Researchers Unravel Mystery of Physics-Connection Between Glass Formation and Crystallization

Glasses are amorphous solids, meaning that they are neither fluids nor crystals. They are one of the greatest puzzles of condensed matter physics. How glass is formed has been a matter of debate over years. Is it due to the freezing of thermal motion of some regions? Or is it due to the clusters or particles which do not fit to produce a crystal?

Solidification under sedimentation: The dynamic map illustrates the speed of the particles in the model system. Regions with low speeds are marked in red and orange while those with higher speeds are blue. Red dots show where a solid has already formed. Similar maps were taken during glass formation, but due to the overall homogeneity of the process they look somewhat more boring. (ill./©: KOMET336, Institute of Physics, JGU)

Scientists at Johannes Gutenberg University Mainz (JGU) have taken a huge leap in integrating these two opposing views in models. Using an intelligent combination of microscopy and light scattering, the researchers could prove that within a melt of hard spheres, minute compressed regions form constituting hundreds of spheres. These precursors are the start for both crystallization at average undercooling and formation of glass at large undercooling.

The scientists noticed that the movement of particles within these precursors was exceptionally limited and reduced further during undercooling, though their number increased rapidly. With only very little precursors, crystallization might initialize on the surface. On the other hand, an increase in these precursors will increase the surface they block. Furthermore, as the precursor increases with time, the system gets stuck and all additional dynamics stop. This implies that from a specific point in undercooling and time beyond, crystal formation cannot be possible. The findings of this study conducted in the JGU Graduate School of Excellence "Materials Science in Mainz" (MAINZ) have been reported in the Nature Physics journal as a superior online publication.

Both glass and crystal can form from a melt, although they are two different structures. In glass the atoms maintain their muddled state, as seen in liquids, whereas in crystals they maintain an extremely regular lattice structure. The solidification process decides which kind of structure will be formed. Mainz University experiments did not aim at the production of a specific glass, like safety windows or fiber optics applications in communication.

They were focused on superior comprehension of the glass formation process, which is a conventional topic of research in the JGU Condensed Matter Physics group. The scientists viewed the amorphous solids formation, and used an investigational model for hard spheres. The undercooling does not occur by reducing the temperature; it occurs by raising the concentration of the polymer spheres. Crystal formation occurs when greater than 50% of the volume is covered by the hard spheres in the suspension, whereas glass formation occurs at greater than 60%. Such micro-sized polymer spheres systems in a solvent have been subject of scrutiny over the last many years, as they imitate the behavior of ideal hard spheres, which are well understood by computer simulation and theory.

Since the 1990s it is well known that hard-sphere melts have areas of both differing order and density in addition to areas that differ on the basis of atoms motility (regions of dynamic and structural inhomogeneity). Ever since, the function played by these two factors during solidification has been a major area of research and debate by theoretical physicists.

What we have now ascertained is that these regions are in fact identical, thus laying the controversy to rest.

Professor Thomas Palberg, Institute of Physics, Mainz University

Mapping motility within hard-sphere suspensions

Sebastian Golde, an associate of the MAINZ Graduate School of Excellence and Palberg's research group, examined hard-sphere model systems in an optical experiment to understand the processes that take place.

We were able to show that the regions with more densely packed spheres and a little more order coincide with those areas where the hard spheres clearly move more slowly.

Sebastian Golde, Associate, MAINZ Graduate School of Excellence

Hence the long-standing mystery relating to the two diverse areas of inhomogeneity has been solved.

The methodology used is a blend of dynamic and static light scattering.

We analyze how much light of a laser beam directed at the sample is scattered in a given direction. This tells us the sample structure. But we also analyze how it flickers after scattering. This tells us how fast the particles move.

Sebastian Golde, Associate, MAINZ Graduate School of Excellence

Golde's device was designed by Dr. Hans Joachim Schöpe, who shifted to the University of Tübingen recently. Using an intelligent imaging system, Golde could obtain dynamic maps with extraordinary resolution a little lesser than the pioneers. Similar to the camera which produces an image, the outcome is a photo that captures the task of the dynamics within different regions. The scientists noticed that as time increased, tiny dense areas with slowly-moving spheres were produced.

The speed of their formation decided if there was sufficient time remaining for the crystals formation before jamming occurred. Since the speed of formation of the precursor is dependent on the hard-sphere concentration, crystallization occurs at low concentrations of hard spheres. However, at higher concentrations the system solidifies into a glass as these compressed areas get arrested quickly.

In other words, glass results when so many crystallization precursors are formed that they in effect arrest each other. For us, this means that an unexpected and fascinating link has been found between the two solidification scenarios. Arguably, this was one of the most important missing pieces of the puzzle.

Professor Thomas Palberg, Institute of Physics, Mainz University

The discoveries are supposed to be extremely general, but the examination should be done to other model systems too to support the analysis of coinciding dynamical and structural inhomogeneities being accountable for formation of glass.

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