Liquid crystal, surface-induced ordering

Keywords Atomic force microscopy Liquid crystal Surface-induced ordering Presmectic ordering Landau-de Gennes... 

The first indication that the surface-induced order in the isotropic phase of confined liquid crystals can be detected by deuteron NMR was the spectrum of selectively deuterated 4 -n-pentyl-4-cyanobiphenyl (5CB) in the spherical cavities of a PDLC material. Far above Sfi C, where the bulk turns into the isotropic phase, the spectral line (a single one) of the PDLC was considerably broader than in the bulk at the same temperature. An even more definite and surprisingly clear evidence of surface-induced order was later obtained from... 

Thin surface-induced ordered liquid crystalline layers can also be observed by measuring their birefringence, which is a sort of ellipsometry method, used to analyze the polarization properties of directly transmitted waves. The method has been used in the pioneering linear optics experiments at solid-liquid crystal interfaces [16-24] and later in [25,26]. 

An isotropic liquid crystal is partially ordered close to the orienting solid surface. The surface induced perturbation of the order in an isotropic phase is localized in a thin layer with the thickness of the nematic correlation length Since the values of the nematic correlation length in the isotropic phase are typically only a few nanometers, ellipsometry represents a perfect tool for the study of interfacial order. 

The sensitivity of deuteron NMR to the molecular orientational order and to director field configurations turned out to be extremely useful in studies of liquid crystals confined into snbmicrometer pores. Moreover, the large surface-to-volume ratio of these composite systems render the interfacial and surface phenomena, induced by the liquid crystal-surface interactions, accessible even to an essentially integrative technique like NMR. Since the discovery of polymer dispersed liquid crystals (PDLCs) in 1986 [4], NMR of selectively deuterated liquid crystals was used to discriminate unambiguously among various director structures in cavities, resulting from an interplay between elastic forces, morphology and size of the cavity, and surface interactions. These structures include the escaped-radial, planar axial, planar-polar, and... 

A quite different scenario is observed, when the surfaces induce a large degree of nematic order. In this case, a very large spatial gradient of the order is created in the confined liquid, which is energetically unfavourable. At some separation, it is more favourable to phase-transform the isotropic liquid crystal into an ordered state, which is the capillary condensation of the nematic phase. 

Vilfan M, Apih T, Gregorovic A, Zalar B, Lahajnar G, Zumer S, Hinze G, Bohmer R, Althoff G (2001) Surface-induced order and diflusion in 5CB liquid crystal confined to porous glass. Magn Reson Imag 19 433-438... 

Abstract Surface-induced ordering of 4-n-octyl-4 -cyanobiphenyl (8CB) near the isotropic-nematic phase transition was investigated using temperature-controlled atomic force microscopy (AIM). The glass surfaces in contact with liquid crystal were modified by an adsorbed silane surfactant, a deposited 8CB mono-layer, or a deposited 8CB trilayer. 

In this paper, we explore the effect of different surface treatments upon surface-induced ordering of the liquid crystal 4-n-octyl-4 -cyanobiphenyl (8CB). Cyanobiphenyls have been intensively studied [11-13]... 

Surface-Induced Ordering of Liquid Crystal on Modified Surfaces... 

J. Tarczon and K. Miyano, Surface-induced ordering of a liquid crystal in the isotropic phase. 11. 4-methoxybenzylidene-4 -n-butylaniline [MBBA),y. Chem. Phys., 73,1994 [1980). 

The interaction of GB liquid crystals with surfaces, particularly with the aim of investigating surface-induced ordering and the details of anchoring and structuring [35], has beoi explored for both generic [14,36-40] and specific substrates like graphite [41]. 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

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