Wetting of liquid-crystal surfaces and induced smectic layering at a nematic-liquid interface: An x-ray reflectivity study

Phys. Rev. E 77, 031607 – Published 14 March 2008
Masafumi Fukuto, Oleg Gang, Kyle J. Alvine, Benjamin M. Ocko, and Peter S. Pershan

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 ΔTμ 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 ξ 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 ξ(TTNA)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 ΔTμ dependence of the PFMC film thickness, L(ΔTμ)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.

DOI: http://dx.doi.org/10.1103/PhysRevE.77.031607

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  • Received 17 December 2007
  • Revised 6 February 2008
  • Published 14 March 2008

© 2008 The American Physical Society

Authors & Affiliations

Masafumi Fukuto1,*, Oleg Gang2, Kyle J. Alvine3, Benjamin M. Ocko1, and Peter S. Pershan3,4

  • 1Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
  • 2Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
  • 3Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
  • 4Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

  • *fukuto@bnl.gov

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