Bubble nucleation from gas cavities - a review

This review is concerned with the nucleation of bubbles in solutions supers aturated with a gas, in particular the bubble nucleation that occurs at spe cific sites, as a cycle. A classification system for the kinds of nucleatio n that occur is defined and discussed in order to place this specific form of nucleation into a better defined context. It is noted that in the absenc e of pre-existing gas cavities, bubble nucleation requires exceedingly high levels of supersaturation. It is argued that the nucleation observed in mo st instances, which is often at low levels of supersaturation of 5 or less, is invariably associated with the existence of metastable gas cavities in the walls of the container or the solution bulk, prior to the system being made supersaturated. Here, the nucleation energy barrier for each gas cavit y is very much lower than for the classical case, given that less interfaci al free energy is needed for the cavity to grow to the critical size when t he system is made supersaturated. Once a system contains gas cavities with radii of curvature greater than the critical nucleation radius, bubbles are produced in a steady fashion without the need to scale a nucleation energy barrier. This non-classical form of nucleation is the main focus of the pa per. Issues concerning the formation of these gas filled cavities, and thei r stability are examined. (C) 1999 Elsevier Science B.V. All rights reserve d.