Thin-Thick Coexistence Behavior of 8CB Liquid Crystalline Films on Silicon

The wetting behavior of thin films of 4-n-octyl-4^′-cyanobiphenyl (8CB) on Si is investigated via optical and x-ray reflectivity measurement. An experimental phase diagram is obtained showing a broad thick-thin coexistence region spanning the bulk isotropic-to-nematic (T_IN) and the nematic-to-smectic-A (T_NA) temperatures. For Si surfaces with coverages between 47 and 72±3  nm, reentrant wetting behavior is observed twice as we increase the temperature, with separate coexistence behaviors near T_IN and T_NA. For coverages less than 47 nm, however, the two coexistence behaviors merge into a single coexistence region. The observed thin-thick coexistence near the second-order NA transition is not anticipated by any previous theory or experiment. Nevertheless, the behavior of the thin and thick phases within the coexistence regions is consistent with this being an equilibrium phenomenon.