Stripe domains in a nearly homeotropic nematic liquid crystal:: A bend escaped state at a nematic-smectic-A transition

We report an observation and mechanism of spontaneous periodic modulations of the nematic director close to the temperature T-NA of a nematic-to-smectic-A phase transition if the surface alignment slightly differs from a pure homeotropic one. Stripe domains appear in the nematic phase about one degree above T-NA and persist into the SmA phase. The instability of the homogeneous state with respect to stripe domains is shown to be related to a very large bend constant which is much larger than the twist and splay elastic constants. The instability mechanism consists of reduction of the highly energetic bend deformation, induced by small surface director tilts, at the expense of a spontaneous periodic splay-twist modulation. Using smallness of the twist-to-bend and splay-to-bend elastic constant ratios, the critical condition of the instability and the modulation period are found analytically. Both the experimentally obtained and theoretically predicted domain period scales very closely to a square root of the cell thickness.