Missing Link Between Glass Formation And Crystallization Discovered, Putting Controversy To Rest

First Posted: Jul 02, 2016 05:28 AM EDT

The researchers from Johannes Gutenberg University Mainz (JGU) in Germany studied the glass formation process, which they find the link between the glass and crystallization.

The findings were printed in the journal Nature Physics. It was authored by Sebastian Golde, Thomas Palberg and Hans Joachim Schope from Johannes Gutenberg University Mains in Germany. The researchers have reconciled controversies of two opposing views on how the glass forms. These questions such as "Is it because some regions freeze their thermal motion? Or is it because there are particles or clusters which do not fit to form a crystal?

The researchers used an experimental model system for hard spheres. They demonstrated that within a melt of hard spheres there were small compacted regions formed, which comprised of a few hundred spheres. These so-called precursors are the early stage for both crystallization at moderate undercooling and glass formation at large undercooling. The team observed that the motility of particles within these precursors was extremely limited. This decreased further with undercooling while their number rapidly increased.

With only a few precursors, the crystallization may still start at the surface. On the other hand, the more of these precursors, the more of their surface they block. The more the precursor increases in time, the system then gets jammed and all further dynamics cease. This means that from the particular point in undercooling and time onwards, the crystal formation is no longer possible.

The team discovered that as time proceeded, even more, small dense areas with slow-moving spheres were generated. Their formation speed tells whether there is enough time left for the formation of crystals before jamming happens. Since the precursor formation speed is linked to the hard-sphere concentration, the crystallization occurs at low concentrations of hard spheres. On the other hand, at higher concentrations, these packed regions become quickly arrested and the system solidifies into a glass.

Golde explained that they were able to show that the regions with more densely packed spheres and a little more order agree with those areas where the hard spheres clearly move more slowly. He further explained that this means that the long-standing enigma concerning the two different regions of inhomogeneity has been resolved.

Since the 1990s, it has been known that hard-sphere melts contain both regions of differing density and order as well as regions that vary in terms of the motility of the atoms. These involve the regions of structural and dynamic inhomogeneity. The theoretical physicists have long been debating about these two factors that occur during the process of solidification. Professor Palberg said that what they are now ascertained is that these regions are in fact identical, thus laying the controversy to rest.

Palberg concluded that the glass results when so many crystallization precursors are formed that they in effect arrest each other. This means that an unexpected and fascinating link has been found between the two solidification scenarios. He further said that this was one of the most important missing pieces of the puzzle.

The glass is defined as a non-crystalline amorphous solid that is transparent. It has numerous uses such as in technology, practical and decorative. It has a non-crystalline structure at the atomic scale and exhibits a glass transition when heated towards the liquid state.

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