Effect of a Strong Electric Field on the Reentrant Nematic to Smectic Ad Phase Transition

We report experimental studies on the effect of a strong electric field on the nematic-smectic A d (N-A d ) and A -reentrant nematic (A d -N R ) phase transitions in a binary mixture. Both the transition temperatures increase with field. The A -N transition point changes much more rapidly than that of A d -N transition, indicating that the A d phase will get bounded at a sufficiently high field. We have included the effect of the electric field in our molecular model of reentrant phases in highly polar compounds, in which the mutual orientation of near neighbour molecules changes from an antiparallel to a parallel configuration as the temperature is lowered. It is argued that the cross-over temperature increases with field which accounts for the observed trends. For suitable parameters, it is also predicted that either a nematic-nematic transition line can branch off from the A d -N R line or a smectic A-smectic A transition line can meet the A d -N R line at an appropriate field. In the latter case, the A d -N R line has an associated slope change which is consistent with the experimental data.