Stripe Instabilities and Disclinations in Freely Suspended Films of Lyotropic Chromonic Liquid Crystals under Oscillatory Shear Flow

Lyotropic chromonic liquid crystals have received a wide interest in recent years because their fundamental viscoelastic properties are relatively not well understood and due to their utility for new technologies in optics and biological sensors. These materials are composed of self-assembled plank-like molecules that form liquid crystal phases uniquely characterized by a huge viscoelastic anisotropy. We study the structural response of a thin, freely suspended nematic film of the chromonic DSCG to oscillatory shear flow. Oscillating shear flows are generated via a suspended magnetic needle driven by an oscillating magnetic field. At shear strain amplitudes below ~0.15, we observe the formation of stripe instabilities with a spatial frequency that depends on shear rate. In this regime, the instabilities are cyclic—twisting and untwisting in response to the cyclic shear. At amplitudes larger than ~0.22, we see the formation of disclinations that store elastic energy and advect away from the needle in a manner reminiscent of elastic turbulence. Rather than exhibiting cyclic reversibility, these disclinations are persistent and grow with the number of shear cycles. At shear strains in between, the instabilities first begin to persist between cycles before ultimately proliferating and transitioning into disclinations. Understanding these materials under large and unsteady strains is a step toward bulk processing, to enable applications such as paintable polarizers and other deformable optics.