Cavitation homogeneous system, effects

In order to understand the way in which cavitational collapse can affect chemical transformations [5-7], one must consider the possible effects of this collapse in different systems. In the case of homogeneous systems, there are two major effects. 

The most pertinent effects of ultrasound in solid-liquid reactions are mechanical, which are attributed to symmetrical and/or asymmetrical cavitation. Symmetrical cavitation (the type encountered in homogeneous systems) leads to localized areas of high temperatures and pressures and also to shock waves that can create microscopic turbulence (Elder, 1959). As a result, mass transfer rates are considerably enhanced. For example, Hagenson and Doraiswamy (1998) observed a twofold increase in the intrinsic mass transfer coefficient in the reaction between benzyl chloride (liquid) and sodium sulfide (solid). In addition, a decrease in particle size and therefore an increase in the interfacial surface area appears to be a common feature of ultrasound-assisted solid-liquid reactions (Suslick et al., 1987 Ratoarinoro et al., 1992, 1995 Hagenson and Doraiswamy, 1998). 

As previously explained, the effects of ultrasound on homogeneous systems are dominated by the enormous changes in pressure and temperature created at hot spots on implosion of the cavitation bubbles. However, in two-phase systems a number of other factors must be taken into consideration. The relative contributions of these phenomena have not been conclusively established in any one case. It appears that effects seen in liquid/liquid systems are principally due to emulsification which occurs when the shearing stresses on the liquid are greater than the interfacial surface tension. In a number of cases this enormous increase in surface area of... 

Cavitation is a huge challenge for the service life of homogenization system due to the abrasion induced, but is also effective in disrupting emulsion droplets. 

In homogeneous liquid systems, sonochemical effects generally occur either inside the collapsing bubble, — where extreme conditions are produced — at the interface between the cavity and the bulk liquid —where the conditions are far less extreme — or in the bulk liquid immediately surrounding the bubble — where mechanical effects prevail. The inverse relationship proven between ultrasonically induced acceleration rate and the temperature in hydrolysis reactions under specific conditions has been ascribed to an increase in frequency of collisions between molecules caused by the rise in cavitation pressure gradient and temperature [92-94], and to a decrease in solvent vapour pressure with a fall in temperature in the system. This relationship entails a multivariate optimization of the target system, with special emphasis on the solvent when a mixed one is used [95-97]. Such a commonplace hydrolysis reaction as that of polysaccharides for the subsequent determination of their sugar composition, whether both catalysed or uncatalysed, has never been implemented under US assistance despite its wide industrial use [98]. 

One of the most important aspects inherent in sonochemistry concerns synthesis and treatment of organic and inorganic materials. Effects of ultrasound on chemical transformations were studied in three directions sonochemistry in homogeneous liquid system, sonochemistry in heterogeneous liquid-liquid or liquid-solid several times as well as sonocatalysis. Thus, the cavitation concentrates sound energy to affect the synthesis from soluble precursors. Chemical reactions are usually not observed in sonicated solid-solid and solid or gas systems. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. The extraordinary local temperatures and pressures so created result in both sonochemistry and sonoluminescence, which provide a unique means for fundamental studies of chemistry and physics under extreme conditions. The chemical consequences of acoustic cavitation are far reaching, both in homogeneous liquids and in mixed-phase system. In the latter, cavitation can have dramatic effects on the reactivities of both extended solid surfaces and on fine powder slurries through microjet and shock wave impact (on large surfaces) and interparticle collisions (with powders). The applications of sonochemistry are diverse and stiU emerging, especially in the areas of mixed phase synthesis, materials chemistry, and biomedical products. 

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