Simulating the pitch sensitivity of twisted nematics of patchy rods

TL;DR

This study uses large-scale simulations to explore how microscopic helical particles influence the cholesteric pitch in liquid crystals, revealing that pitch sense can spontaneously invert with changes in particle density.

Contribution

It provides the first simulation evidence that the cholesteric helix sense can change spontaneously with density, without the need for confining geometries.

Findings

Pitch sense is sensitive to packing fraction, molecular pitch, and chiral interaction amplitude.

Spontaneous pitch inversion occurs at fixed molecular chirality when density varies.

Twist elastic constant increases linearly with density, indicating no anomalous stiffening.

Abstract

Stiff, elongated biomolecules such as filamentous viruses, DNA or cellulose nanocrystals are known to form liquid crystals often exhibiting a helical supramolecular organization. Little is known about the microscopic origin, size and handedness of the helical pitch in these, so-called cholesteric phases. Experimental observations in chiral lyotropics suggest that long-ranged chiral forces of electrostatic origin acting between the mesogens are responsible for such organization. Using large-scale computer simulation we study the sensitivity of the pitch imparted by soft microscopic helices and confirm that the helical sense is sensitive to a change of packing fraction, magnitude of the molecular pitch and amplitude of the chiral interactions. In particular, we find evidence that the cholesteric helix sense may change spontaneously upon variation of particle density, at fixed molecular chirality. These pitch inversions have been reported in recent theoretical studies but simulation evidence remains elusive. We rationalize these sudden changes in the supramolecular helical symmetry on the basis of detailed measurements of the mean-torque generated by the twisting of the helices. The simulation methodology employed does not require confining the twisted nematic in a slab geometry and allows for a simultaneous measurement of the pitch and the twist elastic constant. We find that the twist elastic constant increases almost linearly with density suggesting that twisted nematic shows no signs of anomalous stiffening due to pre-smectic fluctuations at higher packing fraction.