When ice melts, the process usually starts around defects or near the surface, where the crystalline structure is relatively easily transformed into liquid water. Without such encouragement, spontaneous nucleation must take place inside homogenous bulk ice by thermal agitation, but the true mechanism of this phase transition at the molecular level is not well understood. Kenji Mochizuki and colleagues now present molecular dynamics simulations that identify the spatial separation of a defect pair into its constituent components as the crucial step. The initial formation of defect pairs occurs easily and often, but only after accidental disruption of the surrounding hydrogen-bond network and pair separation can defects persist and grow, rapidly turning ice into liquid water.
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