Mol. Cryst. Liq. Cryst. 1990, Vol. 192, pp. 239-243
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SURFACE POLARIZATION AND DOMAIN STRUCTUFES I N
T H I N NEMATIC LAYERS
,
0 .LAVREhTMVICH V.PERG~~SHCEIK,
AND V. SERGAN
I n s t . of Phys. Acad. of Sci.,252650, Kiev, USSR
A b s t r a c t A new e l e c t r o o p t i c a l e f f e c t occuring i n
nematic, which proves e x i s t e n c e of s u r f a c e p o l a r i z a t i o n i n l i q u i d c r y s t a l , i s discovered experimentally.
'Phe e l e c t r o o p t i c a l e f f e c t s i n nematics, caused by t h e
d i e l e c t r i c , f l e x o e l e c t r i c and hydrodynamical mechanisms,
have been c l a r i f i e d t o s a t i s f a c t o r y e x t e n t . S i m i l a r
e f f e c t s a l s o can be induced by t h e s u r f a c e p o l a r i z a t i o n of t h e nematic, caused by assymmetric molecule
o r i e n t a t i o n s on t h e boundary.
However, one d i d not
manage t o d i s t i n g u i s h them unambiguously from t h e background. The p r e s e n t paper gives account of t h e observ a t i o n and i n t e r p r e t a t i o n of a new e l e c t r o o p t i c a l e f f e c t
which proves t h e e x i s t e n c e of s u r f a c e p o l a r i z a t i o n .
We s t u d i e d t h e homeotropically o r i e n t e d l a y e r s
(Fig.1 a , b ) of penthylcyanobiphenyl (5CB) with p o s i t i v e
The e x t e r n a l DC e l e c t r i c
d i e l e c t r i c anisotropy d B
f i e l d was d i r e c t e d v e r t i c a l l y , which enables u s t o exclude t h e d e s t a b i l i z i n g i n f l u e n c e of d i e l e c t r i c and
t h e m a j o r i t y of electrohydrodynamical e f f e c t s . I n o r d e r
t o s e p a r a t e s u r f a c e p o l a r i z a t i o n from f l e x o e l e c t r i c
mechanism, we employed two types of c e l l s of t h i c k n e s s
d = 1 0 - 1 0 0 ~ md i f f e r i n g i n t h e o r i e n t i n g covers and the-
.
240/[676]
0. LAVRENTOVICH ET AL.
FIGURE 1. Surface i n s t a b i l i t y geometries i n A( l e f t s i d e ) and C- ( r i g h t s i d e ) c e l l s : a , b ) below
t h r e s h o l d , c , d ) above t h r e s h o l d . Surfaae p o l a r i z a t i o n i s marked by arrows.
FIGURE 2. Textures of t h e anode s u r f a c e i n s t a b i l i t i e s i n A-cell: a ) homeotropic t e x t u r e , below
t h r e s h o l d ; b) domains on t h e lower e l e c t r o d e ; c)
domains on t h e upper e l e c t r o d e . D i r e c t i o n of t h e
f i e l d a r e shown by t r a d i t i o n a l symbols.
SURFACE POLARIZATION AND DOMAIN STRUCTURES
[677]1241
r e f o r e , i n t h e type of s u r f a c e p o l a r i z a t i o n . I n t h e Ac e l l s , t h e Sn02 e l e c t r o d e s were covered by a l a y e r s
of s i l i c o n elastomer, which i s h y d r o p h i l i c and s o favours t h e o r i e n t a t i o n of p o l a r CN groups of 5CB molec u l e s towards t h e s u b s t r a t e . I n t h e C-cells,
addi t ional l a y e r s of l e c i t h i n were coated on t h e elastomer,
and opposite d i r e c t i o n of molecules would be expected. 2
A s soon a s f i e l d i s a p p l i e d , t h e c e l l i s e n l i g h t e ned and domains a r e formed (Fig.2). The e f f e c t occured
s i m i l a r l y i n t h e c e l l s of both t y p e s and appeared a t
v o l t a g e U g r e a t e r t h a n some c r i t i c a l Uc -- 1 V ( ~ i g . 3 ) .
FIGUIIE: 3.
Threshold v o l t a ge v s c e l l t h i c k n e s s f o r
A-cells ( 0 ) and C-cells
50 d, ~m
For U*Uc t h e domains look a s "cloudy" p a t t e r n s s i m i lar t o those described by Monkade e t a1,3 and f o r UP
(2+3)U,, t h e y look a s d i s t i n c t rounded domain whose
geometry i s shown i n Fig.lc,d. Changing t h e microscope
focusing it i s p o s s i b l e t o f i n d t h a t t h e d i s t o r t i o n s
a r e l o c a l i z e d n e a r t h e anode i n A-cells and near t h e
cathode i n C-cells. This p e c u l i a r i t y i s c r u c i a l f o r
r e v e a l i n g t h e n a t u r e of t h e e f f e c t s .
F i r s t l y l e t us d i s c u s s t h e p o s s i b l e f l e x o e l e c t r i c
mechanism. It i s known3 , t h a t a eurface tilt of t h e
d i r e c t o r n can generate a f l e x o p o l a r i a a t i o n pf=e,'iTdiv&
e r o t z s a n d t h u s lowers t h e coupling energy (-PfE)
3
with applied f i e l d . However, t h e signs of f l e x o c o e f f i c i e n t s e,, and e 3 a r e t h e same i n t h e c e l l s of both
t y p e s , t h u s t h e p o l a r i t y of f l e x o i n s t a b i l i t i e s must
25
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0.LAVRENTOVICH ET AL.
242/[678]
be t h e same i n A- and C-cells, This conclusion contrad i c t s t h e data.
The. d a t a can be explained t a k i n g i n t o account t h e
s u r f a c e p o l a r i z a t i o n of 5CB, which b r i n g s t h e s i m i l a r
c o n t r i b u t i o n (-PsE) i n t h e s u r f a c e d e n s i t y of t h e f r e e
energyO3 Inasmuch as t h e d i r e c t i o n of Ps a r e d i f f e r e n t
i n A- and C-cells, f o r s u f f i c i e n t l y l a r g e \Psi one
should expect t h a t t h e anode i n s t a b i l i t y must occur i n
t h e A-cells, and t h e cathode i n s t a b i l i t y must be developed i n t h e C-cells (Fig.1). A t l e a s t i n one of t h e s e
c e l l s t h e d e s t a b i l i z i n g moment cannot be caused by
any e f f e c t o t h e r t h a n s u r f a c e p o l a r i z a t i o n ; both f l e x o e l e c t r i c and d i e l e c t r i c moments make s t a b i l i z i n g e f f e c t , a s w e l l a s t h e s u r f a c e anchoring r e s p o n s i b l e f o r
t h e i n i t i a l homeotropic o r i e n t a t i o n and bulk e l a s t i c
curvature, I n o r d e r t o e s t i m a t e IPJ v a l u e s we use an
expression t h a t follows from t h e t h e o r y of one-dimen4
sional surface polar i n s t a b i l i t y :
--
-
2
R =d&K / 4 X , Kg3 i s bend e l a s t i c conwhere e=e +e
1 3'
33
s t a n t , W i s anchoring energy, "+I1 and 'I-"
correspond
t o A- and C-cells, r e s p e c t i v e l y . S u b s t i t u t i n g K33'
10-~d~n
db
, -14, e = 1 , 5 * 1 0 - ~ d ~ n ~
(Ref
/ ~ .5), and s p e c i a l l y measured v a l u e s YA=2* l ~ - ~ d ~ n / c m
WC=5*
,
10-~dyn/crn
one o b t a i n s 1 PsA( = 6 * 1 0 - ~ d ~ nand
~ /1~
Psc( =4-10-~dyn1/2 ,
Let us compare t h e v a l u e s obtained t o t h e maximum
p o s s i b l e Ps ,ma,
Assuming t h e 5CB molecule d i p o l e s t o
be o r i e n t e d i n t h e boundary l a y e r of t h i c k n e s s 5 uni, i s molecular
formly, we o b t a i n Ps,mau = f pN, where U
d i p o l e moment, N i s t h e molecular d e n s i t y of 5CB. Sinand
- ~ p - 4 D, we
ce 1-100 8 ( ~ e f . 2 ) , ~ = 2 . 5 - 1 0 ~ ~ c m
SURFACE POLARIZATION AND DOMAIN STRUCTURES
[679]1243
have
i n agreement with our e s t i m a t i ons of PsA and PsC.
I n conclusion we emphasize t h a t t h e e l e c t r o o p t i c a l
e f f e c t observed i s n o t a s s o c i a t e d I ) with t h e e l e c t r o hydrodynamical i n s t a b i l i t i e s : any motions of t h e subs t a n c e i n t h e c e l l s d i d n o t occur up t o t h e v o l t a g e U=
IOV, which i s much h i g h e r t h a n Uc; 2 ) with t h e c u r r e n t
c a r r i e r i n j e c t i o n from e l e c t r o d e s , which was blocked
by t h e elastomer l a y e r s ; 3 ) with probable inhomogeneit i e s of t h e c e l l s : t h e domains were formed simultaneously on t h e whole c e l l s u r f a c e , and t h e i r c e n t e r s
were n o t connected with any d i s t i n g u i s h e d p o i n t s and
changed t h e i r l o c a t i o n s a s t h e f i e l d was switched on
and taken away. More d e t a i l study w i l l be published
elsewhere.
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and G. Durand,
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