Acoustic cavitation phenomenon

High power ultrasound applied to an alkoxide/water mixture makes it possible to obtain nanostructured materials. The process is driven by the acoustic cavitation phenomenon. The cavities act as nanoreactors, where the hydrolysis reaction starts. The products (alcohol, water, and silanol) help continue the dissolution of that immiscible mixture. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Acoustic cavitation is empiricaiiy known to be induced much more efficientiy and repro-ducibiy in a standing wave fieid than in a progressive one aiso suspended particies exposed to an uitrasound standing wave are known to be driven by an axiai force to concentrate in nodai or antinodai pianes. This phenomenon has received substantiai attention in recent years. 

Sonochemistry (chemical events induced by exposure to ultrasound) occupies an important place in organic chemistry. The chemical effects of high-intensity ultrasound were extensively smdied in aqueous solutions for many years, but is now applied to a variety of organic solvents. The origin of sonochemistry is acoustic cavitation the creation, growth, and implosive collapse of gas vacuoles in solution by the sound field. Acoustic cavitation is the phenomenon by which intense ultrasonic waves induce the formation, oscillation, and implosion of gas... 

Sonochemistry is the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz-10 MHz) [4]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation. Let us first address the question of how 20 kHz radiation can rupture chemical bonds (the question is also related to 1 MHz radiation), and try to explain the role of a few parameters in determining the yield of a sonochemical reaction, and then describe the unique products obtained when ultrasound radiation is used in materials science. 

More complex transformations have been observed for crystalline silicon under acoustic cavitation in water sparged with Ar at temperatures of between 10 and 20°C [137]. Spectroscopic investigation reveals that Ar, which is bubbled continuously through the liquid phase, is ultrasonically excited via mechanolumi-nescence, i.e. light emission produced by mechanical action on the Si surface. This phenomenon also triggers physico-chemical transformations at the solid-liquid interface (Fig. 28), thus causing stress and defects as well as an increase in... 

Cavitation is a rather general term used to describe the rupture of a liquid when subjected to sufficiently ]arge negative pressures. Furthermore cavitation is a group of phenomena which are associated with the occurence of cavities or bubbles in liquids. From the practical point of view we know two main research areas of cavitation, namely the hydraulic cavitation and the acoustic cavitation. The hydraulic cavitation is an unwanted phenomenon and occurs in pumps, turbines, ship propellers and hydrofoils for example. 

While soundwaves are commonly used for communication through various media, some unique events occur in liquids when soundwaves interact with the medium. In particular, when ultrasound passes through a liquid medium, it strongly interacts with small gas bubbles that exist in the liquid. Such interaction between ultrasound and gas bubbles leads to a phenomenon known as acoustic cavitation [31, 32]. 

The possibility of using sound energy in chemistry was established more than 70 years ago. By definition, sonochemistry is the application of powerful ultrasound radiation (10 kHz to 20 kHz) to cause chemical changes to molecules. The physical phenomenon behind this process is acoustic cavitation. Typical processes that occur in sonochemistry are the creation, growth and collapse of a bubble. A typical laboratory setup for sonochemical reactions is shown in Fig. 8.17. More details of sonochemistry and the theory behind it can be found elsewhere. - ... 

The particular and interesting characteristics of ultrasound waves in chemical reactions arise from the physical phenomenon known as acoustic cavitation. Cavitation is the production, growth, and collapse of microbubbles in a liquid when a large negative pressure is applied to it [10,11]. As can be seen in Figure 3, the formation of cavitation bubbles is initiated during the rarefaction cycle. 

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