Diffusion and viscosity in a supercooled polydisperse system

TL;DR

This study uses molecular dynamics simulations to analyze how viscosity and diffusion in a supercooled polydisperse Lennard-Jones liquid depend on temperature, particle size, and mass, revealing super-Arrhenius behavior and decoupling phenomena.

Contribution

It provides detailed insights into the temperature-dependent dynamics of polydisperse supercooled liquids, highlighting size-dependent diffusion and viscosity decoupling, and characterizing particle motion types.

Findings

Viscosity and diffusion follow Vogel-Fulcher-Tammann behavior.

The system is classified as a strongly fragile liquid with D ≈ 1.4.

Diffusion shows non-Stokesian behavior at low temperatures.

Abstract

We have carried out extensive molecular dynamics simulations of a supercooled polydisperse Lennard-Jones liquid with large variations in temperature at a fixed pressure. The particles in the system are considered to be polydisperse both in size and mass. The temperature dependence of the dynamical properties such as the viscosity ($\eta$) and the self-diffusion coefficients ($D_i$) of different size particles is studied. Both viscosity and diffusion coefficients show super-Arrhenius temperature dependence and fit well to the well-known Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range investigated, the value of the Angell's fragility parameter (D $\approx 1.4$) classifies the present system into a strongly fragile liquid. The critical temperature for diffusion ($T_o^{D_i}$) increases with the size of the particles. The critical temperature for viscosity ($T_o^{\eta}$) is larger than that for the diffusion and a sizeable deviations appear for the smaller size particles implying a decoupling of translational diffusion from viscosity in deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian behavior at low temperatures where a highly nonlinear dependence on size is observed. An inspection of the trajectories of the particles shows that at low temperatures the motions of both the smallest and largest size particles are discontinuous (jump-type). However, the crossover from continuous Brownian to large length hopping motion takes place at shorter time scales for the smaller size particles.