Surface-Induced Smectic Ordering

Let the director of the nematic phase be perpendicular to a flat interface. Then we can anticipate two effects. First, the polar surface layer due to the break of symmetry n 7 —n appears. Second, due to a correlation in position of the centers of mass (parallel to the interface) in a surface adjacent to the layer the translational invariance is broken and the surface induces the smectic A order. 

In the framework of the Landau theory the smectic order decays with distance from the interface according to an exponential law 

FIGURE 3.4. Oscillations of the force acting between two mica cylinders separated by a homeotropically oriented nematic (5CB) layer [17]. 

This smectic ordering is experimentally observed at the nematic-solid and nematic-air interfaces. X-ray reflectivity measurements showed the existence of the smectic order at the free surface of 80CB (l.xv) both in the nematic and the isotropic phase [14]. Other nematic compounds also exhibit smectic ordering at the free surface [15]. However, the free surface seems not to be favorable for the formation of polar layers. As usual, in polar compounds like 8CB (l.iii), bilayers are formed at the surface, probably due to the favorable orientation of the alkyl chains outward from the liquid crystal. In such a case, polar heads form dimers much easier [16]. 

Solid substrates also induce the smectic order in nematic liquid crystals. This has been shown by direct measurements of an attraction force acting between two mica substrates separated by a nematic layer [17]. The force oscillates with a decreasing gap. Fig. 3.4, the period being equal to the molecular length. It means that smectic layers are pushed out of the gap one after the other (insert to Fig. 3.4). The total thickness of the smecticlike surface layer is about 100 A. The other direct confirmation of the surface-induced smectic order is X-ray measurement [18] which revealed both smectic A and smectic C ordering in surface nematic layers being in contact with an evaporated SiO layer. 

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Whereas in simple nematic liquid crystals like 5CB only a weak pre-nematic force is observable due to surface-induced nematic order, more complex forces are expected for materials that exhibit a tendency to form smectic layers. An example is 8CB, which has only a narrow ( 5 K) nematic phase, which is followed by a smectic-A phase. In the isotropic phase of 8CB there is therefore a kind of natural tendency to form smectic layers under appropriate external perturbations. 

In Fig. 3.18 we notice that the minima are not located on a zero background force, but rather on an attractive (negative) baseline. The attraction cannot be explained by the elastic model only and is to be attributed to the gradients in the smectic order induced by the surfaces. 

Kocevar K, Musevic I (2002) Surface-induced nematic and smectic order at a liquid-crystal-sUanated-glass interface observed by atomic force spectroscopy and Brewster angle ellipsometry. Phys Rev E 65 021703... 

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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