Shear-induced ferrocholesteric-to-ferronematic transitions in magnetic field

Abstract

We explore the effect of magnetic field and shear flow on the helicoidal structure of a ferrocholesteric liquid crystal (the suspension of magnetic nanoparticles in a cholesteric liquid crystal) with finite orientational coupling between magnetic nanoparticles and cholesteric matrix. We consider the geometry in which the helicoid axis is perpendicular to the magnetic field and the shear plane. It is shown how the interplay between magnetic and hydrodynamic forces affects the untwisting of the initial helical structure of the ferrocholesteric, inducing orientation transitions into the ferronematic phase with different orientational and magnetic ordering.We find the field dependences of the ferrocholesteric helix pitch and obtain diagrams of orientation transitions. It is established that the increase in the anchoring energy brings to the reduction of threshold fields necessary for the untwisting of the structure. We study the magnetic characteristics of a deformed ferrocholesteric state and analyze the ability of shear stresses to magnetize a ferrocholesteric liquid crystal. Two regimes of the magnetization vector behavior that depend on the magnitude of the anchoring energy have been found.