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The ultrasound wave is a longitudial wave (P wave) which propagates parallel to its vibration direction (see fig. below). As ultrasonic wave passes through a homogeneous liquid medium, molecules rapidly vibrate from their statistically averaged positions, resulting in alternate occurrence of compressional and rarefactional cycles. While the molecules become denser at the compressional cycle, they become sparse at the rarefactional cycle. Where the distance of these molecules become too large to remain in a liquid state any longer, cavities form very rapidly.
As the expansion rate of cavity at the rarefactional cycle exceeds its contractional rate at the compressional cycle, cavities in the bulk solution continue to grow during the rarefactional cycle. This phenomenon is called 'cavitation.' However, the cavities collapse at compressional cycle because it is not easy for them to overcome high pressured condition. It takes place via three stages: seed-forming stage (or nucleation stage), cavity-growing stage, and cavity-collapsing stage. The cavity tends to collapse symmetrically and spherally in homogeneous liquid, whereas it collapses nonsymmetrically and nonspherally in nonhomogeneous liquid.
Cavitation Bubble Maximum Cavity Bubbles collapse in New bubble
growth compression cycle growth
It is known that cavitation is governed by various factors, such as acoustic frequency and amplitude, temperature, pH and viscosity of solution, and type and concentration of dissolved gases. There are two different types of cavitation: transient cavitation (innertial cavitation) and stable cavitation (non-innertial cavitation). The transient cavitation is characterized by complete destruction of cavities. This phenomenon is known to take place when ultrasonic power is greater than 10W/cm2. On the other hand, the stable cavitation takes place by relatively low ultrasonic energy. Therefore, cavities are not fully destroyed but split into many small cavities which become seeds for new cavities.
Unusually high temperature and pressure are developed at/near the surface of each growing cavity when it is destroyed. At the same time, extremely high power is released shortly and this power can be an energy source for boosting chemical reacions at/near the surface. However, as the cavity destruction takes place in nano seconds and is limited only to such a very narrow zone (film) at/near the cavity surfaces, such high heat and power energy generated by the cavity destruction cannot be transferred to bulk solution. Therefore, it seems reasonable that this can be regarded as an adiapatic change. Unusually high temperature and pressure at the exploding cavity surface raise the kinetic energy of reactants, activate reactants, and also boost reaction rates due to better mixing effect. Technically, a minimum pressure required for cavitation to take place in the water is about 1,500 atm. However, this pressure could be further lowered to 20 atm where plenty of gas bubbles exist or where the surface extension of bulk solution is significantly lowered for any reason(s).
When very fine particles are abundant in the solution, cavitation markedly increases the number of collision between particles, enhancing their inherent reaction rates. Immobile reactants or products adhered to cavity surfaces can be effectively removed from the surfaces and (re)mobilized by cavitation. As a result, both overall reaction rate and product yield tend to increase.
Some benefits expected from cavitation are summarized as below:
1) chemical reactions can be accelerated
2) the initiation of chemical reactions becomes shorter and easier if the reactions are exothermic
3) organic matters can be decomposed very rapidly and effectively
4) lower grade reagents than those used by traditional methods could be used for the same purpose
5) initiating agents are not necessary for synthesizing reactions