# Hydrodynamic instabilities in active cholesteric liquid crystals

## TL;DR

This paper investigates how active stresses affect cholesteric liquid crystals, revealing instabilities, defect dynamics, and flow patterns, with implications for understanding active soft matter systems.

## Contribution

It introduces the effects of active stresses in cholesteric liquid crystals, analyzing instabilities, defect behavior, and flow patterns, which were not previously characterized.

## Key findings

- Extensile stresses cause linear instability in the cholesteric ground state.
- Contractile stresses are hydrodynamically screened, leading to finite thresholds.
- Active flows near defects resemble those in active nematics.

## Abstract

We describe the basic properties and consequences of introducing active stresses, with principal direction along the local director, in cholesteric liquid crystals. The helical ground state is found to be linearly unstable to extensile stresses, without threshold in the limit of infinite system size, whereas contractile stresses are hydrodynamically screened by the cholesteric elasticity to give a finite threshold. This is confirmed numerically and the non-linear consequences of instability, in both extensile and contractile cases, are studied. We also consider the stresses associated to defects in the cholesteric pitch ($\lambda$ lines) and show how the geometry near to the defect generates threshold-less flows reminiscent of those for defects in active nematics. At large extensile activity $\lambda$ lines are spontaneously created and can form steady state patterns sustained by constant active flows.