The phenomenon of re-entrance

When a compound exhibits both nematic and smectic phases, then, as a rule, the nematic occurs at a higher temperature. Exceptions to the rule were discovered by Cladis in certain strongly polar materials. The first observations were on binary mixtures of two cyano compounds over a range of composition the sequence of transitions on cooling was as follows. 

These observations stimulated a great deal of interest, and much more 

A number of other more elaborate models, which take into account attractive forces and hard core repulsions, dipole-induced dipole interactions (which, interestingly, can favour a parallel configuration at small intermolecular separations ), etc., have been proposed sub- 

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