THIN COLLOIDAL CRYSTALS : STRUCTURES AND OPTICAL PROPERTIES

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THIN COLLOIDAL CRYSTALS : STRUCTURES AND OPTICAL PROPERTIES

B. Pansu, P. Piera_ski

To cite this version:

B. Pansu, P. Piera_ski. THIN COLLOIDAL CRYSTALS : STRUCTURES AND OP- TICAL PROPERTIES. Journal de Physique Colloques, 1985, 46 (C3), pp.C3-323-C3-324.

�10.1051/jphyscol:1985326�. �jpa-00224643�

JOURNAL DE PHYSIQUE

Colloque C3, suppl6ment au n03, Tome 46, mars 1985 page C3-323

T H I N C O L L O I D A L CRYSTALS

:

STRUCTURES AND O P T I C A L PROPERTIES

B. Pansu and P. ~ieraiiski

Laboratoire de Physique des SoZides, Universitg de Paris-Sud, B&t. 510, 91405 Orsay, France

Resume

-

Quand on c o n f i n e des c r i s t a u x c o l l o i d a u x dans un c o i n de f a i b l e m e u r , on observe une s e r i e de t r a n s i t i o n s s t r u c t u r e l l e s : l e nombre de couches mais aussi l e u r s t r u c t u r e c r i s t a l l i n e v a r i e n t . Parce que l ' i n d i c e de r e f r a c t i o n v a r i e dans 1 'espace, 1 es c r i s t a u x c o l l o i d a u x minces se comportent optiquement comme des reseaux de phase e t presentent des couleurs vives.

A b s t r a c t

-

When c o l l o i d a l c r y s t a l s a r e c o n f i n e d i n a narrow wedge shaped gap between a p a i r of g l a s s f l a t s , we observe a s e r i e s o f s t r u c t u r a l t r a n s i t i o n s . These t r a n s i t i o n s c o n s i s t i n a change i n t h e number o f l a y e r s and a l s o i n t h e i r c r y s t a l l i n e s t r u c t u r e . Because t h e r e f r a c t i v e index v a r i e s i n space, t h i n c o l l o i d a l c r y s t a l s o p t i c a l l y a c t as phase g r a t i n g s and show very b r i g h t c o l o r s .

C o l l o i d a l c r y s t a l s present many advantages f o r research on two-dimensional systems.

Thanks t o s p e c i f i c i n t e r a c t i o n s o f t h e polymeric p a r t i c l e s w i t h glass surfaces, i t i s p o s s i b l e t o t r a p one o r a few l a y e r s o f these p a r t i c l e s , i n aqueous suspension, i n a narrow wedge shaped gap between a p a i r o f glass f l a t s . The observation o f t h e microscopic s t r u c t u r e o f t h i n c o l l o i d a l c r y s t a l s i s easy w i t h an o p t i c a l microscope and s c a t t e r i n g o f a l a s e r bean i s a good t o o l t o study l o c a l o r d e r /I/.

I n our experiments /2/, a t h r e e dimensional r e s e r v o i r o f c o l l o i d a l suspension imposed i n t h e wedge a h i g h enough pressure so t h a t t h e spheres were organized i n t o c r y s t a l - l i n e l a y e r s p a r a l l e l t o t h e glass surfaces. We used suspensions o f monodisperse p o l y - s t y r e n e b a l l s o f diameter

+

⁼ 1.1 pm i n water p u r i f i e d by i o n i c exchange r e s i n s . For these b a l l s , t h e t h r e e dimensional c r y s t a l l i n e s t r u c t u r e i s fcc, The a v a i l a b l e t h i c k - nes D o f t h e wedge v a r i e d l i n e a r l y i n space from a few t o

l o 5

A. Samples were l i t w i t h white, non p o l a r i z e d p a r a l l e l l i g h t and observed u s i n g a transmission o p t i c a l microscope. We observed a s e r i e s o f c o l o r e d s t r i p s p a r a l l e l t o t h e edge o f t h e wedge /2/. The f i r s t s t r i p corresponded t o a monolayer o f spheres organized i n a 2D hexagonal network. As t h e gap t h i c k n e s s increased, new s t r i p s appeared. T h e i r s t r u c t u r e was determined w i t h h i g h e r m a g n i f i c a t i o n o b j e c t i v e s . The second s t r i p c o r - responded t o a stack o f two l a y e r s w i t h a 2D square s t r u c t u r e . The t h i r d one c o r r e s - ponded t o a stack o f two l a y e r s w i t h a 20 hexagonal s t r u c t u r e . As t h e gap thickness

increased, t h e s e r i e s o f s t r u c t u r e s we observed was :

where a stack o f n square (resp hexagonal) l a y e r s i s w r i t t e n n d r e s p na). The e x i s t e n - ce o f such a s e r i e s has been explained i n terms o f packing e f f i c i e n c y /3/ : t h e pressure a t t h e bottom o f t h e tube imposed i n t h e wedge s t r u c t u r e s o f maximum volune density. For some i n t e r v a l s o f the gap thickness, t h e square packing (0) becomes more dense than t h e hexagonal one ( A ) .

Using a low a p e r t u r e o b j e c t i v e , each s t r i p associated w i t h each s t r u c t u r e showed very b r i g h t c o l o r s (a c o l o r photograph i s shown i n r e f /4/ f i ~ u r e 43). The c o l o r w6s u n i f o r m i n t h e s t r i p s no, I A and ?A. On t h e c o n t r a r y t h e s t r i p s nA (n ), 3) were composed o f domains o f m d i f f e r e n t c o l o r s : m = 2 f o r 3A, 3 f o r 4~

...

We n o t i c e d t h a t i n a1 1 t h e samples one o f t h e c o l o r o f t h e s t r i p nA i s s i m i l a r t o t h e unique c o l o r o f t h e s t r i p no. The existence o f these c o l o r s nay be explained by the d i f f e r e n c e i n t h e r e f r a c t i v e index o f p o l y s t y r e n e (n2 = 1.59) and water ( n l = 1.33). Because Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985326

C3-324 JOURNAL DE PHYSIQUE

the difference between n 2 and n l i s small, l i g h t rays a r e supposed t o propagate without deviation. An optical ray which crosses a sphere s u f f e r s a phase s h i f t compared t o a ray which propagates only through water. The spectrum (therefore the color) of the transmitted l i g h t depends on the phase s h i f t s suffered by t h e rays.

The s t r i p 3A f o r instance i s composed of two types of domains

: ABA

and

ABC.

In these stacks, the number of spheres crossed by the l i g h t i s d i f f e r e n t

:

the colors are d i f f e r e n t . On the contrary there i s only one type of stack in the s t r i p

3 0

(ABA) then i t s color i s the uniform. As we a r e concerned with forward s c a t t e r i n g and normal incidence, t h i s color i s similar t o the color of the domains ABA i n the s t r i p 3A.

In conclusion, t h i s experiment has revealed t h a t in a wedge geometry thin colloidal c r y s t a l s show a s e r i e s of structural t r a n s i t i o n s which consists i n a change in the number of c r y s t a l l i n e layers and in t h e i r bidimensional structure. From an optical point of view, they a c t a s phase gratings.

REFERENCES

/ I /

RUTH

J . , Oral communication a t the Workshop on Colloidal Crystals, Les Houches (1984)

/2/ PANSU B., P I E P ~ ~ ~ K I

P.,

STRZELECKI

L.,

J . Physique 44 (1983)

/3/ PANSU ,B.,

P I E R A ~ ~ S K I

Pi. and Pa., J. Physique 45 (1984)

/4/ PIERANSKI

P.,

Contemp. Phys. 4 (1983) no 1.