Droplets, cholesteric

At constant temperature, when the polymer concentration is increased, anisotropic droplets appear and coalesce. Layers of the cholesteric structure can be observed Inside the droplets. 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

To form cholesteric liquid crystalline polymers, one either polymerizes cholesteric monomers or mixes low molecular mass cholesteric liquid crystals with polymers. In the latter case, two components may be mixed homogeneously or in such a way that the polymers act as a matrix while the small molecular mass cholesteric liquid crystals are in droplets, known as the polymer-dispersed liquid crystals (PDLC) (Doane et al., 1988) or the nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition, there are many polymers in nature exhibiting the cholesteric phase such as PBLG, cellulose, DNA, etc. 

Fig. 4.4.1. Lehmann s diagrams depicting the rotation phenomenon in open cholesteric droplets heated from below. (After Lehmann. )...

An example of this type of thermomechanical coupling appears to have been observed by Lehmann in cholesteric liquid crystals very soon after their discovery. He found that droplets of the material when heated from below seemed to be rotating violently, but from optical studies he concluded that it was not the drops themselves but the structure that was rotating. Fig. 4.4.1 shows a few of the many sketches that he made depicting his observations. Leslie s equations offer a simple explanation of the phenomenon because of the absence of mirror symmetry, an applied field, which is a polar vector, can result in a torque, which is an axial vector. 

There are two classic approaches for encapsulation emulsification [17] and phase separation [18]. The main difference between the two methods depends on how to make and process encapsulated droplets. In the emulsification method, water is used as a solvent to dissolve a polymer and to form a viscous solution, and cholesteric liquid crystals are mixed with the aqueous solution. By a shearing device like a propeller blade, small droplets of micrometer scale are formed and finally emulsified. The resulting emulsion is then printed on a plastic film and dried by evaporation of water. The disadvantage of this method is the broad size... 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

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. 

When L/p I, the cholesteric does not differ much from the nematic phase. No wonder therefore that optical observations for weakly twisted cholesterics reveal thick (nonsingular) and thin (singular) line defects —disclinations similar to that in the nematic phase. Moreover, in droplets of the so-called compensated cholesteric mixtures with extremely small Ljp one can observe point defects [6] which, from the topological point of view, are allowed only in a nematic phase. 

Both the Poincare and Gauss theorems can be applied to structures in cholesteric droplets provided that the surface anchoring is sufficiently strong. 

Figure 5.15. Cholesteric textures in spherical droplets with tangential director anchoring at the boundary. Top A monopole configuration with a point defect N =l in the field / of normals to the cholesteric layers and an attached nonsingular line k — 1, stable when R/p 1 (microphotograph in crossed polarizers). Bottom A boojum configuration with an isolated k — 2 surface point defect at R/p 1 (no crossed polarizers). The insert shows the director field at the surface of the droplet.

Cholesteric droplets have been extensively studied during the last decade, especially after Crooker and Yang suggested to use polymer-dispersed cholesteric liquid crystals for reflective color displays [63], Lavrentovich and Nastishin [64], [65] reported on an intriguing phenomenon liquid crystal droplets dispersed in an isotropic matrix (glycerin with lecithin) divided into smaller ones when one decreases the temperature of the sample, and passes from the cholesteric to the smectic A phase through the TGB phase. The reader is referred to the recent reviews [66]-[68], and to the contribution of Crawford, Svensek, and Zumer in this book for more details about dispersed liquid crystals. 

The effect of the applied electric field on the droplet structure will be discussed here only substances with negative dielectric anisotropies x (12.19) will be of interest. In this case perpendicular alignment is favored (12.18), so that the helical axes tend to align with the field (applying an electric field to a cholesteric with positive Xe would result in destabilizing the chiral order). 

J. Bezic and S. Zumer, Structures of the cholesteric liquid crystal droplets with parallel surface anchoring, Liq. Cryst. 11, 593 (1992). 

H.-S. Kitzerow and P.P. Crooker, Electric field effects on the droplet structure in polymer dispersed cholesteric liquid crystals, Liq. Cryst 13, 31 (1993). 

When smectic A or cholesteric phases are in equilibrium with their isotropic phase, the mesophase is present in the preparation in the form of batonnets or droplets, and extends here and there over larger domains, with a definite distribution of textures. Fans lie in the vicinity of the isotropic interface, whereas planar textures are rarely in contact with it, and polygons are observed along zones separating fans from planes. 

Cholesteric rodlets and spherulites having parallel layers can be devoid of inner defects, but this does not prevent their growth. However, surface points or surface lines are present. Some -n disclinations, resulting from germ coalescence, are frequent, but disappear by confluence with the isotropic interface, as for nematic droplets. 

The distribution of defects in mesophases is often regular, owing to their fluidity, and this introduces pattern repeats. For instance, square polygonal fields are frequent in smectics and cholesteric liquids. Such repeats occur on different scales - at the level of structural units or even at the molecular level. Several types of amphiphilic mesophase can be considered as made of defects . In many examples the defect enters the architecture of a unit cell in a three-dimensional array and the mesophase forms a crystal of defects [119]. Such a situation is found in certain cubic phases in water-lipid systems [120] and in blue phases [121] (see Chap. XII of Vol. 2 of this Handbook). Several blue phases have been modeled as being cubic centred lattices of disclinations in a cholesteric matrix . Mobius disclinations are assumed to join in groups of 4x4 or 8x8, but in nematics or in large-pitch cholesterics such junctions between thin threads are unstable and correspond to brief steps in recombinations. An isotropic droplet or a Ginsburg decrease to zero of the order parameter probably stabilizes these junctions in blue phases. 

The liquid-crystalline nature of nucleic acids in vivo. In lower organisms, the nucleic acids are naked and exist for much of the time as concentrated droplets of cholesteric phase. In electron micrographs of the nuclei of some dinoflagel-lates, for example, the characteristic bands of nested arcs, Bouligand Pat-... 

Churchill D, Cartmell JV (1971b) Display device containing minute droplets of cholesteric liquid crystals in a substantially continuous polymeric matrix. US Patent 3,600,060 Cole KS, Cole RH (1941) Dispersion and absorption in dielectrics-I alternating current characteristics. J Chem Phys 9 341-351... 

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