Dynamics of a rod in a homogeneous/inhomogeneous frozen disordered medium: Correlation functions and non-Gaussian effects

TL;DR

This study uses molecular dynamics simulations to analyze how a rigid rod moves in disordered 2D media, revealing glassy slowdown effects, scaling behaviors, and non-Gaussian dynamics that are weakly influenced by medium heterogeneity.

Contribution

It provides detailed insights into the translational and rotational dynamics of a rod in disordered media, highlighting the effects of medium structure and confirming Mode Coupling Theory predictions.

Findings

Rotational dynamics are significantly slowed in glassy media.

Two-step decay in correlation functions indicates glass-like behavior.

Large scale deviations from Gaussianity are independent of medium heterogeneity.

Abstract

We present molecular dynamics simulations of the motion of a single rigid rod in a disordered static 2d-array of disk-like obstacles. Two different configurations have been used for the latter: A completely random one, and which thus has an inhomogeneous structure, and an homogeneous ``glassy'' one, obtained from freezing a liquid of soft disks in equilibrium. Small differences are observed between both structures for the translational dynamics of the rod center-of-mass. In contrast to this, the rotational dynamics in the glassy host medium is strongly slowed down in comparison with the random one. We calculate angular correlation functions for a wide range of rod length $L$ and density of obstacles $\rho$ as control parameters. A two-step decay is observed for large values of $L$ and $\rho$, in analogy with supercooled liquids at temperature close to the glass transition. In agreement with the prediction of the Mode Coupling Theory, a time-length and time-density scaling is obtained. In order to get insight on the relation between the heterogeneity of the dynamics and the structure of the host medium, we determine the deviations from Gaussianity at different length scales. Strong deviations are obtained even at spatial scales much larger than the rod length. The magnitude of these deviations is independent of the nature of the host medium. This result suggests that the large scale translational dynamics of the rod is affected only weakly by the presence of inhomogeneities in the host medium.