Reorientation mechanisms in smectic A liquid crystals

The reorientation mechanism of smectic A (SmA) layers in a field is non-trivial; at least two different mechanisms have been reported. In thin samples reorientation is achieved through complete disordering and reordering of the smectic layers, while in thick samples layers can also appear to rotate continuously as a monodomain. We explore the transition between these regimes, showing qualitatively that the mechanism adopted depends on the relative importance of the sample-cell wall interaction. We observe reorientation of the smectic layers of 4'-n-octyl-4-cyanobiphenyl (8CB) in a magnetic field of flux density 4 T within 4 K of the nematic to Sm-A transition. Using time-resolved small angle X-ray scattering on samples of thickness 170 mu m, 270 mu m or 340 mu m, we determine the transition between the thickness regimes, demonstrating a new multi-step process that is adopted in specific cases. Importantly, the time-resolved experiments allow the dynamics of the reorientation to be probed, and we measure temperature-independent angular speeds of 0.038 +/- 0.002 degrees s(-1) for the continuous rotation process. Finally, we report evidence of apparent random changes in layer spacing (similar to 0.5 angstrom) as a precursor to the rotation of the Sm-A layers in some cases and discuss the possible origin of this phenomenon.