MAGNETIC-FIELD EFFECTS ON A COMPENSATED CHOLESTERIC LIQUID CRYSTAL

By measuring changes in the dielectric constant of a 1.75:1 cholesteryl chloride-cholesteryl myristate (CM) mixture in the presence of magnetic fields H up to 10 kG, the orientation of the molecular and helical axes with respect to the field direction can be deduced, and the possibility of a dependence of the alignment process on the pitch Z of the helix is explored. When the sample thickness is much greater than Z, there is no preferred direction for the helix axis in the bulk, and applying H either ⊥. or || to the sample causes the helix axis to align || H. This effect only occurs at Z>10 μ, and only partial ordering occurs even at Tnematio. The angle δ between the long molecular axis of cholesteryl chloride and a plane ⊥ to the helix axis is reduced by application of H. Since δ is determined by the internal mechanical energy of the cholesteric structure, the decrease in δ becomes more pronounced as Z approaches infinity. A comparison is also made of the effects of electric and magnetic fields on CM; an electric field of 500 V/cm counteracts a magnetic field of the order of 10 kG. The anisotropy of the diamagnetic susceptibility is deduced to be ∼10-9.