The Effects of Ultrasound on Two-phase Systems

With careful consideration of these factors, it is clearly possible to fine tune systems to maximise the efficiency of the cavitation process and an understanding of the effects of these parameters on sonochemical processes is clearly of help when attempting to optimize the rates and yields of these reactions. 

As previously explained, the effects of ultrasound on homogeneous systems are dominated by the enormous changes in pressure and temperature created at hot spots on implosion of the cavitation bubbles. However, in two-phase systems a number of other factors must be taken into consideration. The relative contributions of these phenomena have not been conclusively established in any one case. It appears that effects seen in liquid/liquid systems are principally due to emulsification which occurs when the shearing stresses on the liquid are greater than the interfacial surface tension. In a number of cases this enormous increase in surface area of 

There are also examples where the main effects of sonication arise from improved mass transport within the system. An example of this is the rate increases reported in electrolysis reactions [35]. The reason for this lies in the increase in momentum brought about as the solvent absorbs energy from the propagating sound wave. This phenomenon is independent of cavitation and is known as acoustic streaming . 

Luche and co-workers have undertaken a detailed investigation of the effects of ultrasound on lithium in the presence and absence of chemical reagents [38]. It could be postulated that the primary role of the ultrasound is to disperse the lithium removing small, highly activated particles from the metal. However, it has already been shown that this does not occur to any noticeable extent in THF [36], the solvent in which these investigations were carried out. Alternatively, cavitation must lead to modification of the metal 

Plate 3a-c. Scanning electron micrographs showing the reduction of particle size obtained on sonication of TaS2 a TaS2 before ultrasonic irradiation, b after 10 min irradiation, and c 120 min irradiation (Note change in scale) 

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