Homogeneous vs heterogeneous nucleation, a comparison

Vapor bubbles of sufficient size to serve as effective acoustic scatterers or cavitation nuclei may form in materials having regions of very low interfacial tension when exposed to an acoustic wave. These regions may be simple “hot spots,” i.e., collections of molecules having higher-than-average kinetic energies which therefore have lower surface tension than is found a room temperature, or the interfaces between dissimilar materials, e.g., the lipid coating on a microdroplet of liquid perfluourocarbon used as an ultrasound contrast agent. In addition, the curvature of such interfaces can act to focus an impinging acoustic beam in such a way as to increase its rarefactional pressure above that of the incoming wave. Another factor not often considered is that any foreign material, whether dissolved liquid or minute solid particulate, can act to disrupt the normal molecular structure of the liquid, thereby providing local regions of reduced interfacial tension. The energetics of the nucleation process shows that the rate of nucleation events is highly sensitive to the instantaneous temperature and moderately sensitive to both the magnitude and the duration of the rarefactional pressure of the acoustic wave. These and additional factors influencing nucleation processes will be discussed.