Cano wedge

The Cano wedge can be used to measure the cholesteric pitch. Two substrates are wedged by a small angle 2y. The cholesteric liquid crystal between the substrates is homogeneously aligned. Under polarized microscopy, the equally spaced disclinations appear at the middle of the wedge. The separation of the lines 2d is associated with the pitch P... 

Figure 11 shows the dependence of the critical unwinding voltage Vf on the cell thickness-to-pitch ratio for a cholesteric layer with perpendicular boundary orientation. For these measurements two reusable cells, having thicknesses of 7.8 and 10.9 ym, were filled with various mixtures of the nematic liquid crystal RO-TN-103 and the chiral substance CB 15.We used the Cano wedge technique to determine the pitch of the mixtures and found the simple inverse relationship... 

The Cano-wedge method is an experimental technique to measure the pitch of cholesteric Uquid crystals. It consists of a flat substrate and a hemisphere with a cholesteric Uquid crystal sandwiched between them as shown in Figure 1.22(a). At the cento, the spherical surface touches the flat surface. On both the flat and spherical surfaces there is a homogeneous alignment layer. The intrinsic pitch of the Uquid crystal is Pg. Because of the boundary... 

Figure 5 Plots of helical twisting power (HTP) against the dopant concentrations in 60CB. HTP describes the strength of moiecular twisting, obtained by Cano-wedge method [21] or polarization microscopy.

Berreman and Heffner [59] considered the cholesteric Grandjean texture with tilted director orientation on the boundaries, Fig. 6.16. In the absence of the tilt, the free energy g is minimum at the following thicknesses d of the Cano wedge ... 

Measure of the Cholesteric Pitch Using the Cano Wedge... 

In the cholesteric phase, this local tendency of molecules to twist is satisfied in only one direction of space—let us say the z-axis as shown in Figure 2.7. The pitch p of such a natural helix is usually quite short in pure materials p 1 pm and cannot be deduced from direct observations in a polarizing microscope. In order to measure the pitch, the so-called Cano wedge method [2] can be used. It is very tricky and quite simple to realize. These are two reasons at least for which the Cano wedge experiment deserves to be presented here. 

Figure 2.9. Photograph of the cholesteric texture in the Cano wedge.

Figure 2.10. The texture of a cholesteric liquid crystal in the Cano wedge the number of half-pitches located in the gap varies as a function of the thickness h x).

Figure 2.12. Disclination / between two bands in the Cano wedge texture.

Let us note at the end of this section, that at the boundary between two adjacent bands, in the Cano wedge texture, one finds a x disclination of the cholesteric texture. This disclination is a topological necessity because the director makes a n turn on the Burgers circuit shown in Figure 2.12. 

The Cano wedge method is very convenient for measurements of variations of the cholesteric pitch in the presence of the magnetic field orthogonal to the helix (Figure 2.13). The idea of this experiment, due to de Gennes [3] and... 

Figure 2.15. Variation of the cholesteric pitch versus the magnetic field measured by the method of the Cano wedge by Durand et al. [5]. The liquid crystal used in this experiment was the mixture of PA A with cholesteryl chloride at concentration 0.02%. The continuous line corresponds to the theoretical prediction by de Gennes.

Splitting of / disclination lines has been observed in the so-called Cano wedges one of the disclinations is always a X, i.e., it does not carry any material singularity. The first dislocations near the center of the wedge have a small Burgers vector b = p/2), while b increases for dislocations far from the center. 

Although the phenomenon is similar in appearance to the buckling of dislocations in Cano wedges [98], [99], it cannot be understood solely as a dielectric response [99] of the cholesteric. The behavior of the oily streaks can be explained only if one adds a spedlic surface anchoring term with an amplitude W K/pXo the standard energy density, comprised of the elastic terms (5.7) and the dielectric term---- (/ E). ... 

Another way to assess the elastic constants in the blue phases is to observe their behavior in the Cano wedge configuration, which tends to compress and stretch the blue phase lattice parameter. This strain also raises the free energy and alters the blue phase transition temperatures, as has been shown by Feldman et al. [98]. For more discussion of blue phases in the Cano wedge configuration, see the papers by Stegemeyer and coworkers [99], [100]. 

Figure 8. Schematic diagram of the Cano-wedge technique for measuring the pitch, wherep=2lxa=2lhlL, in a chiral nematic (see text).

Figure 9. Typical Cano-wedge textures for a chiral nematic. The Grandjean-Cano disclination lines occur at the blue-yellow interface. The slightly curved distortion shows how sensitive the technique is to undulations in the glass of the wedge cell used here.

C1/C2 chevrons, ferroelectric devices 644 cadmium selenium TFT address 233 Cano wedge, chiral nematics 347 f, 351 Canon technology, ferroelectric devices 648 capillaiy flow, shear viscosity 143 carbocyclic compounds, charge transfer systems 958 carbocyclic rings, smectogens 412 carbon atoms, intercalated smectics 808 carbon-carbon bonds, dimers 823 carbon-carbon double bonds, chiral smectics 498 carbonaceous phases 693 carbonyl connectors, antiferroelectrics 687 carbonyl groups... 

A cholesteric liquid crystal with pitch of about 0.3 im in wedge geometry showing Grandjean steps (Grandjean-Cano wedge cell). 

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