Distortion of Twisted Orientation Patterns in Liquid Crystals by Magnetic Fields

Abstract

Abstract At rest between parallel plates, cholesteric liquid crystals commonly exhibit a characteristic twisted orientation pattern, the axis of twist being perpendicular to the plates. Also, this orientation pattern appears possible in nematic liquid crystals, since it is consistent with continuum theory(1) for both types of liquid crystal. This paper discusses the influence upon such a twisted orientation pattern of a magnetic field perpendicular to the plates. If one employs a free energy of the form discussed by Frank,(2) the continuum theory equations have solutions relevant to this situation. For nematic liquid crystals, provided that the twist is not too large, our analysis suggests that no distortion of the orientation pattern occurs until the magnetic field strength exceeds a critical value which varies with the amount of twist. For cholesteric liquid crystals, there seems to be two possibilities depending upon the relative magnitudes of two Frank constants. Either distortion always occurs above a critical field strength, or does so only when the distance between the plates is sufficiently small.