Changing States? It's Complicated

Skatebiker via Wikimedia commons. When copper melts the process is more complex than we realised

The process of changing states of matter, for example from solid to liquid, is not as simple as previously thought, according to a paper in Science

It all seems so simple in high school science classes. Apply heat to the solid and eventually “a change in degree becomes a change in state”. The bonds between the atoms or molecules are weakened to the point where the lattice breaks apart to become a liquid or, under sufficiently low pressures, a gas.

However, it turns out that during these transitions, materials can spend time in metastable states. Dr. Amit Samanta of the Lawrence Livermore National Laboratory found that when melting copper and aluminum, the process “occurs via multiple, competing pathways involving the formation and migration of point defects or dislocations.”

Co-author Professor Mark Tuckerman of New York University says the idea of multiple pathways, “is counter to what we've previously known.” One previously unrecognized pathway involves the formation of point defects, small errors in the crystalline lattice. Surprisingly, these defects can move randomly within the lattice. Sometimes, the team found, multiple defects combine to form what they call “disordered defect clusters.”

"The disordered cluster grows from the outside in rather than from the inside out, as current explanations suggest," Tuckerman says. "Over time, these clusters grow and eventually become sufficiently large to cause the transition from solid to liquid."

A different pathway sees defects grow into lines of disorder that the researchers call “dislocations." Liquids run along these dislocations, expanding until the entire solid transitions to liquid.

The findings were made as a result of efforts by Tuckerman and colleagues to produce computer models for complex systems, using phase changes as examples. To explore the models properly, they needed to see what happened in reality, and paired up with Samanta. Having found that even pure metals behave in ways that are vastly more tangled than anyone had thought, the collaborators want to see what goes on when ice turns to water.

Simply by virtue of being a molecule of two elements, rather than one, ice has an added level of complexity. Moreover, being one of the very few solids that contract when melting (the reason icebergs float), ice can be expected to have a particularly interesting story to tell.

“Our findings reveal the importance of nonlocal behavior, suggesting a revision of the perspective of classical nucleation theory,” Amit and his coauthors conclude. Tuckerman adds, “Phase transitions have always been something of a mystery because they represent such a dramatic change in the state of matter."


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