave focusing at the center of a bubble? What is quantitative theory of ionization-potential lowering inside a bubble nearly at liquid density? Why is the vibrational population distribution of OH radicals strongly in non-equilibrium inside a bubble? What are the roles of pulsed ultrasound and liquid surface vibration in acoustic field in the liquid? Is there any effect of a magnetic field on bubble dynamics? Are the extreme conditions inside a dissolving bubble real?