ALIGNMENT INVERSION WALLS IN NEMATIC LIQUID CRYSTAL LAYERS DEFORMED BY AN ELECTRIC FIELD

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1 ALIGNMENT INVERSION WALLS IN NEMATIC LIQUID CRYSTAL LAYERS DEFORMED BY AN ELECTRIC FIELD A. Stieb, G. Baur, G. Meier To cite this version: A. Stieb, G. Baur, G. Meier. ALIGNMENT INVERSION WALLS IN NEMATIC LIQUID CRYSTAL LAYERS DEFORMED BY AN ELECTRIC FIELD. Journal de Physique Colloques, 1975, 36 (C1), pp.c1-185-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1975 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published 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.

2 JOURNAL DE PHYSIQUE Colloque Cl, supplément au n 3, Tome 36, Mars 1975, page Cl-185 Classification Physics Abstracts ALIGNMENT INVERSION WALLS IN NEMATIC LIQUID CRYSTAL LAYERS DEFORMED BY AN ELECTRIC FIELD A. STIEB, G. BAUR and G. MEIER Institut fur angewandte Festkorperphysik der Fraunhofergesellschaft, Freiburg, Germany Résumé. Nous avons observé les murs d'inversion de l'orientation moléculaire dans un nématique soumis à un champ électrique déstabilisant. Dans des échantillons planaires, différents types de murs peuvent exister. Ces murs se décomposent en deux lignes de disclinaison, généralement à partir d'une irrégularité. La vitesse de décomposition dépend linéairement de la tension appliquée. Nous proposons des modèles pour la structure des murs, et le procédé de décomposition. Abstract. Alignment inversion walls in nematic liquid crystal layers deformed by an electric field have been observed. Different types of walls in layers with planar boundary conditions could be distinguished. Beginning at some irregularity, the walls split into pairs of disclination lines. The speed of the splitting depends linearly on the applied voltage. Models for the structure of the walls and for the splitting process are given. 1. Introduction. In nematic liquid crystal layers 360 degrees [7]). By lowering the frequency of the alignment inversion walls have been observed, as electric field into the positive dielectric regime, the well as point and line defects. Walls with a structure domains are continuously transformed to alignment similar to Neel and Bloch walls in homogeneously inversion walls. After their generation the walls are aligned layers have been described by Helfrich [1] and predominantly parallel to the direction of the preobserved by Nehring and Saupe [2]. ceding domains. The walls can also arise if a low Another kind of wall can be generated in nematic frequency field is suddenly applied to the planar layer, monocrystalline layers by inducing a torque by an The threshold for the Fresdericksz deformation was external field. Alignment inversion walls of this kind about 2.5 V. were first observed by Leger [3] in magnetic fields and by de Jeu et al. [4] in electric fields. 3. Observations. Alignment inversion walls gene- We investigated the structure of walls of the second rated by the method described above are shown in kind in high electric fields, where the walls were figure la and b. The liquid crystal layer is seen through continuously pinched as well as discontinuously split the glass plates, which are parallel to the figure into pairs of disclination lines. plane. Before applying the electric field, the layer in the right part of la was homogeneously planar and in the 2. Experimental. The liquid crystal was confined left part twisted by an angle of 180 degrees. The two in a glass, sandwich cell with a thickness of about 50 p. areas were separated by a disclination line of strength \. Planar boundary conditions were achieved by rubbing This line after application of the electric field can be the plates with aluminium oxide powder. The electric seen as a bright line in the center of the figure. Each field was applied to the liquid crystal by tin dioxide region embodies a special type of wall. In the lower coatings on the glass plates. part of lb a third type of wall, which was generated As liquid crystal we used WI from Merck [5}, a in a region with an initial twist of 360 degrees, can be room temperature nematic mixture of phenyl benzoates. seen. This twist, as well as the corresponding disclina- The dielectric anisotropy Ae = e - e x of this mixture tion line of strength 1 disappeared, if the low freis positive below a frequency f 0 and negative above quency field was high enough. it [6]. In our mixture we found f 0 = 13, khz at 25 C The central planes of all three wall types were _,»2 fotf^f perpendicular to the glass plates (the figure plane) in A s = 1' 1 for f > f order to minimize the elastic energy. The lateral 0 ' dimension of each wall in figure 1 was about 20 u. The walls were generated by the following method : an The type of wall to be seen in the right part of electric field with a frequency near/ 0 is applied to the figure la has been described by the authors [8] and liquid crystal layer, which can be homogeneously a model of its structure, has been proposed. The planar or twisted planar (we used twists of 180 and profiles of the opposite but equivalent, deformations Article published online by EDP Sciences and available at

3 Cl-186 A.. STIEB, G. BAUR AND G. MEIER The walls of the third type ( X walls) again separate deformations of opposite S profiles. In contrast to both other types, small loops of these walls have.a circular shape and after a further shrinking disappear with some sort of whirling movement. After switching off the electric field, these walls leave behind a pair of disclination lines of strength one [7]. At high fields, in the order of 105 V/cm, the walls are pinched to narrow films. In figure 2 this effect is shown for S walls. The walls are no longer exactly perpendicular to the boundary plates because of some hydrodynamic flow in the layer. A couple of diffuse lines near the upper and lower glass plates can be seen. A connecting interface between the two lines and a vertical thread at the former kinks of the wall are observable. FIG Nematic layers with alignment inversion walls in an electric field (10 V150 p, polarizer parallel to the vertical rubbing direction, scale 180 : 1) ; a ) region with S walls in the right part of the figure separated by a sharp disclination line from a region with C walls in the left part ;6) regions with S walls in the upper part and T walls in the lower part of the figure. on both sides of such a wall at voltages just above the threshold have a shape similar to a tilted S. We would therefore like to call these deformations S deformations and the corresponding walls S walls. A Iayer with this type of deformation gives a conoscopic interference pattern which has a characteristic asvmmetrv. The change of deformation in the layer can thus be.observed. If the voltage is abruptly switched off, the interference pattern as well as dust particles indicate a countermovement of the director, before the layer begins to relax to its final state. The conoscopic interference pattern of the region in the left side of figure l a is centrosymmetric and no tilt movement could be observed after switching off the field. We call this type of deformation a C deformation- and the corresponding walls C walls with respect to a model given later. The C walls have no kinks at the extremal points relative to the rubbing direction in contrast to S walls, but pairs of points, where the focal lines are disturbed, can be observed. The closed loops of C walls shrink and disappear in a similar way to that of the S walls. FIG Pinching of S walls in high electric fields of about 105 V/cm. A grey interface, joining the two lines near the upper and lower glass plate, is visible (scale 180 : 1). In contrast to the continuous pinching, a discontinuous splitting effect of the different wall types into a pair of sharp disclination lines can occur. This effect is shown in figure 3 for an S wall. The splitting process begins at points where a disclination line of half strength touches the wall. The disclination line invades the wall by forming a kink and proceeds along the wall, replacing it by a pair of threads. Besides this effect the splitting can occur spontaneously at an U FIG.3. - Splitting of an S wall into a pair of sharp disclination lines (10 V150 p, scale 180 : 1).

4 ALIGNMENT INVERSION WALLS IN NEMATIC LIQUID CRYSTAL LAYERS Cl-187 irregularity in the wall with a probability increasing with the applied voltage, as has also been observed by L6ger [9]. The splitting velocity of the different walls depends linearly on the applied field. The velocities for S, C and T walls parallel to the rubbing direction are given in figure 4. At a voltage of about 7 V (independent of the layer thickness) the splitting remains static, at lower voltages the wall is reformed from the disclination pair. FIG Structure of walls lying perpendicular to the rubbing direction L0 ; orientations of the director are indicated by o (parallel), o (tilted) and 0 (perpendicular to the figure plane) : a) S wall between opposite S deformations, b) C wall between opposite C deformations, c) T wall between opposite S deformations. FIG. 4. -Velocity of wall splitting in a W1 layer of 50 y thickness (S wall : +, C wall : 0, T wall :-; 24 OC). If the S walls and T walls are split up, then between the disclination lines an area with a C deformation appears. The reciprocal effect is observed at C walls, where an S deformed region appears between the disclination line pair. If the voltage is switched off, the S deformed regions relax to planar layers and the C deformed regions to areas with a twist of 180 degrees, which are separated from the planar parts of the layer by a disclination of strength 3. In this state the S and C walls completely disappear. 4. Discussion. - A model suggested by the authors [8] for the structure of those parts of the S walls which are not parallel to the rubbing direction L,, is shown in figure 5a. Because of the high elastic bend constant of W1 compared with the twist constant (k,/k, % 3), the bend walls are replaced by twist walls. The transitions to the surface orientations are achieved by additional twists in the boundary layers, with twist axis perpendicular to the glass plates. In the case of the S walls, both wall edges at the glass plates are twisted by an angle of 90 degrees, with opposite sense in the two boundary layers. The structure of the walls which arise if a field is applied to a region twisted by an angle of 180 degrees is now discussed. Because of the two different tilt possibilities in the central part of the twisted layer, opposite deformations with C-like profiles arise in the field. Two opposite C deformations separated by a C wall are shown in figure 5b. In the central zone of the layer, where the orientation of the director is perpendicular to the glass plates, the twist axis in the wall is oriented parallel to L,. In the boundary edges of the wall, the surface orientations at the glass plates are again adjusted by twists with axes perpendicular to the glass plates. In this case, however, the screw sense of the twists is the same in the two boundary layers. After switching off the voltage, the C deformation can only relax back to its initially twisted state if the director is tilted out of its metastable position in the L,-E plane by a fluctuation. The C deformation is relaxed much more rapidly by a successive spreading of the transition twist at the wall edges over the whole C region. The structure of the T walls, which arise in a layer with an initial twist of 360 degrees, is shown in

5 Cl-188 A. STIEB, G. BAUR AND G. MEIER figure 5c. In the central part of the layer a twist wall is again assumed. The transitional twists at the boundary plates now sum up to 360 degrees. The initial twist in the regions outside the walls disappears with increasing field as soon as the director reaches a direction perpendicular to the glass plates. In contrast to the S and C walls, this type of wall cannot completely disappear in the fieldless state. After the relaxation of the adjacent S deformations to planar layers, the wall leaves behind a narrow stripe with a twist of 360 degrees, which results from the twisted boundary edges of the T wall. This stripe is enclosed by disclination lines of strength one [7].. The different types of walls can adopt all directions relative to L,. In this case the structure of the walls can be constructed from the models in figure 5, if the central part of the wall is thought to be fixed in space while the boundary plates are turned in their plane by the corresponding angle. In order to minimize the elastic energy, the screw sense in the walls is changed at suitable points. These points can be observed as disturbances in the focal line pattern of the walls (see Fig. l). The splitting process is nearly the same for the three wall types because of the similar twist structures of the central parts of the walls far away from the boundary plates. The small differences between the splitting velocities of the different wall types are due to the boundary edges of the walls. In figure 6 a schematic representation of the splitting process is given for an S wall, lying parallel to the rubbing direction L,. The small figures in each row are to be thought of as being one behind the other. They represent successive cuts through the director field, as one proceeds perpendicularly through the' wall. In a the twist wall between opposite Frtedericksz deformations is illustrated. In the central plane of the wall, which is parallel to L, and E, the director is still parallel to L,. By the influence of the disclination a sudden change of the director field in the wall occurs, resulting in an elimination of the wall. After this change, the twist on both sides of the central plane is relaxed and the molecules are aligned parallel to the electric field (see 6b). FIG Splitting process of a twist wall lying parallel to the rubbing direction L. in an electric field E : a) twist wall between opposite Frkedericksz deformations, b) rupture of the director field, c) C deformation between the disclination pair, d) relaxed state after switching off the field. The wall is now replaced by two separate disclination lines of strength 5 near the upper and lower glass plates. They are allowed to move independently of each other by a successive change of the tilt near the glass surface. In this manner an area with a C deformation is created between the two disclination lines (Fig. 6c). In figure 6d the state after switching off the field is shown. The S deformation has now relaxed to a homogeneous planar layer while the former C region has again a twist of 180 degrees. Similar effects as described above have also been observed in planar cells with other substances of positive dielectric anisotropy and in Schadt-Helfrich cells. References [l] HELFRICH, W., Phys. Rev. Lett. 21 (1968) [5] STEINSTRASSER, R., Angew. Chemie 84 (1972) 636. [2] NEHRING, J., SAUPE, A., J. Chem. Soc. Faraday Trans. I1 68 [6] DE JEU, W. H., GERRITSMA, C. J., VAN ZANTEN, P., and (1972) 1. GOOSSENS, W. J. A., Phys. Lett. 39A (1972) 355. [3] LEGER, L., SoZid State Commcm. 10 (1972) 697 ; Solid State [7] STIEB, A. et al., to be published. Commun. 11 (1972) 1499 ; Mol. Cryst. Liqu. Cryst. 24 [8] STIEB, A., BAUR, G. and MEIER, G., Berichte der Bunsen- (1973) 33. Gesellschaft 78 (1974) 899. [4] DE JEU, W. H., GERRITSMA, C. J. and LATHOUWERS, Th. W., [g] LEGER, L., private communication. Chem. Phys. Lett. 14 (1972) 503.