The Thermodynamic Origins of Chiral Twist in Monolayer Assemblies of Hard Rod-like Colloids

TL;DR

This study uses computer simulations to uncover how entropy and microscopic interactions drive the spontaneous chiral twisting in monolayer assemblies of hard rod-like colloids, revealing new thermodynamic mechanisms.

Contribution

It demonstrates that entropy-driven effects at the microscopic level determine the chiral twist in colloidal monolayers, challenging existing continuum theories.

Findings

Straight rods can spontaneously twist into chiral monolayers.

Chiral rods induce assemblies with handedness depending on their geometry.

Polymer and rod entropy both influence the preferred chiral twist.

Abstract

The propagation of chirality across scales is a common but poorly understood phenomenon in soft matter. In this work, we use computer simulations to study chiral monolayer assemblies formed by hard rod-like colloidal particles in the presence of non-adsorbing polymer and characterize the thermodynamic driving forces responsible for the twisting. Simulations show that straight (achiral) rods assemble into monolayers with a spontaneous twist that is either left- or right-handed, while helical (chiral) rods lead to assemblies with preferential chiral features that depend on their handedness and curliness. The onset of chirality in these monolayers can be traced back to small clusters formed at the initial stage of the self-assembly. In these microscopic monolayers, entropy drives twisting in ways that differ from the assumptions on which existing continuum theory is built. Depending on the geometry of the constituent rods, the preferred chiral twist can be driven by entropy gain of the polymers, or of the rods, or both. In addition, the variation of the polymer entropy with twist depends on changes in both the surface area and the volume of the monolayer. Rod fluctuations perpendicular to the monolayer also play an important role in stabilising the twisting.