Cano-wedge method

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.

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. 

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.

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