Distribution of Diffusion Constants and Stokes-Einstein Violation in supercooled liquids

TL;DR

This study directly analyzes the distribution of diffusion constants in supercooled liquids, revealing how dynamic heterogeneity causes Stokes-Einstein violation and how pinning particles influences this breakdown.

Contribution

It provides the first direct calculation of diffusivity distributions in supercooled liquids and links mobile particles to SE violation, also exploring effects of random pinning.

Findings

Distribution shifts from Gaussian to bimodal with decreasing temperature

Mobile particles violate the SE relation, while less mobile ones obey it

SE breakdown increases with higher random pinning concentration

Abstract

It is widely believed that the breakdown of the Stokes-Einstein (SE) relation between the translational diffusivity and the shear viscosity in supercooled liquids is due to the development of dynamic heterogeneity i.e. the presence of both slow and fast moving particles in the system. In this study we \emph{directly} calculate the distribution of the diffusivity for a model system for different temperatures in the supercooled regime. We find that with decreasing temperature, the distribution evolves from Gaussian to bimodal indicating that on the time scale of the typical relaxation time, mobile (fluid like) and less mobile (solid like) particles in the system can be \emph{unambiguously} identified. We also show that less mobile particles obey the Stokes-Einstein relation even in the supercooled regime and it is the mobile particles which show strong violation of the Stokes-Einstein relation in agreement with the previous studies on different model glass forming systems. Motivated by some of the recent studies where an ideal glass transition is proposed by randomly pinning some fraction of particles, we then studied the SE breakdown as a function of random pinning concentration in our model system. We showed that degree of SE breakdown increases quite dramatically with increasing pinning concentration, thereby providing a new way to unravel the puzzles of SE violation in supercooled liquids in greater details.