Eur. Phys. J. E (2017) 40: 50
https://doi.org/10.1140/epje/i2017-11536-2

Regular Article

Hydrodynamic instabilities in active cholesteric liquid crystals

¹ Department of Physics, University of Warwick, CV4 7AL, Coventry, UK
² Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 7-9, 55128, Mainz, Germany
³ Dipartimento di Fisica e Astronomia, Università di Padova, Via Marzolo 8, I-35131, Padova, Italy
⁴ Centre for Complexity Science, University of Warwick, CV4 7AL, Coventry, UK
⁵ SUPA, School of Physics and Astronomy, University of Edinburgh, JCMB Kings Buildings, Mayfield Road, EH9 3JZ, Edinburgh, Scotland, UK
⁶ TIFR Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, 21 Brundavan Colony, 500 075, Narsingi, Hyderabad, India
⁷ Department of Physics, Indian Institute of Science, 560 012, Bangalore, India

^* e-mail: G.P.Alexander@warwick.ac.uk

Received: 18 January 2017
Accepted: 23 March 2017
Published online: 24 April 2017

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 ( 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 lines are spontaneously created and can form steady-state patterns sustained by constant active flows.