Academic journal article The Science Teacher

Crystal-Liquid Interface

Academic journal article The Science Teacher

Crystal-Liquid Interface

Article excerpt

"Imagine you are a water molecule in a glass of ice water, and you are floating right on the boundary of the ice and the water," proposes Emory University physicist Eric Weeks. "So how do you know if you are a solid or a liquid?"

Weeks's lab recently captured the first images of what is actually happening in this fuzzy area of the crystal-liquid interface. The lab's data, published in the Proceedings of the National Academy of Sciences (PNAS), make the waves between the two states of matter visible for the first time. "The theory that surface waves move along the crystal-liquid boundary--the intrinsic interface--dates back to 1965 and is well established," says Weeks, associate professor of physics. "What we have done is found a way to take a picture of the intrinsic interface, measure it, and show how it fluctuates over time."

The visual evidence shows that the fuzzy region between the two states is extremely narrow, Weeks says. "The transition from completely organized to completely disorganized goes very quickly, spatially." Weeks's lab uses tiny plastic balls, each about the size of a cell nucleus, to model states of matter. Samples of these colloids can be fine-tuned into liquid or crystal states by changing the concentrations of the particles suspended in a solution.

"Water molecules are too small to study while they are fluctuating," Weeks explains. "We used the plastic spheres to resize an experiment to a scale that we could observe. You lose some of the detail when you do this, but you hope it is not the critical detail."

The experiment took a great deal of trial and error, says Jessica Hernandez-Guzman, a graduate student in physics and the lead author of the PNAS article. …

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