HAL Id: jpa-00215910
https://hal.archives-ouvertes.fr/jpa-00215910
Submitted on 1 Jan 1975
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
MACROSCOPIC FEATURES. a)
NematicsPRESENTATION OF A FILM ENTITLED
”TEXTURES OF NEMATIC AND CHOLESTERIC LIQUID CRYSTALS”
Y. Bouligand
To cite this version:
Y. Bouligand. MACROSCOPIC FEATURES. a) NematicsPRESENTATION OF A FILM EN- TITLED ”TEXTURES OF NEMATIC AND CHOLESTERIC LIQUID CRYSTALS”. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-173-C1-176. �10.1051/jphyscol:1975133�. �jpa-00215910�
Abstract. — Different situations given by nematic and cholesteric liquid crystals have been filmed and are briefly reviewed in this paper. We also publish some of the most significant paragraphs extracted from the commentary. Nematic and cholesteric textures are observed in samples of M. B. B. A., pure or to which has been added a twisting substance. The organic matrix of the body- wall of Crabs is a fibrous analogue of a cholesteric liquid crystal. The geometry and the optical properties of these mesomorphic liquids and their biological analogues are compared in polarizing microscopy.
This colour film (16 mm, optical sound, 41 min) is an which can be compared with those appearing in true introduction to liquid crystals, illustrated by obser- cholesteric liquids.
vations of the classical M. B. B. A. with a pola- Three main kinds of defects occur in highly twisted rizing microscope. This film has been realized by cholesterics (with a small helicoidal pitch) : translation Mr. J. Painleve (*). dislocations, focal curves (or lines of flare, with refe-
The isotropic-nematic transition occurs in several rence to the trumpet shape of the stacked laminae pictures. The defects of the director arrangement are around these lines) and disclinations ( + % and - %).
depicted in certain situations such as nematic droplets, These latter can be associated according to certain nuclei, disclinations, etc... The cholesteric layering is regular patterns easy to recognize : edge-dislocations, observed in M. B. B. A. to which has been added elementary pinches, quadrilaterals and zigzags (Fig. 1).
different amounts of a twisting substance (cholesterol A single cholesteric mesophase can show very different benzoate leading to a left-handed twist; Canada balsam textures from place to place, between slide and giving a right-handed twist). The helicoidal arrange- coverslip. The textures are accurately defined by the ment is studied in a biological analogue of choleste- nature of their singularities. For instance, in the fan- rics : the organic matrix in the body-wall of the shore- shaped textures, the three kinds of lines occur : discli- crab, Carcinus maenas. In such a material, the chitin nations, curves of flare and dislocations. In polygonal (a polymer of acetyl-glucosamine) and the associated fields, one only finds lines of flare and dislocations, proteins form a fibrous network, with the geometry of Screw- and edge-dislocations are the only defects of continuously twisted plywood. The crab cuticle is planar textures. There is no sharp limit separating these hardened by a deposit of calcite, which has been different textures. The distribution of defects can be removed from the organic matrix by a slight acid considered as the texture frame. In a cholesteric liquid, solution, before slicing the ultrathin sections for during streaming, the whole configuration changes electron microscopy. rapidly, but the nature of the texture is not altered.
The cholesteric layering in the crab cuticle is more or T h e film i s d i vid ed into six parts and their titles are : less distorted and shows several kinds of defects,
— Solid crystals, a brief review.
(*) Directeur de l'Institut de Cinematographie Scientifique, — Liquid crystals, definitions.
38, avenue des Ternes, 75016 Paris, France. — T h e pure nematic State.
MACROSCOPIC FEATURES, a) Nematics
Classification Physics Abstracts
7.130
PRESENTATION OF A FILM ENTITLED
« TEXTURES OF NEMATIC AND CHOLESTERIC LIQUID CRYSTALS »
Y. BOULIGAND
Histophysique et Cytophysique, E. P. H. E., Laboratoire de Zoologie, E. N. S., 46, rue d'Ulm, 75230 Paris, France
Résumé. — Différentes situations données par les cristaux liquides nématiques et cholestériques ont été filmées et sont rapidement revues dans cet article. Nous publions également certains des paragraphes les plus significatifs extraits du commentaire. Les textures nématiques et cholestériques sont observées dans des échantillons de M. B. B. A. pur ou additionné d'une substance torsadante.
La matrice organique de la carapace des Crabes est un analogue fibreux d'un cristal liquide cholesté- rique. La géométrie et les propriétés optiques de ces liquides mésomorphes et de leurs analogues biologiques sont comparées au microscope polarisant.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975133
Cl-174 Y. BOULIGAND
FIG. 1. - Example of a fan-shaped texture near the isotropic transition (M. B. B. A.
+
Canada balsam) ; crossed polars.a ) Elementary pinch ; b) Zigzag ; c) Edge-dislocation ; d ) Quadrilateral ; e ) Isotropic phase.
- The cholesteric liquids and the helicoidal nematic Several chemicals pass directly from solid crystal to nematic
state. state, omitting the smectic state before turning isotropic.
For other substances, the nematic state is omitted and the - The cholesteric textures. transitions are solid, smectic, isotropic.
- Dynamics of textures. We only study in this film the nematic liquids given by a rather
The following paragraphs extracted from the commentary of the film describe the most significant situations.
The first pictures show the growth of potassium bichromate crystals in a saturated solution, during evaporation. The definition of solid crystals, their optical properties and their three-dimensional arran- gement are briefly reviewed. -We then give the main definitions of liquid crystals.
a Liquid crystals, also called mesomorphic materials, or mesophases, are birefringent liquids.
Certain chemicals with elongated molecules give true solid crystals which can be melted at a temperature ( t t ) giving a snzectic birefringent liquid.
The molecules are parallel in a small sample of the smectic liquid. Their centers of gravity are distributed on parallel surfaces. The different layers of equal thickness can slide with respect to the others.
complicated chemical : methoxy-benzylidene-butylaniline, or M. B. B. A. The rod-like molecules move freely one with respect to the other and keep a constant direction.
This substance is nematic at ordinary temperatures and the isotropic transition occurs by a slight increase of temperature or by a small addition of toluene. D
Observations of nematic droplets, their iridescent fringes ; a model is presented.
(< Nematic spherulites observed between crossed polars lead to
different pictures, according to the orientation of the polar axis with respect to the microscope axis.
When the poles are superimposed, the revolution symmetry leads to a Maltese black cross.
If the polar axis lies perpendicular to the microscope axis, then black brushes occur for molecules parallel to the polarizer or the analyzer.
Oblique orientations are very frequent and lead to more complicated pictures. D
Pictures of the thick threads in nematics.
The molecules are generally normal to the layering. In each
r( The word nematic refers to the presence of threads in such layer, the molecules move and can be compared to people in a
crowded party, with everyone trying to speak to everyone else. liquids. This thick thread is the locus of molecules lying either vertical or almost vertical in the medium.
The transition from the solid crystal to the smectic liquid A small shift of the coverslip shows the of the medium occurs at a temperature ( t l ) . and leads to strong deformations of the threads. ))
At a higher temperature (t2), the smectic liquid transforms
into another birefrihgent liquid of the nernatic type. In this kind ~ ~ ~textures, and mobile three-dimensional l ~ i ,
of liquid crystal the molecules lie parallel but do not give parallel
layers. reconstructions.
At a higher temperature (t3), the liquid becomes isotropic, the cr In a nematic liquid to which has been added a very small molecules being distributed randomly. amount of cholesteryl benzoate, leading to a slight twist, one
NEMATIC AND CHOLESTERIC TEXTURES A FILM Cl-175 often gets beautiful sets of nuclei between slide and cove~slip.
Each centre is surrounded by two or four dark brushes.
In a nematic liquid crystal lying between slide and coverslip, the molecules are distributed here and there like these match sticks arranged in a continuous way. In this case, the matches are seen in projection along concentric curves. The centre shifts regularly from the right to the left and reciprocally during the continuous tilt of the model.
Other continuous arrangements lead to more or less spiralized systems seen in projection on the screen, and sets of concentric hyperbolae, the centre position depending on the angle of the model
...
Systems of only two dark brushes arise from other arrange- ments of molecules called disclinations. One kind of disclination involves trajectories of the molecules in the form of confocal parabolae. D
Such nuclei are due to thin threads streched verti- cally between slide and coverslip. They correspond to complementary odd disclinations desyibed by Frank (1958). The nuclei can be analysed between crossed polars.
cc If one rotates the slide about the microscope axis, the brushes can keep a constant direction in certain cases, or rotate more or less rapidly in the same sense or in the opposite sense. These characters allow us to determine the nature of the different nuclei..
.
The polarizer can also be rotated, the slide and the analyzer remaining a t rest. The different changes observed in these conditions also allow analysis.
The cholesteric arrangement and the biological analogues.
Certain substances like cholesteryl benzoate in solution in a nematic substance lead to a structure which can be compared to a helical plywood.
In each plane, the molecules lie parallel. Their direction rotates by a small angle, very regularly from one level to the next one.
I n oblique section, this arrangement leads to bow-shaped patterns.
This structure shows a periodicity. Parallel molecules are found in equidistant planes.
A similar geometry has been observed in certain biological materials. The organic matrix of the crab cuticle for instance is made of a fibrous network (of chitin and protein) arranged in a twisted plywood structure and leading to bow-shaped patterns in oblique thin section.
The sense of the concavity in the bow-shaped patterns depends on the angle made by the model with the axis of the camera. The helicoidal layering is left handed in this model, as in most liquid crystals and biological analogues.
In this perspex model built by Kenchington and Flower, with numerous successive layers, the bow-shaped series appears almost continuous on the oblique sides of the pyramid.
A vertical section of this model can be tilted giving bow-shaped patterns. Patterns of the opposite sense appear when the model is tilted in the opposite direction.
The layering in a helicoidal plywood can be more or less distorted, giving laminae arranged in domes and basins both in cholesteric liquid crystals and in certain biological analogues.
In the microscope fibrils and molecules will be observed a t the same level for a given focus, and in that plane the orientation will turn regularly from the centre to the periphery.
The appearance of the fibril arrangement in this plane can be simulated by superimposed disks, where parallel lines have been drawn. The direction of lines is slowly and regularly rotated from one disk to the other, constantly in the same sense.
Such an arrangement leads to the bow-shaped patterns whorled into a double spiral ...
In the crab body-wall, after decalcification, one can see in the thin sections the bow-shaped series forming a spiralized system.
This section is observed with a phase contrast microscope.
The spirals observed in phase-contrast change to concentric rings, alternatively dark and bright, when the slide is examined between crossed polars.
The fibril direction is constant on the edges of the concentric disks we have formerly described.
Each concentric band between two consecutive disks gives a bright or a dark ring according to the fibril direction in this band, with respect to the polarizer and the analyzer planes.
In the same way, a helicoidal nematic passes from spiralized patterns to concentric rings, when one uses phase contrast first, followed by polarizing microscopy.
The study of the molecule orientations is improved by the use of a quartz first order retardation plate inserted between the crossed polars. )>
Images of mobile fringes (polarizing and phase- contrast microscopy).
cc A preparation of a nematic substance made helicoidal by addition of cholesteryl benzoate shows basins and domes.
The retardation plate gives red and blue rings in alternation corresponding to the orientations at 4 5 O to the edges of the screen.
The rotation of the microscope stage continuously changes the radius of the coloured rings.
This is called the phenomenon of cc mobile' fringes D. They are displaced either towards the periphery or the centre of these systems
...
A suitable section of a decalcified crab cuticle also gives the mobile fringe effect, when the preparation is rotated between crossed polaroids. The fringes give the effect of either appearing or disappearing in the centre of concentric systems. Some of these arrangements show a spiral caused by a dislocation superimposed on the axis of the concentric layers.
Such defects are also observed in the mobile fringes given by cholesteric liquid crystals. Multiple spirals arise in several places..
.
c< The oblique layers can be observed a t high magnification in
phase-contrast. The sharp lines correspond to the locus of hori- zontal molecules. Brownian motion is clearly seen with the aid of a polarizer.
By rotating the polarizer, the motion can be seen in successive strips of parallel orientation. s
Examples of cholesteric textures.
The Nomarsky method of interference contrast microscopy gives beautiful pictures with a high relief. In this region of the preparation, the laminae lie almost vertical and are distributed radially around an isotropic drop. The laminae tend often to lie normally to the isotropic interface.
Numerous defects can be observed and particularly discli- nations
...
The medium is very fluid despite these defects in the layer arrangement..
.
Streams and currents in the mesoohase occur when the coverslip is shifted. The layers become oblique or horizontal and the distances between layers seem to be more or less increased. )>
The following pictures show the distribution of textures in a liquid with a very tight cholesteric layering.
cc One distinguishes a first zone called cc fan-shaped texture followed by a set of polygons, and then a uniform area called cc planar texture D. The word cc texture is used to describe these different aspects of one single mesophase ...
Represented in vertical section between slide and coverslip the layers are vertical in fans, they are oblique in polygons, and they are horizontal in the planar texture, where the layers are thicker in general.
The fans can be separated from the planes by a texture called
Cl-176 Y. BOULIGAND cellular arrangement, where the laminae are either vertical in the
walls or horizontal in the cells. The interiors of cells appear blue in this preparation.
The number of layers seen in the walls decreases in the cells lying in the vicinity of the plane texture.
The polygonal fields show singularities arranged in two lattices at the slide and coverslip levels.
At a higher magnification, we can see that these singular lines are due to a deviation of the layers.
The boundary between a polygonal field and a plane texture can move as the temperature is progressively changed.
Polygons may appear in a plane texture in such a way. The film is accelerated. This phenomenon resembles a volcanic eruption.
The fluid character of a cholesteric mesophase and the stability of the different textures.
<< When a polygonal field is squashed and released, the laminae are first made horizontal and then return to the polygonal arran- gemen t.
The oily streaks indicate the motion of the coverslip. The oily streaks are horizontal edge dislocations. When the motion stops, nuclei appear along these streaks which become less and less visible. As the polygone are restored, this initial order is lost.
The polarization colours with a slightly thicker preparation differ and the polygons are larger, taking longer to stabilize. The film is taken at normal speed.
The polygons are generally stable, but in some cases they are able to flow slowly, whilst remaining as polygons.
This is a speeded up streaming of a polygonal field. When, by contrast, a polygonal field is well stabilized, a square lattice results ...
The laminae of a fan texture are vertical and are seen with their true thickness. Tliey lie generally perpendicular to the sur- face, which separates them from the isotropic liquid.
They show edge-dislocations or quadrilateral shapes and very often lozenges whose angles are precisely 600 or 120°.
The fluid character of the medium is quite obvious. The layers keep their normal orientation to the surface even during the streaming ...
In the equilibrium zone, between the isotropic liquid and the cholesteric phase, a lot of droplets are free when very small or sandwiched between slide and coverslip when larger. The laminae tend to lie normally to the interface except at two diametrically opposite points.
The fan-shaped textures can be squashed, but they rebuild rapidly.
One easily recognizes the disclinations, the zigzags, the ele- mentary pinches, the quadrilaterals and the edge-dislocations, in the flowing texture. D
The dynamics of an edge-dislocation lattice.
(( These edge dislocations are observed in a planar texture, when a drop of Canada balsam enters into contact and dissolves in the nematic substance.
A twist appears in the liquid and its intensity shows a gradient.
Referc [l] BOULIGAND, Y., J. Physique 33 (1972) 715.
[2] BOULIGAND, Y., J. Physique 34 (1973) 603.
[3] BOULIGAND, Y., J. Physique 34 (1973) 101 1.
[4] BOULIGAND, Y . , J. Physique 35 (1974) 215.
[S] BOULIGAND, Y., J. Microscopic 17 (1973) 145.
[6] BOULIGAND, Y., C. R. Hebd. SPan. Acad. Sci. 261 (1965) 3665.
[7] BOULIGAND, Y., J. Physique Colloq. 30 (1969) C4-90.
[8] BOULIGAND, Y., Tissue and Cell 4 (1972) 189.
[9] BOULIGAND, Y., SOYER, M.-O., PUISEUX-DAO, S., Chromo- soma 24 (1968) 251.
The cholesteric layering is very tight in the vicinity of the Canada balsam, looser at a distance, and the liquid remains nematic further away.
So, edge dislocations become more and more numerous near the interface.
Each edge-dislocation introduces a supplementary twist of 360°.
The edge-dislocations are seen in projection as lines parallel to the interface.
The equilibrium between the isotropic liquid and the meso- phase is displaced towards the latter.
Singularities are seen here and there on the edge-dislocations in which they form local level variations
...
The optics of such systems is made complicated by the twist leading to a very high rotatory power. Each interval with its defined colour corresponds to an integral number of rotations of 360°.
The film is now speeded up and this allows us to observe the dynamics of an edge dislocation network.
These arrangements can be very regular. The last pictures are more accelerated. The motion is due to the diffusion of the Canada balsam which causes the increase of twisting. )>
Remarks : Several pictures equivalent to those appearing in this film have been published in a series of papers dealing with the cholesteric textures [l-51, with various biological analogues [6-91, and with synthetic polymerized liquid crystals [IO]. The last pictures of the film resemble very much those published by Kel- ker [l l]. The model of nematic droplets floating in the isotropic phase is that suggested by Parcdi and Dubois- Violette [12]. This film is very different from that worked out by P. Chatelain and R. Cano and which we strongly recommend [13]. We have to mention the beautiful pictures of screw-dislocations filmed by J. Rault in a mixture of para-azoxyanisole and choles- terol propionate. Some similar micrographs have been published by this author [14]. P. E. Cladis has filmed the behavior of disclinations arising from the meeting of a cholesteric droplet and a larger mesomorphic area.
One of the models of our film has been built by Ken- chington and Flower [l 51.
Acknowledgments. - We thank deeply Professor F. C . Frank and Dr. C. A. Neville for considerable help and useful discussions. The English text has been read by Professor Frank (Bristol University) ; thanks are also due to G. Hamon and M.-Br. Bouligand for technical assistance and model preparation. We are grateful to the DtICgation Gtntrale 51 la Recherche Scientifique et Technique for financial support.
[l01 BOULIGAND, Y., CLADIS, P. E., STRZLECKI, L., LIBBERT, L., Mol. Cryst. Liqu. Cryst., in the press.
[l11 KELKER, H., Mo1. Cryst. Liqu. Cryst. 15 (1972) 347.
[l21 PARODI, O., DUBOIS-VIOLETTE, E., J. Physique Colloq. 30 (1969) C4-57.
[l31 CHATELAIN, P., CANO, R., Les propriktt5s optiques des cristaux liquides des types nkmatiques et cholest6riques (film). Service du film de recherche scientifique, 96, bd Raspail, 75006 Paris (1968).
[l41 RAULT, J., J. Physique 35 (1974) 213 ; Phil. Mug. 28 (1973) 11.
[15] KENCHINGTON, W., FLOWER, N. E., J. Microscopy 89 (1969) 263.
