5 Interfaces i n Nematic Liquids
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RICHARD
WILLIAMS
R C A Laboratories, Princeton, N. J.
Since the physical properties of nematic liquids are highly anisotropic, the differences between regions of a nematic liquid which are differently oriented become comparable to those between different phases or different compounds in a system of ordinary liquids. This can give rise to interfaces in a single-phase nematic liquid system at the boundaries of differently oriented regions. When an electric field is applied to a suitable specimen of a nematic liquid, a domain structure appears. This is believed to indicate the underlying preferred orientation of molecules in the liquid. From observations of the domains, two kinds of interfaces have been found in nematic liquids. These are illustrated by photographs of the corresponding domain structure.
TVTematic l i q u i d s are f o r m e d b y c e r t a i n organic c o m p o u n d s w h i c h h a v e l o n g r o d l i k e molecules. T h e essential a n d c h a r a c t e r i s t i c feature of t h e n e m a t i c phase (3) is t h e o r d e r i n g of molecules; a l l are oriented so t h a t t h e i r l o n g axes are a p p r o x i m a t e l y p a r a l l e l . T h i s r e l a t i v e m u t u a l o r i e n t a t i o n is preserved over distances a l w a y s large c o m p a r e d w i t h t h e size of molecules a n d sometimes over macroscopic dimensions. W h i l e there is long-range order w i t h respect t o t h e o r i e n t a t i o n of molecules, there is n o n e w i t h r e spect t o t h e positions of t h e i r centers of mass. I n fact, t r a n s l a t i o n of molecules takes place as i n a n o r d i n a r y l i q u i d . A n i m p o r t a n t result of t h i s c o m b i n a t i o n of r i g i d i t y w i t h respect t o r o t a t i o n a n d f l u i d i t y w i t h respect t o t r a n s l a t i o n is t h a t the l i q u i d takes o n t h e properties of a u n i a x i a l c r y s t a l h a v i n g a u n i q u e s u s c e p t i b i l i t y t o o r i e n t a t i o n b y e x t e r n a l electric a n d m a g netic fields (3, 9, 10,11, 12). A f u r t h e r o r d e r i n g effect is p r o d u c e d b y t h e walls of t h e container. T h e o r i e n t a t i o n of t h e c o m m o n axis of t h e m o l e cules i n a n e m a t i c l i q u i d m a y be d e t e r m i n e d b y t h e w a l l o u t t o m a c r o s c o p i c distances f r o m t h e w a l l . C o m b i n e d o r i e n t i n g effects of t h e c o n t a i n e r w a l l a n d a n e x t e r n a l electric field give rise t o a d o m a i n s t r u c t u r e . T h i s p e r m i t s t h e o b s e r v a t i o n of c e r t a i n interfaces w i t h i n n e m a t i c l i q u i d s , discussed below. 61 Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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T h e arrangement of molecules i n n e m a t i c l i q u i d s imposes a h i g h degree of a n i s o t r o p y o n t h e b u l k properties, s u c h as dielectric constant, m a g n e t i c s u s c e p t i b i l i t y , o p t i c a l a b s o r p t i o n , a n d v i s c o s i t y (8, 9). Therefore, t h e d i f ferences between regions of a n e m a t i c l i q u i d w h i c h are differently o r i e n t e d become c o m p a r a b l e t o those between different phases or different c h e m i c a l c o m p o u n d s i n a s y s t e m of o r d i n a r y l i q u i d s — f o r example, i n the n e m a t i c l i q u i d , p-azoxyanisole, at 117°C. t h e refractive i n d e x (1) for t h e o r d i n a r y r a y is 1.561 a n d t h a t for t h e e x t r a o r d i n a r y r a y is 1.849. T h e difference between these is greater t h a n between t h e refractive indices of w a t e r a n d benzene, w h i c h is a b o u t as great as t h a t between a n y t w o c o m m o n l a b o r a t o r y solvents. W e m a y expect, t h e n , t h a t regions of differing o r i e n t a t i o n or m o l e c u l a r a r r a n g e m e n t w i l l be s e n s i t i v e l y detected b y o p t i c a l o b s e r v a tions a n d t h a t interfaces between differently oriented regions w i l l h a v e some of t h e properties of o r d i n a r y interfaces i n systems of t w o phases. T h e s e i n c l u d e i n t e r f a c i a l t e n s i o n a n d p o s s i b l y electrical properties. In w h a t follows, o p t i c a l observations of n e m a t i c l i q u i d s i n electric fields are used to d e m o n s t r a t e t h e existence of s u c h interfaces a n d t o show some of t h e i r properties. W h e n a t h i n l a y e r of a n e m a t i c l i q u i d is i n a n electric field, a d o m a i n s t r u c t u r e appears w h i c h is r e a d i l y v i s i b l e u n d e r a microscope or even to t h e u n a i d e d eye (2, 5, 6, 7, 8). A n example of t h i s s t r u c t u r e i n p-azoxyanisole
Figure 1.
Domains in p-azoxyanisole liquid crystal
Vertical line about 14 of the way in from the right border is the edge of a strip of transparent conductive coating on the glass plates between which the liquid is contained. To the right of this there is no field. To the left there is a 1-kc. ax. field of 2500 volts/cm. directed perpendicular to the plane of the page. When the field is removed, the domain pattern disappears in 10 or 20 milliseconds. Speciman thickness 50 microns. Temperature, 125°C. Viewed in unpolarized light
Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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is s h o w n i n F i g u r e 1. T h e characteristic feature is a periodic macroscopic s t r u c t u r e w h i c h appears w h e n a n electric field is a p p l i e d across t h e l i q u i d . T h e t i m e constants for t h e appearance a n d disappearance of t h e d o m a i n s as t h e field is a p p l i e d a n d r e m o v e d are o n t h e order of m i l l i s e c o n d s . The reason for t h e f o r m a t i o n of t h e d o m a i n s is n o t y e t k n o w n w i t h c e r t a i n t y . I f t h e field is r e m o v e d a n d t h e n a p p l i e d a g a i n , t h e o r i e n t a t i o n of t h e d o m a i n s is t h e same at a g i v e n p o i n t . T h i s a n d t h e s h a r p changes of d o m a i n o r i e n t a t i o n w h i c h coincide w i t h v i s i b l e boundaries w i t h i n t h e l i q u i d lead t o t h e h y p o t h e s i s t h a t t h e d o m a i n o r i e n t a t i o n is r e l a t e d i n some w a y t o the u n d e r l y i n g o r i e n t a t i o n of t h e molecules a n d serves as a n i n d i c a t o r of t h i s . A possible o r d e r i n g m e c h a n i s m was discussed b y W i l l i a m s (10,11,12). If these are, indeed, t h e result of m o l e c u l a r o r d e r i n g , t h e w a l l s between a d jacent d o m a i n s are interfaces i n t h e sense defined above. I n a d d i t i o n , a n a b r u p t change i n t h e o r i e n t a t i o n of t h e d o m a i n s s u c h as t h a t seen i n t h e u p p e r left corner of F i g u r e 1 indicates a change i n t h e o r i e n t a t i o n of the l i q u i d o n a larger scale t h a n t h a t of t h e d o m a i n s themselves. W e a t t r i b u t e t h i s t o a b r u p t changes i n t h e o r i e n t a t i o n of the n e m a t i c axis. A l l these effects are sensitive to the surface properties of t h e c o n d u c t i n g glass plates between w h i c h t h e l i q u i d is c o n t a i n e d b u t appear to be p r i m a r i l y properties of the n e m a t i c l i q u i d . Crystal
Boundaries
in
p-Azoxyanisole
I f we accept t h e o r i e n t a t i o n s of t h e d o m a i n s as i n d i c a t o r s of t h e l o c a l o r i e n t a t i o n of t h e n e m a t i c axis (9), t h e n regions where t h e d o m a i n s are a l l p a r a l l e l correspond to i n d i v i d u a l single c r y s t a l s of t h e n e m a t i c l i q u i d . W i l l i a m s (10, 11, 12) suggested t h a t t h e n e m a t i c axis coincides w i t h t h e l o n g axes of t h e d o m a i n figures. F o r t h i s discussion i t is o n l y necessary t h a t t h e d o m a i n s be oriented a t some constant angle w i t h respect t o t h e n e m a t i c axis. F i g u r e 2 shows w h a t is believed t o be a p o l y c r y s t a l l i n e specimen of n e m a t i c p-azoxyanisole. S u c h a specimen is r e a d i l y o b t a i n e d b y heating above the nematic-isotropic transition temperature a n d q u i c k l y cooling. T h e specimen first appears to be criss-crossed b y m a n y fine d a r k lines, t h e " t h r e a d s " f r o m w h i c h t h e n a m e " n e m a t i c " derives. T h e y are present before t h e field is a p p l i e d a n d are u n c h a n g e d b y a p p l i c a t i o n of the field. W h e n t h e electric field is a p p l i e d , t h e d o m a i n s become v i s i b l e . I t c a n be seen f r o m F i g u r e 2 t h a t these a l l lie p a r a l l e l w i t h i n a g i v e n area e n closed b y a t h r e a d . O n crossing t h e t h r e a d i n t o the adjacent area, t h e d o m a i n s are a g a i n a l l p a r a l l e l , b u t there is a n a b r u p t change i n t h e i r o r i e n t a t i o n , u s u a l l y b y a b o u t 90°. W e i n t e r p r e t t h e threads as c r y s t a l boundaries of i n d i v i d u a l l i q u i d c r y s t a l l i t e s w h i c h h a v e n u c l e a t e d at different places a n d g r o w n u n t i l t h e y t o u c h . T h e o r i e n t a t i o n s of t h e i n d i v i d u a l l i q u i d crystallites are i n d i c a t e d b y t h e d o m a i n o r i e n t a t i o n s . O v e r a p e r i o d of several m i n u t e s the s m a l l l i q u i d c r y s t a l l i t e s merge a n d grow larger u n t i l there is often o n l y one w i t h i n t h e field of v i e w . T h i s is s i m i l a r to t h e be-
Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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h a v i o r of o r d i n a r y solids, where the i n d i v i d u a l c r y s t a l l i t e s of a p o l y c r y s t a l l i n e mass merge a n d g r o w i n t o larger crystallites if the m a t e r i a l is h e l d n e a r the melting point.
Figure 2.
Domain pattern in a specimen of p-azoxyanisole formed by cooling from the isotropic phase
Complicated thread structure is present both with and with out the electric field and appears to be the boundaries of individual liquid crystallites. Domain pattern appears only in an electricfieldand serves as an indicator of the orientation of the nematic axes of the crystallites. Unpolarized light {(
Experimen
n
tal
T h e p - a z o x y a n i s o l e a n d p - m e t h o x y c i n n a m i c a c i d were o b t a i n e d f r o m t h e Κ a n d Κ C h e m i c a l C o . T h e y were used w i t h o u t f u r t h e r p u r i f i c a t i o n a n d after r e c r y s t a l l i z a t i o n . R e s u l t s were t h e same i n b o t h cases. The m a t e r i a l s were m e l t e d b e t w e e n 1 X 3 c m . pieces of glass, w i t h t r a n s p a r e n t c o n d u c t i v e coatings of t i n oxide o n t h e i r i n n e r faces. T h i s glass is o b t a i n able f r o m the C o r n i n g G l a s s C o . E l e c t r i c a l connections to the c o n d u c t i v e coatings are m a d e t h r o u g h c o n d u c t i n g s i l v e r paste to s t r i p s of a l u m i n u m f o i l cemented t o t h e glass. A t r a n s p a r e n t h e a t i n g stage for m i c r o s c o p i c o b s e r v a t i o n was m a d e f r o m a 5 X 8 c m . piece of glass, h a v i n g a t r a n s p a r e n t
Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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c o n d u c t i v e c o a t i n g . T h i s was c o n n e c t e d t o a V a r i a c p o w e r s u p p l y . The a r r a n g e m e n t of t h e glass pieces c o n t a i n i n g t h e m e l t e d l i q u i d is i l l u s t r a t e d i n a n o t h e r a r t i c l e (9). T h e glass pieces are cleaned i n toluene a n d a l c o h o l before u s i n g . S u r f a c e t e n s i o n p r e v e n t s t h e m e l t e d m a t e r i a l f r o m r u n n i n g o u t f r o m between t h e plates. D o m a i n figures m a y be o b t a i n e d w i t h a n y a.c. v o l t a g e source g i v i n g 10 v o l t s or more. T h e p i c t u r e s s h o w n were o b t a i n e d u s i n g a H e w l e t t - P a c k a r d 2 0 2 - C o s c i l l a t o r a t a f r e q u e n c y of 1.0 k c . T h e specimen o n t h e h e a t e d stage is observed b y t r a n s m i t t e d l i g h t , w h i c h m a y be either p o l a r i z e d o r u n p o l a r i z e d . D o m a i n p a t t e r n s appear a t a n y t e m p e r a t u r e w i t h i n the n e m a t i c range of t h e l i q u i d s . Interface
between
Domains
in p-Methoxycinnamic
Acid
I n five n e m a t i c l i q u i d s i n v e s t i g a t e d , the d o m a i n s t r u c t u r e is s i m i l a r t o t h a t s h o w n i n F i g u r e 1. T h i s s t r u c t u r e persists i n d e f i n i t e l y as l o n g as t h e field is a p p l i e d . T h e b e h a v i o r of n e m a t i c p - m e t h o x y c i n n a m i c a c i d (3) is different a n d i l l u s t r a t e s some i n t e r e s t i n g features of t h e interface or w a l l between domains. W h e n a n electric field of several t h o u s a n d v o l t s / c m . is a p p l i e d t o a t h i n specimen of the n e m a t i c l i q u i d c o n t a i n e d between glass plates h a v i n g t r a n s p a r e n t c o n d u c t i v e coatings, a d o m a i n p a t t e r n forms. A t t h e i n s t a n t t h e field is a p p l i e d t h i s p a t t e r n is s i m i l a r t o t h a t i n F i g u r e 1, as s h o w n i n F i g u r e 3. W i t h i n a few seconds t h i s p a t t e r n evolves t o t h a t s h o w n i n
Figure 5. Domain structure in nematic p-methoxycinnamic acid at instant the field is applied Material nematic from 173.5° to 190°C. Initial pattern similar to typical pattern shown in Figure 1. Picture taken with specimen between crossed polarizers. Magnification same as Figure 1. 1-kc. a.c. field
F i g u r e 4 a n d after a b o u t 12 seconds t o t h a t i n F i g u r e 5.
U l t i m a t e l y the
p a t t e r n disappears e n t i r e l y , e v e n t h o u g h t h e field r e m a i n s o n .
This ap
pears t o be t h e a c t i o n of a n i n t e r f a c i a l t e n s i o n c o n t r a c t i n g t h e d o m a i n w a l l s to minimize their length.
T h e o p t i c a l properties i n d i c a t e t h a t t h e n e m a t i c
axis i n p - m e t h o x y c i n n a m i c a c i d is p a r a l l e l t o t h e glass.
T h e r e is some
Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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ORDERED FLUIDS A N D LIQUID CRYSTALS
Figure 4»
Domain structure of same specimen as in Figure 3
Picture taken 4 seconds after electricfieldwas applied
Figure 5.
Domain structure of same specimen as in Figure 3
Picture taken 12 seconds after electricfieldwas applied
macroscopic periodic m o l e c u l a r o r d e r i n g w h i c h gives t h e d o m a i n s t r u c t u r e . W i t h t h i s s t r u c t u r e t h e i n t e r f a c i a l or d o m a i n w a l l energy gives a stable s t r u c t u r e w i t h l o n g t h i n d o m a i n s . W h e n the field is a p p l i e d , there is r e o r i e n t a t i o n of t h e l i q u i d . W i t h t h i s o r i e n t a t i o n t h e d r i v i n g force p r o d u c i n g the d o m a i n s t r u c t u r e is u n d o u b t e d l y different f r o m t h a t w i t h t h e o r i g i n a l o r i e n t a t i o n . A p p a r e n t l y the i n t e r f a c i a l energy is too h i g h t o preserve the o r i g i n a l geometry, a n d the t r a n s i t i o n t o a c i r c u l a r interface takes place. F i n a l l y even these disappear. T h e changes w h i c h are p r o d u c e d here b y r e o r i e n t a t i o n of molecules i n a single-phase s y s t e m are of a k i n d u s u a l l y p r o d u c e d b y t h e a d d i t i o n t o a s y s t e m or the r e m o v a l f r o m i t of c h e m i c a l l y different molecules. F o r ex-
Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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a m p l e , consider a s y s t e m c o n s i s t i n g of a large n u m b e r of bubbles i n c o n t a c t m a d e b y a e r a t i n g a w a t e r s o l u t i o n of a surface a c t i v e m a t e r i a l . a r r a n g e m e n t m i g h t be stable for a l o n g t i m e .
Such an
I f t h e surface-active m a t e r i a l
c o u l d be s o m e h o w g r a d u a l l y r e m o v e d f r o m t h e b u b b l e s , t h e s t r u c t u r e w o u l d change shape a n d finally collapse since a large n u m b e r o f bubbles is a n u n stable c o n f i g u r a t i o n f o r p u r e w a t e r .
I n n e m a t i c l i q u i d s t h e differences i n
p h y s i c a l properties o w i n g t o r e o r i e n t a t i o n of molecules w i t h i n a o n e - c o m p o n e n t s y s t e m are c o m p a r a b l e t o those p r o d u c e d i n o r d i n a r y systems b y i n t r o d u c t i o n of n e w c h e m i c a l species. Conclusions O p t i c a l e x a m i n a t i o n of t h e effects of a p p l i e d electric fields p r o v i d e s a sensitive t e c h n i q u e for i n v e s t i g a t i n g the s t r u c t u r e of n e m a t i c l i q u i d s . T w o d i s t i n c t k i n d s of interfaces exist i n p u r e n e m a t i c l i q u i d s .
O n e corresponds
to t h e c r y s t a l l i t e boundaries i n o r d i n a r y p o l y c r y s t a l l i n e solids.
T h e other
is t h e interface between d o m a i n s w h i c h shows effects of i n t e r f a c i a l t e n s i o n in p-methoxycinnamic acid.
I n other n e m a t i c l i q u i d s t h i s effect is n o t
found. Acknowledgmen
t
T h e a u t h o r is i n d e b t e d t o A . W i l l i s f o r assistance w i t h some of t h e experiments.
Literature
Cited
(1) Chatelaine, P., Pellet, O., Bull. Soc. Franς. Mineral. Crist. 73, 154 (1950). (2) Elliott, G., Gibson, J . G., Nature 205, 995 (1965). (3) Gray, G. W., "Molecular Structure and Properties of Liquid Crystals," Chap. 2, 4, Academic Press, London, 1962. (4) Gray, G. W., Jones, B., J. Chem. Soc. 1954, 1467. (5) Heilmeier, G., J. Chem. Phys., in press. (6) Kapustin, A. P., Dmitriev, L . M . , Kristallografiya 7, 332 (1962). (7) Kapustin, A. P., Vistin, L . K . , Kristallografiya 10, 118 (1965). (8) Porter, R. S., Johnson, R., J. Phys. Chem. 66, 1826 (1962). (9) Saupe, Α., Maier, W., Z. Naturforsch. 16a, 816 (1961). (10) Williams, R., J. Chem. Phys. 39, 384 (1963). (11) Williams, R., Nature 199, 273 (1963). (12) Williams, R., Heilmeier, G., J. Chem. Phys., in press. RECEIVED March 10, 1966.
Porter and Johnson; Ordered Fluids and Liquid Crystals Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
