Intrinsic cavitation

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

Gent proposed a quite different model for craze nucleation. The local hydrostatic stress Go (concentrated by the presence of flaws) was supposed to decrease Tg of the glass to the ambient temperature. Upon reaching the rubbery state, the polymer will cavitate easily to form voids. The main problem with this mechanism is the large stress concentration factors that are necessary for it to operate at room temperature it also cannot easily account for the time dependence of craze nucleation under constant stress. It provides a possible explanation, however, for the nucleation of crazes II In view of the fact that the work of Kausch and Dettenmaier on craze nucleation and intrinsic crazing will be treated in detail in Chapter 2, it will not be discussed further here... 

There is much evidence that the intrinsic crazing of PC is related to the existence of a general mode of cavitational plasticity in glassy polymers. Examples have been given for polymers which behave very similar to PC. In particular PMMA and pre-oriented PS exhibit an almost identical behavior when stretched at high stresses and strains in a temperature region close to Tg. 

The author is well aware of the fact that many aspects which have been treated in the extensive literature on extrinsic crazing have not been considered in this article and that more information is needed for a comprehensive account of the observed craze phenomenon. For instance the recent work on the intrinsic crazing of PC and on related phenomena which has been re wed here has primarily been based on structural considerations. It is believed that future work on the kinetics of craze formation and on the underlying molecular dynamics of the system may contribute considerably to a more detailed account of this phenomenon. Nevertheless, it is hoped that this work has opened up some new paths which may lead to a better understanding of the phenomenon of cavitational plasticity in polymers. 

The hrst point concerns the intrinsic properties of the materials constituting the blends. It is evident that, for the blends under consideration, we have mixed PP that suffers very high cavitation with increasing amounts of PA6 that deforms plastically with particularly low volume strain. Although the PA6 phase does not support the same amount of strain in the blends as the PP matrix (PA6 particles has shown little deformation in the micrographs), it is presumable that the lower tendency to cavitation of the blend is partly controlled by the more isochoric nature of the PA6 component. 

In any medium, cavities, voids, and density fluctuations exist. It is believed that these induce cavitation, leading to molecular rupture. In solid polymers, the microvoids present intrinsically are responsible for cavitation when they are subjected to a hydrostatic pressure in the manner of an impulse. One of the main causes of microvoid generation in polymer materials is the interatomic bond rupture when they are subjected to mechanical and thermal stresses. Extensive studies showing microvoid formation in stressed polymers have been carried out (Zhurkov et al., 1972). 

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). 

ParametBrs Influencing Cavitation. The intrinsic rate of a polymerization reaction is determined by the initiation, propagation, and termination rate. However, only the initiation rate is influenced by ultrasound. The amoimt of... 

At negative pressure, these scenarios differ with respect to the shapes of the LDM and of the liquid-vapor spinodal curve Ps T) scenario (i) predicts a monotonic LDM and a minimum of Ps as a function of temperature, whereas scenarios (ii) and (iii) predict a turning point in the LDM and a monotonic spinodal. In an experiment, it is difficult to reach the spinodal rather, the liquid will break before by nucleation of vapor bubbles (cavitation). Usually, impurities favor heterogeneous nucleation, and lead to irreproducible results. But for a pristine system, nucleation will occur homogeneously, at a well-defined pressure threshold / cav(T), which is an intrinsic property of the liquid. 

This approach was continued in a series of internal reports one of which,issued in 1973 (38), explored the conditions governing the formation of the two types of boundary separating the full film from the cavitated regions within a hydro-dynamic or a hydrostatic bearing. These were known as the reformation and cavitation boundaries respectively. The former has both form and position and depends on the transportation of fluid in bulk. Cavitation boundaries on the other hand are essentially special, are dependent on pressure and have no intrinsic velocity. 

For the development of sustainable polymer processes, ultrasound is an interesting technology, as it allows for polymerizations without the use of initiator. The radicals are generated in situ by cavitation events [116, 117], which make possible a dean and intrinsically safe polymerization reaction. As a result of the high strain rates outside the bubble, cavitation can also induce chain scission [118,119], which provides an additional means to control the molecular weight of the polymer produced. In Sections 21.3.1 and 21.3.2 the physical background of ultrasound-induced cavitation and radical formation will be described. Subsequently (see Section 21.3.3), an overview of the several types of ultrasound-induced polymerizations will be given, namely bulk, predpitation, and emulsion polymerization. 

Here, we note only that the properties computed from these models are extremely sensitive to how the molecular cavity (solute/continuum interface) is constructed, and that nonelectrostatic solvation effects are typically (though not always ) neglected in the PCMs such as COSMO, GCOSMO, lEF-PCM, and SS(V)PE that are derived from Poisson s equation for continuum electrostatics. In contrast, such effects are built into the empirical SMx models and are crucial to accurate prediction of solvation free energies. " It is unclear how the neglect of nonelectrostatic effects might impact the calculation of anion VDEs cavitation effect should cancel, but dispersion effects may not, as the anion is intrinsically more polarizable. One may hope that these effects will disappear if a sufficiently large number of explicit solvent molecules is included as part of the QM solute. 

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