A New View of Phase Transitions

Vizualizing how matter changes from one state to another, on an atomic level

The Heisenberg Uncertainty Principle states that we can’t know both the position and velocity of the smallest particles at the same time, making it impossible to track an electron’s trajectory. Even if there were a way around this, science does not have the experimental tools necessary to visualize atomic motions in real time; they’re just too small.

Because of these limitations, little is known about the pathways of atomic reorganization during phase transitions, such as water freezing into ice or evaporating into mist. Researchers at Duke, however, are catching glimpses of how atoms likely behave thanks to model systems at a large enough scale that they can be visualized with an optical microscope.

In a recent study published in Soft Matter, Benjamin Yellen, associate professor of mechanical engineering and materials science at Duke University, and his colleagues, Josh Socolar and Patrick Charbonneau in Duke’s physics and chemistry departments, respectively, use a single layer of microscopic colloidal particles suspended in a thin liquid medium to watch phase transitions in action. By varying the strength and direction of an external magnetic field, Yellen and colleagues can adjust the strength and direction of the particle interactions, closely approximating the bonding interactions at work on the atomic scale.

“This allows us to ‘melt’ the crystalline structures that the particles have formed by turning the magnetic field off, and then to ‘freeze’ them again by increasing the field strength,” said Yellen. “It’s a nice system for doing these types of studies because we have the ability to control the field in a manner that’s similar to being able to control the temperature.”

Besides watching crystalline structures grow and anneal, Yellen and his graduate students Ye Yang, Lin Fu and Catherine Marcoux used the technique to create the first-ever visualization of how particles in a compound alloy behave during a martensitic phase transition. Martensitic phase transitions occur when heated materials cool rapidly, causing certain patterns of crystalline deformation. These phase transitions play a role in shape memory materials, refrigeration materials, and are even used to create harder forms of tempered steel. 

Yellen hopes that being able to see how atoms move during phase transitions will allow scientists to understand these transformations at a deeper level, opening new doors for tuning material properties far beyond steel.

“People have speculated that martensitic transformations could occur in various ways and they have performed computational models to predict what would happen, but they’d never seen it before,” said Yellen. “Perhaps the ability to watch these transformations in our model system may help materials scientists learn how these microscopic motions relate to the diffraction patterns they observe.”

This work was supported by the Triangle Materials Science and Research Engineering Center.