Coupling between structural relaxation and diffusion in glass-forming liquids under pressure variation

TL;DR

This paper combines theoretical ECNLE modeling and molecular dynamics simulations to study how pressure influences structural relaxation and diffusion in glass-forming liquids, revealing a pressure-dependent fragility behavior and a pressure-independent coupling between relaxation and diffusion.

Contribution

It introduces a combined ECNLE and MD approach to analyze pressure effects on relaxation and diffusion, highlighting a universal coupling and pressure-dependent fragility trends.

Findings

Relaxation time linearly couples with inverse diffusion constant.

Fragility increases with pressure according to theory, but decreases above 1000 bar in simulations.

Theoretical predictions align well with simulation results and prior studies.

Abstract

We theoretically investigate structural relaxation and activated diffusion of glass-forming liquids at different pressures using both the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory and molecular dynamics (MD) simulation. An external pressure restricts local motions of a single molecule within its cage and triggers the slowing down of cooperative mobility. While the ECNLE theory and simulation generally predict a monotonic increase of the glass transition temperature and dynamic fragility with pressure, the simulation indicates a decrease of fragility as pressure above 1000 bar. The structural relaxation time is found to be linearly coupled with the inverse diffusion constant. Remarkably, this coupling is independent of compression. Theoretical calculations agree quantitatively well with simulations and are also consistent with prior works.