Increase in rod diffusivity emerges even in Markovian nature

TL;DR

This study demonstrates that an increase in rod diffusivity with matrix density can occur even in Markovian systems without kinetic constraints, challenging previous explanations based on tube models.

Contribution

It shows that the diffusivity increase is not solely due to kinetic constraints, using a Markovian Monte Carlo model with high aspect ratio rods.

Findings

Diffusivity increases for rods with aspect ratio > 24

Increase occurs even without kinetic constraints

Challenges the tube model explanation

Abstract

Rod-shaped particles embedded in certain matrices have been reported to exhibit an increase in their center of mass diffusivity upon increasing the matrix density. This increase has been considered to be caused by a kinetic constraint in analogy with tube models. We investigate a mobile rod-like particle in a sea of immobile point obstacles using a kinetic Monte Carlo scheme equipped with a Markovian process, that generates gas-like collision statistics, so that such kinetic constraints do essentially not exist. Even in such a system, provided the particle's aspect ratio exceeds a threshold value of about 24, the unusual increase in the rod diffusivity emerges. This result implies that the kinetic constraint is not a necessary condition for the increase in the diffusivity.