# Fickian yet non-Gaussian behaviour: A dominant role of the intermittent dynamics

## TL;DR

This study investigates how small solutes in crowded environments exhibit Fickian diffusion with non-Gaussian characteristics, highlighting the dominant role of intermittent dynamics over decoupling effects.

## Contribution

It identifies intermittency as the primary cause of non-Gaussian diffusion in small solutes, contrasting with the decoupling mechanism, and explores how interaction strength influences this behavior.

## Key findings

- Intermittent dynamics cause persistent non-Gaussianity.
- Gaussian behavior emerges after long-time averaging in decoupling.
- Intermittency occurs only at intermediate solute-solvent attractions.

## Abstract

We present a study of the dynamics of small solute particles in a solvent medium where the solute is much smaller in size, mimicking the diffusion of small particles in crowded environment. The solute exhibits Fickian diffusion arising from non-Gaussian Van Hove correlation function. Our study shows that there are at least two possible origins of this non-Gaussian behaviour: the decoupling of the solute-solvent dynamics and the intermittency in the solute motion, the latter playing a dominant role. In the former scenario when averaged over time long enough to explore different solvent environments, the dynamics recovers the Gaussian nature. In the case of intermittent dynamics the non-Gaussianity remains even after long averaging and the Gaussian behaviour is obtained at a much longer time. Our study further shows that only for an intermediate attractive solute-solvent interaction the dynamics of the solute is intermittent. The intermittency disappears for weaker or stronger attractions.