Effect of a weak electric field on the cubic blue phases of cholesteric liquid crystals.

Abstract

The effect of a weak electric field E on the cubic blue phases (BP's) of cholesteric liquid crystals is considered. A Landau model, modified to include the uniaxiality induced by the field, is used. It is shown for space groups ${O}^{8}$ and ${O}^{2}$, which are believed to characterize BP I and BP II, respectively, that a free crystal will orient itself such that the field is parallel to a fourfold axis (a 〈001〉 direction), in agreement with experiment. For ${O}^{5}$, the stable orientation would be with the field parallel to a threefold axis (a 〈111〉 direction). In addition, in the presence of the field, there is (for all three cases) an induced birefringence. There are also shifts in the wavelengths of the Bragg reflections, and the cubic unit cell is distorted by a field in a 〈001〉 (〈111〉) direction into one having tetragonal (trigonal) symmetry. The ratio of the cell dimensions parallel and perpendicular to the field, the induced birefringence, and the wavelength shifts are, to lowest order, quadratic in E as expected from symmetry considerations. These results provide additional evidence for the generally accepted assignment of ${O}^{8}$ (${O}^{2}$) for BP I (BP II), and for ruling out an ${O}^{5}$ structure for the observed BP.