The formation of a hole which permits coalescence or direct contact of the two phases which it separates.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 613 (https://doi.org/10.1351/pac197231040577)