The Critical Pitch-Thickness Relation at the Cholesteric-Nematic Phase Transition by the Optical Mass Transport Method

Abstract

The optical mass transport (OMT) method was utilized to study the cholesteric pitch as a function of thickness at the cholesteric-nematic phase transition in a wedge-type diffusion cell at two different geometries. We obtained a linear relation between the cholesteric finger print pitch (P_c) and the film thickness (L_c) at the critical cholesteric-nematic boundary layer. The measurements were done by direct observation via an optical microscope. Due to the presence of the concentration gradient, the surface anchoring is weakened and large pitch values within the range of L_c/P_c ≤ 0.15 were established. The thickness gradient as well as the concentration gradient showed systematic linear effects on the data. We found a qualitative agreement between the present experimental data and that of a previous theory.