Cholesteric–nematic transition induced by a rotating magnetic field

Abstract

We study theoretically the unwinding of helical orientational structure of a cholesteric liquid crystal under rotating magnetic field action. We analyze the non-stationary rotation regime of the orientational structure at the initial stages of the cholesteric helix unwinding in weak magnetic fields. For small deformations of the orientational structure we analytically obtain the dependence of the pitch of the cholesteric helix on the strength and rotation velocity of a magnetic field. For the stationary rotation regime in a magnetic field, the orientational phase diagram of the cholesteric–nematic transition is constructed. We study numerically and analytically the dependences of the cholesteric helical pitch on the strength and rotation velocity of the magnetic field. We reveal that in the narrow range of rotating magnetic field strength, the reentrant cholesteric–nematic–cholesteric orientational transitions take place. We show that in the stationary rotation regime, an increase in the rotation velocity of the field leads to a decrease in the cholesteric–nematic transition field. In the absence of rotation, our results are reduced to the well-known for static magnetic fields.