Effects of ultrasound on crystallization

Both types of US effects (namely physical, which facilitate mixing-homogenization, and chemical, resulting from radical formation through cavitation) influence crystallization by altering the principal variables involved in this physical process (namely induction period, supersaturation concentration and metastable zone width). These effects vary in strength with the nature of the US source and its location also, their influence is a function of the particular medium to which this form of energy is applied. 

This parameter (find) is defined as the time elapsed between the creation of supersaturation and the appearance of crystals, and decreases as supersaturation inoreases. Mathematioal equations for the induotion time that hold for all nuclei forming and growing in a saturated solution have been reported [138], The induction time is usually determined from oonduotivity measurements. Thus, the formation of crystals is signaled by a drop in the solution oonductivity. The crystallization time is taken to be the time where the derivative of the oonductivity with respect to time becomes negative. 

The induotion time is dramatically reduced by the presence of US the effect, however, depends on the particular medium and working conditions. Thus, at an absolute supersaturation of 0.0156 g K2S04/g water, the induction time in the absence and presence of US was found to be 9000 and 1000 s, respectively. Also, the conductivity decreased faster with US than without US. Because the conductivity was proportional to the potassium sulphate oonoentration, this difference suggests that more crystalline matter was formed in the presenoe of US [139]. 

The metastable zone width can also be reduced by the application of US. The apparent order of nucleation or growth is deoreased by US. Based on available evidence, the metastable zone width can be reduced simply by applying a low US power. Thus, US decreases the apparent order of the primary nucleation rate and increases the rate of appearance of the solid. Seemingly, US modifies the meohanism of nucleation itself as its presence strongly reduces the apparent order of nuoleation. 

The increase in the nucieation rate can be ascribed to the presence of shock waves in the soiution. in fact, the waves can raise the probabiiity of coiiision of a moiecuie and a moiecuiar aggregate. 

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