Cholesteric liquid crystal flow properties

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... 

Cholesteric liquid crystals are compounds that go through a transition phase in which they flow like a liquid, yet retain much of the molecular order of a crystalline solid. Liquid crystals are able to reflect iridescent colors, depending on the temperature of their environment. Because of this property they may be applied to the surfaces of bonded assemblies and used to project a visual color picture of minute thermal gradients associated with bond discontinuities. Cholesteric crystals are potentially a simple, reliable, and economical method for evaluating bond defects in metallic composite structures.f Materials with poor heat-transfer properties are difficult to test by this method. The joint must also be accessible from both sides. ... 

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. 

In this context, literature [90] states that at room temperature, acetoxypropyl cellulose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When subjected to specific conditions of shear flow, the cellulose derivative cholesteric liquid crystal suffers transformations, such as cholesteric helix and cholesteric-to-nematic transition. The films prepared from anisotropic solutions of termotropic acetoxypropyl cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated by the molecular orientation of the solution under shear stress. Thus, liquid crystalline solutions give rise to films with anisotropic mechanical properties the films are brittle when stretched parallel to the shear direction and ductile when stretched perpendicular to it. 

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. 

The flow properties of other liquid crystals, such as chiral nematics (i.e.. cholesterics). smectics C, and hexagonal phases, are even more poorly understood. 

Liquid crystals are wonderful materials. In addition to the solid crystalline and liquid phases, liquid crystals exhibit intermediate phases where they flow like liquids, yet possess some physical properties characteristic of crystals. Materials that exhibit such unusual phases are often called mesogerrs (i.e., they are mesogenic), and the various phases in which they could exist are termed mesophases. The well-known and widely studied ones are thermotropics, polymeries, and lyotrop-ics. As a function of temperatirre, or depending on the corrstituerrts, concentration, substituents, and so on, these liqirid crystals exist in many so-called mesophases— nematic, cholesteric, smectic, and ferroelectric. To understand the physical and optical properties of these materials, we will begin by looking into their constituent molecules. ... 

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