4 January, 2010
Broader aspects and grand challenges of the slow dynamics and glass transition problem
Abstract
The usual viscous liquid slow-down phenomenon, ending in the glass transition, is a popular problem with many aspects to challenge the theoretician and experimentalist alike. However it is only one aspect of a broader problem in which disordered elements of a condensed phase system slow down and finally grind to a halt. While the familiar liquid problem is associated in most people’s thinking with metastability, there are systems with such slow dynamics that are not metastable, though their ergodicity-breaking may have all the hysteresis and time-dependent features of the glass transition. In some systems the relaxation time behavior is more interesting than others, being super-Arrhenius in character (“fragile”). In some systems, usually the same ones, the relaxation process is more interesting than in the others, being stretched exponential rather than exponential. In the same systems the dynamics are “heterogeneous”, both in space and time but this is mainly because the dynamics are exponentially sensitive to subtle changes in the structure or thermodynamics. One of the biggest challenges is to understand what it is that makes these systems “heterogeneous”, causing breakdowns in such familiar relations as the Stokes-Einstein equation.
Some of the most pronounced breakdowns occur in systems like water in which an underlying phase transition or its supercritical aftermath is suspected. The fact that some very fragile liquids are now being revealed, by special vapor deposition processes, to have new amorphous phases of ultrastable character, accessible only below their normal glass temperatures, raises the question of whether these hidden transitions might be the source of the fragile liquid anomalies. Thus understanding the true nature of these “Ediger phases” emerges as one of the grand challenges. Belonging to the relaxation time domain counted in kiloyears as they do, this challenge would seem to be one that is considerably beyond the reach of computer simulations, which adds an extra gravity to the problem.
The Ediger phases apparently transform back to the normal viscous liquid across a planar front, as in a first order phase transition, implying an “all-or-nothing” cooperative processes. But many of the “polyamorphic” transitions that have received so much attention in recent times now seem to be closely linked to crystallization. Thus another of the emerging grand challenges concerns the question of whether or not any of the viscous liquid anomalies under discussion can occur in systems that are already in their configurational ground states. The atactic polymers, for instance, are thought to be incapable of crystallization. Are they, or their low molecular weight cousins, fragile in character? Can liquid (or fragile rotator) phases that are thermodynamically incapable of first order transitions to a lower free energy state, actually exist? Are they anomalous in their dynamics?.
Finally, complex systems that unambiguously exhibit all-or-nothing transitions between distinct states, do exist, but at the mesoscopic level. These are the small single domain proteins that fold and unfold reversibly, and seem to exist in near-critical conditions in closed stability domains that are bounded at both high and low temperatures and also at high and negative pressures. Even these turn out to have a lower free energy state, the fibril state. However, this state is normally only accessed on time scales so long that human life based on their catalytic activity remains viable on the 100 year time scale. The extension of these phenomena to systems not of natural origin, for instance heteropolymeric derivatized phosphazenes, is the ultimate grand challenge.
References and suggested reading
- C. A. Angell, Glass formation and glass transition in supercooled liquids, with insights from study of related phenomena in crystals J. Non-Cryst. Solids 2008, 354, 4703-4712 http://arxiv.org/abs/0712.4233
- R. Richert, Heterogeneous dynamics in liquids: fluctuations in space and time J. Phys. Condens. Matter 2002, 14, R703-R738
- M. D.Ediger, Spatially heterogeneous dynamics in supercooled liquids Ann. Rev. Phys. Chem. 2000, 51, 99-128
- L. Xu, F. Mallamace, Y. Z. Yan, F. Starr, S. Buldyrev, H. E. Stanley, Appearance of a fractional Stokes–Einstein relation in water and a structural interpretation of its onset Nature Physics 2009, 565-568
- S. F. Swallen, K. L. Kearns, M. K. Mapes, Y. S. Kim, R. J. McMahon, T. Wu, L. Yu, M. D. Ediger ,Organic glasses with exceptional thermodynamic stability and kinetic stability. Science 2007, 315, 354-356
- C. A. Angell, Formation of Glasses from Liquids and Biopolymers. Science 1995, 267, 1924-1935
- C. M. Dobson, Protein folding and misfolding. Nature 2003, 426, 884