Alignment shear viscosity

Isotropic solutions exhibit a monotonic increase in shear viscosity with increasing concentration. The viscosity increases to a maximum when the isotropic to anisotropic transition is approached. Upon formation of the anisotropic phase, the viscosity begins to decrease, after which the viscosity increases strongly as the concentration continues to increase (Fig. 6). In the isotropic state, the hydrodynamic volume is large because of the random polymer orientation. This restricts the polymer diffusivity and causes an increase in viscosity. In the anisotropic phase, the aligned polymer leads to a small hydrodynamic volume and a decrease in viscosity as rotational diffusion is much easier with a net orientation. 

Fig. 7 Shear rate dependences of the flow birefringence An right scale, open symbols) and of the alignment angle x closed symbols, left scale) for 3/3 mM tetradecyltrimethylammonium bromide and sodium salicylate (Ci4TAB/NaSal), corresponding to a total weight concentration c = 0.15 wt. %. Regimes I, II, and III were determined from steady shear viscosity. Figure adapted from [61], courtesy J.R Decruppe...

The overall situation is summarised in figure 21, which shows that behaviour ranges from simple suspensions and emulsions, where the extensional viscosity is only a small multiple of the shear viscosity, to the cod-stretch situation of a possible thousand-fold difference. The behaviour at the highest (relative) extension rates is shown in figure 22, where a concentrated rod-hke pol)uner—say xanthan gum—is stretched very quickly, see [6]. Both shear and extensional viscosities flatten out as the polymer chains are completely untangled and aligned, and show and approximately forty-fold difference. 

An alternative to reorientation of the sample or the magnetic field is the application of shear during the NMR measurement [130-134]. For liquid-crystalline samples with high viscosity, such as liquid crystal polymers, the steady-state director orientation is governed by the competition between magnetic and hydrodynamic torques. Deuteron NMR can be used to measure the director orientation as a function of the applied shear rate and to determine two Leslie coefficients, and aj, of nematic polymers [131,134]. With this experiment, flow-aligning and tumbling nematics can be discriminated. Simultaneous measurement of the apparent shear viscosity as a function of the shear rate makes it possible to determine two more independent viscosity parameters [131, 134]. 

Shear viscosities under flow alignment can be studied using commercial viscosimeters. 

The anisotropic shear viscosity coefficients V2> 3 t]i2 have been determined only for a few liquid crystals. In contrast there have been many investigations of the shear viscosity coefficient under flow alignment. The reason for this might be that the effort required to determine. the anisotropic coefficients is greater by far. Viscosity coefficients determined under flow alignment are often used to estimate the switching times of liquid crystal displays. For basic research they are less important. 

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