Transcript Document

Wetting of Liquid Crystal Surfaces and Induced Smectic Layering at the Nematic-Liquid Interface*
Masafumi
1Condensed
1
Fukuto,
Oleg
2
Gang,
3
Alvine,
Kyle J.
Benjamin M.
1
Ocko,
and Peter S.
3,4
Pershan
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)
• 8CB only (no PFMC).
•Bragg-like peak at qz = 2/d = 0.2 Å–1 due
to surface-induced smectic layering
(spacing d = 31.4 Å).
Wetting of Liquid Crystal Surface
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.
• Growth of smectic layering as T  TNA+.
Relative election density r(z)/r
z1 = z0 + d/4
T – TNA [K]
z [Å]
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]
of smectic e-density oscillation
g 1 [Å]
qz [Å–1]
Amplitude decay length x [Å]
x = decay length for the amplitude
Adsorption G = g1 – g1(8CB) [Å]
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.
Induced Smectic layering
g 1 [Å]
XR: LC-vapor interface
Fresnel normalized reflectivity R/RF
Abstract
System and Experimental Setup
XR: LC-liquid-vapor interface
• Liquid crystal (LC) subphase: 8CB
Smectic-A
Nematic
~33.5 °C
Isotropic
~40.5 °C
T – TNA [K]
Growth of induced smectic layering (8CB)
• Wetting liquid: PFMC
T-controlled inner
and outer cells
t = (T – TNA )/TNA
PFMC film on 8CB
at T = Tres + DTm
DTm [K]
Saturated vapor
• Wetting film thickness
controlled via T-offset
DTm > 0 between
sample and reservoir.
PFMC liquid
reservoir at Tres
Thickening of
wetting film
XR data
points
(PFMC)
• As in the previous observation at the nematic-vapor interface,
x(T)  x||(T) is consistent with the critical adsorption of smectic
layers at the nematic-liquid interface.
T – TNA = 19.3 K (I)
Fresnel-normalized reflectivity R/RF
• NSLS Beamline X22B: Liquid surface diffractometer, l = 1.53 Å
qz [Å–1]
Fresnel-normalized reflectivity:
 iq z z
e


Model for average electron density profile (relative to r for bulk 8CB):
r z  1 
 z 
 1  erf 

r
2
 2 
qz [Å–1]
qz [Å–1]
2
Induced
 8CB-vapor interface
8CB
 smectic
 z  z0 A exp z  z0  x sin2 z  z0  d 
layering
L   z  L 
 z 
 erf 
  erf 
  PFMC film ( = 0 for no film)
L
2   2 
 2 
Relative election density r(z)/r
Rqz 
d  r z 
  dz 
RF qz 
dz  r
• 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.
z1 = z0 + d/4
z1
z1
• 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.
• 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.
z [Å]
z [Å]
g1 
z1
 dz r z 
r 
L = 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.
1/ 3


A
e
ff

L
 6n QDTm T 


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.
DTm [K]
z [Å]
G = g1 (PFMC/8CB) – g1(8CB)
at z1 = z0 + d/4, is a measure of the
surface order parameter for the
smectic correlations in nematic.
T – TNA [K]
1
z
A1 = r(z1)/r = A exp(–d/4x)
1st smectic layer peak amplitude A1 = A exp(-d/4x)
X-ray Reflectivity (XR) Measurements
T – TNA = 3.7 K (N)
The density oscillation amplitude
of the first smectic layer right
below the interface, i.e.,
A1 = A exp(-d/4x)
T – TNA = 0.7 K (N)
r(z)/r
DTm [K]
2. Effective thickness Leff = G/L based on the adsorption G obtained from the
extracted e-density profile, where:
T – TNA [K]
L = rPFMC/r = 1.59
DTm [K]
T – TNA
Subphase
x||
Aeff
0.7 K
Nematic
208 Å
(1.3 ± 0.4)  10–20 J
3.7 K
Nematic
68 Å
(1.6 ± 0.4)  10–20 J
19.3 K
Isotropic
23 Å
(2.3 ± 0.9)  10–20 J
(extrapolated)
• 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).