1. Acoustic Cavitation

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.