The Various Sites for Sonochemical Reactions

In order to understand the way in which cavitational collapse can effect chemical processes one must consider the possible effects of this collapse in different types of liquid system. 

The sudden collapse of the bubble also results in an inrush of liquid to fill the void. So powerful is this inrush that it will produce shear forces in the surrounding bulk 

It is cavitation in a heterogeneous medium which is the most studied by sonoche-mists. When produced next to a phase interface, cavitation bubbles are strongly deformed. A liquid jet propagates across the bubble towards the interface at a velocity estimated to hundreds of metres per second. At a liquid-liquid interface, the intense movement produces a mutual injection of droplets of one liquid into the other one, i. e. an emulsion (Fig. 3.3). Such emulsions, generated through sonication, are smaller in size and more stable than those obtained conventionally and often require little or no surfactant to maintain stability. It can be anticipated therefore that Phase Transfer Catalysed (PTC) reactions will be improved by sonication. Examples are provided later in this chapter. 

In the case of solid interfaces which are in the form of coarse powders, cavitation collapse can produce enough energy to cause fragmentation and activation through surface area increase. For very fine powders the partides are accelerated to high velocity by cavitational collapse and may collide to cause surface abrasion (Fig. 3.5). For some metal powders these collisions generate sufficient heat to cause particle fusion. 

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