Chaining of hard disks in nematic needles: particle-based simulation of colloidal interactions in liquid crystals

TL;DR

This study uses particle-based simulations to reveal that colloidal disks in nematic liquid crystals experience a directional depletion attraction, leading to chaining, which challenges traditional quadrupolar interaction theories.

Contribution

The paper introduces a novel simulation approach to directly measure colloidal interactions in dense nematic liquid crystals, highlighting the significance of depletion interactions over elastic quadrupolar forces.

Findings

Disks exhibit short-range attraction along the director.

Chaining occurs parallel to the director due to depletion forces.

Depletion interactions dominate over quadrupolar elastic attraction.

Abstract

Colloidal particles suspended in liquid crystals can exhibit various effective anisotropic interactions that can be tuned and utilized in self-assembly processes. We simulate a two-dimensional system of hard disks suspended in a solution of dense hard needles as a model system for colloids suspended in a nematic lyotropic liquid crystal. The novel event-chain Monte Carlo technique enables us to directly measure colloidal interactions in a microscopic simulation with explicit liquid crystal particles in the dense nematic phase. We find a directional short-range attraction for disks along the director, which triggers chaining parallel to the director and seemingly contradicts the standard liquid crystal field theory result of a quadrupolar attraction with a preferred ${45^{\circ}}$ angle. Our results can be explained by a short-range density-dependent depletion interaction, which has been neglected so far. Directionality and strength of the depletion interaction are caused by the weak planar anchoring of hard rods. The depletion attraction robustly dominates over the quadrupolar elastic attraction if disks come close. Self-assembly of many disks proceeds via intermediate chaining, which demonstrates that in lyotropic liquid crystal colloids depletion interactions play an important role in structure formation processes.