Enhanced particle diffusion in fluctuating binary environments

TL;DR

This paper models how a particle's diffusion is affected by fluctuating local environments, showing that slow switching between states can enhance diffusion and increase dynamic heterogeneity in complex media.

Contribution

It introduces a two-state Langevin model capturing the impact of environment fluctuations on particle diffusion, highlighting the role of switching times in dynamic heterogeneity.

Findings

Enhanced diffusion occurs during slow environment switching.

Decoupling of diffusion coefficient from average friction is observed.

Dynamic heterogeneity increases with decreasing switching rate.

Abstract

We investigate single-particle diffusion in a two-state Langevin model where the friction coefficient randomly switches between low-friction (liquid-like) and high-friction (glassy-like) states. The dynamics are governed by the ratio between the friction switching time $\tau$ and the intrinsic velocity relaxation time $\tau_0$. For fast switching ($\tau/\tau_0 \lesssim 1$) the motion is homogeneous and Brownian, whereas for slow switching ($\tau/\tau_0 \gg 1$) the particle exhibits intermittent dynamics and an enhanced diffusion coefficient. Analysis of the single-particle overlap function $Q(t)$ and the dynamic susceptibility $\chi_4(t)$ reveals decoupling of the diffusion coefficient from the average friction upon cooling, which coincides with increasing temporal dynamic heterogeneity. This minimal model provides a transparent framework for understanding single-particle transport in media with fluctuating local mobility, including supercooled liquids and phase-separated soft materials.