Wetting of Liquid Crystal Surfaces and Induced Smectic Layering at the Nematic-Liquid Interface*

1. Smectic-A Nematic Isotropic • Wetting liquid: PFMC ~40.5 °C ~33.5 °C • Liquid crystal (LC) subphase: 8CB • Two methods used to determine the PFMC wetting film thickness produce thickness values that are in agreement with each other: 1. XR fitting parameter L for the thickness of the “box” layer. 2. Effective thickness Leff = G/fL based on the adsorptionG obtained from the extracted e-density profile, where: G = g1 (PFMC/8CB) – g1(8CB) fL = rPFMC/r8CB = 1.59 z1 = z0 + d/4 = center position of 1st smectic layer • The observed 1/3 power law behavior, L  (DTm)–1/3, is consistent with the complete wetting of the liquid crystal (8CB) surface by fluorocarbon (PFMC) liquid. n(T) = molar density of wetting liquid (PFMC) Q = latent heat of wetting molecule (PFMC) Effective Hamaker constant:Aeff A measure of how strong the van der Waals attractions between the wetting and subphase molecules are relative to those between wetting molecules themselves. Wetting of Liquid Crystal Surfaces and Induced Smectic Layering at the Nematic-Liquid Interface* Masafumi Fukuto,1 Oleg Gang,2 Kyle J. Alvine,3 Benjamin M. Ocko,1 and Peter S. Pershan3,4 1Condensed Matter Physics and Materials Science Department and 2Center for Functional Nanomaterials, Brookhaven National Laboraroty, Upton, NY 11973 3Division of Engineering and Applied Sciences and 4Physics Department, Harvard University, Cambridge, MA 02138 *Work supported by U.S. DOE under DE-AC02-98CH10886 (BNL) and by the NSF Grant No. 03-03926 (Harvard) Abstract XR: LC-vapor interface Induced Smectic layering Wetting of Liquid Crystal Surface We present a synchrotron x-ray reflectivity (XR) study of the interfacial behavior of a bulk liquid crystal 8CB surface that is coated by a thin wetting film of an immiscible liquid, perfluoromethylcyclohexane (PFMC). The thickness of the wetting film was controlled by the temperature difference DTm = T – Tres between the sample and a reservoir of bulk PFMC. Interfacial electron density profiles have been extracted from the x-ray interference between the PFMC-vapor interface and the surface induced smectic order. The observed DTm dependence of thickness of the PFMC film, L (DTm)-1/3, is consistent with complete wetting. The liquid crystal side of the nematic-liquid interface is characterized by a density oscillation whose period is equal to the smectic layer spacing and whose amplitude decays exponentially towards the nematic subphase. The results indicate that the homeotropic orientation of the 8CB molecules is preferred at the PFMC-8CB interface and that the observed temperature dependence of smectic layer growth is consistent with a critical adsorption mechanism, independent of the PFMC film thickness. • 8CB only (no PFMC). • Bragg-like peak at qz = 2p/d = 0.2 Å–1 due to surface-induced smectic layering (spacing d = 31.4 Å). • Growth of smectic layering as TTNA+. Note: Each smectic layer consists of a “bilayer”-like assembly of 8CB molecules; the cyanobiphenyl groups point toward the central plane of the smectic layer, and the –CH3 terminated alkyl chains point away from the central plane. g1 [Å] z1 = z0 + d/4 T – TNA [K] x= decay length for the amplitude of smectic e-density oscillation Fresnel normalized reflectivity R/RF Relative election density r(z)/r x|| (3.54 Å) t –0.67 = longitudinal correlation length (along the smectic layer normal) for the smectic order in bulk nematic. PFMC film thickness [Å] Amplitude decay length x [Å] g1 [Å] Adsorption G = g1 – g1(8CB) [Å] qz [Å–1] z [Å] System and Experimental Setup XR: LC-liquid-vapor interface T – TNA [K] Growth of induced smectic layering (8CB) T-controlled inner and outer cells t = (T – TNA)/TNA DTm [K] DTm [K] Thickening of wetting film (PFMC) Saturated vapor • As in the previous observation at the nematic-vapor interface, x(T) x||(T) is consistent with the critical adsorptionof smectic layers at the nematic-liquid interface. DTm [K] PFMC film on 8CB at T = Tres + DTm XR data points • Wetting film thickness controlled via T-offsetDTm> 0 between sample and reservoir. PFMC liquid reservoir at Tres T – TNA [K] The density oscillation amplitude of the first smectic layer right below the interface, i.e., A1 = r(z1)/r = A exp(–d/4x) at z1 = z0 + d/4, is a measure of the surface order parameter for the smectic correlations in nematic. T – TNA = 0.7 K (N) T – TNA = 3.7 K (N) T – TNA = 19.3 K (I) X-ray Reflectivity (XR) Measurements • NSLS Beamline X22B: Liquid surface diffractometer, l = 1.53 Å A1 = A exp(-d/4x) fL = rPFMC/r = 1.59 T – TNA [K] Fresnel-normalized reflectivity R/RF • For nematic 8CB subphase, the amplitude A1 is roughly: • i) independent of whether the interface is in contact with vapor (i.e., no PFMC) or with PFMC liquid. • ii) independent of the thickness of the PFMC wetting film on top. r(z)/r 1 1st smectic layer peak amplitude A1 = A exp(-d/4x) z qz [Å–1] qz [Å–1] qz [Å–1] Fresnel-normalized reflectivity: z1 = z0 + d/4 z1 z1 Model for average electron density profile (relative to r for bulk 8CB): Relative election density r(z)/r Induced 8CB smectic layering DTm [K]  8CB-vapor interface  • Just as at the 8CB-vapor interface, 8CB molecules in contact with the fluorocarbon liquid PFMC are oriented homeotropically (normal to interface), with the -CH3 end pointing toward PFMC. • The extracted Hamaker constant is slightly larger for isotropic 8CB (with very short-ranged surface smectic order) than for nematic 8CB (with extended surface smectic order).  PFMC film (fL = 0 for no film) z [Å] z [Å] z [Å]