Imagine bringing that bathtub to the brink of freezing. A little bit one way or the other and the whole thing freezes or liquefies. But right on the cusp, blocks of ice coexist with pockets of liquid. The coexistence of two phases, on the edge of a phase transition, is called phase separation. The phases break apart—but stay connected. The connection between the two phases takes the form of a balanced cycling back and forth: Molecules in ice patches melt into adjacent pools of liquid. Molecules of liquid swimming by an ice patch lock onto a surface and freeze. That cycling, in which neither
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