Thin smectic

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

Figure 10.26 Viscosity versus shear rate of cholesteryl myristate in a cone-and-plate rheometer as a function of shear rate. At high temperatures, T > 83 C, the sample is a low-viscosity, Newtonian isotropic liquid. At intermediate temperatures, 83 > T > 78°C, the sample is a shear-thinning cholesteric. At low temperatures the sample is a shear-thinning smectic. (From Sakamoto et al., reprinted with permission from Mol. Cryst. Liq. Cryst. 8 443, Copyright 1969, Gordon and reach Publishers.)...

B. Zappone, E. Lacaze, H. Hayeb, M. Goldmann, N. Boudet, P. Barois, M. Alba, Self-ordered arrays of linear defects and virtual singularities in thin smectic-A films. Soft Matter 7, 1161-1167 (2011)... 

J.-P. Michel, E. Lacaze, M. Alba, M. de Boissieu, M. Gailhanou, M. Goldmann, Optical gratings formed in thin smectic films frustrated on a single crystalline substrate. Phys. Rev. E 70, 011709 (2004)... 

Free suspended films of smectic liquid crystals can show a change in the crystalline order from the quasi-two-dimensional to the three-dimensional one [11]. Very thin smectic B films (number of monolayers n < 10) are two dimensional with the hexatic in-plane order, and with (low temperatures) or without (higher temperatures) a tilt of molecules. With increasing n (n > 20) the transition to various three-dimensional crystalline structures is observed. 

There are a number of reasons for the interest in free-standing smectic membranes. First, thin smectic membranes are models of two-dimensional fluids (SmA and SmC) and crystals (SmB). By varying the film thickness, one therefore can study the crossover from three- to two-dimensional behavior as well as the influence of surfaces on the morphology and the phase behavior. 

A very different model of tubules with tilt variations was developed by Selinger et al.132,186 Instead of thermal fluctuations, these authors consider the possibility of systematic modulations in the molecular tilt direction. The concept of systematic modulations in tubules is motivated by modulated structures in chiral liquid crystals. Bulk chiral liquid crystals form cholesteric phases, with a helical twist in the molecular director, and thin films of chiral smectic-C liquid crystals form striped phases, with periodic arrays of defect lines.176 To determine whether tubules can form analogous structures, these authors generalize the free-energy of Eq. (5) to consider the expression... 

Figure 8.16 Illustration of symmetry of Soto Bustamante-Blinov achiral antiferroelectric smectic LC with finite number of layers. Such systems can be studied using DRLM technique with thin freely suspended smectic films, (a) With even number of bilayers, film has local C2 symmetry, and therefore no net electric polarization, (b) With odd number of bilayers, film has local Cnv symmetry and is therefore polar, with net spontaneous electric polarization in plane of layers.

Note 4 A thin sample of a smectic C phase with the layers parallel to the sample surfaces gives schlieren textures with centers that have four brushes. However, a smectic C phase formed by odd-membered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes. 

Decher et al. [420] have developed the idea of using freely suspended smectic liquid crystals and have used it to form thin hlms on solid substrates. A thermotropic smectic liquid crystal is drawn across an aperture in a solid support and is capable of bridging the aperture (which can be up to 15 mm in diameter) with a him which can be between two and several hundred layers thick. The him consists of a smectic structure with the layers lying in the plane of the him. The him is formed a short distance above a solid substrate and the apparatus is constructed so that a difference of pressure between the two sides of the him can be used to force the him down in contact with the substrate. These authors have thus formed good quality hlms up to an area of about 1 cm2. In the work described the material used was ethyl-4 -n-octyloxybiphenyl-4-carboxylate. 

Little work seems to have been done on thin oriented layers of lyotropic liquid crystals although there is one recent report of preparation of such a layer of the lecithin-water lamellar phase (JO). As indicated by Brochard and de Gennes (II), theories of the hydrodynamics of thermotropic smectic materials can be adapted to describe oriented layers of lamellar liquid crystal in lyotropic systems. 

The rheology of lyotropic smectic phases under shear has some similarities, as well as some differences, with respect to that of thermotropic smectics discussed in Section 10.4. In particular, the shear viscosity rj shows a power-law shear-thinning region as a... 

Liquid crystals can be classified into lyotropic and thermotropic systems. The rheology of thermotropic liquid crystals is less documented, but in general, nematic liquid crystals were found to show Newtonian flow, whereas smectic and cholesteric liquid crystals demonstrated more or less pronounced plug flow. Plug flow is typical for non-Newtonian, shear thinning liquids. ... 

They observed that the sample with a polyion backbone formed a smectic A phase with a focal conic fan texture and a perpendicular structure. On the other hand, the material based on the neutral backbone formed a nematic phase with a schlieren texture. Once again, the presence of charges in the polymer severely influenced the polymorphism of the compounds. The authors showed the potential of these LC systems in the fields of photomemory, optical storage, and light drive display, especially because these amphiphilic polymers yield excellent azodense LC thin films [96]. 

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