Cavitation/implosion mechanism

Cavitation/implosion is the most effective mechanism known in ultrasonic cleaning. Cavity formation needs a negative pressure that is generated in the incompressible fluid. The isothermal compressibility of CO2 in the conditions of this experiment is about 2-4 orders higher than that of water at room temperature (Table 2). In other words, CO2 at the current experimental conditions seemed to be no longer an incompressible fluid nor did it induce cavitation/implosion. 

Cavitation per se is not a form of corrosion however, it is closely related. Cavitation is mechanical in nature. It is the mechanical removal of material resulting from the implosion of vapor bubbles in the liquid. In so doing the protective film is removed which permits corrosion to take place, primarily in the form of pitting. There is also the possibility that corrosion will promote cavitation damage by dissolving a matrix phase that holds hard particles. Once these hard particles are loosened cavitation can take place. 

Cavitation has three negative side effects in valves—noise and vibration, material removal, and reduced flow. The bubble-collapse process is a violent asymmetrical implosion that forms a high-speed microjet and induces pressure waves in the fluid. This hydrodynamic noise and the mechanical vibration that it can produce are far stronger than other noise-generation sources in liquid flows. If implosions occur adjacent to a solid component, minute pieces of material can be removed, which, over time, will leave a rough, cinderlike surface. 

The second mechanism proposed for aerosol generation is based on the piezoelectric crystal operating at low frequency and imparting vibrations to the bulk liquid. This results in the formation of cavitation bubbles, which move to the air-liquid interface.The internal pressure within the bubbles equilibrates with that of the atmosphere, causing their implosion. When this occurs at the liquid surface, portions of the liquid break free from the turbulent bulk liquid, resulting in droplet formation. The dependence of atomization on cavitation phenomena has been demonstrated for frequencies between 0.5 and 2.0 MHz.Boguslavskii and Eknadiosyants combined these theories with-their proposal that droplet formation resulted from capillary waves initiated and driven by cavitation bubbles. 

Much of the work done in recent years on polymer mechanochemistry has made use of the high elongational strain rates observed around collapsing cavitation bubbles in sonicated solutions, as outlined in the section on mechanoluminescence [27]. In addition to the distinctive features of sonochemically-induced mechanical reactivity described above, further attention needs to be paid to the sonication conditions in the case of mechanochemical catalysis, because catalyst lifetime and turnover number are reduced by sonochemical byproducts. Implosion of cavitation bubbles is essentially an adiabatic process which leads to formation of local hotspots within the bubble in which temperature and pressure increases drastically. The content of cavitation bubbles pyrolyses under these extreme conditions and results in formation of reactive species, such as radicals and persistent secondary byproducts acidic byproducts may also form from the degradation of the substrates [75]. Chemical impurities deactivate the reactive catalyst partially if not completely. Recent studies in our group have shown that heat capacity of gas... 

According to the depassivation-repassivation mechanism the metal loss is due to an oxidation reaction. The implosion of the cavitation bubbles on a passive surface... 

Cavitation corrosion or cavitation-erosion is associated with the implosion of bubbles in a corrosive environment. In a negative pressure zone caused by high velocity the local pressure falls below the vapor pressure and forms bubbles. The bubbles spread in the liquid and implode in higher pressure zones on the metal surface. Material cavitation consists of a complex combination of mechanical stressing and electrochemical reaction ... 

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