Change To The Nematic Liquid Crystal Phase By The External Field

In nematic liquid crystal, an external field can induce a para-nematic phase(uniaxial or biaxial phase) at the temperature higher than bulk phase transition temperature from isotropic to normal uniaxial or biaxial nematics. Decreasing temperature, system may enter into a normal nematic liquid crystal phase from para-nematic via either first order or second order phase transition. Increasing the external field, system may change from para-nematic to nematic continuously.In order to study the influence of external field to the above phase transition, we consider, in this paper, a biaxial-shape rigid liquid crystal molecule which is abstracted into three perpendicular rods. The interaction between two molecules is the superposition of the interactions between rods on different molecules. The internal energy of molecule is the addition of molecular interaction energy and the energy in external field. Using mean field theory, the orientation distribution function, partition function and the free energy of the system are obtained. Minimizing the free energy with respect to order parameters, two non-linear equations are derived and corresponding numerical solutions are obtained. The phase diagram in the plane of reduced temperature and the molecular stucture parameter shows that, because of the external field, the uniaxial-shape liquid crystal molecule system may be induced a para-nematic phase at temperature higher than the temperature of phase transition from isotropic to uniaxial nematics. Increasing the biaxiality of molecules, system may. also be induced a para-biaxial phase at temperature higher than the corresponding phase transition temperature in zero field. Decreasing temperature, system may change from uniaxial to biaxial nematics continuously.