Surface ordering at the air–nematic interface. Part 1. A neutron reflection study of translational order

Abstract

Neutron reflection was used to measure the build‐up of layers at the air surface as the smectic phase was approached from higher temperatures in a nematic liquid crystal. The liquid crystal was 4‐octyl‐4′‐cyanobiphenyl with a deuterated core and hydrogenous tails. The development of surface smectic layers in the nematic phase was followed by measuring specular reflectivity and monitoring the pseudo‐Bragg peak from the layers. The pseudo‐Bragg peak therefore corresponded to the formation of long‐range smectic layers at the surface as the transition temperature is approached. The results were consistent with the continuous growth of smectic layers at the surface and the amplitude of the smectic density wave decayed with increasing distance from the surface. The depth over which the smectic layering occurred at the interface was found to be ∼450 Å at a quarter of a degree above the nematic to smectic A bulk transition. The smectic order parameter at the surface was ∼0.1 which is much less that found for typical homeotropic alignment agents.