Blue phases BPIII

Blue phases exist in a narrow temperamre region between the isotropic and cholesteric phases. As temperamre is decreased, the order of appearance of the blue phases is BPIII, BPII, and BPI [15-17]. Whether a chiral liquid crystal has a blue phase depends on its molecular stmcmre and chirality. The blue phases can be identified by an optical microscope under reflection mode. BPI and BPII have bright and colorful multi-domain crystal plate textures, while BPIII has a dim uniform foggy texmre [5,18]. Therefore, BPIII is also called the fog phase. As will be discussed later, BPI and BPII have cubic crystal structures while BPIII has an amorphous stmcmre. 

Figure 13.15 Optical rotatory power of blue phases. (1) single crystalline BPI, (2) single crystalline BPII, (3) polycrystaUine BPI, (4) polycrystaUine BPII, (5) BPIII, (6) isotropic phase. Sov. Phys. JETP.

As the temperature increases, up to three types of blue phases BPI, BPII, and BPIII may exist [14]. BPIII is believed to possess amorphous stmcture. BPI (Figure 14.2(a)) and BPII (Figure 14.2(c)) are composed of double-twist cylinders arranged in cubic lattices. Inside each cylinder, the LC director rotates spatially about any radius of the cylinder. These double-twist cylinders are then fitted into a three-dimensional stmcture. However, they cannot fill the full space without defects. Therefore, blue phase is a coexistence of double-twist cylinders and disclinations. Defects occur at the points where the cylinders are in contact (Figures. 14.2(b) and 14.2(d)). BPI is known to have body-center cubic stmcture and BPII simple cubic stmcture. 

This chapter reviews the present understanding of blue phases. Blue phases are distinct thermodynamic phases that appear over a narrow temperature range at the helical-isotropic boundary of highly chiral liquid crystals. In the absence of electric fields, there can be three blue phases BPI and BPII, both of which have cubic symmetry and BPIII, which possesses the same symmetry as the isotropic phase. Figure 7.1 shows schematically the phases in both nonchiral and chiral nematics. For nonchiral nematics, including racemic mixtures (with equal numbers of left- and right-handed versions of the same molecule) and even weakly chiral nematics, the nematic (or weakly chiral) phase heats directly to the isotropic phase. When the chirality is high, however, as many as three blue phases may appear. 

An explanation of the nomenclature should be made here. First, chiral nematic molecules need not come from cholesteryl derivatives, so we use the term chiral nematic instead of cholesteric when referring to liquid crystal materials. The chiral nematic/cholesteric phase itself we will call helical. Second, blue phases got their name from their blue appearance in early investigations. Blue phases are not always blue, however we now know that they may reflect light of other colors, including near infrared. Finally, BPIII was known as the fog phase or the gray fog phase in early publications. Although these terms are descriptive of this phase s appearance, BPIII seems to have survived. 

Figure 7.1. Schematic picture of the temperature region near the nematic (N)-isotropic (ISO) phase transition. Top Nonchiral molecules have only nematic and isotropic phases. Bottom Chiral molecules have helical (H) and isotropic phases, and, depending on the chirality, up to three blue phases (BPI, BPII, and BPIII). BPI and BPII are cubic BPIII has the same symmetry as the ISO phase.

Figure 7.3. Heat capacity CpjR versus temperature T in cholesteryl nonanoate (CN). Helical phase (CH), blue phases (BPI, BPII, BPIII), isotropic phase (I) (from Thoen [26]).

We now turn to BPIII, which has been the most enigmatic of the blue phases. Called variously the grey phase [14], the fog phase [70], the blue fog [103], and BPIII [53], the last name seems to have survived. This phase, which is amorphous and not cubic, has been reviewed in preliminary fashion by Crooker [17], Seideman [19], and Wright and Mermin [20], but at the time of these reviews (1989-1990) only the initial experiments had been performed and theoretical attention was just beginning. 

As shown in Figure 7.4, BPIII is the highest temperature blue phase, appearing either between BPII and the isotropic phase, or, at higher chiralities, between BPI and the isotropic phase. Like BPI, but unlike BPII, it is characterized by a temperature range which increases monotonically with increasing chirality. 

Like BPI and BPII, BPIII selectively reflects circularly polarized light [52], [53]. Unlike the cubic blue phases, however, the spectrum is quite broad ( 100 nm) [132]. Also, while BPI and BPII exhibit several Bragg peaks (corresponding to various crystal planes), BPIII exhibits only one peak. 

As can be seen from Figure 7.3, there is a small heat capacity peak between BPIII and the cubic blue phases and a much larger peak between BPIII and the isotropic phase. From the heat capacity data, BPIII therefore appears to be closer in structure to BPII than to the isotropic phase, in contrast to the visual appearance. 

The pretransitional fluctuation model assumed that BPIII is simply a manifestation of pretransitional fluctuations in the isotropic phase at the blue phase-isotropic boundary [3], [4], This idea was discounted [56] by the fact that the observed BPIII scattering [53] is several orders of magnitude too large for pretransitional fluctuations. In any case, the calorimetry data [26] rule out the pretransitional fluctuation model. 

Figure 7.21. Generic phase diagram showing temperature T versus chirality q for blue phases. Universal features include the survival of BPI and BPIII, but not BPII, at high chirality and termination of the BPIII-isotropic transition at critical point c.p.

When an electric field is applied to the BPiii (foggy) phase with the broad selective reflection peak typical of this less ordered phase decreases in intensity and, at some threshold voltage, is replaced by a sharp peak at longer wavelength [43]. Thus, the transition to a new phase oecurs. The symmetry of the new phase has not yet been established. For a system with a<0 [53], the increase in the field results in a considerable increase in the selective reflection peak. An explanation of such behavior and many other examples of field effects in blue phases can be found in a comprehensive review by Kitzerow [54]. 

Later it was found that there are three distinct type of blue phases. - The lower-temperature modifications, BPI and BPII, possess cubic lattice of different symmetries (O and CF, respectively). The high-temperature modification, BPIII, appears on cooling from the isotropic phase as an amorphous "blue fog." Freeze fracture electron micrographs of BPIII appear to show a disordered packing of filamentary objects. Textures of the different blue phases are shown in Figure 6.23. 

The structure of the BPIII phase actually resembles the L3 (so-called sponge) phase of lyotropic liquid crystals/ and the smectic blue phases observed and studied recently. Both the sponge and smectic blue phases are optically isotropic, and the smectic blue are optically active as well. Theoretical arguments show that the reason for the defect structure is the negative value of tire saddle-splay elastic constant, K24, which makes the defects energetically favorable. A sketch of the sponge phase and of the smectic blue phase is shown in Figure 6.24. 

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