Factors influencing cavitation

This section examines variabies influencing the cavitation phenomenon in a manner that can be adjusted to fuifii specific purposes. 

The presence of particulate matter — especially that of trapped vapour gas nuclei in their crevices and recesses — has also been found to lower the cavitation threshold. This is required under these conditions are of paramount importance in the clinical field, where ultrasound Is widely used in applications such as high-intensity focused ultrasound (HIFU), 

Cavitation requires the formation of a liquid-gas interface. Thus, one might expect the use of a solvent of low surface energy per unit area to lower the cavitation threshold. Although the phenomenon is not as simple as it may seem, the addition of a surfactant to an aqueous solution oertainly facilitates cavitation. 

Cavitation bubbles do not enclose a void. During the expansion phase of cavitation bubble generation, vapour from the surrounding liquid will permeate the interface. This produces 

The sonication intensity is directly proportional to the square of the vibration amplitude of the ultrasonic source. As a rule, increasing the intensity increases the sonoohemioal effeots however, the ultrasonic energy a system can take is limited. Thus, cavitation bubbles, which are initially difficult to create at the higher frequencies as a result of the shorter duration of rarefaction cycles, are now possible by virtue of the collapse time, temperature and pressure on collapse being mutually dependent. However, the sonication intensity cannot be increased indefinitely as the maximum possible bubble size is also dependent on the pressure amplitude. As the pressure amplitude is increased, bubbles may grow so large on rarefaction that the time available for collapse will be inadequate. In fact, it has been unequivocally established that  

---

The factors influencing cavitation are the solution s viscosity, surface tension, vapor pressure, and the presence of contaminants. In addition, the applied forcing frequency and amplitude can contribute to the efficiency of cavitation. 

These questions are not easily answered because of the large number of factors to be considered. Thus, optimizing the sonication conditions entails carefully examining the variables influencing cavitation, namely ... 

Studies were made of chemical and physical factors influencing time dependent near-bond failure in NR/steel bonded joints. Chemical studies revealed no evidence to indicate that chemical modifications were substantially weakening the rubber adjacent to the bond. Video observations suggested that a cavitation-like process, probably arising from dilatational components in the stresses near an interface, could lead to time dependent mechanical failure near the bond. 10 refs. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

It should be noted that acoustic irradiation is a mechanical energy (no quantum), which is transformed to thermal energy. Contrary to photochemical processes, this energy is not absorbed by molecules. Due to the extensive range of cavitation frequencies, many reactions are not well reproducible. Therefore, each publication related to the use of US generally contains a detailed description of equipment (dimensions, frequency used, intensity of US, etc.) [709]. Sonochemical reactions are usually marked )))), in accordance with internationally accepted usage [708], For successful application of US, the influence of various factors can be summarized as follows [710] ... 

The observed results shown in Figures 7 and 8 are in general agreement with the predictions of Buknall and Drinkwater (8). They suggested, based on the influence of stress on the creep behavior of ABS polymers that crazing (cavitation) should be the dominant factor under impact conditions. Their predictions were based on low strain (< 5% ) observations and are clearly substantiated by the data shown in Figures 7 and 8. 

TAME, DIPE and ETBE were investigated in the study by Kim et al. [ 109]. They showed similar elimination rates. If pyrolysis were the main elimination mechanism, the volatility and thus the partitioning into the gas phase during cavitation would be of much influence. MTBE, ETBE, TAME and DIPE have different vapor pressures (345, 183, 105, 222mbar, respectively [109]), however, under the studies conditions no decisive difference in ehmination rates could be observed—another hint for the predominance of the radical-induced elimination path over direct pyrolysis. The addition of ozone to the reaction mixture resulted in an enhancement of the elimination rate for ETBE by the factor of 1.5 and for TAME of 2.1 [110]. The mixture of different ethers proved to yield a slower reduction rate than the ethers alone. 

In addition to the important factors previously considered which influence sono-chemical reactions, a few others need to be considered as well. Among these are surface tension, viscosity, and solubility. The viscosity of a liquid increases as the pressure is increased or the temperature is decreased. Solvents with higher viscosity require higher amplitudes (or power) for cavitation to occur. In other words, cavitation becomes difficult to induce in high viscosity liquids. This is a situation that enhances the ultrasonic effect, and hence higher viscosities should normally lead to greater rate enhancements. 

The values given in Table 1 refer to the cavitation characteristics of the pure solvents. When reagents are present, other factors may have some importance, such as the structure of the solution or diffusion effects. The temperature at which all the factors have a balanced influence corresponds to the optimum, which does not necessarily coincide with Tmax- The indications given above... 

Pawlak A, Galeski A (2005) Plastic deformation of crystalline polymers the role of cavitation and crystal plasticity. Macromolecules 38 9688-9697 Peterlin A (1971) Molecular model of drawing polyethylene and polypropylene. J Mater Sci 6 490 Popli R, Mandelkem L (1987) Influence of structural and morphological factors on the mechanical properties of the polyethylenes. J Polym Sci B Polym Phys 25 441 Read D, Duckett R, Sweeny J, Mcleish T (1999) The chevron folding instability in thermoplastic elastomers and other layered material. J Phys D Appl Phys 32 2087-2099 Resconi L, Cavallo L, Fait A, Piemontesi F (2000) Selectivity in propene polymerization with metallocene catalysts. Chem Rev 100 1253... 

---