Atomic scale imaging reveals an easy way to crystallize

September 23, 2022

(Nanwerk NewsWhat do clouds, televisions, pharmaceuticals, and even the dirt under our feet have in common? They all have or use crystals in some way. Crystals are more than just luxurious gemstones. Clouds form when water vapor condenses into ice crystals in the atmosphere. LCDs are used in a variety of electronic devices, from televisions to instrument panels. Crystallization is an important step for drug discovery and purification. Crystals also form rocks and other minerals. Their critical role in the environment is to focus on materials science and health science research.

Scientists still have to understand exactly how crystallization occurs, but the importance of surfaces in promoting the process has long been recognized. Research from Pacific Northwest National Laboratory (PNNL), University of Washington (UW) and Durham University sheds new light on how crystals form on surfaces.

Their results were published in science progress (“The hydroxide films on mica form charge-stabilized minute phases that circumvent nucleation barriers.”). Aluminum hydroxide, shown here in orange, undergoes inter-structural fluctuations before forming an ordered crystal. (Illustration by Nathan Johnson, Pacific Northwest National Laboratory)

Previous studies of crystallization have led scientists to form the classic nucleation theory – the dominant explanation for why crystal formation or nucleation begins. When the crystals are intended, they start out as small, ephemeral clusters of only a few atoms. Their small size makes it very difficult to detect clusters. Scientists managed to collect only some pictures of such processes.

“New technologies make it possible to visualize the crystallization process like never before,” said chemist Ben Leg, PNNL’s Department of Physical Sciences. He’s partnered with PNNL Fellow Battelle and UW Associate Professor James De Yoreo to do just that. With the help of Professor Kesslon Wojchowski of Durham University in England, they used a technique called atomic force microscopy to observe the nucleus of the aluminum hydroxide metal on the surface of mica in water.

Mica is a common mineral found in everything from drywall to cosmetics. It often provides a surface for other minerals to nucleate and grow. But for this study, its most important feature was its extremely flat surface, which allowed the researchers to detect just a few groups of atoms as they formed on mica.

What Legge and de Yorio observed was a crystallization pattern that was not expected from classical theory. Instead of a rare event in which a cluster of atoms reaches a critical size and then grows across the surface, they saw thousands of fluctuating clusters merging in an unexpected pattern with gaps that persisted between crystal “islands”.

After careful analysis of the results, the researchers concluded that while some aspects of the current theory were correct, their system eventually followed a non-classical path. They attribute this to the electrostatic forces from the charges on the mica’s surface. Since many types of materials form charged surfaces in water, the researchers hypothesize that they have observed a diffuse phenomenon and are excited to look for other systems in which this non-classical process might occur.

“Assumptions from classical nucleation theory have far-reaching implications in disciplines ranging from materials science to climate prediction,” Di Yorio said. “The results of our experiments could help produce more accurate simulations of these systems.”