Effect of Gas and Particulate Matter

It has also been found that the presence of particulate matter, and more especially the occurrence of trapped vapour-gas nuclei in the crevices and recesses of these particles, also lowers the cavitation threshold. The way in which nucleation occurs at these sites (and from similar sites on the vessel walls) is shown in Fig. 2.10. 

During the rarefaction cycle of the acoustic wave, as the pressure in the liquid decreases the liquid gas interface becomes increasingly more convex, its angle of contact decreases, until, at sufficiently low pressure it breaks away from the surface to produce a bubble of radius, Rj. 

Attempts to totally remove all particulate matter (i. e. ultrafiltration) have not been completely successful in that the theoretical limit for water s tensile strength ( 1500 atm) has not been achieved. One of the largest experimental threshold values 200 atm) is that found by Greenspan [11]. 

In the absence of all moieties (i. e. gas or particulate matter) it is possible to deduce an equation relating the critical pressure (P ) which must be exceeded to create a bubble (or void) of radius in an ultrapure liquid (Eq. 2.18). Provided vapour pressure is neglected then  

The fact that the pressure is negative implies that a negative pressure must be applied to overcome the cohesive forces of a liquid. Those readers interested in the derivation of this equation should turn to Appendix 2. 

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