Phase diagrams of binary mixtures of hard rods in an external orientational field

We study a binary mixture of hard spherocylinders with the same diameter (D) and different length (L2>L1) in the absence and presence of a static external electric (magnetic) field on the basis of the Onsager theory. In the zero field, we find that the usual isotropic–nematic phase transition takes place only between the length ratio (L2/L1) of 1–2.9, the reentrant phenomenon occurs for L2/L12.7, and the nematic–nematic phase transition starts to be stable for L2/L1>2.9. In the external field, the isotropic–nematic transition is replaced by a weakly ordered nematic–nematic transition. The increasing external field strength shifts the transition towards lower densities and narrows the density gap terminating the first order transition at a critical point. The critical field strength is found to increase rapidly with increasing length ratio.