External field induced tricritical phenomenon in the isotropic-nematic phase transition of hard non-spherical particle systems

The effect of static external held is studied on the isotropic-nematic phas e transition of a system of hard non-spherical particles (rods or platelets ) with negative anisotropic polarizability (susceptabilities). On the basis of Onsager theory, the phase coexistence curve is calculated numerically w ithout any approximation. It is found that a weakly ordered nematic phase ( uniaxial planar) is in coexistence with a highly ordered biaxial nematic ph ase which ends at a tricritical point. In the limit of infinite field stren gth, the orientations of the particles are confined in a plane perpendicula r to the field and continuous isotropic-nematic phase transition takes plac e.