Abstract
We report the results of a synchrotron x-ray reflectivity study of bulk liquid-crystal surfaces that are coated by thin wetting films of an immiscible liquid. The liquid-crystal subphase consisted of the nematic or isotropic phase of 4-octyl- 4;{'} -cyanobiphenyl (8CB), and the wetting film was formed by the fluorocarbon perfluoromethylcyclohexane (PFMC), a volatile liquid. The thickness of the wetting film was controlled by the temperature difference DeltaT(micro) between the sample and a reservoir of bulk PFMC, contained within the sealed sample cell. Phase information on the interfacial electron density profiles has been extracted from the interference between the scattering from the PFMC-vapor interface and the surface-induced smectic order of the 8CB subphase. The liquid-crystal side of the nematic-liquid (8CB-PFMC) interface is characterized by a density oscillation whose period corresponds to the smectic layer spacing and whose amplitude decays exponentially toward the underlying nematic subphase. The decay length xi of the smectic amplitude is independent of the PFMC film thickness but increases as the nematic-smectic- A transition temperature T(NA) is approached, in agreement with the longitudinal correlation length xi(parallel) proportional, variant(T-T(NA))(-0.7} for the smectic fluctuations in the bulk nematic. The results indicate that the homeotropic orientation of the 8CB molecules is preferred at the 8CB-PFMC interface and that the observed temperature dependence of the smectic layer growth is consistent with the critical adsorption mechanism. The observed DeltaT(micro) dependence of the PFMC film thickness, L proportional, variant(DeltaT(micro))(-1/3) , implies that PFMC completely wets the 8CB surface and is dominated by the nonretarded dispersion interactions between hydro- and fluorocarbons. The complete wetting behavior of PFMC is nearly independent of the degree of interfacial smectic order in the subphase.
Highlights
Fluid interfaces and films often provide an ideal platform for exploring fundamental aspects of condensed matter
We report the results of a synchrotron x-ray reflectivity study of bulk liquid-crystal surfaces that are coated by thin wetting films of an immiscible liquid
The decay length of the smectic amplitude is independent of the PFMC film thickness but increases as the nematic–smectic-A transition temperature TNA is approached, in agreement with the longitudinal correlation length ʈ κT − TNA−0.7 for the smectic fluctuations in the bulk nematic
Summary
Fluid interfaces and films often provide an ideal platform for exploring fundamental aspects of condensed matter. Due to its simplicity and relative lack of defects, the interface between a liquid crystal and an ordinary liquid should be well suited for elucidating the effects of contact material on liquid crystals17͔. Interference between the reflections from the liquid-vapor interface and the underlying liquid crystal provides more definitive phase information on the induced smectic order than was available in the previous measurements on the free surface of other liquid crystals7,9,10͔. The surface of a liquid crystal near a bulk phase transition provides an opportunity to study how the wetting behavior may be influenced by the variable degree of structural order just below the interface. Our results indicate that PFMC completely wets the 8CB surface and that the thickening of PFMC films with reduced chemical potential is nearly independent of the degree of induced smectic layering on the 8CB side of the interface.
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