In
fluid physics, undercooling is the lowering of the
temperature of a
liquid beyond the
freezing temperature and still
maintaining a liquid form.
Normally, freezing occurs when the molecules of the container walls impose an
ordering on the liquid's molecules causing them to arrange themselves into a
crystalline structure and begin to grow.
Without the container, however, the onset of freezing
(nucleation) has a good chance of not happening.
But, at some lower temperature nucleation will
occur and very rapid freezing takes place
(recalescence).