Fragile Glasses Associated with a Dramatic Drop of Entropy under Supercooling

Abstract

We perform kinetic Monte Carlo simulations of a distinguishable-particle lattice model of structural glasses with random particle interactions. By varying the interaction distribution and the average particle hopping energy barrier, we obtain an extraordinarily wide range of kinetic fragility. A stretching exponent, characterizing structural relaxation, is found to decrease with the kinetic fragility in agreement with experiments. The most fragile glasses are those exhibiting low hopping barriers and, more importantly, dramatic drops of entropies upon cooling toward the glass transition temperatures. The entropy drops reduce possible kinetic pathways and lead to dramatic slowdowns in the dynamics. In addition, the kinetic fragility is shown to correlate with a thermodynamic fragility.

Large heat-capacity jump in cooling-heating of fragile glass from kinetic Monte Carlo simulations based on a two-state picture

“…The close correlation of these features with fragility is also clearly demonstrated. This is made possible by the capability of the DPLM to simulate both strong and fragile glasses, with respective characteristic properties already demonstrated to be consistent with experimental trends [29].…”

“…A more realistic temperature dependent φ v have been studied in Ref. [29] which in general gives similar qualitative features. We focus mainly on G 0 = 0.01 and 1.…”

“…We have also directly identified quasi-voids, each consisting of localized and fragmented free volumes, in colloidal experiments at very high packing fractions [35]. Using the Metropolis algorithm, a particle can move to an adjacent void at the following rate [29]…”

“…where E is the change in the system energy due to the hop, T is the bath temperature, w 0 = 10 6 is the attempt frequency, and k B = 1 is the Boltzmann constant. It has been shown that a smaller (but finite) G 0 leads to fragile glasses while a larger G 0 to strong glasses [29]. By tuning G 0 , a wide range of fragility can be realized.…”

“…The number of accessible configurations with energetically favorable interactions then drop dramatically as T decreases. This results in a drastic slowdown in the dynamics, and those systems are thus fragile [29].…”