Lattice of disclinations: The structure of the blue phases of cholesteric liquid crystals

We argue that the basic mechanism stabilizing the blue phases is the appearance of double cholesteric twist. The free-energy cost of the disclination lattice that inevitably accompanies double twist becomes small near the clearing point. We use the director picture and the Oseen-Frank equations in a computer calculation of the free energy for three specific models, with $O^5$, $O^2$, and $O^8$ symmetry. Disclinations are treated as having an isotropic core. Comparison of the results of the computation with a number of experimental quantities is given. In a final section, a mean-field theory of disclinations is presented. It is argued there that the biaxiality, which is a characteristic of Landau theories of the blue phase, can be considered as an "escape" of the core of the disclination, forced by the implicit requirement of analyticity.