Foggy phases

BPIII Unknown periodicity and liquid-like molecular correlations. Three-dimensional photonic bandgap crystals showing optical activity but no birefringence. BPin is lower symmetry foggy phase strongly scattering light... 

As has been mentioned in Section 1.1.3, there exist three thermodynamically stable blue phases BP I, BP II, and BP III (or foggy phase). The structure of the former two is already established, BP I is a body-centered cubic phase (symmetry group O or I4i(32) and BP II is a simple cubic... 

When an electric field is applied to the BP III (foggy) phase, the broad selective reflection peak, typical of this less-ordered phase decreases in in-... 

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]. 

The dynamics of the BP,u (foggy) phase have some features related to the field-induced changes in the size of the domains that this phase forms. 

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. 

The visual appearance of BPIII is foggy, quite unlike the structured appearance of BPI or BPII [14]. Since it appears only at higher chiralities, it is often bluish or grayish in color. At very high chiralities, it may be invisible—indistinguishable from the isotropic phase—to the naked eye. Visually, therefore, BPIII appears to be closer in structure to the isotropic phase than to BPI or BPII. 

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