Why do phase changes need a nucleation site? : r/askscience

Saw a GIF of a plastic bottle of water undergoing a rapid phase, I guess it was supercooled. I'm just wondering why a specific nucleation site is required for this to start and not just the first part to drop below 0 degC act as the 'starting point'. Cheers! Freezing (just like any phase change) requires some degree of undercooling because ice formation would require a new ice–liquid interface, and surfaces cost energy because they leave some bonds unsatisfied relative to the bulk. This factor is important only for the tiniest clusters of the new phase because the ratio of surface area to volume is so large; once nucleation occurs, growth can follow with no problems. Thus, if there's already a defect present, nucleation may occur easily—within a cavity, for example—followed by regular growth. I discuss the thermodynamics and kinetics of freezing with much more math here. Is this effected by whether the solid forms a crystal structure or not? Thanks for the explanation! I'll give the li

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• Curtis Chong