Collective Diffusion of Colloidal Hard Rods in Smectic Liquid Crystals: Effect of Particle Anisotropy

TL;DR

This study uses computer simulations to explore how particle shape affects layer-to-layer diffusion in smectic liquid crystals of colloidal rods, revealing cooperative motion and the impact of anisotropy on dynamics.

Contribution

It demonstrates how particle anisotropy influences diffusion barriers, cooperative motion, and heterogeneity in smectic liquid crystals, linking particle shape to dynamic behavior.

Findings

Stringlike clusters of inter-layer rods move cooperatively.

Barrier height increases with particle anisotropy.

Longer rods exhibit more heterogeneous and non-Gaussian dynamics.

Abstract

We study the layer-to-layer diffusion in smectic-A liquid crystals of colloidal hard rods with different length-to-diameter ratios using computer simulations. The layered arrangement of the smectic phase yields a hopping-type diffusion due to the presence of permanent barriers and transient cages. Remarkably, we detect stringlike clusters composed of inter-layer rods moving cooperatively along the nematic director. Furthermore, we find that the structural relaxation in equilibrium smectic phases shows interesting similarities with that of out-of-equilibrium supercooled liquids, although there the particles are kinetically trapped in transient rather than permanent cages. Additionally, at fixed packing fraction we find that the barrier height increases with increasing particle anisotropy, and hence the dynamics is more heterogeneous and non-Gaussian for longer rods, yielding a lower diffusion coefficient along the nematic director and smaller clusters of inter-layer particles that move less cooperatively. At fixed barrier height, the dynamics becomes more non-Gaussian and heterogeneous for longer rods that move more collectively giving rise to a higher diffusion coefficient along the nematic director.