Parameters Affecting Cavitation

Sonochemistry is strongly affected by a variety of external parameters, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, choice of ambient gas, and choice of solvent. These are important considerations in the effective use of ultrasound to influence chemical reactivity, and are also easily understandable in terms of the cavitational hot-spot mechanism. A summary of these effects is given in Table II. 

Sonication using ultrasonic cleaner baths remains a popular extraction approach particularly for controlled-release products. In sonication, an ultrasonic wave of 20-40 kHz generated by a piezoelectric transducer is used to produce the formation and collapse of thousands of microscopic bubbles (cavitations) in the water bath to facilitate the break up of the solid particles and the subsequent dissolution of the API. Note that parameters such as the wattage power of the sonicator, presence of the perforated tray, depth of the water level, bath temperature and the number of sample flasks sonicated might all affect the extraction rate. For... 

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. 

A series of fundamental studies on sonochemistry by Feng Ruo and his collaborators has been undertaken over a number of years. Their studies have focused on how the parameters of an ultrasonic field, such as sound intensity, frequency, shape of wave, etc., affect the cavitation yield which was detected by different methods. 

This is a simplification of the process occurring in a curing resin-hardener system and a detailed discussion may be found in Pascault et al (2002), Williams et al (1997) and Inoue (1995). The main parameter that it is important to control in the reactive phase separation is the diameter of the elastomer particle. This is because the toughness of the resulting network is controlled by the energy-absorbing mechanisms such as particle cavitation and rubber bridging of cracks. Also of importance is the limitation of the effect of the rubber dispersed phase on the critical properties of the cured epoxy resin such as the stiffness and Tg. This will be affected by the extent to which the rubber dissolves in the matrix-rich phase. 

Various known factors affecting cavitation in conventional-scale systems are altered in microscale systems, such as flow dimensionless parameters, component geometries, liquid contaminants (nuclei sources), and surface roughness and chemistry (surface energy). The differences arise from various causes including microfabrication constraints, typical choice of device materials, and the Reynolds number. Subsequent sections will discuss each effect in detail. 

Three main dimensionless parameters most likely dominating cavitation in microsystems are the cavitation (cr), Reynolds (Re), and Weber (We) numbers. Although early work on cavitation scaling [4] has suggested the We number as a possible parameter affecting cavitation, it has been found that nuclei together with viscous... 

Because of the extreme conditions during a cavitation event, radicals can be formed. Several parameters affect cavitation and thereby the polymerization reaction, since the radical formation rate is directly influenced by the cavitational collapse. The number of radicals formed due to sonification is a function of the number of cavities created and the number of radicals that are formed per cavitation bubble. The bubble wall velocity during collapse and the hot-spot temperature determine the rate at which radicals are formed, both inside and outside a single bubble. These two parameters depend on the physical properties of the liquid as well as on the physical and chemical processes occurring around the cavity. The most important properties and processes occurring in a cavitation bubble are depicted schematically in Figure 21.10. The number of cavities is determined, for instance, by the impurities in the liquid, the static pressure, the ultrasound intensity, and the vapor pressure. This emphasizes the complexity of the influences on the overall... 

Parameters Which Affect Cavitation 574 2.6 Triazole Derivatives 593... 

The different processes that take place during cavitation, like the growth of the bubbles and their corresponding collapse, can be affected by various parameters the most significant experimental variables are frequency, solvent viscosity, solvent surface tension, solvent vapor pressure, bubbled gas, external (applied) pressure, temperature, and intensity Figure 4 offers a brief description of each of them. 

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