Cholesterics flow/viscosity

SmA-C transitions, ferroelectric 481 SmA-hexatic smectic B transition 291 f SmA-SmC transition 65, 289 f, 326,478 SmA-TGBA-cholesteric, multicritical behavior 368 SmA-SmA critical point 302 SmAj/SmA fluctuations 384 SmC -SmA-TGB, multicritical behavior 368 SmC/C phases, flow/viscosity 470 f small angle neutron scattering (SANS) 684 smart pixels 810,814 smectic mesophase, difftision 590 smectic order parameters, XRD 646 smectic phases 17, 43, 60... 

The flow properties of cholesterics have scarcely been studied at all. Figure 10-26 shows one of the few sets of measurements of the viscosity of a cholesteric-forming small-molecule material, cholesteryl myristate, as a function of shear rate in flow through a eapillaiy at various temperatures (Sakamoto et al. 1969). As the temperature is lowered, cholesteryl myristate passes through isotropic, chiral nematir smectic, and nrystalline phases Figure... 

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 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... 

Fig. 4.5.5. Theoretical variation of the apparent viscosity with pitch P = 2n/q for flow normal to the helical axis of a cholesteric (or twist nematic) at low shear rates. Plot of versus P for twisted PAA. The separation between the...

In experiments with cholesteric liquid crystals (geometry III), extraordinary high viscosity rim is observed, few orders of magnitude higher than the viscosity of the isotropic phase or a non-twisted nematic. It seems surprising because the local structure of nematics and cholesterics is the same. In addition such a flow is strongly non-Newtonian with increasing shear rate (s) rim decreases, as schematically shown in Fig. 9.9. In the case of the Poiseuille flow, the viscosity depends also on the radius of a capillary. 

Five independent viscosity parameters are required to describe the flow of nematics and cholesterics. The set of five may be chosen in various ways.9 one particularly simple set determines resistance to the motions shown in Fig. 4. The first three, riii U2 and ri3, are like the viscosity of ordinary isotropic liquids but depend on the relative direction of shear and director orientation. These three may be regarded as principal axis of a viscosity ellipsoid. The fourth, ni2 is a measure of deviation from true ellipticity, giving a difference in viscosity as directors lean toward or opposite the shear flow. The final viscosity, Yl determines resistance to pure rotation without flow. In some cases a fair approximation to twist cell behavior may be obtained ignoring flow and considering only Yl When numerical computations are undertaken, however, one might as well do the problem correctly and include the flow.11 12... 

The encapsulated cholesteric liquid crystals are suitable for flexible displays with plastic substrates. They have much higher viscosities than pure cholesteric liquid crystals and can be coated on substrates in roll-to-roll process [71,72]. The polymers used for the encapsulation have good adhesion to the substrates and can make the materials self-adhesive to sustain the cell thickness. Furthermore, the encapsulated Ch liquid crystals can no longer flow when squeezed, which solves the image-erasing problem in displays from pure cholesteric liquid crystals where squeezing causes the hquid crystal to flow and to be switched to the planar state. 

The encapsulation process for ChLC is mainly attributed to its transport and optical properties [15]. Firstly, since viscosity of pure ChLC is close to that of water, its fluidity prevents ChLC from being coated on flexible substrates. Secondly, when a cholesteric liquid crystal is pressed, the flow generated inside makes the displayed image erase. Therefore, droplet dispersions by encapsulation act as a protector for its bi-stability and optical properties. The additional advantage is that encapsulated cholesteric liquid crystals are self-sealing the materials confined to the droplets cannot flow through an interface of the droplets. 

The response time of cholesteric liquid crystals is finite and comparatively long because the visible color change occurs in two stages. An instantaneous change in the temperature of a cholesteric liquid crystal causes it to assume a new pitch. However, this requires that some of the material flows to a different configuration. The rate at which such flow occurs is limited by the material viscosity in the direction of flow. 

The flow properties of cholesteric liquid crystals are surprisingly different from those of the nematics. The most important difference is that, in some directions (along the helical axis), the viscosity measured in Poiseuille flow geometries (see Appendix B) is about six orders of magnitude larger than in the isotropic phase, or in the cholesteric phase when the flow direction is perpendicular to the helix axis. In this latter case, the viscosity is similar to that of nematics, although the behavior is somewhat non-Newtonian above a pitch-dependent threshold shear rate. It was found that the shear rate above which the fluid becomes non-Newtonian is inversely proportional to the square of the pitch. The apparent viscosities as the function of shear rate of materials with different pitch values are shown in Figure 4.6. 

This range has been called the "distal" region. Although the layer compression modulus is about two orders of magnitude larger than in tire short-pitch cholesterics (see (4.31)), permeation-type flow similar to that observed in cholesterics was observed near the nematic phase, ° shovdng that the apparent viscosity maybe smaller than in the cholesteric, probably due to defects that cause plastic behavior. This and measurements imder periodic deformations indicate the importance of layer defects, which are hard to regulate. 

The physics of liquid crystals [97] indicate that the viscosity passes through a maximum at the point of tire transition from the isotropic to Ae LC state as a result of ordering of the object and cooperative orientation in flow for nematic systems and a large number of cholesteric systems. The existence of a maximum is usually found in flow of a liquid crystal with an uncontrollable orientation. If the coefficients corresponding to orientation of the macromolecules parallel to the direction of the velocity gradient (%) and parallel to the velocity vector (t] ) are separated out, then a more complex picture is obtained (Fig. 9.23). The maximum in the region of the nematic-isotropic phase transition for p-azoxyanisole is observed for the dependences 11(7) and A break is... 

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