Cholesteric Blue Phases in Mixtures and in an Electric Field

Abstract

Abstract We have determined phase diagrams (using the light microscope) and pitch (using a wedge geometry) for cholesteryl propionate mixed with various nematics and a chiral nematic mixed with cyanohexyloxybiphenyl (6OCB). We have succeeded in untwisting both blue phases, BPI and BPII of the latter mixture in an electric field. We find that with increasing field, BPI transforms to a cholesteric whereas BPII transforms to a nematic before aligning in the field. Furthermore, the voltage to align the blue phases is not very different from that in the cholesteric phase despite the fact that the pitch is quite different in each case. Our model is that blue phases are triggered by the appearance of submicroscopic isotropic regions in the cholesteric. These regions necessitate nematic boundary conditions at the isotropic interface. Because cholesterics and nematics are thermodynamically the same phase (they are completely miscible), a cholesteric-isotropic emulsion occurs to minimize the loss in twist energy. The nematic is the emulsifier, the inside phase is isotropic and the outside phase is cholesteric. The size of the isotropic regions scales with the square of the pitch. This implies the existence of a maximum cholesteric pitch for which an emulsion solution is possible. This is BPI. We speculate that it is stabilized by the entropy of disclination loops. Increasing temperature results in the emulsion turning inside out so that the cholesteric is now the inside phase and the isotropic liquid the outside one. This is BPII. A particular example of this model correctly predicts relative periodicities between the cholesteric phase and the blue phases, as well as the maximum pitch of the cholesteric with a blue phase. It also accounts for our observation of blue phases in an electric field and the habits of materials with both positive and negative dielectric anisotropies in the Cano wedge.