Transformations of physical states such as freezing, melting or evaporation begin with a process known as nucleation. In this step, several tiny clusters of molecules begin to coalesce. Nucleation is a very crucial step in diverse circumstances such as cloud formation or the onset of any neurodegenerative disease.
A group of researchers from UCLA have been successful in getting a view of nucleation – which has never been observed before. It depicts the arrangement of atoms in four-dimensional atomic resolution( three space and one time dimension). The study has been published in the Nature journal and it differs a lot from the statements of the classical theory of nucleation mentioned in textbooks.
Jianwei “John” Miao, a professor of physics and astronomy at UCLA, also the lead author of the study mentioned that this has been a historic achievement as researchers not only could locate the individual atoms but also monitor the real-time motion in 4D. Jianwei Miao is also the deputy director of the STROBE National Science Foundation Science and Technology Center.
Scientists from universities such as the University of Colorado, Boulder, University of Buffalo and University of Nevada, Reno collaborated to build a powerful and effective imaging technique previously developed by the research group of Miao. This is called atomic electron tomography and it uses the highly advanced electron microscope located at Molecular Foundry in Berkeley Lab. It captures a sample using electrons and then the sample is rotated to create amazing 3D images of atoms in a way similar to CAT scan.
For the process, researchers took nanoparticles of an iron-platinum alloy and heated it to 520 degrees Celsius. They took the images after an interval of 9 minutes, 16 minutes and 26 minutes. This temperature marks the transition of alloy between different solid phases.
Even though the alloy looks similar in both phases, a deeper look reveals different 3D arrangements. Due to heating, the structure changes from a haphazard arrangement to a more ordered one. The phase change is similar to the process of solving a Rubik’s cube, where the solved cube has arranged colours unlike the random arrangement of the unsolved one.
Scientists tracked a group of 33 nuclei for observing the change. Through this painstaking process, they found out that nuclear formed irregular shapes as opposed to round shapes predicted by the classical theory. The nuclei do not have a sharp boundary but they possess jumbled arrangement closer to the surface.
Classical nucleation theory also states that once a nucleus reaches a specific size, it only grows larger from there. But the process seems to be far more complicated than that: In addition to growing, nuclei in the study shrunk, divided and merged; some dissolved completely.
These diversions from the classical nucleation theory can open up a new area of study of chemical and biological phenomena.