X‐ray‐diffraction measurements were done on octylcyanobiphenyl (8CB) films on a glass grating with a 9 &mgr;m period. The films were heated from the smectic to the isotropic phase using three different methods. It was found that the cooling rate, which determines the time the molecules are able to respond to the influence of the air and substrate interfaces, plays a critical role in the alignment of the liquid‐crystal molecules. In all cases, the surface tension was strong enough, and the energy cost of an interface with a second alignment was high enough, to produce a single homeotropic alignment in films as thick as 15 &mgr;m. ©1995 American Institute of Physics.