Nonequilibrium steady states in a vibrated-rod monolayer: tetratic, nematic and smectic correlations

TL;DR

This study experimentally investigates how particle shape influences nonequilibrium phase behavior and orientational order in vibrated monolayers of macroscopic rods, revealing shape-dependent transitions among tetratic, nematic, and smectic phases.

Contribution

It demonstrates the significant role of particle shape in determining steady-state phases in vibrated rod monolayers, extending understanding beyond traditional molecular liquid crystal systems.

Findings

Cylindrical particles exhibit tetratic correlations at various aspect ratios.

Thinner particles show nematic ordering with large-scale swirling at small aspect ratios.

Basmati rice-shaped particles display indications of smectic order.

Abstract

We study experimentally the nonequilibrium phase behaviour of a horizontal monolayer of macroscopic rods. The motion of the rods in two dimensions is driven by vibrations in the vertical direction. Aside from the control variables of packing fraction and aspect ratio that are typically explored in molecular liquid crystalline systems, due to the macroscopic size of the particles we are also able to investigate the effect of the precise shape of the particle on the steady states of this driven system. We find that the shape plays an important role in determining the nature of the orientational ordering at high packing fraction. Cylindrical particles show substantial tetratic correlations over a range of aspect ratios where spherocylinders have previously been shown by Bates et al (JCP 112, 10034 (2000)) to undergo transitions between isotropic and nematic phases. Particles that are thinner at the ends (rolling pins or bails) show nematic ordering over the same range of aspect ratios, with a well-established nematic phase at large aspect ratio and a defect-ridden nematic state with large-scale swirling motion at small aspect ratios. Finally, long-grain, basmati rice, whose geometry is intermediate between the two shapes above, shows phases with strong indications of smectic order.