Extended nematic films

If the nematic liquid crystal is spin-coated on a silicon wafer, films whose average deposited thickness is larger than the UB or smaller than the LB are stable with an actual thickness equal to that average thickness. For intermediate average thickness, late time coexistence is observed under a microscope between two films with respective thicknesses UB and LB, just like during 3D phase separation at the common liquid-gas transition, or in binary mixtures (Fig. 5.7] (see also chapter 3]. 

More precisely, the average deposited thickness is measured at short times using ellipsometry, before phase separation, and then the ratio of the areas covered by each film at late times allows calculating their thickness, if mass conservation is assumed. The calculated values agree with the values of the UB and LB provided by ellipsometric profiles of microdroplets. Precise thickness measure- 

The situation in liquids has strong similarities with the one in solid substrates but is more complex. Anticipating on the experimental results reported later, we recover the forbidden range of thickness, with values of the LB and UB quite similar to the ones found on wafers, and the wetting transition. 

In contrast, metastability is the rule. This means that several thicknesses may coexist both above the UB and below the LB (see Fig. 5.8). Note that we use metastability in a sense that is not the common one in phase transitions no thickness is observed inside the forbidden range (LB, UB). But we observe simultaneously several thickness values, while they should correspond to different values of the free energy. 

We always assume in the analysis that true equilibrium is achieved between and only between the films with thicknesses LB and UB, respectively. Such a metastable behavior was previously observed with smectic Blms on water. Note that the smectic elasticity differs from the nematic one. 

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Let us now discuss more specifically experiments on extended nematic films, either spin-coated on silicon wafers or deposited on water or glycerol. Usually, a solution of the liquid cr5 stal in hexane is deposited, allowing a fast spreading on the substrate, followed by a rapid evaporation of hexane. The first experiments with 5CB on wafers reported before have been completed by a systematic investigation of the series of n-cyanobiphenyls on water and glycerol in the nematic range. The aim of the study is to define a common scheme of behavior for the nCB series on the three isotropic substrates, as a base for the theoretical analysis. Specific deviations from the common scheme must also be understood. 

We shall now outline the theory of electrohydrodynamic instabilities proposed by Helfrich and extended by Dubois-Violette, de Gennes and Parodi and Smith et We consider a nematic film of thickness d lying in the xy plane and subjected to an electric field along z. Let the initial unperturbed orientation of the director be along x, and let there also be a stabilizing magnetic field along the same direction. We consider a bend... 

The authors developed a unique form of i-glucan association, nematic ordered cellulose (NOC) that is molecularly ordered, yet noncrystalline. NOC has unique characteristics in particular, its surface properties provide with a function of tracks or scaffolds for regulated movements and fiber production of Acetobacter xylinum (=Gluconacetobacter xylinus), which produces cellulose ribbon-like nanofibers with 40-60 nm in width and moves due to the inverse force of the secretion of the fibers (Kondo et al. 2002). This review attempts to reveal the exclusive superstructure-property relationship in order to extend the usage of this nematic-ordered cellulose film as a functional template. In addition, this describes the other carbohydrate polymers with a variety of hierarchical nematic-ordered states at various scales, the so-called nano/micro hierarchical structures, which would allow development of new functional-ordered scaffolds. 

An interesting area, which involves chiral liquid crystal properties, is that of temperature sensors, used, for example, in the diagnosis of skin cancer, as well as in peripheral blood circulation problems. Another application in the medical field is represented by nematic elastomer films or fibers of liquid crystalline polymers with mesogene in the side chain, that can be used in the manufacture of muscle prosthesis [28]. To optimize their performance for different applications, the current knowledge on the relationship between the structure and properties of liquid crystals should be extended. In addition to further development of liquid crystals and their applications, the liquid crystal theories represent a sound basis for other areas of interest. For example, liquid crystals can be used as model compounds for the study of molecular interactions and of their effects on self-organization in supramolecular chemistry. 

A second example for the synthesis of a functional surface-attached polymer brush is the preparation of monolayers of a liquid-crystalline polymer (TCP) with mesogenic units in the sidechain [43]. Such a system could be of interest for the preparation of alignment layers for liquid-crystal displays (LCD) [44,45]. Alignment layers are key components for the production of LCDs [46-50]. As the orientation of the nematic director of the LC in contact with a surface is energetically degenerate, domains are formed in the LC layer that lower the contrast of the display. Thin films of polymers such as rubbed polyimide film ( alignment layers ) are frequently used to align the liquid-crystalline molecules in order to form extended monodomains. 

Fig. 8.8 Schematic of a crosslinked polymer network. The nematic reactive mesogens have photopolymerisable groups at each end of an extended aromatic core with semiconducting properties. The mesogens are deposited as a thin film by solution processing. They are polymerised and crosslinked either thermally or on irradiation with ultraviolet light...

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