How Glassy Relaxation Slows Down by Increasing Mobility

TL;DR

This study explores how increasing the mobility of fast particles affects the structural relaxation in dense mixtures, revealing a reversal in the effect at a critical density related to the glass transition.

Contribution

It demonstrates that the influence of microscopic dynamics on relaxation is non-monotonic and identifies a universal critical density for the glass transition in mixtures.

Findings

Below critical density, increased fast particle mobility fluidizes the system.

Above critical density, increased fast particle mobility hinders slow particle relaxation.

The critical density aligns with the mode-coupling theory's glass transition density.

Abstract

We investigate how structural relaxation in mixtures with strong dynamical asymmetry is affected by the microscopic dynamics. Brownian and Newtonian dynamics simulations of dense mixtures of fast and slow hard spheres reveal a striking trend reversal. Below a critical density, increasing the mobility of the fast particles fluidizes the system, yet, above that critical density, the same increase in mobility strongly hinders the relaxation of the slow particles. The critical density itself does not depend on the dynamical asymmetry and can be identified with the glass-transition density of the mode-coupling theory. The asymptotic dynamics close to the critical density is universal, but strong pre-asymptotic effects prevail in mixtures with additional size asymmetry. This observation reconciles earlier findings of a strong dependence on kinetic parameters of glassy dynamics in colloid--polymer mixtures with the paradigm that the glass transition is determined by the properties of configuration space alone.