Orientational transitions in a nematic liquid crystal confined by competing surfaces - art. no. 051704

The effect of confinement on the orientational structure of a nematic liqui d crystal model has been investigated by using a version of density-functio nal theory. We have focused on the case of a nematic confined by opposing f lat surfaces, in slab geometry (slit pore), which favor planar molecular al ignment (parallel to the surface) and homeotropic alignment (perpendicular to the surface), respectively. The spatial dependence of the tilt angle of the director with respect to the surface normal has been studied, as well a s the tensorial order parameter describing the molecular order around the d irector. For a pore of given width, we find that, for weak surface fields, the alignment of the nematic director is perpendicular to the surface in a region next to the surface favoring homeotropic alignment, and parallel alo ng the rest of the pore, with a sharp interface separating these regions (S phase). For strong surface fields, the director is distorted uniformly, th e tilt angle exhibiting a linear dependence on the distance normal to the s urface (L phase). Our calculations reveal the existence of a first-order tr ansition between the two director configurations, which is driven by change s in the surface field strength, and also by changes in the pore width. In the latter case the transition occurs, for a given surface field, between t he S phase for narrow pores and the L phase for wider pores. A link between the L-S transition and the anchoring transition observed for the semi-infi nite case is proposed.