Hot Spot Conditions during Multi-bubble Cavitation

1. Introduction Together with the chemical effects of ultrasound, light is often emitted [1-5]. Such sonoluminescence provides an extremely useful spectroscopic probe of the conditions created during cavitation bubble collapse. Acoustic cavitation is the origin of both sonochemistry and sonoluminescence. The collapse of bubbles caused by cavitation produces intense local heating and high pressures, with very short lifetimes. As we will demonstrate in this chapter, in clouds of cavitating bubbles, these hot spots have equivalent temperatures of roughly 5000 K, pressures of about 1000 atmospheres, and heating and cooling rates above 10 10 K/s. In single bubble cavitation, conditions may be even more extreme [6-7]. Thus, cavitation can create extraordinary physical and chemical conditions in otherwise cold liquids. Fundamentally, chemistry is the interaction of energy and matter. Chemical reactions require energy in one form or another to proceed: chemistry stops as the temperature approaches absolute zero. Chemists have only limited control, however, over the nature of this interaction. In large part, the properties of a specific energy source determine the course of a chemical reaction. Ultrasonic irradiation differs from traditional energy sources (such as heat, light, or ionizing radiation) in duration, pressure, and energy per molecule. The immense local temperatures and pressures together with the extraordinary heating and cooling rates generated by cavitation bubble collapse mean that ultrasound provides an unusual mechanism for generating high energy chemistry. Like photochemistry, very large amounts of energy are introduced in a short period of time, but it is thermal, not electronic, excitation. As in flash pyrolysis, high thermal temperatures are reached, but the duration is very much shorter (by >10 4) and the temperatures are even higher (by five-to tenfold). Similar to shock-tube chemistry or multiphoton infrared laser photolysis, cavitation heating is very short lived, but it occurs within condensed phases. Furthermore, sonochemistry has a high-pressure component, which suggests that one might be able to produce on a microscopic scale the same macroscopic conditions of high temperature-pressure "bomb" reactions or explosive shockwave synthesis in solids. Figure 1 presents an interesting comparison of the parameters that control chemical reactivity (time, pressure, and energy) for various forms of chemistry. Figure 1. Chemistry: the interaction of energy and matter. 1.1 ACOUSTIC CAVITATION Ultrasound spans the frequencies of roughly 15 kHz to 1 GHz. With sound velocities in liquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to 10-4 cm. These are not molecular dimensions. …