Biaxial nematic liquid crystal phases of a soft ellipsoid-string fluid

Abstract

We have calculated the phase diagram of a fluid, the molecules of which are composed of a string of partially overlapping oblate soft ellipsoid interaction sites, by molecular dynamics simulation. The molecules are elongated in one direction and flattened in the perpendicular direction so that they become biaxial. We find that when the fluid consisting of 8-site or 9-site molecules is compressed, a discotic uniaxial nematic phase is formed. Further compression causes a transition to a biaxial phase. The same phase behaviour is found by compressing a fluid composed of molecules with 10 or 11 sites, but in this case the uniaxial phase is calamitic. The 9-site and 10-site molecules consequently bracket the Landau bicritical point where there is a continuous transition between the isotropic phase and the biaxial nematic phase. This behaviour is consistent with the Landau–de Gennes theory and mean field theories. However, the molecular dimensions for which the Landau point appears differ from the theoretical predictions of the mean field theories.