ABSTRACT
This review comes timely, as there have been many developments in the recent years to fabricate new types of materials based on using high-intensity ultrasound. Application of ultrasound in physico-chemical processes is environmentally friendly and also allows one to achieve high temperature and high pressure chemistry locally on micro-level with the bulk reactor near the room temperature. This leads to the formation of novel microstructure and nanostructure due to non-equilibrium conditions of cavitation interface. Despite its great potential, research in the area of sonochemical treatment of the solid surfaces has not grown, especially related to the quantitative understanding of the cavitation bubble, solid surface interactions. Solving this fundamental question, the next step will involve the
development of new surfaces in a controlled way. It is shown that there are conceptual solutions for the controlled sonochemical fabrication of mesoporous surfaces and metal sponges; moreover, there are still great prospects in the development of such materials, as it is shown in the Section 11.4: Advanced Functional Materials. 11.1 Introduction
The first report on cavitation concerned the notice that the propeller of submarine was pitted and eroded (Thornycroft and Barnaby, 1895). Since the early work of Lord Rayleigh, it is known that ultrasound may form cavitation bubbles inside liquids, which upon collapse create transiently temperatures around 5000 K and pressures around 1000 atm with cooling rates above 108 K/s (Rayleigh, 1917). Structures may also be formed and quenched far from equilibrium. One of the most basic concepts of sonochemistry is that free radicals are formed as a result of cavitation of microbubbles that are created during the rarefaction (or negative pressure) period of sound waves. The growth of microbubbles can be also described by an unequal transfer of mass across the bubble interface during oscillation (Harvey et al., 1994). It was also shown that bubble-induced microstreaming was one of the factors leading to the well-known ultrasonic cleaning effects in heterogeneous systems (Elder et al., 1954; Elder, 1959). Later on, the understanding about the physical effects of ultrasound in liquid systems was also increased. Thus, the hypothesis of microjets formed during asymmetric cavitation was suggested (Naude and Ellis, 1961). Moreover, the sonolysis of water was found to be possible, thus allowing one to discuss about sonochemistry during ultrasound exposure (Makino et al., 1982). Although ultrasound has been used as a cleaning tool in many chemical and metallurgical labs, the knowledge about influence of ultrasound onto surfaces and metals is poor. Ultrasonic cleaning baths have been introduced in the middle of last century and became popular in many chemical and metallurgical labs for cleaning glassware and forming dispersions. Moreover, during ultrasonic treatment of solid surfaces, the resulting surface morphology is not initiated by purpose but happens in an unwanted way in most of the cases, thus making surface control difficult. So, the rules for the development of certain surface structures have to be found. For example, the oldest findings of cavitation erosion of the propellers
(Silberrad, 1912) initiated studies of bubble dynamics, but even after nearly a century of research, the mechanisms are not yet fully understood. Bubbles are created by high pressure gradients and large flow rates in the liquids near a solid surface. The collapse creates locally extreme pressures and temperatures as well as shock waves and a liquid yet impinging on the surface. This may not only reduce efficiency and lifetimes of turbines or pumps but also may be useful, for example, destroy kidney stones by lithotripsy (Delius et al., 1988). Ultrasonic frequencies range from 20 kHz to 1 GHz and it is known that the threshold for cavitation increases with frequency. Thus, the higher frequencies are typically used for ultrasonic diagnostics whereas the lower ones are for chemistry purposes. They encompass wavelengths between 10 cm and 1 μm and, hence, do not directly interact on the molecular level. The mechanism of bubble formation may be qualitatively understood as follows (Fig. 11.1). In the zone of negative pressure of the sound wave, the small volumes of lower liquid density or containing gas clusters may expand to form certain bubble nuclei. These do not fully collapse during the high pressure phase and are further expanded in the next low pressure phases. This process continues until reaching a maximum critical diameter depending on ultrasound frequency and solvent. In the following very quick cavitation collapse, the surface energy of the cavitation microbubble is converted into mechanical energy, chemical energy, heat, and, under certain conditions, light.
