Measurements of Viscosities in Nematics

J.K. Moscicki [ Datareview in this book 8.2 Measurements of viscosities in nematics ]... 

Measurements of viscosities in nematics Cl.l Shear flow capillary method... 

Taking R(p) 1, this leaves only one temperature-independent constant, ao/rj, left to be obtained by fitting the viscosities in the nematic state below Tni. Since ao is an orientation-independent contribution, the ratios of the Miesowicz viscosities r)c -r)b - a deviate less from unity as ao//y is increased. A value aQ/rj = 0.6 fits the Miesowicz viscosities of MBBA reasonably well the predictions of the theory with ao/// = 0.6 are given by the lines in Fig. 10-9a. Measurements of the Miesowicz viscosities for other liquid crystals are similar enough to those of MBBA that this theory with ao/rj = 0.6 is likely to work equally well... 

The introduction of defects into a smectic sample destroys its fluidity. This contrasts markedly with nematics, for which the presence of defects hardly alters the fluid s viscosity. Of course, this is because in a smectic the direction locally perpendicular to the layers is solid-like, and when defects are present, all directions acquire some solid-like character. Horn and Kleman (1978) measured shearing stresses in defect-containing smectic samples of 8CB and found that the shear stress was given by the equation of a Bingham plastic ... 

Because of the difficulty with which polymeric nematic monodomains are prepared, there are few measurements of Leslie viscosities and Frank constants for LCPs reported in the literature. The most complete data sets are for PBG solutions, reported by Lee and Meyer (1990), who dissolved the polymer in a mixed solvent of 18% dioxane and 82% dichloromethane with a few percent added dimethylformamide. Some of these data, measured by light scattering and by the response of the nematic director to an applied magnetic field, are shown in Figs. 11-19 and 11-20 and in Table 11-1. While the twist constant has a value of around K2 0.6 x 10 dyn, which is believed to be roughly independent of concentration and molecular weight, the splay and bend constants ATj and K3 are sensitive to concentration and molecular weight. 

Few other sets of viscosities exist for polymeric nematics. Yang and Shine (1993) obtained three of the Leslie viscosities for monodomains of poly(n-hexyl isocyanate) (PHIC) from rheological measurements in the presence of an electric field, and they obtained values reasonably consistent with the predictions of the Kuzuu-Doi expressions. From monodomains of the polyion PBZT, poly(l,4-phenylene-2,6-benzobisthiazole) in methane sulfonic acid, some of the Leslie-Ericksen parameters have been extracted via light-scattering and magnetic-field-reorientation studies (Berry 198S Srinivasarao and... 

The new FC technique made possible extensive measurements of the self-diffusion constans Dg and Dj of liquid crystals, parallel ( ) and perpendicular (-L) to the director axis, with v ues as low as 10 m s . The most challenging requirements exist for nematic mesophases because their low viscosity facilitates flow in the nonequilibrium state and so necessitates the fast FC procedure to establish the magic angle rotation of the director... 

We shall now discuss the application of the Ericksen-Leslie theory to some practical problems in viscometry. Probably the first precise determination of the anisotropic viscosity of a nematic liquid crystal was by Miesowicz. He oriented the sample by applying a strong magnetic field and measured the viscosity coefficients in the following three geometries using an oscillating plate viscometer ... 

The flow properties of a cholesteric liquid crystal are surprisingly different from those of a nematic. Its viscosity increases by about a million times as the shear rate drops to a very low value (fig. 4.5.1). One of the difficulties in interpreting this highly non-Newtonian behaviour is the uncertainty in the wall orientation which cannot be controlled as easily as in the nematic case. Some careful measurements of the apparent viscosity // pp in Poiseuille flow have been made by Candau, Martinoty and Debeauvais of a... 

An anomalous increase of A, has been observe but the principal experimental difficulty in determining the critical exponent accurately is that the normal nematic viscosity (in the absence of correlations) is itself strongly temperature dependent and the anomalous part forms only a relatively small contribution. However, careful measurements have indicated a mean field behaviour. 

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