less dependent on the degree of supersaturation than is homogeneous
nucleation. In some systems, copious secondary nucleation can occur in the
vicinity of a crystal of the new phase (this is not seeding: see below).
A finely dispersed material, like water in cells or oil in emulsion
droplets, generally needs far more undercooling for crystallization to occur
than for the same material in bulk. This is because of the limited number of
catalytic impurities: every cell or droplet has to contain at least one of these
for crystallization to occur (unless the temperature is low enough for
homogeneous nucleation), and the smaller the droplets, the smaller the
chance that such is the case. This implies that in O–W emulsions, for
FIGURE14.11 Scheme of the various mechanisms that can be involved in the
initiation of a new phase.