EMORY (US)—When does water stop being water and start being ice? New research shows the fuzzy region between the two phases is extremely narrow and the transition actually happens very quickly.
The phenomenon has been captured in the lab for the first time by Eric Weeks, associate professor of physics at Emory University.
“The theory that surface waves move along the crystal/liquid boundary—the intrinsic interface—dates back to 1965 and is well established,” Weeks says. “What we’ve done is found a way to take a picture of the intrinsic interface, measure it, and show how it fluctuates over time.”
Because water molecules are too small to study while they are fluctuating, Weeks’ 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.
“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’s not the critical detail.”
The experiment took a great deal of trial and error, says lead author Jessica Hernández-Guzmán, a graduate student in physics. “I was looking for that transition,” she says. “I knew what the colloids looked like in a crystal state, and I knew what they looked like as a liquid, but I didn’t know what they looked like in-between. When I finally saw (the transition), I felt like I had won the lottery.”
The researchers confined samples of plastic spheres in wedge-shaped glass slides and loaded them onto a confocal microscope turned sideways, so that gravity gradually changed the concentration gradient. Rapid, three-dimensional digital scans were made to record the Brownian motion of the particles over one hour.
Algorithms were applied to the images to classify the degree of organization of each of the particles. The particles were then digitally colored: from dark blue for the most crystalline, to dark red for the most liquid. The series of images were stitched together and sped up, becoming microscopy movies that reveal the action along the crystal/liquid interface.
“You can watch as the boundary fluctuates,” Weeks says. “The yellow area along the bumpy line is liquid, but almost crystal. The light blue area is crystal, but almost liquid. The zone of confusion is less than two particles thick. By looking at the tiniest scale possible, we can see that the fuzzy region between the two areas is much smaller than we previously thought.”
The research was funded by the National Science Foundation Faculty Early Career Development Program. The lab’s data appears in the Proceedings of the National Academy of Sciences.
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