Liquid Crystal Textures and Phases: A Short Tutorial

The optical properties of liquid crystal phases often directly reflect the symmetry of their structures. Anisotropy of the refractive index, or birefringence, is one of the characteristic physical properties of liquid crystals, and it allows for the visualization of the macroscopic molecular orientation. In thin liquid crystal sample cells placed between two crossed polarizers under an optical microscope, a variety of textures and birefringence colors will be observed. These textures and colors not only look beautiful but also provide a lot of information about the macroscopic structure of the LC phases.Although there are many experimental techniques available to investigate the structure and physical properties of LC phases, microscope observations often give enough information to determine the structure even if a well-aligned domain is not obtained

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The Nematic phase

In a planar cell, where the molecular director aligns parallel to the glass surface, with no rubbing treatment a schlieren texture organized around point disclinations, disclination lines and inversion walls is observed [see Figure 1 (a)].In a homeotropically aligned texture, where the molecular director is normal to the glass surface, no birefringence is observed [see Figure 1 (b)] because the phase is uniaxial [see Figure 1 (c) and (d)] and the optic axis is normal to the substrates. Birefringence is visible when the sample is tilted.Flickering originating from fluctuations of the n-director is often observed in both homeotropic and planar cells.

   
  Figure 1.  Photomicrographs of the Nematic phase (E31 at room temperature).  (a) Schlieren texture in a 20-micron planar cell.  Plus and minus 1 point disclinations and plus and minus half disclinations are apparent.   (b) Homeotropically aligned texture in a 100-micron cell of substrates coated with OTE monolayer.  (c) Conoscopic figure observed on cell (b) under white light and (d) under monochromatic light  at 532nm.  

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The Cholesteric (Chiral Nematic) phase

Textures of Cholesteric (Ch) (or chiral nematic (N*)) phase observed in planar and homeotropic cells are shown in Figure 2. The color observed in (a) is not the birefringence color but the selective reflection color originating from the helical structure of short pitch.Lines observed in (a) are Grandjean-Cano lines which appear because a thickness gradient of the cell.Grandjean-Cano lines are defect lines which form between two regions of different number of helical pitches.In a material with long pitch helix, [see figure2 (b)] not selective reflection color but a color due to birefringence and optical rotation is observed.Network-like defect lines are oily-streak lines.In the homeotropic cell (or a planar cell of material with short pitch helix), focal conic texture in Figure 2 (c) is often observed.It is difficult to distinguish the Sm-A and N* from this texture in this case.If the helical pitch is longer than the resolution limit of the microscope, finger-print textures (helical pitch lines) are observed in such focal conic textures[see Figure 2 (d)].

   
 

Figure 2. Photomicrographs of the cholesteric phase. (a) Textures in a planar wedge cell (YR21, 75°C), thinner in left and thicker in right.Grandjean-Cano disclination lines cross the cell.(b) Texture in a 20-micron planar cell (YR21, 75°C).(c) Focal conic texture observed in a 10-micron homeotropic cell (SCE12, 120°C).The focal conic texture looks similar to that in the Sm-A* phase.However, if the helical pitch is longer than optical wavelength, the finger print texture in (d) is observed even in the focal conic texture.The white bar indicates 100 microns.

 

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The Blue Phases

Textures of the BPII phase observed in a planar cell are shown in Figure 3. The phase is often exhibits selective reflection colors but haslow birefringent in almost all the case. Also the temperature region is often very narrow and hence the phase is difficult to observe.

There are four distinct blue phases are known the BPI, BPII, BPIII and Smectic Blue phase. BPI, BPII and BPIII shows no peak in x-ray diffraction but the smectic Blue phase shows layer ordering.

Figure 3. Photomicrographs of the BP II phase of 8SI*.

 

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The Sm-A and Sm-A* phases

The focal conic texture of the Sm-A phase in a planar cell with no rubbing treatment, and the homeotropic uniaxial texture in a homeotropic cell are shown in Figure 4. The apparent optic axis (fast axis and slow axis of the media) is parallel to the layer normal direction in the Sm-A phase.To obtain the layer normal direction in focal conic textures, attention should be paid to details of the texture, for example the shape of focal conics and layer undulation lines.In many Sm-A materials, the layer normal direction points to the center of the focal conic, i.e. layers orient tangentially in a circular focal conic domain.Layer undulation lines are often observed and smectic layers are perpendicular to the undulation lines. The Sm-A* phase (the Sm-A phase of chiral molecules) exhibits a small electroclinic response under application of an electric field, i.e. apparent optic axis tilts by a small angle under a high electric field. This is a polar effect, the sign of the tilt changing when the applied field is reversed.

Figure 4. Photomicrographs of the Sm-A phase (racemic MHPOBC at 140°C).  (a) Fan-shaped focal conic textures in a 10-micron planar cell and (b) in a 25-micron homeotropic cell.  Inset in (b) is the conoscopic figure indicating that the texture is not isotropic but uniaxial.

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The Sm-C and Sm-C* phases

The focal conic textures of the Sm-C phase and the schlieren texture in a homeotropic cell are shown in Figure 5.Since molecules in the smectic C phase are tilted from the layer normal direction, the apparent optic axis is also tilted from the layer normal.Because of this tilt, the Sm-C phase is biaxial and birefringent even in homeotropic cells, as shown in (b). The orientation variation of the tilt direction is observed as a schlieren texture in the homeotropic cell.

Because of the molecular chirality, the Sm-C* phase is generally helical.In surface stabilized cells in which the helix is unwound by surface interactions, domains with tilted apparent optic axes are found (surface stabilized).But if the helical pitch is short or the cell is very thick,the textures of the Sm-C* phase look different.In materials of long helical pitch, helical pitch lines whose spacing corresponds to half of helical pitch are observed while for materials with short helical pitch the texture looks quite similar to that of Sm-A

Figure 5. Photomicrographs of the Sm-C phase (racemic MHPOBC at 118°C).  (a) The broken fan-shape texture with tilted extinctions was observed in a 10-micron planar cell [compare to that in Figure 4 (a)] and (b) the schlieren texture was observed in a 25-micron homeotropic cell.  In this phase, only integer disclinations are allowed to exist.  Plus and minus 1 disclinations are visible in (b).

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The Sm-CA and Sm-CA* phases

The focal conic texture of the Sm-CA* phase in a planar cell and the schlieren texture in a homeotropic cell are shown in Figure 6.Since molecules in this phase are tilted from the layer normal as in the Sm-C phase, the Sm-CA phase is biaxial and birefringent even in the homeotropic cell. Orientational variation of the tilt direction is observed as the schlieren texture in a homeotropic cell as in the Sm-C phase.However, since the molecular tilt direction alternates from layer to layer the principal axis of the index ellipsoid is parallel to the layer normal.The apparent optic axis in a planar focal conic texture is parallel to the layer normal and looks the same as that in the Sm-A phase.Several subtle differences will be found in the detailed observations, but definitive characterization of this phase is often done using a homeotropically aligned sample, freely suspended films, or x-ray measurements (and electrooptic characterization for chiral materials).

Figure 6. Photomicrographs of the Sm-CA phase (racemic MHPOBC at 110°C).  (a) Fan-shape texture with stripe domains along the layers observed in a 10-micron planar cell [see Figure 4 (a) and 5 (a)] and (b) Schlieren texture in a 25-micron homeotropic cell.  In this phase, the wedge-screw dispiration, which looks apparently the same as a half strength disclination, is allowed to exist.  Integer strength (red arrow) and half strength (blue arrow) defects are seen in (b).

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The Sm-AP and Sm-CP phases

In some of the liquid crystalline phases formed with bent-core molecules, the layer structure spontaneously breaks symmetry to become polar even with achiral molecules. The Sm-AP and Sm-CP phase denote polar phases bent-core liquid crystals. A and C indicates whether the molecules tilt from the layer normal or not, and P indicates the phase is polar. Although textural observations are not sufficient to characterize the Sm-AP and Sm-CP phases, they can certainly not be distinguished without texture observations and study under application of electric fields.

              Bent-core LC phases often form circular focal conic domains as shown in Figure 7. As with the smectic phases of rod-like molecules, the layers orient tangentially in these domains.Figure 8 shows the super-molecular structure of these polar smectic phases and their field-induced state accessed by application of an electric field.As shown in Figure 8, the Sm-APF, Sm-APA and Sm-CAPF states are not distinguishable in this observation.

 

 

 

Figure 7. Photomicrographs of the Sm-CPA phase (P10PIMB at 158°C) (a) at zero-field and (b) in an applied electric field (+40V) and the Sm-CPF phase (Gda226 at 115°C) (c) at the zero-field and (d) in an applied electric field (+35V) in 6-micron planar cells.   Circular focal conic domains were grown under application of a square wave voltage (V~80Vpp, 0.8Hz).  The direction of extinctions and birefringence colors are different in each domains (even within one domain as well) which is due to different mixing ratio of + and - chiral layers.  In (c) and (d), not only Sm-CSPF and Sm-CAPF but also bistable domains and monostable domains of Sm-CSPF were found.

 

 

Figure 8.  Illustration of the supermolecular structure of polar smectic phases and their field induced states accessed by application of an electric field.  Yellow arrows indicate the direction of the apparent optic axis and hemispheres beside each structure show extinction brushes of circular focal conic domains in thin planar cells. In ferroelectric phases, several types of surface stabilized states, for example twist, uniform or monostable states are expected to exist as the ground state structure.

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Text and images contributed by Michi Nakata.