Nematics flow/viscosity

The flow viscosity of a nematic phase also determines the spatial and temporal response of the director to an applied field. The bulk viscosity of a nematic phase depends on the direction of flow of each molecule with respect to the director, averaged out over the whole of the sample. Therefore, bulk viscosity is... 

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... 

The anisotropy of the viscosity is an important feature of the rheological properties of nematics the viscosity of the solution has different values as a function of the mutual orientation of the director and the direction and gradient of the velocity (in simple shear flow). It is known (cf. [1]) that the anisotropy of the viscosity is described by the Leslie coefficients aj-Og. In the simplest... 

C. Characteristically, these nematic melts show the persistence of orientational order under the influence of elongational flow fields which result in low melt viscosities under typical fiber formation conditions even at high molecular weights. 

Molecules of nematic Hquid crystals also are aligned in flow fields which results in a viscosity that is lower than that of the isotropic Hquid the rod-shaped molecules easily stream past one another when oriented. Flow may be impeded if an electric or magnetic field is appHed to counter the flow orientation the viscosity then becomes an anisotropic property. 

Analysis of flow curves of these polymers has shown that for a nematic polymer XII in a LC state steady flow is observed in a broad temperature interval up to the glass transition temperature. A smectic polymer XI flows only in a very narrow temperature interval (118-121 °C) close to the Tcl. The difference in rheological behaviour of these polymers is most nearly disclosed when considering temperature dependences of their melt viscosities at various shear rates (Fig. 20). 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

The big difference between normal isotropic liquids and nematic liquids is the effect of anisotropy on the viscous and elastic properties of the material. Liquid crystals of low molecular weight can be Newtonian anisotropic fluids, whereas liquid crystalline polymers can be rate and strain dependent anisotropic non-Newtonian fluids. The anisotropy gives rise to 5 viscosities and 3 elastic constants. In addition, the effective flow properties are determined by the flow dependent and history dependent texture. This all makes the rheology of LCPs extremely complicated. 

The magnitudes of the viscosities (the a ,- s) for a single small-molecule nematic can differ from one another by an order of magnitude or more. As a result, the fluid s resistance to flow depends strongly on the directions of the flow and the flow gradient relative to the nematic director. In a shearing flow, the viscosities o 2 and o 3 determine director torques in the orientations shown in Fig. 10-7b and 10-7c. If the director is oriented in the flow direction... 

A theory by Helfrich (1969) suggests that the viscosity of a chiral nematic phase is high because the cholesteric director is blocked-, that is, it cannot respond to the flow... 

The viscous properties of a smectic A are characterized by the same five independent viscosities that characterize the nematic. As we shall see, however, the elastic properties of the smectic are very different from those of a nematic, and some flows permitted to the nematic are effectively blocked for the smectic. For smectic C, for which the director is tilted with respect to the layers, there are some 20 viscosities needed to characterize the viscous properties (Leslie 1993). Formulas for these, derived using a method analogous to that used for nematics by Kuzuu and Doi (1983, 1984) can be found in Osipov et al. (1995). The smectic phase for which rheological properties are most commonly measured is smectic A, however, and hereafter we will limit our discussion to it. 

---