Lattice Parameter Measurements from the Kossel Diagrams of the Cubic Liquid Crystal Blue Phases

Abstract

The Kossel diagram technique has been used in the study of the blue phases of liquid crystals for some years. In this paper quantitative information extracted from Kossel diagrams of blue phases one (BPI) and two (BPII) is presented. The method used provides absolute measurements of the lattice parameters in these blue phases to an accuracy of about 1%. The measurements reveal a distortion in the BPI structure which is not obvious from qualitative examination of the Kossel diagrams themselves. The equality of the cholesteric phase helical pitch at the transition to BPI and the BPI helical pitch at the transition to BPII is confirmed. The experimental technique involved the use of a high stability temperature controller, reflection microscope, high resolution CCD camera and image digitization system. The Kossel diagrams were calibrated using a reflecting diffraction grating and hence measurements of lattice parameters required only the use of a single wavelength of light.