Non-Gaussian Dynamics in Smectic Liquid Crystals of Parallel Hard Rods

TL;DR

This study uses computer simulations to explore non-Gaussian diffusion and structural relaxation in smectic liquid crystals of parallel hard rods, revealing slow inter-layer dynamics and hopping diffusion behavior.

Contribution

It provides new insights into the non-Gaussian dynamics and heterogeneity of diffusion in smectic phases of parallel hard rods, highlighting differences from freely rotating systems.

Findings

Non-Gaussian layer-to-layer diffusion due to barriers and cages

Very slow inter-layer relaxation over four decades of time

Hopping-type diffusion becomes more heterogeneous at higher densities

Abstract

Using computer simulations, we studied the diffusion and structural relaxation in equilibrium smectic liquid crystal bulk phases of parallel hard spherocylinders. These systems exhibit a non-Gaussian layer-to-layer diffusion due to the presence of periodic barriers and transient cages, and show remarkable similarities with the behavior of out-of-equilibrium supercooled liquids. We detect a very slow inter-layer relaxation dynamics over the whole density range of the stable smectic phase which spans a time interval of four time decades. The intrinsic nature of the layered structure yields a hopping-type diffusion which becomes more heterogeneous for higher packing fractions. In contrast, the in-layer dynamics is typical of a dense fluid with a relatively fast decay. Our results on the dynamic behavior agree well with that observed in systems of freely rotating hard rods, but differ quantitavely, as the height of the periodic barriers reduces to zero at the nematic-smectic transition for aligned rods, while it remains finite for freely rotating rods.