This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The most common scientific approach for thinking about how condensation, freezing and other phase transitions begin is based in classical nucleation theory, which was developed about a century ago. Thousands of experiments have supported a key equation describing how initial ordered seeds, called nuclei, form within disordered matter.
Now, new UCLA-led research published in Nature Materials proposes a revision to classical nucleation theory. The investigators used tiny spheres, or nanoparticles, made of complex materials with varying degrees of atomic order. They devised a method for trapping crystal nuclei at various stages of development during a phase transition.
When the team examined the nanoparticles using an imaging method that maps individual atoms in 3D, what they saw differed from current explanations of the nucleation process. "The crystals were not uniform with a sharp boundary from the disordered atoms around them, as predicted by classical nucleation theory," said corresponding author Jianwei "John" Miao, a professor of physics and astronomy in the UCLA College and member of the California NanoSystems Institute at UCLA. "Instead, we saw a gradient.
Every nucleus had a core of highest crystallinity and then became more disordered as you go from that core to the boundary." The researchers introduced a more complex equation, which they call the gradient nucleation pathways model. It generalizes classical nucleation theory in line with the results of their rigorous experiments. "It seems that the classical theory is actually a special case," Miao said.
"If you substitute that case into our equation, then you get exactly the same results as classical nucleation theory. But experimentally, we never observed a sharp boundary. Instead, we observed this gradient at the atomic scale." The findings represent a new understanding of a fundamental process of nature.
And because nucleation is seen in so many different systems—from droplets forming in clouds to industrial manufacturing—the study could inform further discoveries and new technologies with broad societal impact. The nanoparticles were made up of high- and medium-entropy alloys, combinations of metallic elements in roughly equal proportions. In contrast, more familiar alloys such as steel are dominated by one principal element.
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