Fluctuations and clinicity in tilted smectic liquid crystals.

The overwhelming majority of tilted smectic liquid crystals exhibit synclinic (Sm-C) ordering (a uniform tilt direction in all smectic layers) rather than anticlinic (Sm-C(A)) ordering (a tilt direction that alternates from layer to layer). We propose that polar molecular-scale fluctuations of the interface between smectic layers provide a general entropic mechanism favoring synclinic ordering, and present evidence from simulations of the hard spherocylinder system in support of this hypothesis. We find that the entropy of the synclinic state of L/D=5 spherocylinders is higher than that of the anticlinic state for large tilt angles, and show that this entropy difference can be directly traced to molecular-scale fluctuations of the layer interface. This entropic mechanism may be suppressed in materials exhibiting anticlinic ordering due to a bent molecular conformational preference that quenches interface fluctuations.