Cholesteric order in systems of helical Yukawa rods

TL;DR

This paper models the chiral interactions of helical rods to predict cholesteric phase behavior, revealing how microscopic helix properties influence macroscopic pitch and phase demixing, aligning with experimental observations.

Contribution

It introduces a scaling expression linking microscopic helix parameters to chiral potential and extends a second-virial theory to predict cholesteric order and phase demixing.

Findings

Handedness of cholesteric phase depends on coil strength.

Stronger coiling can invert the macroscopic cholesteric handedness.

Phase demixing occurs with disparate helix parameters.

Abstract

We consider the interaction potential between two chiral rod-like colloids which consist of a thin cylindrical backbone decorated with a helical charge distribution on the cylinder surface. For sufficiently slender coiled rods a simple scaling expression is derived which links the chiral `twisting' potential to the intrinsic properties of the particles such as the coil pitch, charge density and electrostatic screening parameter. To predict the behavior of the macroscopic cholesteric pitch we invoke a simple second-virial theory generalized for weakly twisted director fields. While the handedness of the cholesteric phase for weakly coiled rods is always commensurate with that of the internal coil, more strongly coiled rods display cholesteric order with opposite handedness. The correlation between the symmetry of the microscopic helix and the macroscopic cholesteric director field is quantified in detail. Mixing helices with sufficiently disparate lengths and coil pitches gives rise to a demixing of the uniform cholesteric phase into two fractions with a different macroscopic pitch. Our findings are consistent with experimental results and could be helpful in interpreting experimental observations in systems of cellulose and chitin microfibers, DNA and {\em fd} virus rods.