Pseudo-liquid crystals

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

A. Ajdari, B. Duplantier, D. Hone, L. Peliti, and J. Prost, "Pseudo-Casimir effect in liquid-crystals"]. Phys. (Paris) II, 2, 487-501 (1992). 

Second, in addition to the above, the fact that many expanded porphyrins are highly colored makes their use as dyes an obvious possibihty. Here their planar nature makes them particularly attractive as chromophores for use in liquid crystals and optical data storage applications. Also, these properties could make them of interest as photo-sensors in various clinical or pseudo-clinical situations. For instance, the high affinity by certain sapphyrins for enveloped viruses and cholesterol rich liposomes suggests that expanded porphyrins could be used to detect and/or destroy a variety of unwanted biological targets, including arterial sclerotic plaque. 

On the one hand, these complete and rapid one-step solid-state reactions are due to the pseudo-liquid phase properties of solid-state HPAs. On the other hand, suitable crystal water of HPAs is important to the synthesis of POM nanoparticles. The HPA precursors with about twenty crystal water molecules are the best candidate for the preparation of uniform nanoparticles as observed in the TEM images (Figure 1). HPAs with excessive or lacking crystal water could not become the appropriate precursor. 

Figure 2.19 Pseudo-five fold diffraction pattern of a decagonal phase (65 C from the ten-fold axes Cu - K

The trapping of polycyclic aromatic molecules in n-heptane monocrystalline matrices, prepared by slow cooling, leads to oriented molecules inside the paraffinic crystal lattice, and it appears that the trapping sites for coronene, perylene, and pyrene are similar to those observed earlier for quickly frozen solutions of coronene and designated pseudo-liquid sites and substitutional sites .28 From the former studies it seems that aggregation does not play a dominant role. A nitrogen-pumped laser has been used to excite perylene in n-octane at 4.2 in order to investigate non-linear dependence of fluorescence on excitation intensity in a Shpol skii system. The approximately square dependence of one of the vibronic emission lines on the excitation intensity is consistent with super-radiant emission.80... 

With the octahedral compounds of the same type, Swager " studied the formation of columnar liquid crystals where the molecules in the mesophase can align themselves along the pseudo-C3 molecular axis (Figure 6.25). 

Experimental Evidence of Structural and Pseudo-Casimir Forces in Liquid Crystals... 

There are some indications that the pseudo-Casimir force in liquid crystals can be studied systematically with existing experimental techniques [69]. The lower limit for the separation of the substrate between which the force could be studied is roughly given by a molecular size where the system starts to behave as a continuum. The upper limit depends on the sensitivity of the force apparatus which is in the case of standard apparatuses about 10 pN [70]. This means that these apparatuses are precise enough to detect the pseudo-Casimir force at distances up to ss 40 nm [69]. 

The name pseudo-Casimir force for the fluctuation-induced interaction is due to the analogy with the Casimir effect [63] at T = 0 quetntum fluctuations of the electromagnetic field in a cavity yield a weak yet measurable attraction between the walls of the cavity. Because the force between the walls is determined by a derivative of the free energy of a system rather than by a derivative of its energy, a similar effect is expected above absolute zero where the interaction is not just due to quantum but also due to thermal fluctuations. In liquid crystals, the fluctuation-induced interaction is due to thermal fluctuations of the order parameter field instead of the electromagnetic field. 

The force measured in liquid crystals is a superposition of structmal, pseudo-Casimir, and non-structural forces. The non-structural forces include electrostatic and polarization forces, such as the van der Waals force. The behavior of non-structural forces is well-understood [68]. Recently, the improved... 

Polymer stabilized liquid crystals are formed when a small amount of monomer is dissolved in the liquid crystal solvent and photopolymerized in the liquid crystal phase. The resultant polymer network exhibits order, bearing an imprint of the LC template. After photopolymerization, these networks in turn can be used to align the liquid crystals. This aligning effect is a pseudo-bulk effect which is sometimes more effective than conventional surface alignment. Several characterization techniques... 

An example of recorded curve is shown in fig. 3a. Fig. 3b corresponds to the case of a small molecule liquid crystal (PAA). The intensity transmitted by the sample varies abruptly at the transition temperature the observed difference of transition temperature during cooling and heating is small. Contrary observations are made in a polymer (DDAQ) where there is a relatively large biphasic area. To describe the transition, we trace only the temperature at which the polymer becomes completely isotropic (shown by an arrow). The variations of this temperature can be compared with pseudo-transition temperature as defined by BrochardS and calculated JO,al though deviations are expected for large biphasic separation. 

The orthorhombic crystals have no conformational isomerism, the pseudo- and hexagonal orystals are condis crystals (see Sect. 5.5), the anisotropic liquid has been described as a liquid crystal. ... 

The length of the cellulose molecules, especially its relation to the length of the elementary crystals or fused fibrillar aggregations. Recent studies favour the idea that the cellulosic material is dispersed in a matrix of non-cellulosic material. The ordered cellulosic fibres are organized in a pseudo-liquid crystalline form. This type of model is consistent with many physical properties of these materials (Figure 12.6b). 

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