Frank elastic constants measurements

Measured values of the Frank elastic constants Ku, 1 22. and K33 and their temperature dependence are available in the literature. " The common order of magnitude far from Tni is around 10 N. K n and K33 are close (they are equal in the Landau-de Gennes analysis). For the 5CB, K33 1.3/Tii, while K22 0.5/fn. Similar trends... 

There are other reports on the study of pretransitional dynamics in polymeric and lyotropic nematics. Quantitative measurements of ratios of Frank elastic constants and Leslie viscosities in the pretransitional range of poly-y-benzyl-glutamate polymeric nematic are reported by Taratuta et al. [85]. McClymer and Keyes [86-88] report light scattering studies of pretransitional dynamics of potassium laurate-decanol-D20 system. An interesting study of a magnetic-field induced I N phase transition in a colloidal suspension is reported by Tang and Fraden [89]. 

These three different modes of deformation are demonstrated in Figure 2.21 for a nematic liquid crystal. The fluid-like nature of the nematic phase allows these deformations to take place with relatively little energy cost (compared with a crystalline solid). In these equations, the constants K, K22. and K33 are known as the Frank elastic constants and typically take a value around N/m. Measurement of the elastic constants is often carried out to characterize the switching capabilities of liquid crystal materials (i.e., how well they will perform in a device under the application of an electric field). 

Dynamic fight scattering has often been used in bulk liquid crystals for several different purposes. In most cases, the ratios of the viscoelastic coefficients were determined, according to the dispersion relation (4.8). In principle, from the amplitude of the non-normalised autocorrelation function one can also obtain the elastic constants themselves by using (4.7), but as it is difficult to measure absolute intensities in fight scattering, Frank elastic constants are usually obtained using other methods. 

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