Electric-field-induced phase transitions in frustrated cholesteric liquid crystals of negative dielectric anisotropy.

Abstract

The phase diagram of a large-pitch cholesteric liquid crystal sandwiched between two parallel glass plates with homeotropic anchoring is determined both experimentally and theoretically. Contrary to previous experiments [P. Ribi\`ere and P. Oswald, J. Phys. (Paris) 51, 1703 (1990)], the materials chosen have a negative dielectric anisotropy so that experiments are performed at smaller thickness than the critical value ${\mathit{d}}_{\mathit{c}}$ that is necessary to unwind the helical structure completely at vanishing electric field. The two control parameters are the frustration ratio d/p of the thickness over the quiescent pitch of the cholesteric liquid crystal and the applied voltage V. The main difference with previous experiments is that the cholesteric-nematic phase transition can be second order or first order depending on the sample thickness. There is thus a Landau tricritical point in the phase diagram. Furthermore, we have found a triple point where the nematic phase coexists with both a translationally invariant configuration and a periodic structure. A theoretical model, including two order parameters, allows us to calculate the coordinates of these two particular points as a function of the Frank elastic constants as well as the general aspect of the phase diagram. The agreement between theory and experiment is satisfactory.