Cavitation threshold

Electroorganic synthesis will be covered in section 4.5.4. It is appropriate, however, to make a reference here to the role of u/s in electroorganic processes. Atobe et al. (2000) have reported the effect of u/s in the reduction of acrylonitrile and mixtures of acrylonitrile and methyl acrylate. The selectivity for adiponitrile in the reduction of acrylonitrile was significantly increased under u/s irradiation with a power intensity over the u/s cavitation threshold ( 600 cm ). This favourable influence of u/s can be attributed to the improved mass transfer of acrylonitrile to the electrode interface by the cavitational high-speed jet-stream. 

Cavitation threshold, Intensity of cavitation, rate of chemical reaction Transient threshold Size of the nuclei (cavitation threshold)... 

Choice of liquid Vapor pressure Surface tension Viscosity Chemical reactivity Intensity of collapse Transient cavitation threshold Transient cavitation threshold Primary or secondary sonochemistry... 

Bubble Formation and the Factors Affecting Cavitation Threshold... 

It has also been found that the presence of particulate matter, and more especially the occurrence of trapped vapour-gas nuclei in the crevices and recesses of these particles, also lowers the cavitation threshold. The way in which nucleation occurs at these sites (and from similar sites on the vessel walls) is shown in Fig. 2.10. 

The final factor to be considered here, and known to affect the cavitation threshold, is the temperature. In general, the threshold limit has been found to increase with decrease in temperature. This may in part be due to increases in either the surface tension (a) or viscosity (rj) of the liquid as the temperature decreases, or it may be due to the decreases in the liquid vapour pressure (P ). To best understand how these parameters (a, q, Py) affect the cavitation threshold, let us consider an isolated bubble, of radius Rq, in water at a hydrostatic pressure (Pjj) of 1 atm. 

Let us now consider the effect of solvent viscosity on the cavitation threshold. According to Tab. 2.1, an increase in the solvent viscosity required the application of a... 

Increasing the external pressure (Pjj) leads to an increase in both the cavitation threshold and the intensity of bubble collapse. Qualitatively it can be assumed that there will no longer be a resultant negative pressure phase of the sound wave (since Pj, — > 0) and so cavitation bubbles cannot be created. Clearly, a sufficiently large... 

The first requirement for the level of ultrasonic power required to cause chemical effects in a reaction is that sufficient acoustic energy must be supplied to overcome the cavitation threshold of the medium. Once this has been exceeded then the region of... 

Whilst vapour pressure may be the major solvent factor involved in the degradation process, there could also be a contribution from solvent viscosity or even, yet less likely, from surface tension. It has already been argued (see Section 2.6.2) that although an increase in viscosity raises the cavitation threshold, (i. e. makes cavitation more difficult), provided cavitation occurs, the pressure effects resulting from bubble collapse... 

In Chapter 2 we explained why there existed a cavitation threshold i. e. a limit of sound intensity below which cavitation could not be produced in a liquid. We suggested that only when the applied acoustic amplitude (P ) of the ultrasonic wave was sufficiently large to overcome the cohesive forces within the liquid could the liquid be tom apart and produce cavitation bubbles. If degradation is due to cavitation then it is expected that degradation will only occur when the cavitation threshold is exceeded. This is confirmed by Weissler who investigated the degradation of hydroxycellulose and observed that the start of degradation coincided with the onset of cavitation (Fig. 5.21). 

Fig. 5.37 also implies that there exists a cavitation threshold for this system, since polymerisation only occurs when the intensity is greater than 2 W cm. This is in agreement with earlier work by Lindstrom and Lamm and ourselves. 

Acoustic intensity has a dramatic influence on the observed rates of sonochemical reactions. Below a threshold value, the amplitude of the sound field is too small to induce nucleation or bubble growth. Above the cavitation threshold, increased intensity of irradiation (from an immersion horn, for example) will increase the effective volume of the zone of liquid which will cavitate, and thus, increase the observed sonochemical rate. 

There is other acoustical evidence to support the belief of Sirotyuk and other investigators that stable microbubbles serve as cavitation nuclei in fresh water. As noted by Sirotyuk (ref. 25), numerous experiments have disclosed that the cavitation threshold of water is increased by degassing of the liquid or by the... 

COMPARISON OF CAVITATION THRESHOLDS FOR AGAROSE GELS AND VERTEBRATE TISSUES... 

Solvent properties affect US-assisted digestion, as they impose a cavitation threshold above which sonochemical effects are perceived, as it were, by the medium. Therefore, any phenomenon altering the same solvent property can modify such a threshold. 

For the same reason as above, excess solvent molecules in the cavitation bubble also seriously limit the applicability of many volatile organic solvents as a medium for sonochemical reactions [2,25,26]. In fact, water becomes a unique solvent in many cases, combining its low vapor pressure, high surface tension, and viscosity with a high yield of active radical output in solution. Its higher cavitation threshold results in subsequently higher final temperatures and pressures upon bubble collapse. Most environmental remediation problems deal with aqueous solutions, whereas organic solvents are mostly used in synthesis and polymer modifications processes. 

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